Thursday, October 31, 2024

Hiking and POTA (Parks On the Air) operation from Arches National Park (US-0004)

Figure 1:
Double-O Arch
Click on the image for a larger version.
Earlier this month I had the opportunity to spend nearly a week in Arches National Park in south eastern Utah.  As the name implies, there are a lot of natural arches scattered throughout the area, the result of erosion occurring over millenia, the semi-porous sandstone eventually yielding the the inexorable forces of wind and water.

These trips to red rock country are not new to me:  I've been visiting this part of the state for more than 30 years now, having spent more well over six months cumulative camping, hiking and backpacking in this austere land.  On most of these trips, I have been in the company of other amateur radio operators - and that was the case here, as well.

Compared to other national parks in Utah, Arches is fairly small - on a few 10s of miles long and narrower than this in width.  Compared to some places I go, it's a bit of a "tourist" park meaning that it's fairly crowded with comparatively few developed trails concentrated in a few areas.

Figure 2:
Fins and more fins, backgrounded by the La Sal mountains
as seen along the "primative" trail.
Click on the image for a larger version.
When going to such places, I tend to do about as much hiking as I can - but Arches is comparatively limited, but one of the longer trail systems is that associated with Landscape Arch and Double-O arch.  On this hike I took the "primative" trail, separating me from the madding crowds - a much longer route over occasionally rugged terrain, occasionally requiring a bit of scrambling up or down slick rock:  Just the way I like it!

Over the course of a few hours I made my way from the campground to Double-O Arch where I met the rest of my group who'd taken the other trail where we stayed for a while before splitting again and heading back.  Altogether, I managed about 10 miles (16km) or so by the time I got back to camp.

With temperatures in the mid 80s (about 30C) I set up my radio in the shade of my tent and shade and started operating.

* * *

Equipment:

Antennas:

The evening before, I had a bit of extra time around dinner and I took that opportunity to set up my portable antennas in the cooling evening air.  For this POTA operation, I eventually set up two antennas - the first one being my JPC-7 loaded dipole.

Figure 3:
Operating CW in the shade, on a portable table, using
a cast iron frying pan to keep the paddle in place.
Click on the image for a larger version.
I've discussed the JPC-7 antenna on this blog before (LINK) - and have used it for several POTA operations already with good results.  Since the last POTA operation I'd rewound the loading coils, replacing the original stainless steel wire with silver-plated copper to reduce the losses - I discuss the details about this HERE.  It's difficult to estimate how much improvement this change made, but it's likely in the general area of 3dB or so - only 1/2 "S" unit or so, but it's certainly worth a bit of hassle to improve efficiency on an already-small antenna.

A day after setting up the JPC-7, I also set up the JPC-12 vertical antenna (described here).  This antenna, too, has been refitted with a silver-plated loading coil as well:  With a few extra mast sections, a top-hat and resonant, elevated radials it also makes for an excellent portable antenna - albeit a bit more complicated to set up than the loaded vertical, particularly when changing bands.

Radio and power:

The radio - an older Yaesu FT-100 (with the CW filter from an FT-100D) which was powered by a 100 amp-hour Lithium-Iron Phosphate battery using a paddle from cwmorse.us - (link).  I've used this particular paddle ("Outdoor pocket double paddle with magnets") for several POTA activations and as before, I've used the same cast-iron fry pan for all of them to keep the paddle from sliding around - often ending up with a bit of soot on the side of my hand and wrist!

Figure 4:
The antennas - and solar panel.
There was no audible interference from the
now-modified solar controller.
Click on the image for a larger version.
Operating (mostly) on 20 meters I managed to make about 285 contacts - all but four of them CW with 277 of them counting as POTA contacts.  The operating position was almost as POTA as one gets:  Sitting in a chair, under a shade, surrounded by sand and red rock.

Mixing antennas with solar - with no QRM!:

 Figure 4 shows the "antenna farm".  In the foreground - just left of center - is the JPC-7 loaded dipole, using a studio tripod for support while in the background - to the right of center - can be seen the JPC-12 vertical with tophat.

Also in the foreground is a 200 watt solar panel - but you may be wondering if this would cause QRM (interference) from its controller:  The answer is NO - but this is only true because I've done previous work to add extra filtering to it.  Even with the antenna (particularly the JPC-7) right next to the solar panel with its controller, I could not hear any interference at all - but this is by design as I have taken steps to make it quiet, and you can read about the details to accomplish this HERE in a previous blog entry.

At this camp site there were two other PV systems in operation located some distance away from the antenna, but I could hear those.  For the one closest, I happened to have an FT240-43 toroid on hand and I was able to cram five turns (with connectors) of the cables from the two panels feeding it:  Predictably, this reduced the QRM somewhat (1-2 S-units) - but as noted in the blog entry noted above, ferrite alone will not likely solve such a QRM issue!

Figure 5:
Red and green auroras backgrounding the big dipper.
Click on the image for a larger version.
The "other" PV system - which was even further away - caused minimal interference so nothing was done about it - but since I'd used my only FT240-43 toroid, I wouldn't have been able do anything about it, anyway.

Red Rock + Aurora = More red!

As it happened, the sun did a bit of burping in the days leading up to and during this trip, the result being the repeated appearance of a visible aurora, the first appearing on October 7 when very visible red pillars appeared in the northern sky:  Scrambling to the top of a nearby bluff, we could see a bit of red and green in the sky along with the Big Dipper.

For the next few days we noticed something else:  On the first night, the sky was spectacularly dark - the Andromeda Galaxy being visible - but on the night of the first aurora and for a few nights thereafter it seemed as though we lost a lot of the "deepness" of the sky.  We also noticed that despite the lack of moonlight, we could see the surrounding landscape and make out large objects on the ground without needing additional light.

Figure 6:
Sky glow, lighting up the camp and environs.
Click on the image for a larger version.
We eventually realized that what we were seeing was sky glow.  In other words, the entire sky was glowing dimly:  Not bright enough to be perceived as color, but the cumulative glow of the entire sky was enough to illuminate the landscape in that odd way.

A few days later the aurora was clearly visible again - and that's when the photo in Figure 6 was taken, showing a bit of red behind the clouds to the north and some green glow on the northern horizon.


* * * * *

This page stolen from ka7oei.blogspot.com

[END]


Monday, September 30, 2024

Neon bar-graph VSWR/Power meter using the ИН-13 (a.k.a "IN-13") "Nixie" - Part 2

Figure 1:
Power/VSWR meter using ИН-13 neon bar-graph
indicators.
Click on the image for a larger version
In Part 1 (link) I laid out the requirements of the ИН-13-based neon bar-graph VSWR/power meter.  Admittedly, this is a "buy cool, old tech and figure out what project might use it" scenario - but having one tube always showing the forward power and the other tube showing either reverse power of calculated VSWR was the goal.

In the previous installment we talked about how to generate the high voltage (130 volts or so) for the bar-graph neons, the means to drive precise amounts of current through the tubes using precision current sink circuits, and the "Tandem" coupler to detect forward and reflected power.
 
Mounting the tubes
 
Figure 2:
ИН-13 tubes in the raw.
It is up to the constructor to determine how best to mount
these tubes - and how to connect them to the circuit.
Figure 3 shows how flexible wires were attached as the
wires on the tubes themselves are very easily broken!
Click on the image for a larger version.
In looking at Figure 1 you can see that the ИН-13 tubes are mounted to pieces of clear acrylic, but a quick look at Figure 2 shows that they don't really have a means of mounting, leaving the method to the imagination of the user.

In preparing the tubes for mounting I trimmed the wire leads and soldered flexible wires to them, covering them with "hot melt" (thermoset) adhesive to passivate the connection, making them relatively durable:  The original wires will NOT tolerate much flexing at all and are likely to break off right at the glass "pinch" - which would make the tube useless.   Figure 3 shows how the leads were encapsulated - the thermoset adhesive being tinted with a permanent marker - mainly to add a bit of color.

Laser-cut sheets and markings
Figure 3:
Close-up of the "hot-glue" covered wire
attachments for the ИН-13 tubes.  Also visible
are the black wire loops holding them in place
and the laser-edged markings on the acrylic.
Click on the image for a larger version.

In looking at Figure 1 and 3 you will also notice that there are scales indicating the function and showing scale graduations and the associated numerical values.  I'm fortunate to have a friend (also an amateur radio operator) who has a high-power laser cutter and it was easy to lay out the precise dimensions of the acrylic sheets and also have it cut the holes for the mounting screws in the corners as well.

While it takes a bit of laser power to cut the sheets, a far lower power setting will ablate the surface, yielding a result not unlike surface engraving and when lit from the edges, these ablations will light up with the rest of the sheet remaining pretty dark:  A total of four sheets were cut and "engraved" in this way:  The front sheet for "VSWR" and its markings, the middle sheet for "Reverse Power" and the rear acrylic sheet for "Forward Power".  It was possible to arrange the lettering so that only "VSWR" and "Reverse Power" were atop each other but in subdued light - and with a bit of darkened plastic in front of the display - the markings on the un-lit sheet are practically invisible.  The fourth sheet mentioned was left blank, being the protective cover. 

Edge lighting

Edge-lit displays go back decades - and the idea likely goes back centuries where it was observed that imperfections in glass (later, plastic) would be visible if the substrate was illuminated from the edge.  Since the early-mid 20th century, one could find a number of edge-lit indicators - usually in some sort of test equipment of industrial displays - but they occasionally showed up in the consumer market - usually acrylic or similar with the markings engraved with a rotary tool or - as may be done nowadays, a laser.

While incandescent lamps would have been used in the past, LEDs are the obvious choice these days and for this I selected some "high brightness" LEDs to light the edges of the engraved acrylic sheets.  For the "Forward Power" sheet - which would be that which was always illuminated in use - I chose white while using Green for VSWR and Blue for Reverse Power.  I'd considered Yellow and Red, but discarded the former as it might appear too much light the white under some conditions and past experience has reminded me that - particularly in a dark room - the human eye can't see or focus on fine detail on red objects very easily.

Figure 4:
Six LEDs are epoxied to the edge to evenly light the laser-
etched markings in the acrylic sheet.  The faces of the LEDs
were filed flat to facilitate bonding and improve efficiency.
Click on the image for a larger version.

Figure 4 shows some details as to how the edge lighting is accomplished.  Six equally-spaced LEDs were epoxied to the bottom edge of the display, arranged to be nearly the width of the engraved text.  In writing this entry I observed that photographing edge-lit displays such as this is nearly impossible owing to the variations in illumination (e.g. it's difficult to take pictures of very bright objects in the dark!) but the effect is very even as viewed by the human eye.

The six LEDs were connected as two series strings of three LEDs:  As each LED requires about three volts - and I have only a 12 volt power source - doing so requires only a bit more than nine volts to power the LED arrays.  As the green and white LEDs are also silicon nitride based as well, they take similar voltages.

Not readily apparent from Figure 4 is the fact that the LEDs were modified slightly.  As we are trying to interface a standard T1-3/4 LED to the flat edge of a plastic sheet, it's apparent that the rounded, focused lens makes this physically difficult.  To mitigate this, the top of the LED was flattened with a file and the clear epoxy was removed to just above the light emitting die.  The result of this is that a flat surface is mated to another flat surface for a physically stronger bond and a more efficient coupling of light and a bit of the LED's original directivity in the form of the "lens" is removed from the equation. 

Just prior to mounting the acrylic sheets in the "stack up" some black electrical tape was applied.  This tape was put on both sides of the sheet, extending just above the bottom edge, to reduce the glare from the LEDs and to minimize the possibility of this light coupling into the adjacent sheet.

Mounting the tubes and sheets

As can be seen from Figure 3, the tubes are held in place with loop of solid-core insulated wire - the holes mounting them also "drilled" with the laser.  The "stack-up" of acrylic sheets and the tubes - both of which were mounted on "VSWR" acrylic layer - is held together using 6-32 brass machine screws and spacers with a piece of 1/4" (5.2mm) plywood covered with black felt for the back to provide contrast.

The box and base

As can be seen from figure 1, the entire unit is in a wooden base:  The same friend with the laser cutter also had some scraps of red oak and a simple base was made, decorated with an ogee cut around the perimeter with the router while atop it a simple box with mitered corners - facing at a slight upward angle - in which the display and electronics reside.  On the base itself are two buttons:  One switches between VSWR and Reverse Power and the other between peak and average readings.  These switches have other functions as well, which will be discussed in the third installment when the final circuit and internal workings of the software is discussed.

* * * * *

This page stolen from ka7oei.blogspot.com

[END]







Wednesday, August 28, 2024

Neon bar-graph VSWR/Power meter using the ИН-13 (a.k.a "IN-13") "Nixie" - Part 1

Figure 1:
Power/VSWR meter using
ИН-13 (a.k.a. "IN-13") neon bar-graph indicators.
Click on the image for a larger version.
(Be sure to read Part 2 of this article - LINK).
 
Several years ago I bought some Soviet-era neon bar-graph displays - mainly because I thought that they looked cool, but I didn't have any ideas for a specific project.  
 
After mulling over possible uses for these things for a year or so - trying to think of something other than the usual audio VU meter or thermometer - I decided to construct a visual watt/VSWR indicator for amateur radio HF use.
 
* * *
 
I actually bought two different types of these bar-graph tubes:
  • The ИН-9 (a.k.a. "IN-9").  This tube is 5.5" (140mm) long and 0.39" (10mm) diameter.  It has two leads and the segments light up sequentially - starting from the end with the wires - as the current increases.
  • The ИН-13 (a.k.a "IN-13").  This neon bar-graph tube is about 6.3" (160mm) long and 0.39" (10mm) diameter.  Like the ИН-9 its segments light up sequentially with increasing current but it has a third lead - the "auxiliary cathode" - that is tied to the negative supply lead via a 220k resistor that provides a "sustain" current to make it work more reliably at lower currents.
Note:  It would be improper to refer to these as "Nixies" as that term refers to a specific type of numeric display - which these are not.  Despite this, the term is often applied - likely for "marketing" purposes to get more hits on search engines.

Figure 2:
A pair of ИН-13 neon indicator tubes.  These tubes are
slightly longer than than the
ИН-9 tubes and have three leads
Click on the image for a larger version.
For a device that is intended to indicate specific measurements, it's important that it is consistent, and for these neon indicators, that means that we want the bar graph to "deflect" the same amount anytime the same amount of current is applied to it.  In perusing the specifications of both the 
ИН-9 and  ИН-13 it appeared that the  ИН-13 would be more suitable for our purposes.

This project would require two tubes:
  • Forward power indicator.  This would always indicate the forward RF power as that was that's something that is useful to know at any time during transmitting.
  • Reverse power/VSWR.  This second tube would switchable between reverse power, using the same scale as the forward power display, and VSWR - a measurement of the ratio between forward and reverse power and a useful indicator of the state of the match to the antenna/feedline.
Driving the tubes
  
"Because physics", gas discharge tubes require quite a bit of voltage to "strike" (e.g. light up) and these particular tubes need for their operation about 140 volts - a "modestly high" voltage at low current - only a few milliamps (less than 5) per tube, peak.

Figure 3:
Test circuit to determine the suitability of various inductors and transistors
and to determine reasonable drive frequencies.  Diode "D" is a high-speed,
high-voltage diode, "R" can be two 10k 1 watt resistors in parallel and
"Q" is a power FET with suitably high voltage ratings (>=200 Volts)
and a gate turn-on threshold in the 2-3 volt range so that it is suitable
to be driven by 5 volt logic.  V+ is from a DC power supply that is
variable from at least 5 volts to 10 volts.  The square wave drive, from a
function generator, was set to output a 0-5 volt waveform to
make certain that the chosen FET could be properly driven by a 5 volt
logic-level signal from the PIC as evidenced by it not getting perceptibly
warm during operation.
Generating high voltage from a low is one of the aspects that I tackled in a previous project on this blog when I built a high voltage power supply for the Zenith Transoceanic:  You can read about that here - A microcontroller-based A/B Battery replacement for the Zenith TransOceanic H-500 radio, with filament regulation - link.
 
The method used for this project and the aforementioned Zenith radio is  boost-type converter as depicted in Figure 3.  The switching frequency must be pretty high -  typically in the 5-30 kHz range if one wishes to keep the inductance and physical size of that inductor reasonably small.

As in the case of the Zenith Transoceanic project, I used the PWM output of the microcontroller - a PIC - to drive the voltage converter with a frequency in the range of 20-50 kHz.  For our needs - generating about 140 volts at, say, 15 milliamps maximum, I knew (from experience) that a 220uH choke would be appropriate.  Figure 4, below, shows the as-built boost circuit.
Figure 4:
The voltage boost converter section showing the transistor/inductor, rectification/filtering and
voltage divider circuitry.

Description:
 
Q301 is a high-voltage (>=200 volt) N-channel MOSFET - this one being pulled from a junked PC power supply (the particular device isn't critical) which is driven by a square wave on the "HV_PWM" line from the microcontroller:  R301, the 10k resistor, keeps the transistor in the "off" state when the controller isn't actively driving it (e.g. start-up).  L301, a 220uH inductor, provides the conversion:  When Q301 is on, the bottom end is shorted to ground causing a magnetic field to build up and when Q301 is turned off, this field collapses, dumping the resulting voltage through D301, which is a "fast" high voltage diode designed for switching supplies - a 1N4000 series diode would not be a good choice in this application as it's quite "slow".
 
R304, a 33k resistor, is used to provide a minimum load of the power supply, pulling about 4.25 mA at 140 volts:  This "ballast" improves the ability of the supply to be regulated as the difference between "no load" (the neon bar-graphs energized, but with no "deflection") and full load (all segments of the tubes illuminated) is less than 4:1.  The resistive divider of R302 and R303 is used to provide a sample of the output voltage to the microcontroller, yielding about 2.93 volts when the output is at 140 volts.  The reader will, by now, likely have realized that I could have used R304 as part of the voltage divider - but since the value of this resistor was determined during testing, I didn't bother removing R302/R303 when I was done:  Anyway, resistors are cheap!
 
Setting the current:
 
Having the 140 volt supply is only the first part of the challenge:  As these tubes use current to set the "deflection" (e.g. number of segments) we need to be able to precisely set this parameter - independent of the voltage - to indicate a value with any reasonable accuracy.  For this we'll use a "current sink".
 
Figure 5:
The precision current sinks that drive the neon tubes precisely based on PWM-derived voltage.
Click on the image for a larger version.
 
Figure 5, above, shows the driving circuits for the two tubes using the "precision current sink".  Taking the top diagram as our example, we see that the inverting input of the op-amp (U401c) is connected to the junction of the emitter of Q401 and resistor R406.  As is the wont of an op amp, the output will be driven high or low as needed to try to make the voltage (from the microcontroller) at pin 10 match that of pin 9 - in this case, based on feedback from the sense resistor, R406.

What this means is that as the transistor (Q401) is turned on, current will flow from the tube, through it and into R406 meaning that the voltage across R406 is proportional to the voltage on pin 10.  It should be noted that current through R406 will include the current into the base - but this can be ignored as it will be only a tiny fraction (a few percent at most) of the total current.  It's worth noting that this circuit is insensitive to the voltage - at least as long as such current can be sunk - making it ideal for driving a device like the ИН-13 (or ИН-9) in which its intended operation is dependent on the current rather than the operating voltage.

At this point it's worth noting that the driving voltages from the microcontroller ("FWD_PWM" and "REV_PWM") are not plain DC voltages, but rather from the 10 bit PWM outputs of the microcontroller.  The use of a 10k resistor and 100nF (0.1uF) capacitors (R405 and C406, respectively) "smooth" the square-ish wave PWM into DC.
 
Q401 and Q402 were, again, random transistors that I found in scrapped power supplies, but since there's at least 70 volts drop across the tube, about any NPN transistor rated to withstand at least 80 volts should suffice.  It's also worth noting the presence of R407, which provides the "sustain" current on the "auxiliary" cathode.
Figure 6:
An exterior view of the tandem coupler module.
Visible is the top shield and the three feedthrough
capacitors used to pass voltage and block RF.
Click on the image for a larger version.

RF sensing

For sensing forward and reflected power I decided to use an external "sensing head" that was connected inline with the radio, on the "tuner" side of the feedline.  

For sensing power in both directions I chose the so-called "Tandem" coupler which consists of a through-line sampler in which a short length of coaxial cable carrying the transmit power (T1 in the diagram of Figure 7) passes through a toroidal core - using some of the original cable's braid grounded at just one end as a Faraday shield.  An identical transformer (T2) is connected across the first (T1) for symmetry.

When carefully constructed this arrangement has quite good intrinsic directivity and a wide frequency range.  Figure 6 shows the diagram of this section.

Figure 7:
Schematic diagram of the "Tadem" coupler.  A bidirectional coupler sends power to
separate AD8307 logarithmic amplifiers - one for forward and the other for reverse.
The outputs, expressed in "volts/dB" are sent to the microcontroller.
Click on the image for a larger version.

The RF sensing outputs of the second tandem coupler (T2) then goes through resistive voltage dividers (R606/R607 for the reverse sample and R603/604 for the forward sample) to a pair of Analog Devices AD8307 logarithmic amplifiers - one for forward power and the other for reverse - to provide a DC voltage that is logarithmically proportional to the detected RF power.  This voltage is then coupled through series resistors (for both RF and DC protection) R605/R608 and to the outside world using feedthrough capacitors.

The use of a logarithmic amplifier precludes the need to have range switching on power meter as RF energy from well below a watt to well over 2000 watts can be represented with only a few volts swing.  Looking carefully at Figure 6 one can see a label that notes that the response of the AD8307 is about 25 millivolts per dB - and this applies across the entire power range of a few hundred milliwatts to 2000 watts.

All of this circuitry is mounted in a box constructed of circuit board material and connected to the display unit with an umbilical cable that conveys power and ground along with the voltages that indicates forward and reflected power.

Figure 8:
An inside view of the Tandem Match (sense unit) showing
the coupling lines, internal shielding and AD8307 boards.
Click on the image for a larger version.
Figure 8 shows the as-built "sense unit" and the two coaxial sense lines are clearly visible.  As can be seen, the "main line" coupler is physically separated and shielded from the secondary sense line, using PTFE ("Teflon") feedthrough lines to pass the signals.

The AD8307 detectors themselves can be seen at the left and right edges of the lower half of the unit, built on small pieces of perfboard.  All signals - including the 12 volt power and the DC voltages of the output pass through 4000pF feedthrough capacitors to prevent both ingress and egress of RF energy which could find its way into the '8307 detectors and skew readings.

* * * * *

In a future posting (Part 2) we'll talk about the final design and integration of this project.


This page stolen from ka7oei.blogspot.com

[END]


Wednesday, July 31, 2024

Rewinding the stainless steel coils with silver-plated copper wire on the JPC-7 and JPC-12 antennas

Portable antennas (verticals, loaded dipoles) typically use coils on the lower HF bands to make them electrically "larger" to alow them to be resonated at frequencies well below their physical size - but what about losses in those coils?

While it's "traditional" to use copper wire wire for these coils, there are a number of modern offerings that use stainless steel - and both types have their cheerleaders and detractors, so what's the deal?

Figure 1:
The JPC-12 vertical in the field.

Note:  This post refers to previous entries on this blog about the JPC-7 and JPC-12 antennas that are relevant to this discussion, namely:

  • JPC-7 loaded dipole antenna - link.
  • JPC-12 portable vertical antenna - link.

While some details in this article are specific to these antennas, the general observations may be applied to any HF antenna using loading coils.  I have not (yet?) done A/B field tests with antennas using different (stainless vs silver plated/copper) coils and/or simulations - perhaps a topic for a future blog entry?

* * * * *

In previous posts I have discussed the JPC-12 vertical and the JPC-7 dipole:  To make either antennas usable at frequencies lower than their natural resonance, inductance is required (the "loaded" part) to achieve resonance at the desired frequency - and for their lowest operating frequency - 40 meters - it takes a fair bit of "loading", indeed.

For this, the JPC-7 dipole, which has a "coil-less" resonance of around 22 MHz, has two coils with adjustable taps - one for each element - a slider being used to adjust the amount of inductance:  Higher inductance = lower frequency.

The JPC-12 vertical - made by the same folks - unsurprisingly uses the exact same coil as the JPC-7 - and for the same reason:  To add inductance to make the electrically-short element - a radiator of approximately 150" (381cm) total length (resonant around 18 MHz without any added inductance and using the originally-supplied components) offer a semblance of a match on lower bands.

Having the coil in common, they also share the same trait:  Loading coils wound with stainless steel - and since, when running on a lower band like 40 meters - all of these coils run quite warm at nominal transmitter power (100 watts or so) there are definitely power losses in the coil - but how bad is it?

Wanting to answer this question, I ordered an extra coil from the seller from which I'd bought my JPC-7 and JPC-12 antennas and with that - and the three that came with the two antennas originally - I now had four coils - enough to do direct A/B comparisons on both antennas when I rewound two of them with silver-plated wire.

Why stainless?

The coils originally supplied with the JPC-7 and JPC-12 are wound with 1mm diameter (18 AWG) stainless-steel wire.  Fortunately, an austenitic (non-magnetic, as checked with a neodymium magnet) type of stainless steel is used:  If this wire been magnetic at all things would be much worse in terms of loss.  While the 1mm diameter stainless steel wire is very rugged physically, the fact that it is stainless steel means that its resistance is quite high compared to copper - in this case the end-to-end DC resistance is about 4 ohms, but the RF resistance, taking the "skin effect" into account, is likely to be very much higher.

Using Owen Duffy's online skin effect calculator (link) and assuming 1mm diameter, 316 Stainless, the 4 ohms of DC resistance translate as follows to RF resistance including skin effect:

  • 3.5 MHz = 5.2 ohms
  • 7 MHz = 7.2 ohms
  • 14 MHz = 9.6 ohms
  • 28 MHz = 13.6 ohms
These values would be for the entire coil, but since one uses slightly less than the full number of turns of the coil to resonate at 40 meters, the losses should be lower - but the message is clear:  The less of the coil that you need to use, the lower the loss.   The total length of 1mm wire is estimated to be about 180 inches (457cm) and  by comparison, copper wire of this same diameter and length would have a DC resistance of about 0.1 ohm - or, according to Owen Duffy's calculator, a skin effective resistance of 2 ohms at 28 MHz.

Why stainless steel, then?  Obviously, stainless steel won't oxidize/corrode like many metals - and it may be that in quantity, stainless steel wire is less expensive than silver plated/copper, but in this case I believe that there's another reason.  Other manufacturers of portable antennas (Wolf River, for example) advertise the use of stainless steel for their coils as well, extolling the virtues of the material in regards to its inability to corrode - but I'd be surprised if such corrosion is likely to be the main reason for a hypothetical copper coil's losses in an electrically-short antenna that would make it worse than stainless.

I suspect that the "advantage" of a stainless steel coil is, in fact, related to the fact that it is lossy.  As portable antennas - when used on the lower HF bands - are necessarily smaller than their full-sized counterparts, their radiation resistance will be commensurately lower and this means that the feedpoint resistance may be lower as well when fed with simple matching schemes such as a series coil.

What this means is that rather than somewhere "around" 50 ohms, the feedpoint impedance when using a very low-loss coil may be much lower, resulting in an "unacceptable" VSWR (e.g. >2:1) at resonance:  While this would actually imply greater efficiency due to lower loss, it's "inconvenient" to the user.  While a more versatile means of matching the antenna is possible (multiple coil/capacitors such as a simple antenna tuner or the use of an autotransformer) this complicates construction, operation and can increase cost.

As implied earlier, another method of dealing with low feedpoint impedances is to add series resistance to raise it to something closer to 50 ohms to make radios (and their operators) "happy" - but an ohmic resistance in the signal path (say, the use of stainless steel) means power loss, and power loss means heat!

How hot is it?

Figure 2:
The original loading coil (lower) wound with stainless wire as
seen with a thermal infrared camera.  After 60 seconds at 75
watts (on 40 meters) the coil temperature rose by 110F (61C)
from the ambient 53F (12C) to about 166F (74F)!
Click on the image for a larger version.

I've operated both the JPC-7 and JPC-12 antenna a number of times in the field on the "lower" bands of 40 and 30 meters at 100 watts, using both CW and SSB, and observed that in each case, the coil gets "hot".  As the coil forms are (apparently) molded nylon, this is nowhere near the likely softening point of more than 300F (150C) - and being open to the air to allow convective cooling, and using a mode where the duty cycle is intermittent certainly helps prevent a "meltdown".  (Compared this to PVC - which has a softening temperature in the area of 140-180F or 60-80C)

As a test, I put both the original stainless steel and the rewound silver-plated coils in series on the JPC-12 vertical, putting a jumper across the coil not under test.  I then transmitted 75 watts into the JPC-12 vertical for 60 seconds and measured the temperature of the coil with an infrared thermometer and thermal camera, noting a temperature rise of about  110F (61C) - still not hot enough to risk melting the coil form, but certainly enough to dissuade one from running a 100% continuous mode like SSTV, RTTY or other digital modes on a hot day!  (Note:  On a hot day a temperature rise of 110F/61C may well be enough to soften a PVC coil form.)

The picture in Figure 2 - taken with a thermal infrared camera - shows the heat produced when testing with the JPC-12 vertical.  (Note:  During this test I swapped positions of the two coils to see if there was much difference in the current/heat of the stainless coil owing to differences in current distribution, but as expected, there was not.)  Similar results were observed when operating SSB and CW on the JPC-7 loaded dipole.

At this point I should make something clear:  The reader should not presume that the use of a stainless steel coil is going to result in an antenna that doesn't work, but rather it implies a degree of loss of efficiency.  As I've made many contacts with both the JPC-7 and JPC-12 in their original form, I know that it's perfectly capable of usable performance - but how much better would it be if we were to address coil losses?

Also, once I had seen the loss in the coil, I couldn't "un-see" it and I had to do something about it.

Choice of wire

In order to minimize losses in an electrically-small antenna it is important to reduce resistive losses and the loading coil and reducing the generation of heat produced by it is a good place to start - and copper wire is an obvious choice.  Knowing that the wire used is 1mm diameter - about 18 AWG - there were a lot of choices:  I had some enameled 18 AWG wire already on-hand and I could easily have obtained some tinned 18 AWG "buss" wire as well.  Finding bare copper wire was a bit more difficult, but since we need only make contact on the ends and along the slider, there's no reason for the entire coil to be bare and thus be subject to oxidization:  If I needed to do so, I could have wound the coil with enameled wire and then selectively remove the insulation along the path of the inductor's slider with fine sandpaper.

On a hunch, I did a search and quickly found on Amazon some 1mm (18 AWG) "Silver plated" copper wire of the same diameter described as being used for jewelry - a small spool costing about US$15 with more than enough wire to re-do three of these coils. Footnote 1

Figure 3:
The coil - still with the stainless steel wire.  On the left end of
the slider (the "top") of the coil can be seen the insulator.
Prior to disassembly move the slider to the end opposite the
insulator (maximum inductance) as shown.  When removing
or installing the Allen screw, keep a firm grip on the end with
the insulator to prevent it from rotating and damaging the
insulator itself or the end of the rod that protrudes into it.
Click on the image for a larger version.
The use of silver-plated wire is traditional in RF devices as it has the advantage over copper wire in that as it oxidizes, the result (e.g. silver tarnish) is still a conductive substance, much better than copper oxide - and compared to bare copper it is less (chemically) reactive overall - plus the coil looks very nice!

Rewinding the coil:

The coil form itself - with molded grooves - is quite rugged and lends itself very well to being rewound by hand.  Using a silver-colored "Sharpie" I noted where the original coil's windings started and ended.  I would also recommend taking a photo of it - particularly if you are rewinding the coil of a JPC-12 vertical and do not have a second coil as a comparison.

It is also important to note that one end of the slider is insulated to prevent the shorting the unused turns of the coil itself - something that would surely reduce "Q" and overall efficiency:  It is important to reinstall the slider assembly in the same orientation as before to put the insulated end of the slider rod on the "top" (e.g. the side closest to the top of the vertical or end of the dipole).

When rewinding, first move the slider to the end farthest away from the end with insulator on the rod (e.g. the "bottom" of the coil, with the stud protruding) and cover the spring contact with a bit of tape to keep it with the slider body:  This moves the slider - and the contact spring - well away from the end of the wire that we are going to remove first.  Using an Allen wrench, carefully remove the screw holding the end of the slider bar with the insulator (e.g. the part at the top of the coil, with the female threads):  The end of the wire is tucked under the supporting post and the screw itself goes into the brass slug at the center of the coil with the M10 threads used to assemble the rest of the antenna.  Keep tension on the hardware with a finger as you undo this to minimize the possibility of it being launched across the room.

Figure 4:
This shows the end of the new wire looped around the screw
and the post tightened down to hold it in place as it is wound.
A blade screwdriver is used to push the wire into the groove
below the slider boar to keep it from jumping out of the slot.
Be sure to start the wire in the same place as the original coil.
Click on the image for a larger version.
At some point, the coil of stainless steel wire will unwind itself rather forcefully when it slips out from under the screw (it may be a good idea to wear glasses) as it is under a fair bit of spring tension:  Even if you are prepared for this to happen, it can be startling!  At this point be sure that the contact spring is still on the slider block:  If it is not, look for and find it now!

With the tension released, remove the other end of the slider bar.  At this point, carefully remove the slider bar from the insulated end so that you have just the support post and set the rest of it aside.  At this point you'll have a loose coil of stainless wire to set aside.

Take the end of the new wire and using a pair of needle-nose pliers, bend a loop to go around the screw for the support post and using (just) the support post that was insulated for the slider, secure it in place, under the post.  Lay the wire in the groove and at the point where you marked the coil to begin, lay the wire in that groove and then push the wire into the shallow slot above which the slider moves to hold it in place.

Figure 5:
As the wire is wound, keep pressure on the wire and coil form
with a thumb while rotating the form itself, forcing the wire to
drop into the molded slots.  Continue winding until you get
to where you had previously marked the end of the original
coil - but there's no harm if you add one extra turn.
Click on the image for a larger version.
Keeping the wire under tension - and using a thumb as necessary to hold that tension and push it onto the form - tightly wind the wire onto the form, making sure that it drops into the wire slots.  When you get to where you marked the end of the coil (you can go one extra turn if you like!) push the wire into the slot again (to help hold it in place) and - leaving enough extra to go around the screw on the bottom of the coil - trim it off.  Before putting a loop in the end of the wire to go around the screw, again use a blade screwdriver to push it into the groove to help hold it into place.

At this point I temporarily wrap a the loose end of the coil with a bit of electrical tape to keep it from unraveling while I loosen the post at the top of the coil and align it carefully so that I can plug the slider bar back in and re-mount it and the other post at the bottom of the coil, torquing the screws firmly and being careful to prevent the post with the insulator from twisting as this is done.

Figure 6:
The finishing end of the coil with the wire looped under the
slider rod support and tightened down.  In this picture you
can see how the wire has been pushed into the groove, under
the slider.  To the left of the end of the wire can be seen the
blob of adhesive used to lock the end of the coil into place.
Click on the image for a larger version.

Now, the coil has been successfully re-wound.  While it may not be strictly necessary, I put a dab of "Shoe Goo" - a thick rubber adhesive - on the top and bottom 2-3 turns of the coil near where the wire drops into the slot and connects to the post to "glue" it into place, making sure that it doesn't jump out of its slot.  If you don't have "Shoe Goo" or something similar, some RTV ("Silicone") can work as can epoxy - but cyanoacrylate and polyurethane glues (e.g. "Super" and "Gorilla" glue, respectively) may not work very well - and "hot melt glue" are definitely not recommended as either will likely break loose their bonds across a wide temperature range and changing mechanical stress. 

The trick here is to bridge several turns of wire with the adhesive to lock them into place together as much as adhere them to the coil form.

Results

Figure 7:
The coil rewound with silver-plated wire (upper), under the
marker.  As can be seen, the temperature rose by about 3F
(less than 2C) above the ambient temperature of 53F (12C)
after 60 seconds of key-down on 40 meters at 75 watts.
Click on the image for a larger version.
As expected, the use of lower-loss wire for the coil results in a dramatic reduction of generated heat which no doubt corresponds with an improvement in overall antenna efficiency - The "after" picture (Figure 7) of the coil using the thermal camera after 60 seconds of transmission on 40 meters with 75 watts shows the difference.  As in Figure 2, the original stainless steel coil is on the bottom, but it is the one that is jumpered, putting all of the RF energy into the upper (silver-plated) coil, instead.

Touching the coil immediately after the 60 second key-down, the loss-related heating of the coil wound with silver-plated wire was barely perceptible - a far cry from the original stainless-steel wound coil that was  "hot"!

Electrical comparison of the stainless and silver-plated coils

For capacitors and inductors, one measurement of their departure from the ideal is their "Q" (e.g. "Quality Factor") and for inductors, the majority of this is likely to be the radio of the inductive reactance of the coil (XL) to its ohmic resistance.  I decided to measure the unloaded "Q" (Qu) of the original stainless steel loading coil and the rewound silver-plated coil.  To do this I used a NanoVNA and the method described in W7ZOI's article "The Two Faces of Q" (link) under the section called "Measuring Resonator Q":  I used both methods (#1 using parallel L/C and #2 with L/C in series) to determine the "Q".

Using method #1, for the "Cc " capacitors I used two 1pF NP0 capacitors in series each (0.5pF) which resulted in a 35-45dB through loss at resonance.  I put a high-quality 27pF silver mica capacitor in parallel with the coil under test and measured the -3dB response of the resonance curve.  In this test I set the variable inductor to the mark indicating tuning for 40 meters (around 22 uH) which, with the 27pF capacitor, yielded a resonance in the area of 6.6 MHz for each of the two coils being tested

Assuming that the Q of the series silver mica capacitor (Co) is 1000 (a mediocre value - it's probably a bit higher) the results were:

  • Original stainless steel coil unloaded Qu:  47
  • Rewound coil (silver-plated wire) unloaded Qu: 199

I then used method #2 (with L/C in series) and got:

  • Original stainless steel coil unloaded Qu:  47
  • Rewound coil (silver-plated wire) unloaded Qu: 221

At the risk of being accused of "cherry picking" my results, I'll note that for high "Q" values and where the value of Co is quite small, method #1 is less forgiving in terms of variances and minor losses in the test fixture, so we'll use the value from method #2.  The reader should also note that with a higher Q, deficiencies in the test measurement and effects of the coil itself will result in lower than actual Qu (e.g. you will not get an erroneously higher value of Q) so it is likely that even the higher reading from method #2 on the silver-plated coil is, itself, a bit conservative.

Note:  During testing I observed that just laying the coil on my wooden workbench lowered the Q of the silver-plated coil significantly (15-20%) so all readings were taken with both coils held about 12" (25cm) above it.  I think that there is likely some effect of free-space capacitance that is reducing the reading so I suspect that the "actual" Qu of the silver-plated coil is higher, still.  This same effect was extremely small with the stainless steel coil, further indicative of its lower Qu.  

Comment:  It's worth mentioning that with higher "Q" coils, the physical aspects of the coil itself - namely the ratio of the length versus diameter, spacing between turns, material of the coil form, increasingly affect the Q - both for reasons of geometry (which can affect the amount of wire needed) and less obvious parameters such as distributed capacitance, etc.

Taking these Qu measurements at face value, we can calculate the approximate "R" (resistive) loss of the two coils using the general formula:
  • Q = XL  / R

Or the more general form, knowing the inductance:

  • Q =  2π f L / R

And rewriting this equation for R we get:

  • R =  2π f L /Q

So, for a frequency of 6.6 MHz (which should be representative of 40 meters) and an inductance of 22uH, XL is approximately 912 ohms, so for each of the two coils the apparent "R" value - which would be a combination of conductor loss and skin effect resistance we get:

  • Original stainless steel coil:  R= 19.4 ohms
  • Rewound coil (silver-plated wire):  R=4.1 ohms

The reader should be reminded that for ideal components, at resonance the reactance of the inductor is losslessly canceled out by the reactance of the capacitor so what we are left with - the value "R" mentioned above - will be the ohmic (conductor loss + skin effect) losses of the materials.  This also means that the "R" value will be added to the feedpoint resistance - and the effect of this "R" value is to lose power as heat as we will see below.  It is not lost on me that the loss values appear to be far higher than those obtained from Owen Duffy's calculator if one presumes skin effect to be the main source of loss - which we know is not going to be the case

The ohmic loss mentioned above is not going to be the only source of loss in a real antenna system:  In the case of a vertical, the "ground" losses (ohmic loss of radials, dirt, etc.) and with any antenna, the materials from which it is constructed (wire, telescoping rods which are themselves stainless steel, any balun being used, etc.) will come into play - and for an "electrically small" antenna such as either the JPC-7 or JPC-12 on 40 meters, will dominate and probably be the main points of loss besides the coil.

This goes to show how - in either case - doing anything to physically "embiggen" the size of the antenna - such as making the elements longer (adding drooping wires to the loaded dipole, adding a tophat to the vertical) will reduce the amount of inductance needed and increase the radiation resistance - both things that will contribute to improved efficiency.

With the stainless coil, it gets worse the lower you go!

Out of curiosity I re-did the Qu measurements using a 270pF silver mica capacitor - which lowered the resonant frequency to about 2.2 MHz - and got the following results using method #2: 

  • Original stainless steel coil unloaded Qu: 29
  • Rewound coil (silver-plated wire) unloaded Qu: 277

Given the lower frequency and lower skin-effect losses I fully expected the loaded Qu to be slightly higher - which is true for the silver-plated coil - but initially I did not expect the Qu to go down on the stainless steel coil so I re-did the measurement using method #1 and got about the same results (to within a few percent) - but in retrospect, I realized that this was to be expected.

As QL can be defined as being the ratio between inductive reactance ( XL ) and skin effect and ohmic resistance (R), if "R" remains pretty high and XL lowers with frequency, the "Q" will be lower:  Since the resistance of the stainless steel wire is so high to begin with, it figures significantly in the reduction of Q and thus the losses incurred.

In perusing the forums in the back-and-forth discussions about stainless steel versus silver-plated coils, people have observed a "hotter" coil at the lower frequencies.  At first glance, this makes sense since lower frequency = "more coil" = more lossy wire - but the fact that - at least at HF - the Q of the stainless coil goes down significantly with frequency makes it even worse! 

Update:

I recently got my HP-4191A RF Impedance Analyzer online and did some direct measurements of the coils and capacitors.  Unfortunately, at the 22uH inductance and 2.2 or 6.6 MHz, this instrument doesn't do too well (it wasn't designed to fully analyze such high inductance at those frequencies), but it fares much better with the 27 and 270pF capacitance values at these same frequencies.

Using the '4191A I measured the "Q" of the 27pF at 6.6 MHz as being 237 - but the Q did easily exceed 1000 in the area of 9-11 MHz.  Meanwhile, the 270pF capacitor had a measured Q of 267 at 2.2 MHz and it, too, exceed 1000 at some frequencies.

As the Q of the silver mica capacitors mentioned above were assumed to be 1000, we can now use a known value for the capacitor's Q and recalculate the measured Qu of the L/C combination, which yields, using method #2 with the 27pF capacitor at 6.6 MHz, above as:

  • Original stainless steel coil unloaded Qu:  55
  • Rewound coil (silver-plated wire) unloaded Qu:  766

Similarly, at 2.2 MHz using the 270pf capacitor:

  • Original stainless steel coil unloaded Qu:  32
  • Rewound coil (silver-plated wire) unloaded Qu:  1156

As expected, the measured unloaded Qu of the stainless steel coil didn't change by a huge amount, but the calculated Qu of the silver-plated coil certainly did! 

If the capacitor Q values are taken at face value, we can come up with new values of "R" for the coil loss at 6.6 MHz:

  • Original stainless steel coil:  R= 16.6 ohms
  • Rewound coil (silver-plated wire):  R=1.2 ohms

 * * *

Testing with the JPC-12 vertical and JPC-7 loaded dipole.

As noted earlier, the rewound coil was initially tested on the JPC-12 loaded vertical on 40 meters - mostly because it uses only a single coil and at that time I had rewound only one with silver-plated wire.  While I was at it I decided to see if I could detect any difference in the current flowing through the coil at a given RF power output related with the use of the original (and lossy) stainless steel coil and the silver plated coil.  Again, figure 7 shows this rewound coil with a thermal infrared camera just after a 60 second key-down at 75 watts, the temperature rise being just 3F (<2C).

Let us now consider the measured resistive losses of the coil (let's say 20 ohms for the stainless coil, 4 ohms for the silver-plated one) at 75 watts - the power at which we observed the temperature rise.  As we know the approximate current to be expected (about 600mA at 20 watts as measured with a known-accurate thermocouple-type RF ammeter) we can calculate the apparent losses at 100 watts which would equate to about 40 watts for the stainless coil and 5.7 watts for the silver-plated coil.  What this means is that nearly half of the power is lost in the stainless steel coil - but this still represents less than 1 "S" unit of loss. Footnote 2

Note:  Judging by the ratio of the temperature rise between the two coils (3 degrees F for the silver-plated coil and 110F for the stainless) we would expect far greater difference in power loss between the two coils (more than 30-fold difference, so I'm likely missing something here).

Update:  Based on the revised "Qu" measurements mentioned above (e.g. Q=766 with R=1.2 ohms at 6.6 MHz) the calculated thermal loss for the silver-plated coil is estimated to be 2.2 watts rather than 5.7 watts.
 
Once I had two silver-plated coils and two stainless steel coils, I could do a direct comparison on the JPC-7 loaded dipole. The JPC-7 is more or less a pair of JPC-12 vertical on their sides, fed with a balun - but rather than having the ground (radial) system to "push" against when radiating RF, it - being a dipole - used both elements against each other and the "ground" under - unlike the vertical where the ground/radial participates directly in current flow - is somewhat less affecting of the impedance, although the proximity of the ground does have the effect of lowering feedpoint resistance and resonant frequency.  (As we are concerned only with "feeding" the antenna, we will ignore the antenna pattern.)  

With the original stainless steel coils, the feedpoint resistance at resonance is "close enough" to 50 ohms to keep a radio without a tuner happy (it's actually lower than 50 ohms as noted below) - but consider that this means that each half of the dipole is closer to 25 ohms, the two being in series with each other:  With two coils' losses now in the mix - and the fact that a given loss of a coil in a 50 ohm circuit as a percentage was about half that of the same amount of resistance in a 25 ohm circuit - the losses are arguably worse, but "split" between the two elements.

While I didn't have the opportunity to use the thermal infrared camera to measure the temperature rise of the stainless coils on the JPC-7, they both got rather hot to the touch after key-down at 75 watts, indicating a roughly comparable amount of loss as did the original stainless steel coil on the JPC-12 vertical:  As with the vertical there was little change in temperature of the silver-plated coils.

Using a NanoVNA and minimal coax length  Footnote 3 I set up the JPC-7 as per the the manufacturer's instructions on 40 meters:  From the feed point there were two mast sections, the coil and then the telescoping rod on each side.  Carefully setting the coils and the element lengths to yield the lowest "R" value (e.g. at resonance), I then noted the "feedpoint" resistance at resonance (where reactance, or "J" = 0) using the stainless steel and then the silver plated coils:

  • Stainless steel coils:  38 Ohms (1.32:1 VSWR)
  • Silver plated coils:  15 ohms (3.4:1 VSWR)

It's worth noting that these "feedpoint" readings were taken with the supplied 1:1 balun inline along with a short length of coaxial cable so the above readings are NOT precisely those of the actual feedpoint resistance:  There is likely a bit of loss and transformation occurring in the aforementioned set-up so the absolute numbers above may not reflect the actual feedpoint resistance itself.  I also observed that on the JPC-7, the (normalized) 2:1 VSWR bandwidth was lower with the silver-plated coil - an expected effect with higher Q resonator coils.

Note:  On higher bands (e.g. 20 meters and up) the feedpoint impedance was much closer to 50 ohms with either coil and it's likely that nothing special will need to be done to keep a radio "happy".

One might be tempted at first to think that because of the higher VSWR,the silver plated coil constituted an antenna that was "worse" - but that would be wrong - this actually indicates the opposite.  What this measurement shows us is that the apparent total resistance of the two silver plated coils at 40 meters was 23 ohms less (about 11.5 ohms for each coil) than that of the silver plated coil - and this increased resistance is what accounts for the power being lost as heat.

This realization still leaves us with the problem that if we take away much of the loss of the coils we lower the feedpoint resistance which means that we can end up with a rather high VSWR - of over 3:1 - meaning that many radios won't be particularly happy with the situation without throwing a tuner into the mix.  This leaves us with several options:

  • Pretend we didn't see this and continue using the stainless steel coils.  This is an obvious choice and I can attest that both the JPC-7 and JPC-12 antennas do work pretty well despite the loss of the coil, but personally, I can't "un-see" the lossy nature of these coils, so that's not an option for me.  As a "portable" antenna is all about compromise of performance, I prefer to minimize the deleterious effects of as many aspects of this "compromise" as I reasonably can.
  • Use an antenna tuner.  Placing a tuner at the antenna is the preferred choice as it will minimize mismatch losses that will result if the tuner is placed at the far end of the cable feeding the antenna (e.g. in the radio.) Whether the magnitude of mismatched loss of the cable when the tuner is placed at the distal (radio) end of the feedline to match the lower-loss silver-plated coil is worse than using no tuner at all with the stainless steel coil cannot be easily answered without knowing the properties of the coax used and how a specific tuner works under the impedance conditions that it might see.
  • Rework the balun.  The JPC-7 has a 1:1 balun (one that isn't very "balanced" - but that's another topic) but it is clear that you could  choose a balun that inherently provides a suitable transformation - but more than one such balun would be required to cover all bands.
  • Autotransformer.  A tapped autotransformer used to be a common "thing" many years ago for matching short verticals (e.g. mobile installations) to deal with the low feedpoint resistances at resonance - often well under 20 ohms for a low-loss coil.  These devices seem to be less common these days, but if you look carefully they may still be found on the surplus market - namely the Atlas MT-1 and Swan/Cubic/Siltronix MMBX, both of which offer selections of impedances that will easily yield 1.5:1 VSWR or better at any likely feedpoint resistance at and below 50 ohms.  I have tested the Atlas MT-1 (by putting two units back-to-back) and found a single unit to have about 0.2dB of loss on 40 meters which theoretically represents about 5% power loss.  (Useful articles about RF autotransformers may be found in the November 1976 issue of "Ham Radio" magazine - link and the December, 1982 QST - link.)

As mentioned previously, the losses of the stainless steel coil are "about an S-unit" on the lower bands so the user would have to weigh the benefits of the potential losses incurred by matching a silver-plated coil and additional matching versus just using the stainless steel coil and getting a more convenient match and just "eating" the losses.

Conclusion:

The reader should not go away thinking that antennas using loading coils wound with stainless steel wire don't work:  They do - and can be quite effective - but... 

In my measurements, the losses added by the stainless steel coils amounted to roughly "an S-unit" (more or less) in a worst-case situation for the vertical antenna and somewhat more than this for the loaded dipole.  I have very successfully used both antennas with their original stainless steel coils for portable, remote and POTA operations with good results.  The difference of "about an S-unit" may be an issue for marginal situations using SSB, but it's less likely to be a problem for CW or digital modes under the same band conditions and distances where the signal margins are more favorable for weak signals.

As electrically-small HF antennas will often have lower feedpoint resistance than their full-sized counterparts this means that intentionally using low-loss coils can shift the impedance well below 50 ohms, complicating the matching of the radio to it - particularly in the case of the loaded dipole:  The use of a radio's built-in antenna tuner - particularly with a long length of coax - may well incur losses greater than those of the lossy stainless steel coil without a tuner.

I'm guessing that the use of stainless steel wire for the coils is at least partly a result of it "simplifying" the operation of a portable antenna by resistively (lossily!) providing a feedpoint resistance closer to 50 ohms.  From a standpoint of operational simplicity and cost (both avoiding more complicated matching arrangements) the use of stainless steel - and simply "eating" the power loss - may be a reasonable compromise for most users.

But, it's not as simple as that.  The above is certainly true for the loaded dipole where the feedpoint resistance ends up being quite low (15 ohms on 40 meters) but for the vertical - where more variables are at play (e.g. lengths of radials, length of vertical resonator) one can easily attain a good match (<2:1) to 50 ohms even with the lower loss of the silver plated inductor coming into play.

All of the above should also point to something else:  In my respective articles about the JPC-7 and JPC-12 antennas I noted that performance could be improved by making them electrically "larger" (e.g. the addition of a top hat to the JPC-12 and "droop" wires on the JPC-7) which both reduces the amount of loading inductance and likely increases the feedpoint resistance - both of which contribute to improved efficiency.

Should you toss or rewind your stainless steel loading coil in favor of something using lower-loss material?  If you are trying to eke out every last bit of efficiency from your portable antenna and are prepared to deal with the possibility of slightly more complicated matching requirements (at least on the lower HF bands like 40 and 30 meters) to deal with potentially low feedpoint resistance - then perhaps.  If you operate a lot of SSB, operate using high power (>= 100 watts) and/or high duty cycle, it may well be worth doing what you can to reduce at least one of the sources of loss of these types of portable antenna systems and a potential failure point due to heat.

* * * * *

Footnotes:

  1. This silver-plated jewelry wire that I used is varnished, so it's not actually bare - but this poses no problem with this project:  The protective coating is pierced when the new wire is clamped under the posts and the slider easily "bites" through it, so there is absolutely no need to strip it.  The varnish on the rest of the coil offers protection from oxidation and while silver oxide is a reasonably good conductor, unoxidized silver is much better, so the coating is left intact.
  2. The term "S Unit" is occasionally used in this article, but always with a bit of "hand waving" indicative of its ambiguity.  An "official" international definition of an S Unit is a 6 dB difference in signal level according to IARU Region 1 Technical Recommendation R.1 (where "S9" = -73dBm into 50 ohms - link).  While U.S.-made radios and many SDR programs use this definition by default, Japanese radios are often calibrated with 3 dB S-units meaning that for these radios, smaller amounts of signal change are more strongly indicated.  The reader should always note that while modern SDR-based receivers often do have reasonably good relative signal indications (e.g. the S-meter moves as it should for given changes in signal level) this is likely not true for older, analog radios.
  3. For both transmitter and VNA testing, minimal coax length was used.  For the former, a very short (15cm) coax jumper was used, connected directly between the radio and the antenna feed, the radio being powered by battery.  For the VNA, the instrument was connected similarly - the 15cm coax for the JPC-12 and hanging directly from the JPC-7's balun - to minimize possible effects of common-mode RF currents on the antenna.  In real-world operation this would be emulated by using an effective common-mode choke as close to the antenna feed as possible. 
Related articles:
  • Observations, analysis and field use of the JPC-7 portable "dipole" antenna - link.
  • Observations, analysis and modifications of the JPC-12 vertical antenna - link.
  • "The Two Faces of Q" by Wes, W7ZOI - link.
  • About Q-factor of RF inductance coil - link.
  • High-Q RF Coil Construction Techniques by Serge Stroobandt, ON4AA - link.

   * * * * *

This page stolen from ka7oei.blogspot.com

 

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