Fence Wire Antenna

[K9ACT]

I have been working with an antenna modeling program called MMAMA-gal and think I have learned something very interesting.

I made my 160 meter dipole out of fence wire for the simple reason that I am a tightwad.  I have a half mile spool left over from sheep fencing and buying $75 worth of copper wire seemed unnecessary.

Furthermore, I have been using a fence wire antenna with ladder line from 160 to 20 meters with good enough results.

My analyzer insisted that my antenna was about 30 ohms no matter what I did or how high it was.  I do not have a tuner for 160 so just living with this was not an attractive option.  A 2:1 SWR is not the end of the world but it seemed like something else was going on.

After downloading MMAMA (free) and learning to use it, I kept getting the same sort of results.  Nothing would make it work right.

I eventually noticed that one can choose the wire type and when I switched from "no-loss" to Cu and Fe, things began to fall into place.

What is most interesting about iron wire is that the feed point Z is nearly a constant 100 ohms, no matter the height above the ground within very wide limits.  You have to get down to a few feet before it changes much.

I decided to believe the simulator and ignore the MFJ analyzer which still insisted on the 30 ohms.  I built a transformer that steps down the 100 ohms to 50 ohms at the feed point and Voila! analyzer now says:
SWR is 1:1, R=50 and X = 0.  It looks more like a dummy load than my dummy load.

On air testing shows zero reflected power, no heating of coil and good sig reports.

When steel wire is mentioned, we automatically think of resistivity as the problem but it also turns out that the permeability of iron is about 1000 times greater than copper. I  have no clue how this effects an antenna other than the fact that I now have a 160 meter dipole that does not care how high it is.  I have checked it from 5 feet to 50 feet and the numbers do not change more than a few percent.

So, can someone elaborate on this to help understand what is going on?

I would also be interested in hearing from anyone running other simulators to try this and confirm the results.

Jack

 

[N2DTS]

Or is it the resistance in the wire?
Since its designed for high voltage no current, it could be acting like a sort of dummy load in the air.

At power and high current points, that might give quite a bit of current, and loss?

Brett

[WA1GFZ]

When you consider skin effect I bet the resistance really effects the Q. I have seen rhombic pictures that used steel wire as a termination resistor.
The coax stubs on my 40 meter delta loops hold the VSWR under 2:1 across the whole 40 meter band.

[Tom WA3KLR]

Wire conductors with a permeability greater than one have additional increase in the skin effect, not good news.

[KM1H]

It is the RF resistance that kills the signal and it goes up with frequency.

The military uses stainless steel wire in a 2-30 mHz broadband antenna meant for rapid deployment, forward operations, and frequency hopping. They drive it with enough power to do the job since the loss is calculated in.

The fence wire may be 10dB down but we all know 100W or even 10W can get thru at the right times.

Back in my real cheap days I used #18 Copperweld type electric fence wire for Beverages. That stuff turns to rust in a season around here so I built up a little Rustoleum applicator as I drew the 1000-1500' wire thru it. Lasted fine the 6 years I lived at that QTH.

Carl
KM1H

[KD6VXI]

Wire conductors with a permeability greater than one have additional increase in the skin effect, not good news.

Would that be the "negative skin effect" Tom Rauch was caught propagating

Of course, with an antenna like this, you wouldn't need toaster wire parasitic suppressors, wouldja

(tongue in cheek, of course).

--Shane