RadioBanter - View Single Post

FBMboomer[_2_] · November 19th 14, 07:57 PM posted to rec.radio.amateur.antenna

On 11/19/2014 1:37 PM, wrote:
On Wednesday, November 19, 2014 5:48:31 AM UTC-6, Wimpie wrote:

When the quarter wave or 5/8 lambda antenna is say 0.25lambda above
ground, you can use the two ray model using the direct ray and the
reflected ray to get a good impression. The reflection (versus
elevation angle) you can calculate using the Fresnel Equations for
media interfaces (using p-polarization).

I remember when I was using a 40m GP at slightly higher than a quarter
wave up, I always had to increase the ground qualities when modeling, if I
wanted the modeled test results to match the real world results I saw when
comparing to other antennas. And even then, they often came up a tad short
in the models. If I used "average", the modeled vertical results would
be quite stunted and way below the results I saw in the real world.

Instead of "average" ground quality, I would have to use "very good"
or "excellent", or whatever the exact names they called those in the
programs.

This area has pretty decent ground conductivity according to the maps.
Not sure how that would apply to people with a lesser ground quality.
Their modeled results may be closer to their real world results.

I found one thing when comparing a full size 40m radiator. The ground
mount version with 32 radials, was no where near as good as the same
32 ft whip elevated at 36 ft, with only four sloping radials.
It was like a whole new antenna when elevated.

After using it a while, I rigged up a 24 volt relay at the feed point
so I could add a small matching coil for 17m 5/8 wave GP use.
I could take it in or out of the circuit from the shack by plugging or
unplugging a 24v wall wart. :/ Worked out very nice.

You are not the first ham to discover that elevated radials work so much
better than those in the ground. I guess in theory, 120 radials in the
ground would work OK, but who wants to do that when elevating four
radials does better.

I was pretty shocked when I first read about this in the attached pdf