The Morrow CM-1 receiver - A relic of the cold war's effect on amateur radio

Figure 1:
The front panel of the Morrow CM-1 CONELRAD monitor.
The meter indicates relative signal strength and below it is the
on/off/volume control.  The dial - with a pair of "CD"
markings, is a smooth-tuning reduction drive - which is also
very accurate.  Shown here, it's on and tuned to a local station.
Click on the image for a larger version.

If you were an amateur (Ham) radio operator in the late 1950s and early 1960s, you  had a broadcast radio turned on in the background whenever you were on the air - or at least you should have!

The reason for this was spelled out in the FCC rules:  Beginning January 1, 1957, Section 12.192 required all amateur radio stations to monitor a broadcast station - at a minimum of ten minute intervals - to see if it transmitting, while they (the amateur) was on the air.  If the station had gone off the air, it was then required that the amateur determine if this was because of a CONELRAD alert - and if so, immediately cease transmitting.

What was CONELRAD?

CONELRAD, short for "CONtrol of ELectromagnetic RADiation", was a system - part of the U.S. Civil Defense - where, if an imminent attack of Soviet bombers was detected, ALL radio stations - commercial (AM , FM and TV) and amateur - were to go off the air to prevent their signals from being used as directional signals for navigation by the enemy.  The exception to this were some of the AM broadcast stations which were to ultimately occupy either 640 or 1240 kHz - but only for a few minutes at a time, the frequencies to be used by several different stations during that period - in a "round-robin" sort of system.  The "musical chairs" of transmitters, frequencies and locations was intended to make it difficult for the Soviet bomber to use them for navigation.

If you have seen an AM broadcast receiver intended for the U.S. market from the time period when the CONELRAD system was in existence (from about 1951 to 1963) you may have noted that at 640 and 1240 kHz there are triangular marks - often with the letters "CD" in them (the Civil Defense logo) - on the dial signifying the two CONELRAD frequencies - and these symbols are clearly visible on the tuning dial in Figure 1.  The intent of this is that civilians would tune to those frequencies to receive civil defense instructions during the "on" periods of the broadcast station near them.

Figure 2:
Rear of the CM-1.  The terminals connect to normally-open
contacts of the relay (if fitted) and the wire antenna can be
seen emerging from the chassis.  Between the two is a
potentiometer for adjusting the S-meter and relay sensitivity.
Click on the image for a larger version.

How it worked

As you might imagine, this system was cumbersome.  All stations had to be alerted in some way and "key" stations - perhaps notified via a wire service - would turn their transmitters off-on-off and on again in five second intervals and then transmit a 1 kHz tone for 15 seconds:  As transmitters of the day weren't designed to be "short-cycled" in this way, it was hard on the equipment - sometimes leading to failures.  There was also the requirement that some transmitters - and their antenna systems - change from their normal frequency to either 640 kHz or 1240 kHz CONELRAD channel - a further complication - to participate in the on/off "round robin".

Needless to say, this system had several well-publicized false alarms as well as failures when tested over the years that it was extant.  Eventually, because of the implementation of ballistic missiles rather than piloted bombers, its reason for existing ceased in 1963.

How it affected amateur radio operators

As noted earlier, if you were an amateur radio operator, you were required to monitor a local broadcast station, while you were on the air, at least every ten minutes:  If the radio station went off the air, the idea was that you were to check for a CONELRAD alert and if it was happening, immediately stop transmitting.

Figure 3:
Top of CM-1 chassis as seen from its left side.  In the fore-
ground is the tuning capacitor, IF transformers and amplifier
tube and main filter capacitor.  The 6C4, if installed, would
be plugged in the empty socket just visible behind the tuning
capacitor.  It's well laid-out - even the audio transformer is at
an angle to minimize coupling to the AC power transformer.
The antenna wire is lightly coupled to the preselector circuit.
Click on the image for a larger version.

It's difficult to know these days how well the average amateur radio operator adhered to this rule, and for those that did, doing so certainly added a bit of complexity and awkwardness to their operating style.  A number of article appeared in the amateur radio magazines at the time describing how to "automate" the detection of the local radio station going off the air - typically by monitoring the AGC voltage of an inexpensive radio and sounding a buzzer or turning on a light.

There were also some receivers designed for just this purpose.

The Morrow Radio Mfg. CM-1

The Morrow CM-1 was one such receiver.  The receiver pictured was found among the effects of a good friend - and fellow amateur radio operator - that passed away several years ago, sitting dusty on a shelf in his basement.  After careful inspection, I was surprised to see that it had no bad capacitors:  The components used in its construction - particularly the capacitors - are all good-quality Cornell Dubilier (all are ceramic disk types other than the main filter) - seemed to be a cut above those found in a typical, cheap domestic radio of the time and it worked perfectly the first time it was turned on - and I haven't even bothered checking the tubes:  Even the main electrolytic capacitors is still in excellent shape - likely a result of the receiver having spent the past six-plus decades in a cool, dry basement.

Figure 4:
Schematic of the Morrow CM-1.  Despite the somewhat unusual tube line-up, it functions
the same as a typical "All American Five" superheterodyne receiver of the era, complete
with a 455 kHz IF.  The main difference is that it has provisions for connecting to an external
alarm or light based on the presence/absence of a signal using the (optional) 6C4 tube and the relay.
Click on the image for a larger version.

A "brief" circuit description  (Refer to Figure 4)   While many "AA5" receivers use a resonant loop - either a ferrite stick or a coil wound on the back panel of the chassis for an antenna - the CM-1 uses just a short piece of wire, depicted in the upper-left corner of the diagram connected to C1, one half of the tuning capacitor and inductor "L1".  This and the 50-100pF series capacitor "lightly" couple a short piece of wire to the resonant circuit.  This works, but its effects can be swamped if a long piece of wire (more than a few feet/meters) is used which can spoil image rejection an cause the radio to overload on strong signals. The 6EA6 converter doubles as both the local oscillator - its frequency determined by the other half of the tuning capacitor C2 and L2 - and mixer as it converts and amplifies the received signal to the Intermediate Frequency (IF) of 455 kHz via its plate through transformer T1 which forms a selective band-pass filter.  Following T1 is a 6BJ6 which functions as an amplifier at 455 kHz and this goes to T2, another 455 kHz IF transformer, and then to the 6AV6 diode/detector/amplifier tube.   The secondary output of transformer T2 applies the 455 kHz signal to a diode section and since its cathode is grounded, it causes that part of the transformer to go negative with the rectified DC voltage as well as the detected audio:  The received audio is then coupled via a 0.01uF capacitor and 10 MegOhm potentiometer to the triode section of this tube for amplification where the audio is then coupled to the plate of the 6AS5 - its grid biased slightly negative by the 220 Ohm resistor in the cathode - and amplified.  Functioning as a "Class A" single-ended audio amplifier, transformer T3 couples its high-impedance plate to the low-impedance speaker. Referring back to the bottom of the secondary of T2 where the audio is tapped, the negative DC voltage that is also produced there is in proportion to the amount of signal getting to the 6AV6 detector tube and ultimately, the strength of the signal to which the radio is tuned.  This voltage is smoothed to remove the 455 kHz IF by a 250pF capacitor and then sent to one of the grids of the 6EA6 converter tube through the antenna-tuning inductor, L1 and also to one of the grids of the 6JB6 IF amplifier via the secondary of T1 and when the voltage gets more negative, its sensitivity is reduced.  In this way the "AVC" (Automatic Volume Control) is formed so that both weak and strong stations produce similar amounts of audio. This same AVC voltage is also sent to the (optional) 6C4 tube:  If this voltage is very negative - as it would be with a moderately strong signal - this tube is "cut off", but if the signal disappears and the AVC voltage is less negative, it will conduct and thus turn on the (optional) relay. For the signal level meter, a voltage divider using a 15k resistor  and 2k potentiometer (seen at the bottom of T1) sets the voltage threshold on one side of the 1 mA meter - the other side of which is connected to the cathode of the 6JB6 IF amplifier tube along with a 1k resistor to ground.  There are two mechanisms of action here for driving the meter:  If there is more signal into this tube, it conducts more current and the voltage goes up across the 1k resistor, but a higher signal will also result in a more negative AVC voltage which will negatively bias the tube and reduce the current.  It's this latter aspect that prevails:  A more-negative AVC will reduce the tube's conduction and also the voltage across the 1k resistor and when this voltage is lower than that across the 15k-2k voltage divider, the meter will move up-scale.   The final section is the power supply which consists of a transformer to isolate the circuit from the mains.  The high voltage is half-wave rectified by a "modern" (for the time) selenium diode and then filtered by one section of a two-part capacitor which is then decoupled by a 750 ohm resistor in series which then goes to another capacitor.  In this way, the voltage is reduced somewhat and better-filtered than it would be with just a single stage of smoothing.

The CM-1 is a compact, tube (valve) type superheterodyne receiver from this time period that was designed to indicate when a station went off the air.  As can be seen from the photo, it looks more like a piece of ham gear than the AM broadcast band receiver in that it has a "nicer" geared (and accurate) tuning dial and a meter than indicates the relative strength of the signal to which it is tuned.

Internally, he CM-1 itself is mostly an unremarkable receiver:  Electrically, it's a variant of the "All American Five" (AA5) superhet that was produced by the millions over about four decades, but with a few interesting differences.  Sporting a power transformer, all of the tubes use six volt filaments and with the isolation, it - unlike most later, cheaper versions of the "AA5" - it does not have a "hot" chassis (e.g. one that is connected to one side of the line cord) - but with a solid-state (selenium) rectifier rather than the typical 35W4 and a rather different six volt tube line-up than a typical AA5.  A 6BE6 as the converter, 6BJ6 as the IF amplifier, 6AV6 as the detector/amp and interestingly, a 6AS5 (a tube typically used in car radios) for the audio output.  As can just be seen in Figure 5, the 6AS5's "getter" shows discoloration and Figure 3 reveals that there is a slight stain around the audio transformer where some of its wax wicked onto the chassis due to heat, both indicating that this receiver has quite a few "on" hours!

An interesting departure from a typical AA5 is its antenna connection.  Most AA5s had either a ferrite loopstick or a large coil of wire wound on the cardboard/masonite/phenolic back cover of the receiver that doubled as an RF pre-selector, but this receiver simply has a piece of wire capactively coupled to the tuning network connected to one of the capacitor's gangs.

According to the manual, only 2-3 feet (50-95cm) of wire is necessary for reception and testing of this CM-1 indicated that this was true - but it also means that one must resist the temptation to connect it to a longer wire:  Doing so simply overloads the receiver and wrecks image rejection, particularly since the front-end preselector tuning is overcoupled to the antenna itself, effectively bypassing it altogether.

During testing - with only the 2' (50cm) wire antenna, when placed outside I found that the receiver was capable of picking up even the weaker signals across the broadcast band - but since it is a wire rather than a coil, it's non-directional and is far more prone to pick up local "E-field" noise energy than the typical "H-field" loop which is not only directional, but offers a degree of rejection of such noise.  Practically speaking, one could probably add a loop antenna, connecting to the wire and the grounded screw (one of the two used for the relay) on the back panel to improve reception in today's modern electrical environment.

Figure 5:
The other side of the top of the chassis.  In the foreground is
the power transformer and next to it is the 6AS5 audio amp.
The socket next to the 6AS5 is for the (optional) relay, driven
by the (also optional) 6C4.  The antenna tuning coil is visible
next to the tuning capacitor, almost against the front panel.
Click on the image for a larger version.

When I first removed the receiver from its chassis I thought that it was missing two tubes as evidenced by empty sockets, but I then realized that it had a selenium rectifier accounting for there being only four tubes and immediately wondered why there were extra sockets.  A bit of "Google-Fu" found the manual online and I learned why:   This receiver could optionally be fitted with a relay (also requiring another tube - a 6C4 to detect the loss of signal and drive the relay) that was intended to operate a sounder, a light or some other means of indicating loss of signal.  I find it interesting that this relay and its driver tube were omitted as shipped from the manufacturer, but I suspect that this was a cost-saving measure - and it may have been enough for most amateur radio operators using this receiver to simply glance at the front-panel meter occasionally to see if a signal was still there.

Is it still useful?

Certainly, the reason raison d'etre for this device ceased to be when, in 1963, the FCC rule requiring the amateur radio operator to monitor broadcast stations while operating was rescinded and removed from FCC Section 12.192, but since it's rather compact and has a speaker that dominates the top of the case means that it still works as a pretty nice AM broadcast receiver.  Whether or not one might think an AM-only receiver is still useful is another matter altogether!

Figure 6:
Bottom of chassis of the CM-1.  High-quality components -
including all Cornell Dubilier capacitors - are used
throughout.  The orange object left of center is the selenium
rectifier and the oscillator coil is just right of center.
Click on the image for a larger verison.

The usefulness and practicality of this receiver is probably on par with most other vintage radios that one might collect and own:  They are not likely to be "everyday drivers" and more likely to sit on a shelf with other old radios - but this one, at least, has a particularly interesting bit of cold-war history behind its existence.

* * * * * * *

 This page stolen from ka7oei.blogspot.com

[END]

A new push-pull switch/potentiometer for the Heathkit HM-102 VSWR/Wattmeter

Figure 1:
The Heathkit HM-102 HF VSWR/Power meter.  This meter -
often found at swap meets and hamfests for low cost - is still
as useful as it was when new - and is easily repaired today,
decades after its introduction.
Click on the image for a larger version.

A few weeks ago I got my hands on a(nother) Heathkit HM-102 VSWR HF Wattmeter/SWR bridge.  This device was available from the early 1970s and into the early 1980s and was a common sight in ham shacks in the U.S.¹

This unit consists of two parts:  The front panel meter/switch and the "bridge" section and it included a fairly long cable, giving the user the ability to use it as a single, stand-alone unit with the bridge mounted in the enclosure with provided screws or to have the meter and the bridge separated by several feet - often more convenient in a busy ham shack with lots of cables that cannot be conveniently routed to where the meter resides.

Repair needed!

The unit depicted in Figure 1 had belonged to a (silent key) friend of mine and due to an unfortunate set of circumstances, it was submerged for some time when the basement of his house flooded after his passing.  Amazingly, the meter movement survived unscathed - aside from a few small particles of dust that were easy to blow/brush out after carefully popping off the front  - but the rest of it needed a bit more attention.

The case itself and the knobs - once worked over with a bit of "Formula 409" cleaner - were in excellent condition, as was the "SWR/2000/200" switch that selected between displaying the VSWR and forward power after it had been cleaned.  The circuit board comprising the bridge and the SO-239 coaxial connectors looked a bit "crusty" and would require a few parts, but a few minutes with an old toothbrush, some dish soap and an ultrasonic cleaner did wonders in making it look better.

What did not fare well was the combination front/panel SWR "Set" switch and potentiometer.

SWR measurement by the HM-102 and other single-meter bridges

Using a single-meter VSWR bridge is a multi-step process:

• Transmit at a relatively low power.  If the VSWR of the load is uncertain, using a low power setting very good idea as is prevents possible damage to the radio and it simplifies measurements by minimizing interaction with the radio's protection circuit. ²
• Set selector switch to "SWR" and put the potentiometer in "set" mode.  This bypasses the wattmeter's calibration, allowing the user to set the meter and is accomplished on the HM-102 by pulling on the "Set" switch.
  
• Set the meter to full scale.  This sets the meter so that the full scale is relative to the current forward power to allow a direct comparison of the reflected power and thus the VSWR.
• Take the meter out of "set" or "calibrate" mode.  With the forward power calibrated, the meter will now directly read SWR on the appropriate scale.  On the HM-102, this is done by pushing on the "Set" switch and the VSWR can be directly read from the meter's scale.
  

For the HM-102, the "set/calibrate" function uses a "push-pull" switch on a potentiometer, and as Figure 2 shows, this assembly did not survive.  This type of switch was ubiquitous in the 50s through the 70s, found on radios and televisions as well as many other consumer electronic devices, but it has all but disappeared in recent decades and I thought that finding a suitable replacement might be difficult.

Figure 2:
The remains of the original push-pull switch/potentiometer.
The aluminum shaft had seized/galled to the aluminum
bushing and efforts to free it resulted in its eventual
destruction - although it was likely already a lost cause.
Click on the image for a larger version.

While I figured that it was a long shot, I initially tried several things:  Some "PB Blaster" as well as some heating/cooling cycles, but it became apparent that even if I had been able to free the potentiometer shaft that had seized to the bushing, both the potentiometer and the switch itself were likely to have been destroyed by my attempts to free it, if not having been submerged in water.

For a few days I wasn't sure what I might do to find a replacement:  I rummaged around in my collection of potentiometers and while I found one or two "push/pull" units with an integrated switch, none of them were anywhere near the needed 200k-Ohm value.

The Internet to the rescue

On an impulse, I decided to check the parts suppliers like Mouser and DigiKey and was surprised to find out that these devices were still readily available.  On a whim, I also checked Amazon for a "push-pull potentiometer switch" and was greeted with dozens of options:  I'd managed to overlook the possibility that such switches would still be sold as replacement in other gear - namely certain brands of guitar amplifiers.

A bit of further searching showed that the two most common values of these potentiometers seemed to be 500k and 250k - the latter being "close enough" to the original 200k part that it shouldn't really matter ³.  While most of these devices have shafts for splined knobs - which would have sufficed even with the setscrew knob of the HM-102 - I did find on Amazon a Bourns unit (P/N:  PDB183-GTR02-254A2 - which I also found at Digi-Key) that had a "smooth" 1/4" shaft and ordered that .  The cost:  About US$13 or so, including shipping - cheaper that the other parts suppliers, and faster delivery, too.

Figure 3:
The new push/pull potentiometer/switch installed in the re-
furbished HM-102.  This is the Bourns P/N
PDB183-GTR02-254A2 - still readily available.
Just visible in the photo is a 1.3 MegOhm resistor across the
terminals to bring the maximum value closer to the 200k-
Ohms of the original device and this may help to make it
slightly less "touchy" during adjustment.
Click on the image for a larger version.

When it arrived a few days later it was a perfect fit with the shim/spacer of the original switch/potentiometer.  As this potentiometer was intended for stereo audio, it has two independent sections - and the switch itself was a DPDT - so only half of the device needed to be wired up.

Getting it working again

The only other casualties were on the bridge's PC board:  The 2-20 pF trimmer capacitor was seized and the 50k board-mounted trimmer potentiometer was too "flaky" to be trusted.  It so-happened that I had an exact replacement for the capacitor and found a very similar trimmer potentiometer in my collection that was 60k - close enough.  Spending a few minutes with a dummy load, I was then able to null the VSWR bridge and calibrate the meter as described in the manual.

A quick test showed that the power meter was within specifications and the VSWR bridge's balance was good from at least 80 through 10 meters when spot-checked.

Conclusion

At first, I thought that this vintage HM-102 might have been parted out and/or consigned to the trash can, but with only a bit of work and a few parts, it was resurrected and cleaned up.  Aside from a few scuffs revealing the fact that it had, in fact, been used for some time, it works well and it looks very good for a piece of electronics that is a half-century old and had been nearly drowned!

 * * *

Footnote:

1. The HM-102 - which is designed to work from 1.8 through 30 MHz - appears to have been introduced around 1972 when the kit cost $29.95 while in 1981, the kit cost $49.95 and was apparently discontinued after that.  There was also a VHF version for 6 and 2 meters - the HM-2102 - that looks very similar and in terms of mechanical design and much of the above discussion could be applied to it as well - and it even uses the same potentiometer/switch as the HM-102.
2. Modern radios have "fold-back" circuitry to automatically reduce power if they "see" a high VSWR.  It is best to set the transmitter to low power if the VSWR is unknown (say, 5-10 watts on a 100 watt radio) as even the best protection circuit isn't completely able to protect the radio under all conditions - particularly if one is using an external antenna tuner and forgetting to reduce the power level.  Running a low power output during tuning prevents the fold-back circuit from activating as much:  If the radio had been set to high power, the output power will rise rapidly as one attains a better match (e.g. when using a manual tuner) and the user will have to frequently re-set the SWR to full scale to obtain an accurate reading and possibly confusing the operator - but at low power, this complication is minimized.
   
3. The original potentiometer/switch was 200k, but the modern replacements are only readily available as 250k devices with a logarithmic taper.  This higher value and possibly the logarithmic taper may make this control - which is simply placed in series with the front-panel meter - a bit "touchier" than the original when it is adjusted at some power levels.  Placing a 1.2-1.5 MegOhm fixed resistor (the precise value is unimportant) across the potentiometer connections will reduce its maximum value to about 200k and make this adjustment slightly less "touchy".  If you have a meter like this but can find only the 500k version of the potentiometer, go ahead and use it anyway - just place a 330k fixed resistor across its two connections to limit its maximum value to about 200k:  It, too, may be a bit "touchy", but it will work.
    

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This page stolen from ka7oei.blogspot.com

[END]

Making LED headlights RF-quiet

TL;DR

 If you have LED headlights that are causing RF interference, ferrites alone will probably not be enough to completely solve the problem:  You will probably need to put the offending switch-mode LED controller in "RF Jail" as described below.  While VHF/UHF is the target of the efforts noted below, they should be equally effective at HF.

* * * * *

We live in a world of RF - and an increasing amount of this is from devices that are not intended to produce radio-frequency energy.  These devices have proliferated in the past several decades and surround us at home, work and in our cars.

Figure 1:
The vehicle in question with LED headlights -
which are now RF-quiet on all bands.
Click on the image for a larger version.

Generally speaking, modern internal combustion vehicles are in RFI terms "pretty quiet" - especially compared to their electric and hybrid counterparts - and someone in such a vehicle will experience less RF noise when they are "out and about" in a rural-ish area than they will at home.  For amateur radio operators, this is a good thing as mobile operation often includes weak signals - whether this is on HF, or on VHF/UHF with weak signals from distant repeaters or during simplex operation, with intervening terrain.

Effective mobile operation is therefore contingent on a vehicle that is intrinsically "RF-quiet", but this also means that any accessories that you might add to this vehicle also be RF-quiet as well.  These days there are any number of things that you might throw in your car that can spoil an otherwise-clean RF environment and the short list includes USB chargers¹ , GPS receivers and extra lighting ², to name but a few.

Real-world case - Aftermarket LED headlights

A friend of mine recently installed aftermarket LED headlights in his older Honda CRV ³.  It took a while to correlate the cause, but he eventually noticed that when the headlights were turned on, he lost significant weak-signal sensitivity on 2 meters:  Around town and with stronger repeaters, the effect wasn't really noticeable, but when the repeater was distant - or when communicating simplex (e.g. direct - with no repeater) with a weak/distant station - turning on the headlights dramatically reduced received ⁴ signal quality and range.  As it happens, he has more than one VHF/UHF radio in his vehicle and although both were affected, the one with its antenna mounted to the front fender - much closer to the LED headlights - was more severely impacted.

LED headlights typically consist of two modules:  The LED module itself and the "controller" - which itself is a switch-mode power supply to regulate the current to the LEDs as well as switching arrays of LEDs for high/low beam - and even color temperature in some models.  Sometimes these units are combined, but in this case, they were separated with a short cable, the "controller" being separate from the LED module that is mounted in the headlight housing.

Even knowing that it was futile, we tried putting clamp/snap-on ferrites on the cables to/from the LED modules and its controller, but all that we could manage was a slight reduction in interference that was hard to quantify, taking the problem from being "terrible" to just "awful".  This was not unexpected:  Under the very best conditions ferrites alone may provide 15-20dB of reduction in conducted energy (2-3 "S" units) but at VHF/UHF getting anywhere near that much attenuation is very difficult - and measurements indicated that even if we did achieve 15dB reduction across the board, the "jamming" of weak signals by the headlights' switch-mode controllers would still be significant:  It would be like taking an "10-over S-9" interfering signal down to just "S-8" - still pretty bad!

Methods of filtering

As noted earlier, simply putting ferrite devices on the conductors can reduce the amount of conducted energy, but their effect is typically limited - likely 15-20dB in the best case when this is the only method employed and ferrites alone are likely to be effective only when the interference is very slight to begin with.  Ferrite devices - such as beads - simply add inductance (and thus loss or impedance) to RF energy while leaving DC and low-frequency signals alone, but these devices have limitations:  Properties such as self-resonance and the permeability of the magnetic material vary wildly with frequency and high levels of attenuation are difficult to attain - particularly at high frequencies (e.g. VHF/UHF) where even short conductors carrying RF currents can radiate with reasonable efficiency - particularly when the noise-generating device and the receive antenna are in close proximity.

As discussed previously on this blog ⁵ one sure-fire way to quash such interference is to put the offending device in "jail" - that is, enclose it completely in a metal box and use both inductive and capacitive filtering on each and every wire to prevent RF energy from being conducted and the metal enclosure can prevent direct radiation from leads themselves - particularly important at VHF/UHF but less-so at HF.  Done properly, this method can be "completely" effective ⁶ in preventing interference.

Figure 2:
Simplified diagram of the method of filtering.  The inductance - provided by the ferrite beads ("L")
provide "choking" impedance to the RF currents being carried on the wires from the LED controller
(the "noisy device") before they connect to the feedthrough capacitors ("FT") in order to maximize
their efficacy.
Click on the image for a larger version.

Figure 2 shows a very effective method of dealing with this problem and it involves inductance ("L") and capacitance in the form of "feedthrough" capacitors (marked "FT").  The inductance is in the form of a ferrite bead installed on each of the conductors between the "noisy device" and the capacitors that increase the impedance at radio frequencies on that conductor.  The capacitors are then used to shunt the remaining RF energy to the local "ground" which, in this case, is the partition on which the feedthrough capacitors are mounted (this will be discussed shortly) and also the metal enclosure in which the noisy device is mounted.

The intent here is to prevent RF currents flowing through to the "external connections" where the wires themselves will act as antennas to radiate the RFI generated by the noisy device.  By shunting RF to the partition - and the metal box itself - the remaining RF energy will be minimal and confined within the enclosure.  The above configuration is easily capable of attenuating RF energy from HF through UHF by 30dB or better (more likely 50-60dB at some frequencies) - a value far higher than ferrite alone.

The metal box containing the electronics and filtering offer another important benefit:  As even short conductors a few inches/cm long can radiate at VHF/UHF, placing the noisy device and its conductors within the metal enclosure will prevent this.

An important feature of the design is that the inductances ("L") are located between the noisy device and the feedthrough capacitors.  As these inductances (ferrite beads) offer 10s to 100s of Ohms of impedance to the RF signal, this allows the very low impedance of the feedthrough capacitors at those same frequencies (which is likely an Ohm or less at higher frequencies) to more-effectively shunt that energy.  If the ferrite beads - or some sort of inductor - had not been installed, the shunting of the low-impedance RF energy from the noisy device would have been far less effective.

Figure 3:
An assortment of feedthrough capacitors.  The top two
rows are of the "screw-in" type, typically mounted to chassis-
walls and bulkheads while those on the bottom row are the
"solder-in" type as used in the partition in this project.
Click on the image for a larger version.

In this case, we were preventing RF from a noisy device from leaving the enclosure - but if we were trying to protect a sensitive device FROM RF energy from a nearby transmitter we would place the inductances on the conductors coming from the outside world as well to allow the capacitors to better-perform their function.

Feedthrough capacitors and the partition

A bit more needs to be said about "feedthrough capacitors".  Even if you are "into" electronics, you may not have seen these devices for the simple reason that they are a bit esoteric - and, perhaps, they are not quite as prominent as they have been in the past.  Figure 3 shows an assortment of feedthrough capacitors:  The top two rows are chassis-mounted types that are held in a pre-drilled hole by a nut while the three on the bottom row are of the "solder-in" type.

A feedthrough capacitor has a wire that passes through its center with the "capacitance" surrounding this wire over the length of the of its body and the other "plate" of this capacitor is the body of the feedthrough capacitor itself.  By being constructed this way, there are no wires or leads between the "capacitor" part  and either the signal or ground wires and as such, any series inductance - which would reduce the efficacy of the capacitor - is minimized.

Figure 4:
Solder-in type feedthrough capacitors soldered
to the brass partition.  This large sheet of metal
provides a low-impedance RF path to the
common "ground" (e.g. case) to contain RF
entirely within the metal case.
Click on the image for a larger version.

Compared to a "normal" capacitor with wires, such a capacitor is far more effective at bypassing RF energy to "ground" and it also suffers much less from parasitic issues like self-resonance - a property in which the capacitor and its internal inductance form a resonant circuit can cause it to practically "disappear" from the circuit (e.g. cease to be effective) at certain frequencies.  For this application - where it's particularly important to reduce RF interference at VHF and UHF - the use of feedthrough capacitors is - along with some inductance - a nearly foolproof method of attenuating such energy without resorting to surface-mount components and/or a specially-designed PC board. ⁷

Figure 4 shows nine feedthrough capacitors soldered to a brass partition (the soldering to the plate is on the opposite side) and as can be seen from the photo, these are capacitors that have a wire that runs through them.  As such, they have no "ground lead" aside from the outside of the body of the device and all of them are tied together on the same piece of metal.  This method assures a low impedance RF path between all of the capacitors and since the partition itself is bolted to the aluminum case (see Figure 5), it, too, is well-bonded.

Putting it in the box

Figure 5:
The LED controller in the box w/filtering.  By
containing RF currents within the box, both
common-mode and differential RF currents
on the leads are reduced to near zero.
Click on the image for a larger version.

To eliminate direct radiation from even the very short leads, the LED's controller which can cause problems at VHF/UHF, ferrite beads ⁸ and feedthrough capacitors are contained within a box as seen in Figure 5.

Toward the top of the image we see the switch-mode controller for the LED headlights, bolted inside the case (which also helps dissipate heat) and farther down we see that all nine wires (three for power, the remaining six to the LED module itself) connect to the feedthrough capacitors on the partition. Each of these wires has its own ferrite bead and these wires go directly to their respective feedthrough capacitors on the brass partition, which is held in the case with screws.

Below the partition (see Figure 5) are the wires that connect to the outside world:  On the right are the three wires that go to the power supply (e.g. the original connector to the headlights) while the gray cable on the left goes to the LED module.  The original LED retrofit had very short leads - on the order of 5" (13cm) for the headlight connector and another set of similar length to the LED module - and this made the installation a bit challenging as there was just enough wire to make the connection between the controller and the capacitors on the partition and install the rather large ferrite beads.

The box containing everything is die-cast aluminum and it's a bit larger than necessary - but it was the only size for which I had two identical cases and also large enough to accommodate the LED controller and the filtering.  As this box is quite a bit larger than the original controller, rather long wires had to be used to allow it to be placed where there was room in the rather crowded engine compartment, somewhat away from the headlights.

Figure 6:
As the original cables were very short (about 5", 13cm) longer
wires had to be spliced to allow placement of the large die-
cast box.  This shows the male headlight connector and the
LED module spliced to the cable and covered with nylon web.
Click on the image for a larger version.

At the opposite end of the wires it was necessary to splice the added cable to the LED unit's headlight connector and LED module and this was carefully done using soldered connections insulated with head-shrinkable tubing, all of which was covered by woven nylon braid for protection and a neater appearance.

The result

With the added length of the cables, there were nooks and crannies into which the die-cast box with the LED controller could be placed within the engine compartment of the Honda CRV.

The real test came when a distant 2-meter repeater was keyed up to cause it to send its ID:  The return signal was very weak and noisy - as hoped and expected - but there was no difference in the way that it sounded when the lights were switched on and off.  While admittedly unscientific, this test tells us pretty much everything that we needed to know:  Whatever RF interference there is that might be escaping the box and its filtering is well below the level at which it can be detected and the problem is considered to be solved!

* * * * *

Footnotes:

1. The topic of "very noisy" plug-in USB chargers effectively "jamming" VHF/UHF reception was discussed on this blog several years ago - see:  "How USB car power power adapters can ruin 2 meter mobile reception" - link and its follow-up article:  "A 'quiet' 5 volt USB car power supply" - link.
   
2. This same friend frequently volunteers in public service events involving runners and cyclists on roads where it is required that yellow/amber lights be used to minimize hazards.  Certain makes/models of these lights have been observed to produce tremendous amounts of RF energy that effectively quashed all 2 meter reception,so they were sent back to the seller until he found a unit that was "quiet".
3. Check your local regulations regarding retrofitting of headlights with equipment other than that of the type provided by the original manufacturer.
4. A low-level increase in the noise floor in the proximity of the vehicle would have absolutely no effect on the transmitted signals, but the result of this interference is that the station in his vehicle became an "alligator" - all mouth, no ears - meaning that he was able to "talk" much farther than he could hear.
   
5. Whereas simple capacitor (shunt) or inductor (series - and this includes ferrite devices) may reliably attenuate an offending signal by 15-20dB or so at best (very generally speaking) combining both types of reactance - "L" (inductor) and "C" (capacitor) - appropriately can provide many 10s of dB of attenuation if done properly - easily 30-60dB for simple circuits.  This greater amount of attenuation is far more likely to be able to put the interference from the device well below the noise floor of the receive system.  This is the technique used in footnote #1 (above) and explained in some detail in the blog entry "Completely containing Switching Power Supply RFI" link.
6. "Completely" eliminating conducted RF is actually impossible, but reducing it by 30-60dB is likely to attenuation it below the level of detection.
7. The use of surface-mount components - like capacitors - with their lower parasitic reactance than their counterparts with leads - can be used very effectively to filter RF, but several cascaded stages of such capacitors and inductors - and careful layout of a PC board - are likely to be required to obtain sufficient attenuation.  "Feedthrough"-type surface-mount capacitors are also available - which have excellent performance - but these, too, require a properly-designed PC board.  With the solder-in feedthrough capacitors, a brass partition was used as it was easier to drill and solder to than one of copper.  If screw-in feedthrough capacitors were used, the partition could have been aluminum.
   
8. The ferrite beads used in this project were Fair-Rite 2643000801.  These use "43 mix" ferrite and are 0.295" O.D., 0.297" long and 0.094 I.D. (7.5x7.55x2.375mm) and are able to accommodate the wires + insulation of the conductors from the LED controller.  This material has a typical impedance of about 94 ohms at 100 MHz and cost about $0.24 each in single quantity at the time of writing.  I used them primarily because they were on hand.
   

 * * * * *

This page stolen from ka7oei.blogspot.com

[END]

Replacing the relays in the Yaesu FT-480R (FT-280)

tl:dr:

Figure 1:
The front panel of my "FT-280R" all-mode 2 meter
transceiver including paper labels showing the "new"
modes.
Click on the image for a larger version.

If you have a Yaesu FT-480R that doesn't work, it's probably the relay(s)!

* * * * *

Back in 1994 I picked up an FT-280R at a local swap meet for a good price - particularly for an all-mode 2 meter transceiver.

Except, of course, there was never such a thing as an "FT-280R":  This radio had clearly been modified, a previous owner attaching pieces of self-adhesive paper with "R" written on it after the model number, and similar "+" and "-" labels added to the MODE switch.

A look under the hood shed some light on this mystery:  It had likely been sent back to Yaesu at some point as an FT-280 - an uncommon radio (perhaps sold only in the Japanese domestic market?) that did not have repeater offsets - and modified to the otherwise-identical FT-480R which did "know" about repeater offsets.

At some point I acquired the manual for the FT-480R (I don't recall if I got it from Yaesu, or if it came with the radio) which the elements of a service manual - including schematics, locations of semiconductors and adjustment/test point and alignment procedures - but it was clear that this radio wasn't exactly like an FT-480R, either, as it contained an additional board with a pair of relays on it that the manual didn't show.

Figure 2:
The "unobtanium" 221D012 relays used in the FT-480.
These relays use a different from factor from modern/
available relays requiring a bit of "adaptation".
Click on the image for a larger version.

It quit working!

As this radio was probably made in 1980 or 1981, it's long in the tooth and about a dozen years ago, it gradually became too unreliable to be useful, requiring more frequent "percussive repairs" to make it transmit or receive and the problem was traced to three relays used to steer important things like the transmit and receive voltages to their respective circuits.  A bit of "cleaning" (piece of paper between contacts, contact enhancer, etc.) would help initially, but even that stopped working.

These relays were Fujitsu 221D012 - low-current, DIP-style DPDT units, but a quick check revealed that their form factor (pin-out, pin spacing, etc.) was not in common with anything else that was in current production.  I did find new-old stock units that some were available - some on EvilBay (who knows what they really are?) and others from RF Parts - but they were quite expensive, on the order of $20 each from RF Parts.

Figure 3:
One of the control boards with two of these relays.  This
board seems unique to the "upgraded" FT-280 as it doesn't
match what is in the FT-480 manual.
Click on the image for a larger version.

Since there were three relays that had gone intermittent, I didn't relish spending $60 (plus shipping) for relays that were likely 40+ years old - and since these were un-sealed, they would likely have (or soon have) the same oxidation issues that made the original relays so unreliable.

"Substitute" relays

As no drop-in relays were available, I found some similar-sized, 12 volt DPDT relays - using a now-common pin-out (I used Hui Ke HK19F-DC12V-SHG relays - but many others share the same pin-out and could have been used) - and set about making a "carrier" board to accommodate them.  While I could have made a PC board to adapt their pin-outs to those of the original Fujitsu, I decided that with just three relays in question, doing so wouldn't be worth the effort.

Figure 4:
Carrier board with relay mounted to it.  The clearance on the
board shown in Figure 3 allowed the relay's position to be
slightly shifted, simplifying construction as the new relay's
pins offset from the original relays'.
Click on the image for a larger version.

Instead, I used a small piece of phenolic prototype board.  For the undocumented board with the two relays, I had a bit of extra space available on either side which allowed them to be offset from the original pins:  Short pieces of wire (26 AWG) were soldered to the proto-board in the locations of the original relays' pins and short wire jumpers were run from there to the locations of the corresponding pins on the new relays.

To "ruggedize" this assembly, the jumper wires were covered with UV-cured resin before the new relays were soldered into place and then each relay assembly was soldered into place on the board.

The third relay - next to the carrier oscillator, on the main RF board - was a different matter:  It was surrounded by components which meant that there wasn't enough room to offset it from the homebrew carrier board, so the new relay had to be placed directly atop the original pins.

Figure 5:
Carrier board and relay used near the FM oscillator on the
main board.  Unlike that in figures 3/4, this relay couldn't
be offset so the pins were folded over and wires used to
connect the relay to the carrier board so that it could sit
directly atop the original location.
Click on the image for a larger version

To accommodate this, wires were soldered through the homebrew carrier board like before, but the pins on the new relay were laid onto their sides and short pieces of 30 AWG wire-wrap wire were used to connect to the proper locations.  Again, this was flooded with some UV-cured resin for both mechanical stability and to prevent the relay's pins from touching the wires on the carrier.  A bit more resin was then used to adhere the new relay to the carrier board, making a solid unit.

With the leads protruding from the bottom of the carrier board straightened,  they were carefully aligned and pushed through the main PCB and soldered into place.  As it turns out, although the "new" relay sits slightly higher than the original, it just clears the bracket for the switches on the bottom panel of the radio.

Testing the radio

With radios of this vintage - with individual wires going everywhere - the bane of the service technician is wires breaking off their soldered connections on switches and/or PC board.  If caught early, the "bend" of the wire will keep the broken-off end very close to where it should be attached - but too often, one spends a lot of time reverse-engineering:  While the schematic is (mostly?) complete, neither it or the parts layout diagrams detail where every wire and connection go - particularly on the front-panel switches.

Figure 6:
A look on the main RF board on the underside
of the radio's chassis.  The two "new" relays are
along the top edge while other relay below and
to the left of center, just above the metal box
with the black label (the FM modulator).
Click on the image for a larger version

The broken wires fixed, the radio worked the first time it was powered up - more or less:  Going through the (somewhat incomplete and ambiguous) alignment procedure in the manual brought the radio back to usable condition - but one "semi-major" problem remains:  None of the top-row buttons (the yellow-brown) work reliably.  A bit of testing revealed that shorting the terminals on the back of the board resulted in their functions working, but a check with an Ohmmeter showed that they had all gone to high resistance despite having been injected with several flavors of "DeOxit":  The button switches themselves will have to be replaced, but I'll have to get with a friend with a 3D printer to come up with a means of mechanically coupling the front-panel buttons with the "new" switches.  Fortunately, the radio is perfectly usable - particularly when using SSB - even if none of these buttons work.

* * * * * *

This page stolen from ka7oei.blogspot.com

[END]

A (simple) WWVB loop amplifier for radio-controlled clocks

Note:

The techniques described below should work - with only minor adaptation - for any "Longwave" time signal used by these radio-controlled (non-GPS) clocks - not only WWVB, but DCF77, MSF, BPC and both JJY signals as well.

* * * * *

Last year I moved a bunch of SDRs (KiwiSDRs, RTL-SDR) and a bunch of network gear to a new shelf in my shack, but this placed them much closer to the wall on which I'd previously mounted the two "Atomic" (e.g. radio-controlled) clocks which had been there - and operating - for years.  Since then, they hadn't been able to reliably synchronize to the 60 kHz WWVB signal out of Fort Collins, Colorado.

Figure 1:
The two radio clocks surrounded by
the four-turn loop, near a number of
pieces of "noisy"equipment.
The top clock is set to UTC and
the bottom for local time.
Click for a larger version.

While annoying, I wasn't terribly surprised.  There are several switch-mode power supplies involved in the aformentioned gear and it's not uncommon for them to operate in the 30-60kHz range, offering the potential of "jamming" the receivers.  As both the location of these clocks - and the nearby gear - is convenient, I wasn't too inclined to move them again and initial efforts to "filter" the switching power supplies didn't really help - but I wasn't surprised about this, either, since it's likely direct coupling of their magnetic fields that is the culprit rather than any electrostatic field as the clocks themselves use ferrite loopstick antennas sensitive to just the H-field.

A solution

 Many years ago a friend came to me to solve a similar problem in a downtown Salt Lake office building where the WWVB clocks in a conference room never synchronized and I constructed the remote loop and amplifier/coupling system, described here:

• Getting "Atomic" (WWVB) clocks to work indoors and in weak signal areas - LINK 

In short, a rooftop loop antenna amplified the signal and it was conveyed into the room with the clocks where it was further amplified and then, using inductive loops placed in the proximity of the clocks.  This is how the WWVB signal was coupled to them.  To my knowledge, this system worked for many years (well over a decade) and for all I know, it may still be in use.

 Why revisit?

I've tackled this type of problem before - but I decided to revisit it as the circumstances are slightly different:  I already had a signal source as noted below plus I wanted to see if I could do this with more commonly-available components in a simpler manner.

While I don't have a WWVB loop on my roof, I do have a dedicated LF E-field whip antenna - a 40 year old LF Engineering LF-400B with integrated low-pass filter.  This antenna has been on the roof wherever I have lived almost continuously since I purchased it in the mid-late 1980s and with a few repairs over the years, it still works well, having been on the roof of my current house for several decades.  Its use for LF reception as described on the following page:

• A (semi)-typical suburban E-field whip receive system for the 630 and 2200 meter amateur bands - LINK 

The fact that I already had an LF/VLF receive antenna system meant that I had a "clean" source for WWVB, and other devices that receive signals below 500 kHz (e.g. LF receivers for 630 and 2200 meter operation and my Blitzortung "Blue" receiver) and I decided to add one more to the list.

Other types of outdoor antennas 

Note that the circuit described here should work well with other types of active antennas including E-field types such as the PA0RDT "Mini-Whip" and the DX Engineering ARAV3 - to name but two.  An amplified loop such as the Wellbrook and similar will work, provided that it is not oriented such that the desired time station's transmitter is not in its nulls.

Buffer/Amplifier

Through back-of-the-envelope calculations I figured that the already-amplified signal from the active whip needed another 15dB or so of boost and it could then be applied to a loop of wire around the WWVB clocks on my wall.  One thing that helps greatly is that the WWVB signal is extremely strong here in northern Utah - on the order of 5mV/meter or so - and connecting an oscilloscope to my LF-400B whip's signal output showed that the amplitude-modulated time code of the 60 kHz signal from WWVB was visible among the many others.

What I needed to do was to tap off the signal (e.g. "bridge" the connection) from the existing coaxial cable without affecting was was being sent to the other devices using it, amplify it. and apply it to the loop - and I did this "tap" using a BNC "Tee" connector on my antenna feed.

The circuit diagram below gives more details:

Figure 2:
The schematic of the loop buffer/amplifier/driver showing the isolation from the power supply
via L1, the buffer circuit of Q1 and the amplifier and loop driver of Q2.
Click on the image for a larger version.

Circuit description

Of high importance is L1, a common-mode choke, liberated from a failed switch-mode power supply somewhere.  This particular unit has an inductance of about 1mH per winding meaning that it has about 377 Ohms of impedance at 60 kHz and helps to prevent a ground loop and the coupling noise from the power mains.  If you replicate this circuit I would strongly suggest that whatever you use for L1 have at least a similar amount of inductance.  On either side of L1 are electrolytic capacitors (C1, C2 - preferably of low ESR types) to offer low impedance and a degree of reinforcement of common-mode rejection through L1 while C2 and C3 provide RF bypassing for the circuit itself.

A buffer amplifier consisting of Q1 - with a high-impedance input, but no actual gain - couples the signal from the existing antenna:  Having several k-Ohm of input impedance, it is unlikely to appreciably load the existing antenna system.  On the feed from the E-field whip, I simply installed a coaxial "T" connector to allow me to bridge across the signal feed rather than split the signal, which would have been complicated owing to the fact that the DC power for the whip was also being carried on that same cable.  The connection to the amplifier in Figure 3 was made using a very short piece of coaxial cable (about 2 feet long - less than a meter) and since this whip is not used for reception above about 500 kHz, neither its presence or that of the added amplifier had any discernible effect on the other received signals.

Coupling from the existing antenna are series components L2 and C4, selected to resonate at about 60 kHz:  The resonance is extremely broad, so finding a capacitor combination precisely equal to the "ideal" value of C4 - according to the formula below actually calculating as 0.007uF (7000 pF) - is unimportant.  This series resonant circuit is probably not essential and a simple coupling capacitor of 0.01uF (10000 pF) could be used (omitting L2 entirely) but I chose built it with L2 to broadly filter off-frequency signals - something that might be important if your E-field whip antenna doesn't have a low-pass filter to remove AM (Mediumwave) signals as mine does as well as to block any stray coupling of HF signals when I transmit.

Figure 3:
The circuit of Figure 2 built on a piece of prototyping board
in the case.  Bifilar choke L1 is on the right with the BNC
connector (J1, input) and output to the loop (J2) on the left.
Click on the image for a larger version.

The buffered signal from Q1 is then passed to amplifier Q2 which is configured to have "about" 15dB of signal gain.  This circuit is, perhaps, slightly more complicated than it needs to be, but with its feedback, it is very stable and tolerant of large signals.  The use of electrolytic capacitors for C5 and C6 is, perhaps, overkill (0.1uF ceramic would probably suffice) but I used them as they were handy.

As the signal from WWVB is quite strong at this location, there is only one stage of amplification shown in Figure 2, but if I lived more distant, greater overall system gain might be required.  Replicating the circuit involving Q2 (R4-R8, C5-C6) and cascading it with the existing amplifier would add yet another block of gain to boost the absolute signal level - but this would presume that whatever active antenna you were using outdoors to pick up the WWVB signal was working well, providing a "clean" signal and that the deficit was just in signal strength at the clocks rather than than signal-noise ratio.

Due to the smallness of the project box that I chose I couldn't mount the bifilar choke "through" the prototype board so it was mounted on the edge to minimize height.  To hold it in place I used UV cured resin along the edge to prevent it from breaking the pin connections mechanically:  UV cured resin is very handy as it's about a strong as epoxy, but it is cured almost instantly meaning that it's able to be handled immediately.  For the BNC connector, the one that I found in my parts bin didn't have its matching mounting nut, but more UV-cured epoxy did the job for that, too!  As can be seen in Figure 3, I didn't bother "mounting" the board in the box, letting it hang about on its own wires.

Indoor Coupling loop

The "coupling loop" - visible in Figure 1 and shown on the schematic - is just a loop of wire - and it is used to inductively couple the signals from the outside antenna to the clocks.  In my case, I measured a rectangle that would encompass both of the wall clocks and found a cardboard box with similar dimensions and on it I wound four turns of 22AWG hookup wire.  Connecting this loop to the amplifier, I used some shielded microphone cable:  Coaxial cable would have been fine as would just some single-pair speaker wire as this frequency is not all that much higher than audio!

Neatly forming the individual conductors, I used small "zip" ties to hold them together and with four screws, attached it to the wall, placing the clocks inside the loop of wire.  Within the loop, signals from the amplifier would be strongly coupled into the ferrite loopsticks in the clocks themselves - but being very small in terms of the 60kHz wavelength, this loop is unlikely to radiate more than a few feet/meter outside it.

To improve efficiency of the coupling loop I wanted to series-resonate it at around 60 kHz as this would increase the amount of energy transferred to the loop from the amplifier somewhat, effectively providing "free" signal gain.  Measuring the inductance of the loop I found that it happened to be about 22uH and using this simple formula, I calculated the value of C7 - the resonating capacitor in Figure 3:

LC = 25330/(F_MHz)²

Where:

LC is the product of the inductance and capacitance (e.g. Capacitance in pF * Inductance in uH)

F_MHz is the desired resonant frequency in MHz (e.g. kHz/1000)

Knowing that we have 22uH of inductance in the coupling loop and a frequency of 60 kHz (0.06MHz) we end up with "LC" being equal to 7036111.  Dividing this value by the known inductance of our coupling loop (22uH) we get  the capacitance, as in (7036111/22) = 319823pF, or 0.319uF.

Figure 4:
The finished amplifier in its box, hanging
out below the loop - connected, and in
service.  (It's just visible in the bottom
of Figure 1)
Click on the image for a larger version.

As 0.33uF (330000 pF) is the closest common capacitor value, I used that for C7.  Again, as with C4 and L2, the resonance is very broad and precision isn't too important.  The article linked near the top of this page goes into more detail on how one would construct and resonate a coupling loop.  Based on this formula, if I wanted to resonate the same loop for use with DCF77 at 77.5 kHz I would have picked a 0.18 or 0.2uF (180000 or 200000 pf) capacitor, instead.  Similar changes could be made to accommodate longwave time signals on other frequencies (e.g. 40 kHz, 50 kHz, 68 kHz).

The formula above can also be used to calculate the value of C4 with the 1mH (1000uH) L2 inductor:  If your interest was for another frequency, such as DCF77 at 77.5 kHz, C4 would be 0.0047uF (4700 pf), instead.

It need not be said that this loop should not be placed very close to whatever outdoor receive antenna you are using - but more than about 10-15 feet (3-5 meters) should suffice:  If they are too close to each other, feedback (oscillation) could occur - but as this loop is only around 0.01% of a wavelength in circumference it does not radiate efficiently at all - and since it's inductive, its signals won't efficiently couple to an E-field antenna, anyway.

In the diagram, C7, the resonating capacitor for the coupling loop, is shown at the amplifier - but it could have been placed at the loop itself.

Power supply  

First off, do not use a switching power supply for this device!

As noted, common-mode choke L1 was used to "decouple" the power supply from the amplifier - and also from the coaxial cable of the LF antenna.  To power this loop amplifier I would strongly recommend using ONLY a transformer-type DC power supply and NOT any type of switching power supply for the simple reason that the switching power supply will be comparatively noisy, and it - its harmonic - will likely operate at/near the frequency of WWVB or whatever time signal you are trying to receive.

This power supply does not need to be regulated:  Simple capacitor filtering with low-ish ripple (a few hundred millivolts) will suffice and any voltage between about 11 and 16 volts will work which means that about any old "wall wart" in that voltage range - regulated or not - would be fine.

Conclusion

Having had the parts on hand it took only a bit more than an hour to piece this together and almost as long to put it in the box seen in Figure 4.

When I forced both clocks to re-acquire WWVB's signal for syncing they immediately set themselves to the correct time and date - and since it had been the start of daylight saving time the night before but had not been able to synchronize prior to this - they "knew" the new, correct time, too!

* * * * *

This page stolen from ka7oei.blogspot.com

[END]