K9AY project

Just after completing tests on RX antennas, I’ve decided to build the final K9AY antenna setup (control box and shack controller), because of its encouraging results during the test.

I started with some research activities over the Internet and found some really useful links/articles. Without them, I would’t have been able to finish the project. To save your time I am placing the links right below:


The antenna is a diamond-shaped loop, hung from a single point and with a ground rod placed at the bottom. An impedance-matching transformer is connected at the one end of the wire to the ground rod and the terminating resistor is connected at the second end of the wire also to the ground rod. The antenna has a directional pattern, with the main lobe from the feed point end. Its far-field can be adjusted by changing the terminating resistor value.

I am not playing on 160m band, so I decided to make a half-size version of K9AY to easily fit the antenna on the lawn (it should not require preamplifier on 40 and 80m band – noise level should be greater when you connect the antenna to the receiver than with no antenna attached).

K9AY antenna draft

Two loops are placed perpendicular to each other pointed at NW-SE and NE-SW direction in my configuration, but it can also be pointed at N-S/W-E.
In the control box according to the below schematics, there are 5 relays in total – relay no. 1 controls loop selection, relay no. 2 controls resistor position towards the ground rod and the transformer (for example: when NW-SE loop selected, relay no.2 switches the main lob towards NW or SE direction).

Main lobe direction controlled by relays no. 1 and 2 – “1” means +12V applied to the relay.

Relays no. 3-5 control the termination resistor value – combining 3 different resistors with 680ohm one in parallel, results in 8 different resistance termination values.

Termination resistance value controlled by grounding 3 different resistors by relays no. 3-5 – “1” means +12V applied to the relay.
K9AY control box schematics – OK2RR

Please pay attention to the grounding procedure of the control box. The resistor and the transformer (wiring at the antenna side) should be grounded to the ground rod just at the bottom of the antenna.
Relays ground and coax shield should be grounded at the hamshack entrance or at the hamshack grounding bus.

PCB assembly parts list (version assembled below):

  • 2x SPDT relays (G5V-2 12VDC or GS-SH-212T or any similar)
  • 3x SPST relays (PRMA1A12 or R2-1A12 or any similar)
  • 5x 1N4001 diode or any similar
  • 2W carbon resistors: 4k7, 2k4, 1k2, 680Ohm
  • transformer winded on BN73-202 – 2 turns on the coax side and 5-6 turns on the antenna side (5 turns when 75ohm coax used, 6 turns when 50ohm coax used)
  • DIN-8 socket
  • F-type socket


The controller is build of two rotary switches. The first switch controls relay K1 and K2 (pole no. 1 – VCC application to the relays, pole no. 2 – VCC application to the LED – 4 LEDs signalizes reception direction selection) and the second one controls K3, K4, K5 relays VCC application.

The controller has been assembled on a prototype circuit board.

I’ve added a simple power switch and front-end receiver protector unit to prevent the receiver from blowing its front-end by RF during tx-ing.

VCC is +12V from the power supply, negative wire to the ground.

The controller is placed inside the project box (metal case).

I’ve used DIN-8 socket to attach the control cable.

K9AY controller draft





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