Improving Front to Back Ratio of the K9AY Loop – cheaply!

Hello! If you’ve been keen to actually hear some meaningful stations amongst the QRM on 160, you may have come across my article about top-band receiving antennas.

No, you don’t need shitloads of space, a circle of 8ft round will do it if you want to use a half-size k9ay loop, which will give you FOUR separate rx directions and a fighting chance of putting the signal you want in your rig speaker. Or, you could “listen” on your tx antenna: short vertical or whatever, and “work” the neighbours SMPS QRM and Plasmas, etc 😉

K9AY Loop basic layout and size

G7KLJ: The K9AY Loop
The K9AY loop. Half size loops work. They have less gain, but better front-to-back ratio

Most people, even with quite small gardens, can fit one in. Here’s a pdf describing how to make one. Note that the control volts fed up the co-ax is a pants idea (as shown in the pdf). If you do that, you will hear mains hum in one direction. Instead, send the direction control voltage up a separate cable (I use cheap alarm cable). Decouple the control cable (with a common-mode choke near the aerial end) and all will be good. Use a spare alarm cable inner to send up a variable voltage to control a ‘vactrol’ (voltage-dependent resistor) to peak the rear rejection on any frequency you choose to listen to!

Instead, send 12 or 13 volts up the coax (through a choke, of course!) to power a W7IUV pre-amp right at the k9ay loop itself. The pre-amp isn’t really needed for the gain, but it relaxes the requirements for decoupling the coax. You don’t want a quiet aerial that is ruined by noise pick-up (common mode) on the co-ax!


Improving the front-back ratio of the k9ay loop

And no, you don’t need to spend loads of money — a couple of old components from the junk-box, and a spare afternoon — should do the trick!

To truly test how wonderful a k9ay loop antenna is, bearing in mind the sparse and intermittent nature of 160m signals, you might like to experiment with nulling out local plasmas with it, but this gets a bit “old” after a while. Instead, have some fun by checking out the front-back ratio you can acheive on medium-wave signals during the day.

Improve the low-angle front to back on the k9ay loop — without buying loads of expensive kit from DX Engineering, or whoever..

The reason I’m writing this article is that most online guides to building k9ay loops will say put a 390 ohm to 470 ohm termination. Though this works well, it places the null of the loop somewhat “up in the air” — great for nulling out those Pappa Alfas on 1850 , lol, but I digress… 🙂 — normally, in suburban England our primary QRM source is not so much our European cousins, but more likely our troublesome neighbours with plasma tvs, and all sorts of shitty little electronic gadgets that seem to radiate all over 160m. Now, more often than not, these pesky QRM machines are arriving at a very low angle to our k9ay antenna.

What we want, then, to minimize this crap ‘n’ hash, is to move the null of the k9ay loop out of the sky and right down onto the ground. I’ve found, through experimentation and modelling, that simply replacing the 300-600 Ohm termination with a resistor and cap in series does just that.

You should experiment with the values you need to use (depends on desired angle of rejection, local ground properties, shape and size of k9ay and so forth), but a good starting point is to use a 1k (1000 Ohm) resistor in series with a 1000pF capacitor instead of just, say, your 470 Ohm resistor.

Test it out somewhere at the top end of Medium Wave with a semi local signal, switching the loop to receive in the dead opposite direction. If you are lucky, and the station is directly in the null of the loop, you, like me, could see the station reducing from, say 9+10dB when “beaming” at it, to almost nothing in the opposite direction. Play with the values of both R and C til you get the best null. Some station above 1400kHz should do the trick. Try several.

Using my empirical and admittedly not very scientific methods, I reckon that you might see as much as 35dB front-to-back ratio now on your k9ay loop system with the modified termination values. Give it a try!

Imagine… if you could possibly reduce a local plasma by 35dB, and not reduce the wanted signal by hardly anything — and you could do it for pennies — now you can! (Disclaimer: unwanted signal needs to be in the dead opposite direction of the wanted signal!)


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