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Username: Tech237

Post Number: 1861
Registered: 4-2004

Posted on Saturday, April 16, 2016 - 5:30 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

(or How to build an antenna out of recycled stuff)
by Simon, Tech237

This is a simple coax fed antenna that is based on the late Jim MacIntosh VS1AA antenna design.

The VS1AA antenna is a full wavelength of wire at the frequency of interest, similar to a Windom, except it is fed with a single wire of smaller gauge than the top. The original antenna was built for HF at 138ft. long and tuned up on 80, 40, 20 and 10M bands.

In this variation, we go even further away from a Windom and feed the antenna with 75 ohm coax. Yes, you read that right -75 ohm coax. Why? To help with impedance transformation. This antenna could be considered a single-band off-center fed (OCF) dipole.

For this 6m antenna, we need 6m (19ft) of wire. In this version I used a twisted pair out of an old recycled Cat5 cable, shorting the two wires together at both ends. We also need at least 1 (preferably 3) insulators. As mine will be pinned to an inside wall, I am using only one, and that one is the plastic core from a roll of “Doggy Poop Bags” (hey, I have a stack of them, and intend to use more of them to make open wire feed line later).

If you look carefully at the picture to the left, you may be able to see at least one of the holes I drilled in the plastic core to feed the antenna wire through. For an outside antenna, I would suggest good quality insulators and a heavy gauge wire.

Through the holes in one end of the insulator, I threaded the ends of a 5ft length of the twisted pair. The other set of holes were the home for a 15ft length. Once soldered, these wires will be adjusted to 4ft 9inches and 14ft 3 inches.
In the second picture, we can see the two sets of wires inserted, but yet to be soldered. The wire to the right, needs a little more insulation stripped from it first. This is also the shorter of the two wires and will have the shield of the coax soldered to it. If this antenna is to be erected outside it would pay to cover all joints with heat-shrink tubing or some other form of weather protection. The 75 ohm coax I used was recycled, like the rest of this project- It came from the end of an old roll of cable company aerial cable – the kind that goes from the street to the house.

How does it work?
As mentioned earlier, my prototype was installed inside an apartment (darn HOA), with the main leg running approximately north-west, and the shorter leg running north-east. A straight line would have been better, but have to use what room there is.

Initial tests indicated some issues with the antenna as the SWR was almost off the scale. A broken coax joint was easily found and fixed. The joint looked good, but seems the solder had not flowed and the shield was not connected to the short section. SWR came down to less than 2:1 from 50.0 to 53.35MHz. Murphy took a second swing at this point – 6M was DEAD quiet. So we’ll have to wait for 6 to open before I can really try it, but I can tell you it does receive on other bands, so that may be a good sign.

Some years back, I made a version for 2m using aluminum pipe that I mounted vertically. Never did try that one horizontally, but suspect it would have worked OK. Below is a table of sizes for several other bands, including CB.


God made me an athiest, who are you to question his wisdom?

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