9:1 transformer
During my preparations for the PACC contest I made some K9AY loops. It is generally recognized that galvanic separation of the loops and the receiving system is highly recommended to decrease interference, noise, and other strange phenomena. The original K9AY design used an 9:1 autotransformer, and voltages to switch the two relays are fed through the coax. Both on the internet and in ON4UN’s Low-band DXing book, usage of a galvanic separated 9:1 transformer seems to coincide with the loss of the initial, and elegant solution to switch the relays via the coax cable.

Noise/interference from the control box
Reported noise/interference when positioning the controlbox in the ‘NW’ position may be the reason for this. In this position an AC voltage is superimposed on the coax instead of a DC voltage. What actually occurs, is that the mains power supply (220V here), with a lot of ‘superimposed’ interference from connected equipment, is almost directly ‘transformed’ into the receiver when the switch is in the NW position (!) In the SE (+V) and SW (-V) positions rectification with subsequent derippling shortcuts this mains power interference.

I also experienced an increase in noise/interference with the switch in NW position initially, but as far as I could ascertain it disappeared when using the ideas below.

AC/DC blocking choke in control box
I think that the inductance of the original choke (in the original K9AY-article 100 μH) is insufficient for the range 0.5 – 3.5 MHz. In my control box I use a 22 mH (Farnell) choke I had ‘in stock’. Its reactance @ 1.8 MHz (XL = 2*pi*1.8E6*22E-3 = 250 kΩ) is of insignificant influence compared to the original 100 μH choke (which reactance is -of course- 220 times lower).

PG0A’s Law: Listening is feeling
I think that a 1 mH choke will eliminate interference/noise most cases. However, I have not experimented with this. To assess the noise/interference, put your receiver into AM mode, dismount the coax towards the loops from the control box, and place the switch into the NW position. Compare it with the SE (+V), SW (-V), and NE (0V) positions. Besides some clicks (AGC effects due to the change of super imposed voltages which (dis)charge the DC-blocking capacitor in the control box) no (significant) increase of interference/noise may occur.
If you feel this is not the case (PG0A’s Law: listening is feeling ; -) increase the inductance of the choke significantly. With my 22 mH choke, all four positions (NE, NW, SE, and SW) sound equal. This was not the case when I used a 100 μH choke.

Another idea, which I haven’t tried yet, is to use a mains AC filter (e.g. from an old computer -switch mode- supply) in series with the power supply transformer.

LEDs on the switch box
Experiences from last year, while building and testing the loops, urged to have some kind of indication which of, and in what direction, the loops are pointing or to assess ‘remotely’ if anything switches at all. So, I replaced the (rectifying) diodes with LEDs, assuming that the relays draw between 15 – 25 mA of current (well.. they do and so do yours ; -).
By the way, I use DPDT 5V relays to compensate loss of voltage when using large lengths of coax as the loops have to be placed far away from the TX-antenna. And.. I have some ideas to create more than four states on a single coax to switch the Rterm value too in order to ‘optimise’ the nulls.

Below you’ll find the schematic diagram. I also made detailed pictures of the contrapsion, controlbox, switchbox-v1, and switchbox-v2. The difference between v1 and v2 of the switchbox, besides the Rterm switch, is the transformer used.
Switchbox v1 used a toroid and v2 uses a binocular version with teflon isolated wire.


Control box
I have been asked to publish the diagram of the control box. My implementation, depicted below, is identical to the original K9AY version except that the ‘AC/DC’ blocking choke is 22 mH, and I don’t like fuses.


Rocket science?
My initial feelings concerning the aforementioned were that it was not ‘rocket science’ but ‘handy’.
However, my ex-collegue, friend and 160m guru Kees PA0CLN was really surprised with my LED ‘invention’.
“You really should inform Gary about this”, he said. I told him about the galvanically separated transformer, the enormous loss of interference, the single coax solution, and that everything really worked well and so on.
“But how to you feed the relays via the coax cable then?”, he asked.
I promised him to mail him my contrapsion but I thought it would be a nice idea to publish it here : -)

Does it work for you?
Please find out if the above is reproducable, and leave your feedback here.