Random wire antenna for shortwave listening
Following the shack reorganization I wanted a permanent antenna for the Yaesu FRG-7 receiver. After a little research I decided that a random wire would be fine for what I wanted (casual listening of shortwave broadcaster stations right across HF). Further investigation revealed that it would be best to use a 9:1 transformer to match the random wire to the 50 ohm feedline. Such a transformer could be built in a number of ways and a guide to the different approach to winding a transformer can be found here. Basically you can build the the transformer by using:
- A single toroid and using a conventional wire wrapping approach as shown here and towards the bottom of this page.
- A single toroid and using a bifilar winding.
- Or make the transformer by using a binocular core winding, as shown at the bottom of the page here, (in that example multiple binocular cores were used).
I decided to use the last approach, guided by the design of the Par Electronics commercial end-fed SWL antenna, which receives some very good reviews. As I did not have a binocular core ferrite I glued together two ferrite beads and made the winding similar to the way shown by K9AY. You can see the result in the photograph above. The one difference I made with the winding was to put the 9 turns on first and then overwrap the 3 turns. Likely this does not change anything.
The windings ratio to a achieve a 9:1 impedance transformation has to be 3:1. OK1RR describes the theory.
The antenna is a true random wire as I did not measure the length when I set it up, but I estimate it is about 50 ft. It is mounted in a sloping arrangement with the base just outside the shack at the house. The wire rises almost straight up for 7ft and then up on a slope to about 30ft, directly away from the house. Like the Par antenna I have two bolts (stainless steel) for grounding independently the antenna side and the feedline side (see towards the bottom of the photograph). Currently both are tied together and connected to the shack grounding rod. The noise level is not a problem. Reception is very good with signals very strong on the S-meter. I have an ATU designed for shortwave receivers but find this does not change signal reception much now (unlike when I used internal antennas). I may even disconnect it totally and not bother tuning.
This was a simple and straight forward project that allows me to listen to broadcast SW stations whilst in the shack. It has also renewed my interest in shortwave DXing again.