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Random wire antenna for shortwave listening

June 23, 2008

The 9 to 1 transformer

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.

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10 Comments leave one →
  1. June 24, 2008 1:47 pm

    I would like to build a copy of your transformer and I was wondering what ferrite cores you used.Also I wonder if you have tried it on the 80 meter band.Best wishes terry lynch EI3IZ.

  2. va3stl permalink*
    June 24, 2008 8:14 pm

    Terry,

    Thanks for the questions.

    The beads were bought at the local Smiths Falls hamfest (radio rally); a handful for a few dollars. I do not know their exact details. Their size is big enough to push through RG58 or RG59. They were being sold for RF chokes. They certainly appear to be ferrite material as they have that dull, slate type look and the the sharper edges which can scrape the enamel of wire if you are not careful. As for the mix I do not know, could be 43 or 77. All I can say is it works. The glue was Krazy glue. The type that easily bonds fingers together! Superglue will be the UK equivalent. The nine turns was done with enameled wire and the three turns is with plastic sleeved solid wire.

    As for 80m, yes it work well for that. I have listened easily to amateurs on 80m with the FRG7 – which is not very selective in SSB mode. If you are particularly interested in that band I would consider lengthening the wire. Alternatively investigate the K9AY antenna which also uses 9:1 transformers.

    Sorry I cannot be more specific about the beads. This really was a junk box project. It was made from parts I had lying around in the shack.

    Feel free to get back to me if you have more questions.

    Two final points, this is a receive only antenna and consider and implement lightning protection.

    Let us all know how your project goes by posting another comment.

    73
    Alan

  3. Bob VE3MPG permalink
    June 24, 2008 10:07 pm

    Another great article Alan. I’ll be using the PAR Electronics 40m at our field day on the weekend. I also have the 30m and 20m versions so far untested.

  4. va3stl permalink*
    June 25, 2008 5:00 am

    Bob,

    Thanks for the nice feedback.

    I would be interested to see the PAR antenna when I visit the Manotick Amateur Radio Group’s FD event.

    BTW I see the weather forecasts are showing mostly sunny for Saturday and showers for Sunday for this area, so not too bad for FD. Hopefully the thunderstorms will stay away.

    All the best to all FD groups this weekend.

  5. June 25, 2008 7:20 am

    Thanks, Alan for your reply!. I will try a few ferrite cores and let you know how things work out.Best wishes, Terry.

  6. July 27, 2008 8:11 am

    Alan, I got some fb-77-6301 beads from Amidon and wound 2 windings 3 turns and 9 turns, the 9 turns goes to 15 meters of wire plus earth and the 3 turns goes to the receiver.It works perfectly from 198khz to 30mhz.I am very pleased!. Best wishes, Terry lynch.

  7. va3stl permalink*
    July 27, 2008 2:28 pm

    Terry,

    Thanks for the update and great news to here the transformer and antenna work so well for you.
    It is always satisfying to build something yourself and to find it performs as well as you hoped, or better.

    Enjoy the SWLing with your new antenna.

    Best regards
    Alan, VA3STL

  8. Sudipta Ghose permalink
    February 2, 2009 7:38 pm

    I have a very good stock of binocular cores that were used in the B/W TVs. With demise of B/W TV nobody looks for them. In case you want to experiment I can send you a supply free.
    I live in India and SWLing since 60s.
    Regards,
    Sudipta Ghose
    oneghose at gmaildotcom

  9. va3stl permalink*
    February 2, 2009 10:00 pm

    Sudipta,

    That is a very nice offer, thank you very much.
    I am sure any interested readers will contact you and at least pay for shipping.

    73 and very best DX
    Alan
    VA3STL

  10. Joe permalink
    July 20, 2013 3:28 pm

    Hi,

    I built a 9:1 unun for my random length wire SWL antenna by winding 31 turns of 20 magnet wire and then 10 turns of the same wire. One end of the 31 turns went to the wire (approx. 60 feet), one end of the ten turns went to the center of the SO-239. The remaining end of each winding were tied together and went to the common ground. Believe it or not, the thing works!

    I used 31 turns on the primary because I felt that would give just a tad better match. Anyway, it eliminated all of the electrical noise from the house. Neat project to include the grandkids on, one in which they can actually see the before and after difference.

    73
    Joe KB0TXC

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