More on the Random Wire Antenna, Efficiency - 10 Meters

I was playing with my random wire antenna (the magnet wire one, I don't have the spool of leader wire yet) today. I was testing it on the rest of the bands to see if I had any RFI issues on my TV. I also pulled the rest of the cables off the tuner to see if that would have any affect.

Once I pulled the other cables off the tuner and just had the ground to the duct work and the 34 gauge magnet wire I was able to load it on all bands 10-160m. 80 and 160 cause RFI to my TV (not surprising, the antenna is short on those bands). Before I pulled the cables off the antenna would not load on 80m at all. I guess it is a non-issue because I am getting RFI with 80 anyway - that puts the band out of commission.

One very interesting affect I noticed was the efficiency of the antenna on 10 meters. I can get it to a perfect SWR through the tuner, but 10 meters seems DEAD. I switched between my 10m ground plane (no tuner required - resonant) in the attic and the random wire. I found a strong (s8-s9) signal on the ground plane and switched over to the random wire... NOTHING! I could hear the station in there over the noise floor, but absolutely no movement on the meter! This was with the preamp on and the attenuator off. That is pretty alarming.

Though, the bands I need it to play on are lower in frequency. I have the ground plane for 10m so that is an awesome antenna (at least considering my circumstances). I also have a 6m ground plane, so I am set there too. I just need something that can handle the lower bands.

So far the random wire seems to work well on 20, 30, and 40 meters. I have made some pretty good contacts on all three of those bands (good DX on 30 and 40!). I heard some CW at s8-s9 on 17 meters as I was scrolling around. So it seems to be fairly sensitive on that band, though I haven't had any contacts to judge it with.

I do know that 40-10 meters on the random wire don't cause RFI on my TV. That is a GOOD thing. However, I need to find out from my neighbor if I am getting in to their TV at all.

I am planning my attack for setting up the leader wire. I am going to run it slightly different. My plan is to use picture hanging eye rings (the tiny screw in ones) and some sort of bracket to attach to the peak of the roof on the front and also the fascia outside my window at the roof line. That way I can support the wire up higher and it will not touch the shingles (abrasive). Then I will run it out to another tree. This will allow me to get a longer length of wire up in the air, and higher. Maybe I can get 80 meters on it. That would be cool!

Not-So-Random Wire Antenna Theory

The more I look in to the "random wire" antenna the more complicated it gets.

This is supposed to be a 'throw-it-up-in-a-pinch-and-load-with-a-tuner' antenna. So why does it matter?

If you happen to hit a 1/2 wave length with a standard manual antenna tuner you will be in for a trip trying to get it to tune. In most cases it wont. The impedance is simply too high and out of the range of the tuner.

Here are a couple of links I found online while researching this.

The first one is a sub-article by Jack VE3EED over at Hamuniverse. This one is the most comprehensive I have seen. Note the lengths listed in green.

Here is one from Patrick W0IPL. This article is not as comprehensive, but is still a good reference.

One note is there is a bit of variation in the numbers between the two. As far as I can tell nether one takes in to account the velocity factor of the wire used. Keep in mind that the lower the velocity factor the SHORTER the wire.

I wonder if there are any tables on velocity factor of commonly used materials? I will do a little digging on that and see what I come up with.