K9AY 160M / 80M Receiving Loop Antenna System
After operating a couple 160 Meter contests last
Winter, I realized I needed to do something to improve my receiving
performance. It was apparent that many stations were copying me much
better than I could hear them, so something needed to be done.
I knew Beverage antennas were the hot ticket, but I did
not have anywhere near the real estate required for these antennas.
Looking around the Net, I came across several articles about the K9AY Loop
System, which seemed to be a good alternative, and required much less space.
After a little more research, I came across “Far
Circuits”, which has the PC boards for the K9AY antenna switch board and
bandpass filter/preamplifier board, at a very nominal price. A
complete set of chip capacitors is included with the boards.
http://www.farcircuits.net/
http://www.farcircuits.net/preamp1.htm
I ordered the circuit boards from Far Circuits, and the
rest of the parts from Mouser electronics. This was easy, because Far
Circuits provided a list of the necessary Mouser part numbers. For the
outside relay control unit I used a 4”x 4” x2” gray “Carlon”
electric junction box, which is readily available from Lowes or Home Depot.
I chose not to connect the coax shield, coming from the shack, to the loop
ground to prevent the possibility of ground loop noise. The primary of the
mini-circuits 9:1 xfmr is directly connected to an isolated BNC connector.
For the control unit, I ordered a small project box from Mouser
(546-1455N1201), when I placed my order for the other parts. I would
recommend anyone considering this project, to order the Mini-Circuits 9:1
Transformer from Far Circuits, since they have them readily available, and
much easier than a special order with Mini-Circuits.
To help simplify things, I have provided a detailed
wiring schematic below, using the Far Circuits boards. To take
advantage of eight possible termination resistance values, I made a small
diode matrix board using 1N4001 diodes. I purchased a 1P12T switch
(Mouser CK2425), and configured it for 8 positions. Each switch position
For loop direction switching, I used a 3P4T rotary switch (Mouser CK2395),
and used one section to control each of the loop relays (K1 & K2).
The spare section of this switch can be used to light front panel LEDs to
show the direction selected, if desired. The Mini-Circuits 9:1 transformer
data sheet was a little confusing as to which pins are the primary and
secondary, so I terminated and tested it with a network analyzer to make
sure I got it right. I found the side with the “Index Dot” is the 50 ohm
primary. If you attempt to test this transformer with an ohmmeter, it
will show zero ohms on both windings.
For the actual loops themselves, almost anything will
work. Due to space considerations, I installed four 4”X 4” fence
poles (NE SE NW SW), for anchor points for the two loops. Refer to the
pictures below. For the center guys, I used parachute cord, and made
tensioners from small pieces of acrylic that I bought as scrap from a local
plastics supplier. I also made the insulators from this scrap acrylic. As an
afterthought, I should have made the insulators for the sides of the loops
slightly longer, and drilled three holes in them. One end of the
insulator would slip over the eye bolt, with the outter hole used for the
intermidiate guy rope, and the center hole providing the pass-thru for the
loop wire itself. Anything can be used for the center support. I
purchased 6 sections of surplus fiberglass mast on the Net for a reasonable
price. They were only 44” long, so I placed a 3” piece of 1-1/2”
PVC pipe at the top, to obtain 25 feet. If you have a tree in your back yard
you probably won’t need a mast at all J An eight foot copper clad
ground rod was drivin for the loop ground, at the base of the mast.
As for actual performance; All I can say is WOW!
Switching between either my 160M or 80M Inverted Vee and the loops (with
preamp on), the signal strengths appear to be equal, BUT the noise floor is
10-15db lower on the loops. Signals, at or below the noise floor seem to pop
out of the noise, on the loop antenna. On low angle signals, I am seeing a
20 - 30+db FB ratio. I have never heard, yet alone worked Europe on 160
Meters from the West Coast, however the first day after getting the loops
operational, I was able to hear and work Latvia. The same evening,
several other stations were heard from Europe on 75 Meters. Like
someone said “It’s like having a beam on 80 & 160!
Good Luck, and happy hunting!
73’s Frank, K7SFN
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