The JPC-12 antenna (possibly made by BD7JPC) is relatively inexpensive a portable vertical antenna - made in China, of course - that may be found for sale at quite a few places under a few different brand names (including "Chelegance"). The price varies very widely - sometimes well over $200 - but I got mine via AliExpress for about $120, shipped, about a year and a half ago.
Note:
I analyzed the JPC-7 loaded dipole antenna - which is made by the same company and uses many of the same components - and reported on it in previous article, and you may find that discussion HERE.
Silver-plated coils: Since this article was posted I have added an article describing the effects of the stainless-steel coils and how to rewind with silver-plated copper, found HERE.
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| Figure 1: All of the standard components of the JPC-12 kit. Click on the image for a larger version. |
"The perfect is the enemy of the good"
The above statement should be kept in mind when doing any temporary, portable installation. The idea is to have an antenna that will work "well enough" to do the job. It's also likely that in the situation where you are portable, you will not (and cannot!) spend an inordinate amount of time tweaking things to eke out the last decibel.
This is not to say that one should not be mindful of good practices as too much corner-cutting can excessively impact performance and potentially replacing enjoyment with frustration. One should achieve a balance between that which works well and something that will allow more operation than fussing about.
Remember: The more time you spend trying to get that last bit of performance out of your system is less time that you are spending operating - and I'm presuming that operating is your goal.
What is included with the JPC-12
As shipped and as depicted in Figure 1, the antenna comes with these components:
- Aluminum ground stake. This is a pointed stake 9-5/8" (24.5cm) long end-to-end about 1/2" (1.3cm) diameter. This stake has M10-1.5 (coarse) threads on the end - the same as all other male and female threads used on the other mast/coil components of this antenna.
- Four aluminum mast sections. These are hollow tubes with (pressed in?) screw fittings on the ends - one male and the other female, both with M10-1.5 coarse threads that may be assembled piece-by-piece into a mast/extension. End-to-end these measure 13-3/16" (33.5cm) each, including the protruding screw - 12-3/4" (32.4cm) from flat to flat and are 3/4" (1.9cm) diameter.
Figure 2:
The feedpoint for the JPC-12. The upper half (right)
is insulating while the bottom portion is machined aluminum.
Click on the image for a larger version.
- Feedpoint assembly. This has a (correctly-machined!) SO-239 (female UHF connector) - the shield of which connects to the bottom half while the top - which is isolated by a section of fiberglass tubing - is connected to the center pin. On both ends are female M10-1.5 threads to receive the screw from the ground stake (on the bottom) and the "hot" portion of the antenna on top. This piece appears to be well built and is 6-9/16" (16.7cm) long.
- Adjustable coil. This is a piece of what appears to be thermoplastic or possibly nylon with molded grooves for the wire. This unit is connected to the others via a male threaded stud on the bottom and female threads on the top, both being M10-1.5 like everything else.
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| Figure 3: The adjustable resonator coil, wound with 1mm stainless- steel wire. (The markings are mine.) Click on the image for a larger version. |
- Telescoping section. This is a stainless steel telescoping rod that is 13-1/8" (33.4cm) long including the threaded stud (12-7/8" or 32.7cm without) when collapsed and 99-11/16" (8' 3-11/16" or 253.2cm) when fully extended - not including the stud.
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| Figure 4: This is the supplied "radial" kit - a 10-strand chunk of ribbon cable - the ring to be sandwiched on the bottom of the feed. Click on the image for a larger version. |
- Counterpoise/Radial cable. This is in the form of a chunk of 10 conductor ribbon cable terminated with large (0.4", 1cm I.D.) ring lug on one end sized to fit over the M10-1.5 threads. This cable is about 203" (16' 11" or 516cm) long, including the ring lug that is intended to be sandwiched between the bottom of the coil and the ground stake. As noted later in this article, this radial isn't as useful/convenient/versatile as one might initially think.
- Padded carrying case. This zippered case is about 14" x 9" (35.5x23cm) with elastic loops to retain the above antenna components and a zippered "net" pocket to contain the counterpoise/radial cable kit and the instructions. There is ample room in this case to add additional components such as small-diameter coaxial cable - and enhancements to the antenna, as discussed below.
- Instruction manual. The instructions included with this antenna are marginally better than typical "Chinese English" - apparently produced with the help of an online translator rather than someone with intimate knowledge of the English language. The result in a combination of head-scratching, laughter and frustration when trying to make sense of them. Additionally, the instructions that came with my antenna included those for the JPC-7 loaded dipole as well, printed on the obverse side of the manual.
Fully assembled with the originally-supplied components, the length of the antenna is about 13' 5" (411cm) not including the ground spike meaning that it is self resonant - without added inductance - at a bit below the 17 meter band. This means that at 17 meters and above, the tuning can be done solely with adjustment of the telescoping section and the coil can likely be omitted entirely. Below 17 meters additional inductance is required which is obtained by moving the slider of the antenna downwards, requiring all but the last 3-4 turns of the coil to obtain resonance on 40 meters.
Comments:
Build quality
I'm quite pleased about the overall build quality: The design seems to be well thought-out, perhaps inspired by other (similar) products on the market. The individual mast sections seem to be plenty strong and I've seen no indication of the end sections coming loose. I have screwed eight of these sections end-to-end and held them horizontal and noticed very little drooping and no "permanent" bends.
The feedpoint - being a combination of aluminum and plastic - seems to be well-built, the bottom section being machined with a flat to accept the SO-239 connector. The upper section appears to be fiberglass, threaded at the top to accept an aluminum plug into which female threads are tapped to accept threads of the mast sections.
Likewise, the coil itself seems to be well built, the 32 turns of wire set into a spiral groove molded into the body with the coil tap selection having firm, positive action. As noted previously, the wire comprising the coil is, itself, about 1mm diameter (approximately 18 AWG) and is apparently austenitic (e.g. non-magnetic) stainless steel.
While this wire is very rugged, the fact that it is stainless 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
While these values would be for the entire coil remember that less than full inductance is typically used - but the message is clear: The fewer turns of coil you need to use, the lower the loss! The total length of 1mm wire is estimated to be about 180 inches (457cm). By comparison, copper wire of this same diameter and length would have a DC resistance of about 0.1 ohm - or a skin effective resistance of 2 ohms at 28 MHz. Alternatives will be discussed later.
Using the supplied radials - or not!
Noted in most reviews is the nature of the included radial/counterpoise wire - particularly since there is little or no mention of how it is to be used in the included manual. Clearly, the single ring lug is intended to be captured between the bottom of the feedpoint section with the SO-239 connector and the ground stake.
For use as a resonant radial, the length of the this cable (203" or 516cm) is approximately correct for 1/4 wavelength at 20 meters, but this is not really suitable for 40 meters. For best efficacy, the radials should be elevated above the ground by about a foot (25cm) or so so that the 1/4 wave impedance transformation (e.g. the distal end of the radial being open being transformed to a "short" at the antenna end to make it work effectively) but laying it on the ground directly - particularly if it is dry - will usually work quite well.
Being 10 conductor ribbon cable, the opportunity exists to split the wire lengthwise to obtain individual wires to spread radially around the base of the antenna. This wire - with its PVC insulation and rather small gauge conductor (likely 26 AWG) means that it is difficult for it to lay flat unless it is warmed by the sun on hot ground (or with rocks laid on the wire) - plus a large number of connected-together conductors from a split-apart ribbon cable are the makings of a portable rats-nest of wires that cannot easily wound/unwound later.
Most reviewers/users of this antenna - including myself - don't really like the "ribbon cable radial" system and personally, I have never used it - but I keep it in the kit, just in case.
Location of the loading coil
While it might be tempting to place the loading coil immediately above the feedpoint, this is not the suggested location, but rather at the top of the four supplied screw-together mast sections immediately below the telescoping section. This makes sense on several counts:
- This elevates the coil above the ground, making it easier to adjust as it is at more convenient height (about 4' 3" or 130cm above ground).
- Because it is the portions of the antenna that conduct the most RF current are those that will radiate the most, those same sections below the coil - and above the connection to the counterpoise - will emit the bulk of RF energy
- The markings on the coil for 40 and 20 meters assume that you have placed the loading coil in the location described above using the original components in the kit.
From a practical standpoint, placing the loading coil immediately above the feedpoint will also work - albeit with some loss of efficiency - and this may be desirable if the base of the antenna (and radials) itself is elevated - perhaps by being clamping it to a fence post or table. In this case one might place the coil closer to the feed point to keep it at a reasonable (accessible) height rather than needing to access the coil's tuning slider by standing on a ladder or chair.
Augmenting/improving the JPC-12 with optional accessories
A bit of perusal among the goods of the various sellers of the JPC-12 (and the related JPC-7 dipole) will reveal that spare parts: It's a pretty good idea that - if you find that you are using this antenna a lot - to get a few "extra" parts (I strongly suggest an extra telescoping section or two) when things inevitably get worn out or broken. There are also several "accessories" that may be used with the antenna(s) that might be useful - some of which are discussed below - and other components that you can easily assemble and add to the kit.
Improved ground radial system
For a 1/4 wave vertical - and this antenna is exactly that, albeit electrically lengthened with a coil and tophad on the lower bands - half of the antenna is its mirror reflection from the ground. As it is unlikely that most people will ever set up their antenna atop a metal surface or in salt water, a set of wires is typically deployed to locally simulate the needed reflective "ground" surface.
The common advice in years past has been to bury many, many ground radials just below the surface of the ground - advice that is practical in terms of avoiding trip-hazards and to provide a degree of lightning protection - equal or better performance may be had by deploying an array of radials that are odd-order quarter-wave multiples (1/4, 3/4, 5/4, etc.) that are elevated slightly above the ground. Emperical testing (see the linked article below) that as few as three or four elevated, resonant radials can be quite effective - and this number of radials is perfectly manageable in a portable installation.
The accessories described below make it easier to quickly deploy a resonant ground radial system - elevated or not.
The ground radial plate
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| Figure 5: This is the "radial plate" - an add-on accessory. Spade-lug terminated radial wires connect easily under the wing nuts. Click on the image for a larger version. |
As shipped, the radial kit (ribbon cable) included with JPC-12 antenna is perfectly usable - but in the opinion of many (including myself) the supplied radials aren't particularly practical or convenient. From the same seller as the antenna I purchased what is cryptically called a "JPC-12 PAC-12 Network Disk" - seen in Figure 5.
What this really is is an aluminum disk about 4-3/4" (12cm) in diameter with a series of eight wingnuts and screws around the perimeter with a center hole sized appropriate for the M10 stud on the top of the ground rod or one of the antenna elements. This device makes the connection of individual ground radials equipped with spade lugs much more convenient.
In looking at Figure 5, you may have realized that it's sitting atop its protective pouch to keep the screws from tearing up the inside of the carrying case: The rear pocket removed from an old pair of blue jeans!
Using individual wires for the radials
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| Figure 6: Four radials on kite string winders - each long enough for 60 meters - with markers on the wires for the different bands. Click on the image for a larger version. |
Marking the radials' lengths
At various points along the length of each of these wires are pieces of marked heat-shrink tubing to indicate the points corresponding to quarter-wavelengths of the various amateur bands from 60 through 10 meters and only as much wire as needed is unspooled from the cable winder to achieve the desired length for the intended band of operation: These yellow tags can just be seen in Figure 6 among the wire on the winders.
For these marker tags I used heat-shrink tubing cartridges for my Brother label maker - but I could just have easily have written on light-colored tubing with an indelible marker prior to shrinking them. To keep these tags from sliding around I put a dab of "Shoe Goo" (rubber repair adhesive) on the wire and slid the tubing over it before applying heat, locking it into place with much greater tenacity than the compression of the tubing shrinkage alone: Having used these radials in the field a quite a few times, I have yet to have one come loose.
Using elevated radials
From an operational standpoint, just three or four elevated, resonant radials will perform equally to or better to a large number of radials - resonant or not - buried in the ground. The reason for this - alluded to earlier - is the fact that any open-ended conductor that is an odd multiple of a quarter-wavelength long (e.g. 1/4, 3/4, 5/4) will exhibit a low impedance on the opposite (antenna) end - which is exactly what we want.
Simply laying such a length on the "average" ground will tend to diminish this effect somewhat, but elevating it even a short distance above the ground will preserve it. For more information and an analysis of vertical antennas with elevated radial systems see the article "A Closer Look at Vertical Antennas with Elevated Ground Systems" by Rudy, N6LP - LINK. It's worth noting the admonition of the author of this page to avoid the use of radials that are around 1/2 wavelength long and multiples thereof - likely for the reason that the nature of a free-space half-wavelength conductor is not to provide a low-impedance on their proximal end when the distal end is unterminated!
The obvious hazard of elevated radials is that of tripping - of you, the operator, others in the area, or animals, so it isn't necessarily practical in every situation. If it is possible to control access to the area with the antenna - or raise the radial above the height of the average person for much of its length - then this is a good choice.
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| Figure 7: Fiberglass driveway markers modified to mark/hold radials. Click on the image for a larger version. |
In my operation - typically out in isolated areas - I don't have much worry about tripping anyone other than myself so I obtained some 4' (1.2 meter) long fiberglass driveway marker stakes. These bright-orange stakes are about 4" long each (122cm) each - much 1-- long to fit in the antenna case, so eight of them were cut to shorter lengths to allow them to fit in the case, yielding two pieces each - the bottom portion with the sharpened point cut to 11-3/4" (30cm) and the top portion cut to 13-3/8" (34cm). To the bottom portion, I glued (again using "Shoe Goo") a 2" (5cm) long of 8.5mm I.D. stainless steel "Capillary" tubing (found on Amazon) so that the two pieces could be assembled to a single (mostly) non-conductive post about 26" (66cm) long.
Eight of these two-piece posts allow the support of four elevated radials at two points along their length, the radial wire being wrapped once or twice around to form a friction fit to keep them from sliding down. At the distal end, the remaining lump of wire still on the kite string winder is simply wrapped and hung over the post once a slight amount of tension is pulled on it.
Figure 7 also shows something else: I made a drawstring bag (again, from an old pair of blue jeans) that keeps all of these post pieces together and it can accommodate some of the extra mast sections, all while fitting in the original padded antenna case.
Comment:
The reader should be conscious of the fact that for the purposes of this discussion, we are talking about a temporary, portable antenna rather than a permanent installation. In the case of the latter, a different approach (the deployment of many, many radials, perhaps buried) is reasonable - but for a temporary antenna - where less effort to erect and break down is desirable - the use of four elevated, resonant (e.g. 1/4 wavelength) radials is likely to outperform the same number of radials laid atop the ground. In either case, however, the antenna will be usable - and that's the entire point!
On-the-ground radials
The use of elevated radials is arguably most important on the lower frequencies of operation of this antenna - namely 40 and 30 meters - where efficiency of this "electrically small" antenna will suffer due to a number of factors, but maximizing the efficiency of the ground plane is one way to mitigate this. In those cases where it is not practical to elevate the radials, the wires may simply be laid atop the ground.
As these posts are intended for marking driveways they are bright orange, making them stand out, but near the top they have a piece of white reflective tape so that they will show up at night. As I had this type of tape on hand I added a piece to the bottom section as well - just below the stainless steel capillary tube - to make them even more visible - particularly if the bottom and top portions are used separately to mark where an on-the-ground radial might be run to warn against a possible trip hazard.
The use of a "Magic Carpet" (e.g. "Faraday Fabric")
There is no reason why one could not use the aforementioned conductive fabric as part of their ground plane - but you would probably have to construct an additional component to connect to the fabric and use it effectively. For this, a piece of clean aluminum or copper plate laid atop the fabric - possibly weighed down with a rock - should provide a low-impedance connection to it.
While I do own some of this "Faraday Fabric" (obtained from Amazon) I have yet to try it with this antenna - and when I do, I plan to perform an "A/B" comparison. As of the time of this writing I have yet to see a serious, scientific and well thought-out comparison between a simple radial field and the use of just the fabric: Most of these comparisons simply demonstrate that it is possible to get a good antenna match while using the fabric - but as we all know, simply getting a match does not mean that the antenna will work: After all, a dummy load has a great match!
I expect that - at least on the sort of desert ground that I'm likely to encounter - the radials will "win" the contest - although I still plan to do a comparison: I suspect that using both the fabric and radials will offer decent results - even when tuned to a higher band for which the lengths of the radials are not expected to work (e.g. 20 or 10 meters with 40 meter radials.)
Addendum - brief testing with "Faraday Fabric":
I have done some cursory tests with two pieces of "Faraday Fabric" laid out around the base of the antenna which were overlapped by a few inches/cm and rocks laid atop to make sure that they were electrically connected. The connection to the "ground" of the vertical itself was done using an aluminum plate connected to a wire - the continuity verified using an ohmmeter.
The result was not particularly impressive. Using four resonant elevated radials, the resonance of the antenna was distinct and sharp - but with the "Faraday Fabric" there were a number of apparent resonance points - and worse, these shifted in frequency when I grasped the coaxial cable with my hand indicating that the feedline itself was doing some radiating and was participating in the "grounding": This did not occur when using the elevated, resonant radials.
I have yet to do more thorough testing of this antenna with the "Faraday Fabric" to compare its performance to one using resonant radials, but the fact that the resonance of the antenna was skewed and that the feedline seemed to be participating in the "ground" seems to indicate that the fabric, alone, is not equivalent to a proper, low-impedance radial system.
Additional antenna height - both real and "virtual"
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| Figure 8: The accessory top had kit consisting of a machined piece that attaches to the top of the vertical with four telescoping rods. Click on the image for a larger version. |
Any antenna that has to be electrically lengthened with inductance is likely to suffer from efficiency loss as that inductor is unlikely to be comparatively lossy. As the antenna is mechanically "about" 1/4 wavelength on bands above 20 meters, it make sense, then, that the lower bands that it is intended to cover - particularly 30 and 40 meters - need some additional inductance to bring it to resonance. It further follows that anything that may be done to make the antenna "taller" will reduce the amount of needed inductance and minimize these losses.
Tophat capacitance
Another accessory available for this antenna is a small tophat attachment for the telescoping vertical section. Often described as a "PAC-12 Capacity Cap" this consists of what looks like a knurled aluminum knob with five holes drilled around its circumference and yet another hole on the bottom sized to receive the "static ball" (really a short cylinder) atop the telescoping section.
Using a set screw to secure it to the top of the antenna, this kit contains four small telescoping whips (3-1/8" 8 cm long collapsed, 12-1/4" 31c fully extended - not including threads) that screw into the 5/8" (2cm) diameter center disk. Assembled, the end-to-end length of two of the telescoping elements is 25" (98.4cm) which forms a four-spoke "disk" that adds to the effective height of the main telescoping section of the antenna by increasing the capacitance. The idea (and hope) is that this allows the reduction of the amount of inductance needed to bring the system to resonance - and it also allows potential coverage of 60 meters as noted later.
The size and weight of this attachment is, in my opinion, about right: Any larger or heavier, it would likely be too much for the fully-extended main whip to handle and expose it to excessive wind loading. To be sure, one must always be very careful when handling the whip when fully-extended, anyway and adding the tophat increases the risk of damage. Even so, I've had the antenna up in 20+ MPH (32kPH) and despite a bit of swaying, experienced no damage.
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| Figure 9: The top hat kit assembled - but the rods are not extended. Click on the image for a larger version. |
Testing has shown that the addition of the tophat - when the antenna has previously been tuned for 40 meters - lowers the resonant frequency by approximately 1 MHz indicating an increase of virtual height by about 12 percent at that frequency. Even with the tophat the antenna falls short of being able to resonate at any 60 meter frequency with the normal complement of parts included with the antenna.
For the higher bands, the top-hat may be enough to eliminate the need for the coil on 20 meters - or at least greatly reduce the amount of coil and thus the potential loss.
Of course, the use of this top hat means that the existing coil marking scheme (e.g. the paint marks that show approximate slider position for the bands) is meaningless as tuning is changed - but if one is already prepared in the field for this (e.g. using an antenna analyzer, added markings to the coil for the new configuration, a paper template marked with pre-determined coil positions) then this is of little consequence.
This tophat kit is constructed fairly well, using a small grub screw with a supplied Allen key to attach it to the top of the telescoping section, but I noted that the key and the screw weren't well matched and couldn't be tightened too much with the key slipping. Rummaging about in my collection of hardware I found several metric machine screws and a hexagonal brass stand-off with matching threads and I replaced the grub screw with the stand-off, allowing it to be attached firmly to the top of the telescoping section using just my fingers.
Additional mast sections
It should not be surprising to know that you can buy individual mast sections. These are often described as being "dedicated lengthened vibrator for JPC-7 (PAC-12) multiband portable antenna". I purchased two more of these sections when I first purchased the antenna, increasing its fully erect height from 13' 5" (411cm) to 15' 7-5/16" (476cm) and coupled with the tophat and the full inductance of the coil allows the antenna itself to resonate at approximately 4.7 MHz, allowing complete coverage of the 60 meter band.
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| Figure 10: Four more mast sections to be used in a variety of ways - as ground supports, or as the "live" mast itself. Click on the image for a larger version. |
At this extended height and with the tophat, this antenna was used in fairly high winds with gusts of 35 MPH (approx 67 kph) with no issues: Having clamped the ground stake of this antenna to a metal fence post helped keep the antenna vertical and minimize sway certainly helped!
After using the antenna several times, I purchased yet two more mast sections (for a total of eight) - not only to have as spares, but also to elevate the bottom of the antenna still further. Living in the desert west of the U.S. (Utah) it's often the case that there is only sand into which the ground stake can be pushed and it simply isn't long enough to adequately support the antenna: Lengthening the ground rod with the addition of another mast section allows the ground stake to be pushed in farther and support the antenna without burying the feedpoint below ground level or stealing one of the mast sections from the antenna and reducing its height. This is mostly a problem on the lower bands (40 and 30 meters) where one needs as much height as one can get to maximize antenna efficiency.
This extension also facilitates the use of an elevated ground radial system, placing the feedpoint - and the ground radial disk - at a reasonable height.
Maintenance
The telescoping whip(s)
The telescoping whip is certainly the most fragile component included
and it - like any other telescoping antenna - is easily broken if one
is not careful. The "safest" way to collapse one of these things is to
pull it down - section by section - starting from the bottom: One
should NEVER push it down from the top as that is just asking for problems.
As with any telescoping whips that I own, one of the first things that I do when I get it is to make sure that it is clean of dirt and oxidation (particularly if it has been "pre-owned") as this can cause the metal-on-metal - especially when the two metals are the same - to gall and seize up, making it more difficult to extend or collapse. If I do find a section that is hard to move, I carefully examine it, often discovering slight scratches, buffing them out with very fine sand paper (1000 grit or finer) and/or steel wool (size 0000 or finer).
The final step - after cleaning with paint thinner or alcohol to remove any dust - particularly if it was just buffed with steel wool or sandpaper - is to put a light coating of oil on all sections when they are fully extended: I prefer to use a PTFE ("Teflon") based lubricant like "Super Lube" (made by Synco) as it does not dry and become "gummy". Extending and then retracting the whip a few times does a decent job of spreading out the lubrication - even getting inside the individual sections.
Although much smaller, I did a similar thing to the four telescoping whips that comprise the tophat.
I would consider this "cleaning and lubricating" to be a necessary maintenance item when using this antenna, needing to be done occasionally, with constant vigilance toward possible issues every time it is used.
Inductor slider
The adjustable inductor's components are all stainless steel - including the coil wire itself. Besides being known for the fact that this material "stains less" than others, it is also known for galling - that is, developing tiny burrs on the surface and jamming up when it is used against the same type of metal: In the case of stainless screws, nuts and bolts - if these gall, even if you ARE able to remove them without breaking them, they have to be replaced.
While the contact area on the slider is small enough that it is unlikely to gall and get "stuck", I noticed immediately a bit of "roughness" in its movement that indicated excessive metal-on-metal friction: I could not tell if this was on the contact area of where the slider rubbed across the coil's windings or on the sliding rod itself - but it was probably a combination of both.
This "roughness" in movement was relieved with the application of lubricant - the same "Super Lube" used on the telescoping sections - also making the adjustment easier to do.
Improving the coil
As noted previously, the coil is wound with 18 AWG (1mm dia) stainless steel wire. It is suspected that one of the main reasons why this type of wire is used despite its terrible losses - as compared with copper - is that this resistive loss increases the feedpoint resistance - but at least in relatively cool temperatures (below 90F or 32C) I wouldn't worry about running key-down for several minutes at 100 watts - or higher power with a low duty-cycle mode like CW or FT-4.
As it happens, one can juggle the proportions of a vertical antenna a bit to vary the feedpoint resistance - but if you consider that these same coils are also used in the JPC-7 dipole, the reasoning behind the use of stainless steel wire becomes more clear. An electrically-short dipole - such as when the JPC-7 is configured for 40 meters - would ideally have a feedpoint resistance of just a few ohms - but this would not match at all well to a 50 ohm system: Even a very low-loss antenna tuner would have difficulty coping and placing the tuner away from the antenna through a length of coaxial cable would make the situation even worse!
Having said that, testing was done that revealed that on the JPC-17 - while operating on the lower bands (30, 40 meters) a significant amount of power was being dissipated in the coil - enough to raise its temperature by 135F (75C) at 70-100 watts on 40 meters (lower loss on higher bands) - but rewinding the coil with silver-plated copper wire pretty much eliminated that element of loss.
A future article on this blog will detail the rewinding of this coil along with measurements/comparisons between the original stainless steel and rewound coil for both the JPC-7 dipole and this JPC-12 vertical which will eliminate any nagging worries about power handling capability.
Using the JPC-12 vertical in the field
I have used this vertical in the field a number of times, mostly with the "augmented" kit with the extra mast sections, top hat and ground radial plate and elevated radials - typically on 40 and 60 meters SSB, but also for POTA using CW. While the signal reports comparing my signal with that of others using full-size antennas unsurprisingly indicates that this doesn't to as well as the others on the lower bands, conditions have generally been good enough that there was little difficulty in copying my signal.
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| Figure 11: The JPC-12 vertical out in the wild in a slight breeze. The tophat is installed as is a section below the radial plate - along with extra sections to increase height. Click on the image for a larger version. |
As the ground here in Utah is usually rather poor making it difficult to simulate the "other half" of a vertical antenna, it is even more important that the radial system be effective. While I usually configure it to have four radials elevated about 18" (0.5 meters) above the ground, I have also simply laid the radial directly on the sandy soil - or found some convenient sagebrush, scrub oak or some other low plant or small tree to support them off them ground - and have always had pretty good results.
While I like this antenna, I find it to be far less convenient than the JPC-7 loaded dipole in that the vertical takes quite a bit more time to set up, needing a bit of assembly of the various pieces and laying out of the radials. With resonant radials, changing bands - and trying to maintain optimal performance - also makes it a bit awkward by the fact that the radials need to be shortened/lengthened as appropriate/
Practically speaking, having the radials laid out for 40 meters and running on 60, 30 or 15 meters isn't much of a problem, but picking a band that is an even multiple of the radials' base resonant frequency - being an odd half-wave multiple (e.g. 20 or 10 meters with a 40 meter radial) - will not work well as that is the worst possible radial length (other than zero length) to choose: The half-wave length will not provide the low impedance at the antenna and it's also likely that the coaxial cable will become most of the radial, possibly causing a "hot rig" in terms of RF and the related ill effects (RF into the audio, computer/radio crashing, extra noise) from doing so.
If, however, I have a bit of extra time and I want a better signal that I would otherwise get from the loaded dipole or a mobile-mounted HF antenna, I would definitely set up the JPC-12.
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Related articles:
- I analyzed the JPC-7 loaded dipole antenna - which is made by the same company and uses many of the same components - and reported on it in previous article, and you may find that discussion HERE.
- Silver-plated coils: Since this article was posted I have added an article describing the effects of the stainless-steel coils and how to rewind with silver-plated copper, found HERE.
This page stolen from ka7oei.com
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Thank you for a thorough and excellent analysis /guide to this antenna!
ReplyDeleteJust an outstanding review of this antenna. Bravo! -de KE0N
ReplyDelete