[antennaware] vertical modelling
Guy Olinger, K2AV
Mon, 18 Jun 2001 12:18:37 -0400
The old "feed the 80 meter dipole like a 160 vertical" trick, or so it appears.
Just guessing, since your description doesn't tell me enough, that you have an 80 meter dipole up in the air, let's guess 60/70 feet, and you are feeding both inner and outer conductor against ground.
That would be an 1/8 wave vertical on 160, and the feed impedance (given a perfect ground system) would be considerably reduced from the typical 35 ohms of a full size 1/4 wave vertical against perfect ground. Twenty or so ohms would be a fair expected value.
Without spending the 1000$ or so to get equipped with NEC4 (only solution to buried ground wires I know of), you can do some intelligent approximations and make some conclusions with the ordinary modellers available.
Set up the above ground components as usual for your modeling program.
Set up a two level ground system: First get the average radius of your existing radial system. Set up the ground to be "perfect" for that radius and then switch to whatever is "normal" for your area after that. That will do a fair job of telling you what you would have with a very dense radial system for that radius. Any of the EZNEC programs (at least) will allow you to do this. Given the radius of radials, you will not be able to do any better than that.
If your measured impedance is higher, you are measuring additional loss caused by your ground system being less than ideal. If you add a resistive load matching the difference to the base of the vertical element you should get an swr/impedance curve you recognize.
This doesn't quite do what you were asking, but it is a very useful approach for getting a reason to recommend or ignore improvements to the radial system.
Best of luck & 73, Guy.
> From: Arie.Voors@ict.nl
> Date: 2001/06/18 Mon PM 12:19:39 EDT
> To: firstname.lastname@example.org
> Subject: [antennaware] vertical modelling
> For a periode of time now I am trying to modell a vertical with a two spoke top
> hat (80 m dipole) and a radial ground screen.
> The actual (burried) ground screen consists of about 20-30 copper wires with
> lengths between 1/8 and 1/4 wavelengt;
> some without, but mostly with one or two copper ground stake pipes (5 meters
> The measured impedance is about 45 + j 400 ohms.
> The modelled antenna, using a 20 radial ground screen elevated 20 cm above
> ground, gives me an impedance of 22 +j 150 ohms
> I can not beleave that my ground (screen) has such a 'big' impedance of about 23
> My modelling of antenna systems, not using a ground screen, works very well.
> The question is, is it possible with nec-2 to model such a vertical antenna
> by using a ground plane elevated above ground, or do I have to use nec-4 ?
> Is it possible for 'normal' people, not connected to any official company or so,
> to get a PC based nec-4 executable ?
> Another question is, if there are people who have succecfully modelled a
> vertikal antenna with
> a burried ground screen using nec-2 ?
> If so, is it possible for me to recieve a copy of this model ?
> Thanks in advance,
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