February 2011
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The K9AY receiving loop antenna

Not to take anything away from Gary Breed, K9AY, who makes and sells these things under the corporate name AYTechnologies, I decided to make my own K9AY antenna system and controller.  Basically, after looking at the currently available commercial version, I figured I could make a better unit for less money and be happy.

The basis for the K9AY antenna is that it has a steerable null.  The gain around the antenna is close to unity, except for the terminated side of the loop, which has a deep null.  This can be switched around using a combination of relays that change the loops and termination.  This comes in very handy for MW and SW listening, when co-channel stations can create annoying interference and hetrodynes.  I have had good success pulling many stations out of the muck, especially in the AM band using this antenna.

This antenna requires a good ground to work against.  For optimum installations, I would recommend placing two radials under each side of the loops.  This will keep the ground conductivity below the antenna fairly constant, thus the value of Rterm will remain consistent for each band.

My other idea is to add a preamp right at the antenna to overcome transmission line loss and the loss from a 4 port passive receiver coupler.  Something around 10 dB, low noise (obviously), low parts count and rugged.  I decided that a Norton preamp was a good design, with only one active device, a common 2N5109 BJT.  Most of the time, this preamp is switched off and out of the circuit.  There have been several occasions, however, where an extra 10 dB made the difference between no copy and good copy.

This is the schematic of the relay board and preamp combined:

K9AY antenna controller with preamp

K9AY antenna controller with preamp

The parts list is as follows:

Symbol Part Symbol Part
C1 – C5 Ceramic 0.1 uf capacitor R1 2 Kohm ¼ watt
FB-1 Ferrite bead, Amidon FB-43-101 R2 8.2 Kohm ¼ watt
K1 – K3 Omron G6K-2F-Y small signal relay R3 100 ohm ¼ watt
L1 22 uH ¼ watt R4 51 ohm ¼ watt
L2 100 uH ¼ watt T-1 9:1 balun
Q1 2N5109 w/heat sink T-2 Norton feedback trans

The 2N5109 transistor is a CATV unit and it has a 50 input and output, that reduces the number of impedance transformers required. The value of Rterm is determined by which band one wants to operate on.  I used Omron G6K series low signal relays.  Again, because this is a receive only antenna, those relays will work well.

Terminal board connections, TB1:

Terminal Use
1 SW loop
2 SE loop
3 NW loop
4 NE loop

Wire loops go between Terminals 1-4 and 2-3.

Control terminal board connections, TB2:

Terminal Use
1 Preamp power
2 Rterm
3 Rterm ground
4 Ground
5 Relay 2
6 Relay 3

To create a low noise preamp, I decided to use surface mount devices and to try and make all the traces as close to 50 ohm impedance as possible.  I created this SMT printed circuit board:

SMT K9AY board, not to scale

SMT K9AY board, not to scale

From this, I ordered 6 boards from PCB express:



This is the board with all passive components installed:

K9AY loop antenna control board partial

K9AY loop antenna control board partial

This is the board completed:

K9AY antenna control board completed

K9AY antenna control board completed

My current K9AY is an amalgamation of parts removed from various equipment.  The relays are large, 12 VDC units which do not have the best contacts.  It works well enough, but I’d love to get one of these units into the control box at the base of the antenna.  Unfortunately, my antenna field is still in about 18 inches of snow, so it will have to wait until some of the snow melts off.

I would position this antenna as far away from transmit antennas as possible to avoid overloading the preamp and or causing problems with the switching relays.  For the average amateur set up, 75 to 100 feet separation should be more than enough.

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6 comments to The K9AY receiving loop antenna

  • Gary McAuliffe

    Thanks for this. Most homemade K9AY designs are too kludegy for me. I haven’t built one yet because of that. I have rotten conditions with too many local AM’s, poor soils conductivity, and courtesy of our local utility, lots of QRM. I live on a rock in the pacific and miss my old Hammarlund SP600JX and a good antenna. Would you share a good board layout I can make or buy?


  • Hans Sparreboom

    Do you sell these antennas?

  • Paul Thurst

    @Hans, yes, I will contact you offline.

  • Rudy Gubiolo

    hello Paul
    i am interested in k9ay antenna, do you have one to sell ??

  • Ben Witvliet

    Hi Paul,

    Do you sell these circuits (switching and preamp) to make a K9AY receive antenna?

    Ben, PA5BW

  • Ed Johnson W2PH

    Do you offer the stuffed board or complete antenna for the K9AY receive antenna?
    Ed W2PH

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