Magnetic Loop – step by step

Well, the replacement stepper controller chips arrived early this morning. Didn’t take too long to replace them and connect everything up for a test.

The good news is that after a slight adjustment of VR2 the stepper appeared to be controlled by the rotary encoder. It made the right noises.

Try as I may, I wasn’t able to carry out the initial setup procedure as outlined by Loftur. I couldn’t find a peak in the noise.

After applying a portable SWR analyser directly to the loop when I thought I was at the lowest frequency point the loop would tune, I discovered there was a dip around 23 MHz! I was nearer the other extreme of what I think this loop’s range should be. So I’m thinking I may have the stepper motors wired wrongly with the result that the stepper goes in the opposite direction to what the controller thinks it should be.

After switching one pair of wires at the controller end, the stepper made a terrible noise as it hit the end of the capacitor’s travel. Checking it tentatively it didn’t seem to distinguish between clockwise and counterclockwise movement of the encoder. Whatever you did, the stepper turned in the same direction.

Time for further closer inspection of the wiring around the newly re-oriented common mode chokes and their associated bypass capacitors. This kind of behaviour may have something to do with a missing or failed connection in this part of the circuit.

Stepper only appears to turn CCW which turns the VVC CW. I’m now hoping the glitch is pretty obvious in this part of the board or associated wiring.

Magnetic loop progress

Finally found some time to re-orientate the two transformers. I decided to remove the eight bypass 10nF capacitors to give my soldering iron more wriggle room to remove the transformers. The solder on the transformer pads needed some encouragement – in the form of more molten solder – to loosen up. This allowed me to use solder wick to take away most of what was holding them in place. The corners of the transformer are pretty fragile and would not tolerate much heat. Wasn’t too hard to lift them. Much more complicated was cleaning up the holes where the eight capacitors had been. I managed to destroy one of the pads. Decided to stop destroying the PCB and to re-install the bypass capacitors on the other side of the board.

I was actually able to use most of the capacitors I had removed and checked every connection a couple of times at least, especially a couple of the tiny pads for the caps that are near the larger choke pads.

Soldered the transformers back in properly this time, again checking every step over and over again.

After re-orienting the common mode chokes...
After re-orienting the common mode chokes…
...and replacing the caps on bottom of the PCB to avoid damaging fragile pads.
…and replacing the caps on bottom of the PCB to avoid damaging fragile pads.

Taking a few deep breaths now and having a break before reassembling the whole device and connecting it to the radio and the loop for the moment of truth and to find out if the A4975 stepper driver ICs can come back to life after having both their two outputs accidentally connected together! Normally they are connected across one of the stepper motor windings.


Well, because there is still no response from the stepper motor, the answer, unfortunately, is no, they probably need replacement. At least this time they’re not red hot. I have a pair of A4975 chips due to arrive sometime on Monday. Essentially $4.90 a pop, no shipping charge.

And reading the datasheet on the A4975 I see on page 10 mention of the thermal protection circuitry that shuts off the output transistors when the junction temperature reaches +165 degrees C. “This is intended only to protect the device from failures due to excessive junction temperatures and should not imply that output short circuits are permitted.” And I reckon the tiny choke winding is close enough to a short circuit.

At least we are still at three out of four. Looking forward to being able to control the stepper to tune the vacuum variable capacitor and calibrating the controller, and perhaps opening myself up to a new level of complications. At least the well-commented code should be a smart guide.

In the meantime – getting way ahead of myself now – I’m thinking about drilling two holes in the loop to take the stepper wiring, possibly via a length of CAT 5 cable. People seem to think the twisted pairs work well with stepper windings. Not sure what type of connector to use in VVC part and lower down alongside the feed loop SO-237.

I also need to get a couple of plastic pipe ends to fit the PVC tube cover Henrik gave me to weatherproof the stepper and the tuning capacitor.

Then I need to think about a rotator and maybe a tripod roof mounting. And a way of remotely controlling the rotator. Anthony K3NG most probably.

At least it looks like it might be ready to take away to Tooraweenah for a field test in March!!!

Magnetic loop fault finding breakthrough

Maybe my mistake has been not to refer more directly to the actual code. Over the last 24 hours I have been steadily working through the last two years of email conversations. Loftur keeps the group informed of each new version of the firmware and the newly added features. Sometimes he jumps the gun, but then releases a newer version shortly afterwards.

Well, well well! I think I have discovered why the final part of the loop controller is not working and perhaps why the A4975 stepper motor driver ICs are running so hot. I have just been doing some continuity testing working back from the stepper motor connector, K1 and noticed that there is continuity between C11 and C12 and also between C15 and C16, and there is no continuity between the legs of say C16 and C14 connected T2 etc, all of which implies that I have oriented the common mode chokes, T1 and T2 exactly 90 degrees out from where they should be – I think and I hope… They are both surface mount with four points of contact(!!!!), so there might be some interesting re-working to be done. I actually have one spare if I bugger one up. Also I may need to replace the eight 10nF ceramic bypass caps nearby. But this is progress of a kind.

Loop Controller PCB before 'Aha!' moment
Loop Controller PCB before ‘Aha!’ moment

I wonder what made me install them this way. Surely not simply the orientation of the writing on the package!?!?!?, or the oblong shaped pads on the PCB?!?!?!?! Two visual cues to be misled by!

I suppose I should be optimistic about the transformers being okay if they still show continuity… There’s a big lesson here about using the schematic during the construction and not simply populating the board. And maybe even being curious enough to actually turn the component over to see its magic revealed and be inspired to orient it correctly.

The exterior of the common mode choke with two 51uH windings
The exterior of the common mode choke with two 51uH windings
Inside the common mode choke clearly showing the two 51uH windings
Inside the common mode choke clearly showing the two 51uH windings

And now of course when I look back to page 8 of the BOM and building instructions I can clearly see the correct orientation of the transformers. Oh me oh my!

And I have just noticed during my rapid review of the emails on the Loop Controller list that on 8 May last year, while advising someone about testing the sense of the stepper winding connections, Loftur dropped this comment, “Also, have a careful look at the circuitry around the A4975 stepper controllers. One fairly common mistake is to mount a common mode choke incorrectly.” I’ll say!

Magnetic loop controller – home stretch

After a long break I have finally got back to completing the Magnetic Loop Controller designed by Loftur Jonasson that I started almost two years ago now. I am using his original PCB version from June 2014 which indicates how long I have been intending to build this device!

I noticed a small SWR board kit based on the kitsandparts kit offered via the Loop Controller list by Frederik ON5IA.

I ordered a couple, but while I was waiting I checked where I had got up to with my original build.

I completed my original SWR bridge board based on the kit from I worked out I could use the same larger gauge bus wire (as used for the single turn in each ToroidCalc1 transformer) to securely and rigidly mount the SWR board to the inside of the BNC sockets. I discovered later that one of the joints to a BNC centre was intermittent, ie not actually soldered!

With the Controller apparently complete, I connected a 4-pin socket to the four wires coming from the stepper motor on the loop and attached the stepper to the VVC with the shaft couplers. I also spent quite a bit of time confirming the correct way to connect the main PCB to the SWR board, to the serial port, to the Stepper motor as well as all the other connectors and controls mounted on the front and back panels of the enclosure.

While the Teensy appears to be working as expected there was no apparent connection with the radio or the loop!!! So far the score is just one out of maybe four! (I suppose my four targets are 1. Teensy controller, 2. Serial connection with the radio, 3. Connection to the Stepper motor and 4. SWR Autotune.)

I did notice scratchy noise due to the dodgy non-joint on one of the BNC sockets. Also the Controller would occasionally just stop and reboot, especially if the enclosure was tilted slightly!

I checked continuity of the earth/-ve line. Seemed intermittent. Noticed I had used nylon stand-offs to mount the PCB. But there was a connection to the metal enclosure between the PCB ground plane via the heatsink tab of the 5 volt regulator. Decided to install metal stand offs. Needed added height of one washer to match original height for the USB socket to match the hole in the rear panel. Also confirmed that the -ve connected directly to the enclosure via the power socket mount.

Properly soldering the antenna lead eliminated the scratchy noise from the radio.

I had last installed firmware version 3.04. I updated the Arduino IDE and re-installed the Teensyduino libraries and then installed the latest version 3.08 of the Loop Controller firmware. No actual difference with connection to radio or loop.

Reading over the last couple of years of conversations on the Loop Controller Yahoo group I came across one post where someone had experienced problems connecting to their KX3. Loftur replied with a check about the way the TXD and RXD is connected to the serial lead. Mine was wired opposite to his description, as I may have confused the order on the serial header and the serial socket. Flipped the TXD and RXD connections and immediately I was able to see the frequency readout on the LCD update after a change on the KX3. Satisfying! A review of the schematic confirmed I had misread it earlier.

This was with the Elecraft K3 Auto setting on the Controller. The controller responds to changes of frequency on the KX3 but only after there is a change. And the update of the information in the frequency display feels a little laggy and as if the update periods are longish. Switching over to the polling method Elecraft K3 Poll the update is much faster and smoother. It also recognises band changes before a change in frequency. (Perhaps I need to change the KX3 setting for AutoInf back to its default). I am using a standard stereo patch cable between the serial port on the controller and the Acc1 socket on the KX3.

One step at a time! Of my four targets then, it looks like we may have achieved two!!! Now just SWR & Stepper to go.

I suppose I should be able to test the SWR board. I have a dummy load connected to the controller ANT BNC. Perhaps I could even calibrate the Power and SWR???? I have checked the ML.h file in the firmware package and read the section in the BOM and Building Instructions pdf (page 13) about the settings to enable the Power/SWR and Autotune options. #define PSWR_AUTOTUNE had been set to 0, so I set it to 1 and verified the program and re-programmed the Teensy. All pretty speedy. And it works, but because frequency ranges have not been set yet it seems to read most frequencies as out of range.

Definitely making some significant progress today!!!

Now to attack the issue with the lack of connection between the controller and the stepper motor.

I examined the solder joints of the two common mode chokes and tested them both for connection and continuity. I have also noticed that the two stepper driver chips are running very hot, way too hot to keep your finger on them for more than a touch. Also there’s probably a static discharge risk there too. Given their cost is only about $7 for these two ICs, it might be worthwhile replacing them, while taking proper antistatic precautions. It would be good if I could discover any other reason/s there is no linkage to the stepper and more importantly why they get so hot so quickly. I hooked up another NEMA 17 stepper motor directly to the board and there was again no movement, but the chips immediately heated up. With the stepper disconnected the chips still heat up.

Time to read more of the manual. That’s the only thing lacking here – a comprehensive manual pitched at dummies like me. I suspect a lot of the wisdom about using this device is waiting to be discovered in the email discussion group. I am slowly working my way backwards over the last almost 2 years I have been subscribed to the list and so far, I’m up to conversation #214 of 265 conversations, that is I have 214 to go! It would be great if you search the conversations directly from the group page… must see why I can’t do that…

Milk crate tower

Wacky idea #257 of the day
Since they were invented – apparently by an Australian engineer (see below) – milk crates have been used in many other ways besides holding milk containers.
A milk crate chair by Matt Sutter
Everything from furniture to milk crate versions of the loch ness monster.
Milk crate Nessie emerges from Australian backyard swimming pool!
Six interlocking milk crates stacked is 186cm high.
They feel stable but they need a flat level base.
They lend themselves to cable ties to tie together and offer numerous points for guying and attaching halyards.
Not sure how strong they would be at a height of 10 or 12 metres and whether they are structurally strong enough to withstand the forces exerted by guy lines etc. Presumably they would have a high degree of wind loading comparable to a steel lattice tower.
Not sure of the actual plastic material they’re made of – PE extrusion? It would be important to find out how strong it is especially where shackles etc might be connected for guying and attaching pulleys for halyards etc.
If six crates are 1.86m high, 18.6m requires about 60 crates (!) so I guess 9m equals 29 crates in a stack! 10m = 32 crates and 12m is about 40 crates.
It would not be practical to assemble a tower crate by crate from the base up as the upper crates would quickly become unwieldy and too heavy to lift. The same problem would arise with sections of a number of crates at a time. So there would be the same problem of the stresses involved in tilting and lifting any horizontal structure into a vertical position especially at the base pivot point. All crates would need to be tied together on all four sides, top and bottom, so requiring (4n + 4) ties where n is number of crates. I assume the interlocking crates would be solid when tied together.
The crates are bulky and may not be so practical for portable deployment. External dimensions are 36cm x 36 cm and 32cm high so 9 would occupy 108 x 108 x 32cm or 0.37 of a cubic metre so 40 crates for a 12m milk crate tower would occupy about 1.66 cubic metres.
One other downside besides wind loading might be the actual weight of the structure should it collapse. Long-term exposure to UV may also be an issue for a more permanent installation.
Below, a table listing various – mostly unrelated – pros and cons – the biggest one, of course, being that using them in this (or any other way besides storing milk containers) amounts to theft in the eyes of the makers and owners of the crates.
long/short term “borrow” required
requires no machining or preparation beyond cable ties
high wind loading
ready-made solid structure
weight if tower falls
UV exposure?
Here’s a Sydney Morning Herald report featuring the Australian engineer, Geoff Milton who designed the milk crate for Dairy Farmers and how they designed it with holes big enough for potatoes to fall through to discourage theft. This doesn’t explain why they made their internal dimensions the perfect size for storing vinyl LPs!
Coincidentally these milk crates were made by Nally, the same name as the manufacturer of Nally Radio Towers that shut down in 2015 when the owner passed away.

Back in the loop

My main project for at least the last 12 months has been building a solid magnetic loop antenna and its companion automatic loop controller. I’ve been roughly tracking its progress at my magnetic loop antenna project page on this blog.

As usual, life has got in the way, but I want to get back on track and complete the project. To start pumping some RF current through it again, over the weekend I spent a short time playing with the loop on WSPR on 40, 30 & 20m. The tests were too brief but they certainly confirm that the loop is capable of transmitting a signal in spite of the fact the loop is only half a metre above ground and surrounded by metal garden furniture, a steel framed awning and gutters.

I used the WSPR Beacon android app to control my transmitter. There was some discrepancy (tens of Hz) between the actual output frequencies on the app and those shown on WSPRnet. I also found that tuning the loop to each WSPR frequency using the iP30 antenna analyzer was easy and the KX2 gave lower SWR figures.

The brief test became an exercise in understanding theWSPRnet results taking into account propagation and loop orientation which was aligned north-south.

This map view combines all 20 spots of the 1W VK2RH transmissions from grid square QF56oc. The first test was logged at 2017-05-07 01:36 UTC. (I’ve trimmed repeated info from the chart below to improve its fit on the page.)

Time MHz SNR Drift Reporter RGrid km az
 05:24  14.097001  -15  1  VK4ALR  QG56fk  1151  356
 05:24  14.097016  -26  0  VK4TDI  QG62lm  733  14
 04:48  10.140109  -22  0  VK4TDI  QG62lm  733  14
 04:48  10.140094  -23  0  VK7TW  QE37pc  1057  198
 04:48  10.140091  -17  0  VK6XT  OF86td  3086  261
 04:40  10.140095  -27  0  VK7TW  QE37pc  1057  198
 04:40  10.140090  -4  0  VK3WE  QF32se  547  216
 04:40  10.140090  -22  0  ZL1RS  RF64vs  2069  101
 04:40  10.140092  -15  0  VK6XT  OF86td  3086  261
 04:40  10.140091  -16  0  ZL3GA  RE66ho  2130  126
 03:18  7.040121  -24  0  VK3BAL  QF22mc  711  230
 03:18  7.040134  -7  0  VK3AXF  QF33fn  516  235
 03:18  7.040135  -18  0  VK4MOB  QG62ol  734  16
 03:18  7.040130  -18  0  VK3DXE  QF21nv  720  228
 03:18  7.040128  -12  0  VK2TPM  QF56of  14  0
 03:18  7.040129  -14  0  VK7DIK  QE38cu  918  207
 01:36  7.040183  -16  -1  VK3AXF  QF33fn  516  235
 01:36  7.040177  -16  -1  VK2TPM  QF56of  14  0
 01:36  7.040184  -24  -1  VK4MOB  QG62ol  734  16
 01:36  7.040179  -21  0  VK3DXE  QF21nv  720  228

40 metres favoured north-south, while 30 metres was literally an all-rounder and 20 metres was too brief. These results probably say more about propagation than the loop, not to mention the heavy lifting done by all the reporter stations extracting my down to -26 or -27 dB signals from the noise! Impressive all round!

I wonder how many people are using the Sotabeams WSPRlite antenna tester device. Certainly looks tempting, especially for longer term antenna evaluation.

In any case, the main purpose of today’s exercise was to re-start the loop project. The To Do list includes

  • building & installing the SWR bridge into the loop controller,
  • deciding on the best way to couple the stepper motor shaft to the tuning capacitor shaft,
  • and wiring it all together with appropriate coax and control cables.


The Smartlock is an accessory for my SGC SG-239 HF Smartuner, and other ATUs they make. It can be bought ready made or built.

There are indicator LEDs that signal status (TND, l ‘Z’, 2:1, PHZ, FWD, Auto & Man) on the lower section of the PCB of the SG-239 but they are only visible near the unit. As they recommend mounting the unit in a weatherproof container and as close to the feed point as possible, it’s unlikely these will be useable except in testing and servicing.

For reference – here are the indicators and connections on the transceiver end of the SG-239:

B.I.T.E.* Status LED Descriptions – *Built In Test Equipment

TND This LED will light when the tuner has found an acceptable match. It will remain lit until conditions have changed which will cause the tuner to find a different match. (i.e. A new transmit frequency has been detected, or tuner has been reset.)

L ‘Z’ This LED shows the status of the antenna impedance. When lit, the impedance is 50 ohms or less. When off, the impedance is greater than 50 ohms.

2:1 This LED will light when the VSWR is greater than 2:1. It will extinguish when VSWR is less than 2:1.

PHZ This LED indicates the status of the antenna reactance. When lit, reactance is inductive. When off, reactance is capacitive.

FWD This LED indicates the presence or lack of RF power from the radio. When transmitting, the LED will light to indicate RF is being detected. In receive, the LED should be extinguished.

OTHER All LEDs will blink on and off at a rate of 2Hz to indicate the tuner was not able to find a valid match.

The LEDs are very small and quite faint and almost impossible to see on a sunny day.

As the ad below indicates the Smartlock provides two controls that can be used remotely from the tuner and close by the radio – allowing the operator to lock or stop the ATU constantly retuning as the load changes and to reset which forces a retune the next time a signal is transmitted.

The Smartlock also indicates if the ATU managed to tune the antenna and if the lock is on. The lock can be invoked when the antenna is to be used for receive or when there are too frequent changes in the physical environment of the antenna such as when mobile and passing trucks or going under bridges….

Installation requires normal coax and a four conductor cable for power and control.



Catalog #54-63
The SmartLock provides two external controls for the SG-230/237/239 Smartuner. The locking function prevents retuning despite changing antenna loads. The reset function forces the coupler into a retune cycle the next time a signal is transmitted. Tune and lock status is indicated by one green and one red blinking LED. Supplied with 9 feet (2.5 metres) connecting cable. For use with SG-230/237/239 couplers manufactured after July 1, 1993, only.

From the product number, this seems to be the unit described above, on the SGC shop page at

This is the official circuit from the manual for the SG-239 (

Its simplicity and the cost of the assembled unit have inspired a number of people to roll their own.

The colour code of the cable to the Smartuner appears to be:

TND = Green,
HLD/RSET = White
+12V = Red
Gnd = Black

Simplified SmartLock

Phil Salas – AD5X – decided to remove the components making up the voltage regulator part of the circuit.

“I wanted to build a SmartLock to use with my SG-239. After studying the SmartLock schematic, I couldn’t figure out why SGC put in the transistor and zener diode. The ST-TNE input on the SG-239 is just a 1.5K resistor to an open collector transistor to ground. So I eliminated Q1, R3, D1 and C3 on the SGC SmartLock schematic. My final circuit is shown below. I used a DB9S connector to interface with the SGC tuner (I attached a DB9P to the tuner interface wires), and a PowerPole interface for 12VDC. This way I could use a standard DB9 extension cable as necessary for interfacing between the tuner and SmartLock. I used ultra-bright LEDs (3000mcd or so) to provide plenty of visibility.”

Phil Salas’s build of the Smartuner

There is another variation by Luca Molino IV3ZNK that can be seen at

This uses a tiny PCB available on Italian eBay from a seller who wanted 50 € to send one to Australia!

I worked out a way to use a scrap of veroboard to hold the two capacitors and one of the resistors and to manage the wiring between the base and the panel of the enclosure which is probably more efficient and certainly faster. As well I noticed that the PCB seems to be designed for a different kind of DPDT switch where the centre poles are not in the centre!?! So I can add the satisfaction of saving whatever Euros I would have ultimately paid to the greater satisfaction of having nutted out the layout of the veroboard scrap…

Luca used the simplified circuit by Hans Nussbaum DJ1UGA which appears to have in turn have some subsequent input from OE7OPJ (who by the way has a very interesting website at

This circuit can also be seen at

The components required are

2 x 1k5
2 x 100nF
Green LED
DPDT switch
SPST MOM pushbutton

Pretty neat!

text of the italian site:

L’accordatore automatico SGC-239 indubbiamente è un gran bello strumento. Si collega e funziona ottimamente con qualunque radio (HF) semplicemente utilizzando un cavo coassiale e una fonte d’alimentazione a 12 volts. Non è particolarmente esoso di corrente e anche in utilizzo SOTA non va a gravare sull’autonomia giornaliera. Ne ho trovato uno usato sui soliti canali nazionali ad un prezzo decente e non mi sono fatto scappare l’acquisto. Accorda davvero di tutto gestendo potenze da 1,5 a 200 watt. Ottimo per il mio Yaesu 817, ma altrettanto utile per le “normali” radio munite dei canonici 100 watt.  Per poterlo però utilizzare al meglio, si rende necessario l’acquisto di un interfaccia che permette di resettare o bloccare lo stato d’accordo del SGC-239. Tale interfaccia (SMARTLOCK) si trova però in vendita a quasi 100 euro, decisamente troppi per 2 condensatori, 2 resistenze, 2 led e un paio di interruttori. La prima idea è stata quella di prendere lo schema presente sul manuale dell’accordatore e realizzare su basetta forata il circuito. Cosa questa realizzabile, ma che avrebbe dato un idea di “precarietà” a tutta la realizzazione. La scelta a questo punto è ricaduta su un mini circuito stampato realizzato con maestria da Danilo Cramaglia (lo trovate su Ebay come utente Martelloman), che partendo dallo schema elettrico mi ha consegnato quanto riportato in foto:

The SGC-239 automatic tuner is undoubtedly a very nice tool. It connects and works well with any radio (HF) simply by using a coaxial cable, and a power source to 12 volts. It is not particularly power hungry and even practical to use on a routine SOTA outing. I found one used on the usual national channels at a decent price. It really suits around handling power output from 1.5 to 200 watts. Great for my Yaesu 817, but just as useful for “normal” radio equipped with 100 watt “cannons”. 

But in order to make the best use, it is necessary to obtain an interface that allows you to reset or block the status of the SGC-239. This interface (SMARTLOCK), however, is for sale at nearly 100 euro, far too much for 2 capacitors, 2 resistors, 2 LEDs and a pair of switches. The first idea was to use the circuit in the tuner’s manual and build it on perforated board. While this is feasible, it could give an idea of “insecurity” to the whole creation. Instead I chose a mini PCB made with skill by Danilo Cramaglia (Ebay-user Martelloman), which, starting from the wiring diagram handed me what is reported in the picture:

I plan to use this wiring arrangement for the 4 pin plugs, socket and line.

1 Red – +12V
2 Green – Tuned
3 White – Hold
4 Black – Ground and -12V

As well Philip Storr VK5SRP describes his version at

SGC Smartlock control

In the hand book to the SGC SG237 Auto Antenna Tuner there is a circuit for an interface they call the SmartLock and it allows some manual control over the AATU and more importantly, it indicates when the tuner has tuned successfully.


JH2CLV has documented his build.

These photos (from give an idea of the layout which uses an external “chocolate block” connector for the 4 core power and control cable.

Case used is a plastic case – HA1593-KB of TAKACHI.

The main parts are as follows.
② 4P harmonica terminal (Chocolate block)
③ 6P toggle SW – actually a DPDT switch
④ push button SW
⑤ LEDs (red and green)
⑥ resistance (150Omu-330Omu)
⑦ capacitor (optional)
⑧ 9V3 terminal Reg (7809)
⑨ plastic case (TAKACHI / HA1593-KB)
⑩ wiring material


Also has details of changes – design and layout…

(2) inside the box configuration

The shielded 4 core wire is clamped in the cable tie through a pull through bushing. The case uses a diecast aluminum box TD 5-8-3B (55x30x80) by TAKACHI. The front of the enclosure has the Normal / Lock switch (2-pole, double-throw), the Tuned LED (Green), the Locked LED (Red) and the Reset switch (Make contact). It includes the three-terminal regulator. The bypass capacitors on the circuit diagram are omitted here.

(3) box overview and operation

Once tuning is complete the Tuned LED (Green) glows. If the Normal / Lock switch is pointing to the right, the Lock side, the Lock LED (Red) is lit and fluctuating SWR, etc. does not cause automatic re-tuning.

If the switch is set to the Normal side, normal operating conditions prevail, the SG-239 will automatically start tuning if SWR or the band is changed.

If you press the RESET (red) button, the ATU will re-tune.


Also check for detailed description of using the Smartuner with a balanced antenna.

For some good ideas about how to protect the SG-239 see He uses a large-ish electrical junction box and includes windings on toroids…

>So here is my effort successfully completed today…

I used the simplified circuit by Hans Nussbaum DJ1UGA and the front panel layout of Phil Salas AD5X. I managed to squeeze everything into a compact package – a small die cast aluminium enclosure 64 x 58 x 35mm which Jaycar sell for $9.95. And that was the main cost. Everything else was on hand.

I’ve yet to label the panel, but the green LED indicates a successful tune, the red LED is on when the switch is in the Locked position, and the push button switch is the reset. I used 4 pin sockets and line plugs to connect to and power the Smartuner. And I used a DC socket to supply 12V power. I had planned to use power poles but decided that would not fit comfortably in this small enclosure.

The ATU is housed in a plastic lunchbox which can be protected by a larger rubber waste paper bin as recommended in the SGC manual.

Antenna tuning by stealth

One of the most important documents for anyone who wants to know what makes a magnetic loop tick is Leigh Turner VK5KLT’s “An Overview of the Underestimated Magnetic Loop HF Antenna” which can be found on his club website.

Midway through building my version of Loftur Jónasson – TF3LJ / VE2LJX‘s Automatic Loop Controller, I came across Leigh Turner’s impassioned plea to consider this noise bridge antenna tuning design mentioned on page 32 of the “Overview”. As a concluding note VK5KLT states that he considers “The perceived need for a slick and salubrious auto-controller for properly tuning an MLA is oftentimes overrated and exaggerated, IMHO”.

He argued that elaborate microcontroller aided automatic loop tuning circuits are unnecessary and people should consider using this more covert and considerate approach. I think the bridge could be an excellent idea and a simpler way of staying in tune as you change frequency for all sorts of antennas. For a magnetic loop, it still requires a way to remotely adjust the tuning capacitor.

“The circuit goes inline between the rig and the antenna and sends a gated broadband noise signal to the antenna using a directional coupler and a noise bridge. You just listen on the desired operating frequency and watch your RX S-meter for a sharp dip whilst adjusting the loop tuning capacitor.

You simply remotely tune the loop with the aid of the receiver S-meter while you are on the wanted frequency without keying up and TX power output. This makes tuning a breeze without having to move off frequency and have the TX put out any RF power.”

VK5KLT mentioned the MFJ-212 Matchmaker that uses this same approach (and which is still on the MFJ catalog at US$99.95) and also referenced ZL3KB’s April 2001 RadCom article (pp17-21) as an easy and more economical way to replicate the same functionality.

“The distinguishing merit of the novel gated coupler/noise bridge loop tuning method is it’s completely passive and covert in operation; you don’t transmit any TX power whatsoever to attain an optimal loop tune setting. The technique makes for fast, QRM free, safe and easy QSY shifts and netting a frequency.” Leigh Turner adds that it’s even simpler if you use a pan adaptor or a modern SDR receiver as you can see the sharp null on the screen of the band scope display.

Kelvin Barnsdale ZL3KB’s RadCom article describes building and using the wide band noise bridge as a silent antenna tuning indicator. These four pages include circuit, PCB design and layout and full details of BOM and balun/transformer construction.


On 14 May 2001 ZL3KB published a 4-page follow-up pdf document “Instructions for Antenna tuning Noise Bridge” with info supplementary to the RadCom article about construction and operation. This article has an updated circuit and parts layout and refers to an issue B of the PCB. The new board includes places for the LED and dropping resistor R14, and pads for the two 100Ω load resistors and the two diodes D2 & D3.

This is the updated circuit with some updated values.

This indicates parts placement with the updated PCB.

Here is the foil side of the updated PCB

I contacted Kelvin Barnsdale and was lucky enough to obtain the PCB above.

Buddipole vertical for 80m

I spent the pleasant sunny part of the final day of autumn testing a vertical antenna for 80 metres using Buddipole parts for home.

The back garden space here is barely 4 metres by 4 metres and for the moment the chimney is out of reach. While I have dreams of a magnetic loop for 80m, the vertical is more in reach now.

Plan for 80m vertical made from Buddipole components
Plan for 80m vertical made from Buddipole components

I installed a counterpoise wire a few feet shy of 66 feet length hidden on a timber fence that runs down the side of the property. The idea is to connect a short fly lead to connect the hidden counterpoise when the antenna is deployed and then disconnect when it’s all packed away. Buddipole components don’t lend themselves to permanent installations. The counterpoise doesn’t follow the recommended dog leg arrangement and is higher off the ground than the 2 feet suggested.

The purpose yesterday was to establish how speedily the antenna could be assembled and adjusted for a frequency of interest such as a net.

Here’s a list of the items used along with the counterpoise:

9′ telescopic whip
2 x 22 inch antenna arms
low band coil + clip
Buddipole short mast
Buddipole tripod

I was surprised how easily it all went together. The adjustment wasn’t as fiddly as I expected such a short antenna for this band would be, and it appeared to give a usable bandwidth.

The Buddipole Low Band coil showing the coil tap for 80m
The Buddipole Low Band coil showing the coil tap for 80m

Assembly was straightforward. Set up the tripod and mast with only bottom two sections telescoped out. Attach the Versatee horizontally to the top of the mast. Connect the Low Band coil. Leave the red fly lead loose for the moment. Attach two 22 inch antenna arms to a long whip antenna fully extended. Then carefully attach that assembly to the top of the Versatee. I also connected a 1:1 balun between the Versatee and the iP30 SWR Analyser.

The next step is to simply drag the fly lead across the coil turns to identify the best spot to tap the coil. Background noise level rises as you get in the zone. I used the iP30 SWR analyser to narrow it down to a spot 16 turns up from the base of the coil.

This means I was shorting out the bottom 16 turns of the coil. The adjustment is too coarse on a turn by turn basis. You appreciate the value of being able to tap at 1/8 of a turn increments. (The coil is on an octagonal former.)

It took me a few measurements to realise that as I progressed left (from my point of view) I was decreasing the amount shorted out and hence increasing the loading inductance and so lowering the resonant frequency. It’s actually more confusing reading that sentence than understanding it in practice!

My target frequency was 3535kHz and this is a chart of the SWR readings I had when the coil tap was set at what I calculate to be 15 3/4 turns up from the base of the coil.

35051.8:1 35501.1
35301.1 35751.4
3545 1.135901.8
The magic spot for my 80m vertical
The magic spot for my 80m vertical

The 1.0:1 bandwidth was 10 kHz while at 1.5:1 it was in excess of 65 kHz.

From readings at the other possible coil tap points my guess is that at this frequency range each face of the coil moves the resonant frequency by about 4 kHz. One thing to be aware of with the Buddipole hardware is not to accidentally short out adjacent turns of the coil with the coil clip. It’s hard to do but I managed and it will throw your readings.

Next step of course is to make some contacts or at least activate the antenna on WSPR or JT65 to get an idea of whether the signal gets over the fence.

From checking the chart on page 146 of the ‘Buddipole in the Field’ book by B. Scott Andersen, NE1RD, I estimate that my shorting tap at about 16 turns from the base means I’m using about 39-40 uH of loading to achieve resonance at 80m. So that’s a starting point if I wanted to build a more permanent and cheaper vertical installation.

Lightweight portable VHF antennas

One of my favourite sites is Martin DK7ZB’s collection of pages detailing the construction of practical antennas for VHF and UHF.

I first visited the site following a link to designs for lightweight portable yagis that would be suitable for SOTA VHF activations. Under the link ‘2m/70cm-Yagis ultralight’, Martin describes a number of yagis for 2m and 70cm that use thin metre long aluminium welding rods mounted on PVC booms.

“These Yagis are constructed with cheap lightweight materials for electric installations and you can mount and dismantle them without any tools. The boom is made of PVC-tubes with 16mm, 20mm or 25mm diameter, the element holders are the clamps for these tubes.” DK7ZB

What makes the designs particularly attractive is that they can be quickly assembled from a compact (admittedly metre long for 2m) pack you can carry on your ascent, even designs using a 2 metre long boom.

The components of a DK7ZB lightweight 6 element yagi suitable for SOTA or any portable operation
The components of a DK7ZB lightweight 6 element yagi suitable for SOTA or any portable operation

The welding rods – used for TIG welding – are available in Australia in 2.8mm and 3.2mm diameters from welding supplies shops. I’m still on the lookout for 4mm diameter rods. PVC conduit and the mounting clamps are readily available in VK from hardware stores.

I’ve managed to cut a suitable slot in the end of a 3.2mm aluminium welding rod using a Dremel with a thin cutting wheel. One suggested way of attaching the feedline to the driven element is to crimp the lines into thin slots like this.

Also of interest to the portable operator are the J Pole designs based on Wireman 450Ω window feedline. There are dimensions for bands from 2m down to 40m. The J pole is essentially a half wavelength dipole where the high feed impedance is transformed by a quarter-wave length matching section (the tail of the J) tapped at a suitable distance to yield a 50Ω match. Follow the ‘Wireman-J-Pole’ link in the left navigation. These pages remind you that the J-pole can be configured in any way so a 40m J pole in a Zepp arrangement starts to look quite practical if you have just under 10m of 450Ω feedline available. I want to start with the 6m design and see if I can make it robust enough with heat shrink etc for portable work.

Kits for the DK7ZB yagi designs are also available from, Attila Kocis DL1NUX’s website. Both sites are in German and English.