HFU HF Underground
Technical Topics => Equipment => Topic started by: ChrisSmolinski on May 24, 2019, 2157 UTC
-
Two summers ago I played around with a large MW loop antenna: https://www.hfunderground.com/board/index.php/topic,36639.0.html
I'm back at it again. I am using the same PVC frame as before. I am still iterating the design, as a first test I have four turns of wire (recycled antenna wire in fact) and then a smaller pickup loop, also made with antenna wire. It's not the neatest winding (don't look, jFarley, you'll go blind!) so the nulls are not very sharp.
It's tuned with a varactor diode, powered by a 9 volt battery through a potentiometer which works as a voltage divider and 1M resistor, connected to the loop via some 0.1 uF capacitors. The varactor diode capacitance varies from about 25 to 735 pF, which is presently not quite enough to tune down to 530 kHz. The switch throws in an additional 600 pF in parallel to accomplish that. It can also add 1200 pF to turn down into the upper end of the NDB band.
Initial tests were done placing a Sony 7600G portable on the loop windings, coupling the signal to the internal loopstick antenna. I was pleasantly surprised to get an SIO 444 signal from my weak test station, a pirate on 1620 kHz which I suspect is about 10 miles away. With the crossed parallel loop it is SIO 222, and weaker with the non directional sky loop.
I then used the leftover antenna wire to add a coupling turn close to the center of the loop, fed to one of my Squid transformers. The output of that was fed to an AOR 8200, which is not a great performer on MW, but still worked well.
The next steps will be to play around with the windings and see if I can get it to tune the entire MW band with the varactor diode range. Then I may replace the crossed parallel loop with this antenna, so it can be rotated from inside the shack. I'll need to figure out how to run tuning voltage out to the antenna. I have an unused 8 conductor ethernet cable (from the old LZ1AQ amp that used to be in the crossed parallel loop) so I could probably use one of the pairs.
(https://i.imgur.com/tpyGLlF.png)
(https://i.imgur.com/cpwBysK.png)