I've got a book here with some old G.H. Brown data.
This is probably ~1 MHz data. It shows about a 6dB
difference between 2 quarter-wave ground-mounted
radials and 15 quarter-wave ground-mounted radials
for a .05 to .07 wavelength vertical radiator. For a full-
quarter-wavelength, this difference is smaller (~2.5dB).
It would be interesting to see if NEC-4 predicts Brown's
results which were presumeably obtained via carefully
conducted experiments.
Sevick also has some interesting observations regarding
short-verticals with abbreviated radial systems in the last
chapter of his book "Building and Using Baluns and
Ununs". His data seems to suggest that for a reasonable
electrical height, the difference between a sparse system
(four 1/16 wl radials and a more dense system - say
40 1/16 wl radials isn't all that much with minimum
efficiency on the order of 50%). Of course, Sevick's
data shows that the 1/16 wavelength radials are more
efficient than the 1/4 wavelength radials, but I assume
this is a typo in the plot legend.
I would suggest that a worthwhile experiment would
be to put a reference antenna with a lot of radials and
then erect a test antenna several hundred feet away
with a sparse system and then do a bunch of A-B tests
on the air. Take away radials from the test antenna until
the degradation from the reference is no longer bearable :)
73 Mike, W4EF...........................
----- Original Message -----
From: "Jim Lux" <jimlux@earthlink.net>
To: <towertalk@contesting.com>
Sent: Monday, July 04, 2005 5:04 PM
Subject: [TowerTalk] portable verticals with crummy ground radials
> Inspired by the recent article in QEX, I was just doing a bit of
> experimenting with NEC4 simulating portable verticals with severely
> restricted grounding systems. That is, say you put up a 30 odd foot high
> (for 40m) monopole and laid out some wire on or near the ground for
> grounding. Sure, it's not going to perform like a "real" grounding
> system,
> but I was wondering just how much difference there is between, say, a
> couple 10 foot lengths of wire laid out, and a bunch of longer wires. If
> you're setting up a portable operation, there's a lot of difference
> between
> a minute throwing out a couple 10 foot wires, and spending half an hour
> stringing radials around.
>
> At first, it looks like not a heck of a lot of difference. I tried 4
> initial shots:
>
> 4 radials 3 meters long
> 4 radials 10 meters long
> 10 radials 3 meters long
> 10 radials 30 meters long
>
> In all cases, the radials were modeled as AWG16 wire, 3 cm off the ground
> (which NEC4 should handle appropriately, but I'm not sure). I used the
> not
> necessarily valid average 13/.005 ground (Sommerfeld/Norton). I'll
> systematically look at ground property effects later.
>
> Interestingly, the gain tends to run around -1.1 dBi for all cases, with
> maybe 0.2 dB difference between the cases.
>
> Also interestingly, just like the authors of NEC claimed in their report,
> adding a ground stake (or, at least, a connection to ground under the
> antenna) makes things worse. This kind of thing is precisely one of the
> things they wanted to model with NEC4 when it was written. I haven't
> compared my modeling approach with theirs, though, so my results (ground
> stake wise) shouldn't be taken seriously yet.
>
> Anyone aware of any similar modeling or testing that has been done,
> specifically to compare subobtimal or marginal grounding systems.
>
> Jim, W6RMK
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
> Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
> any questions and ask for Sherman, W2FLA.
>
> _______________________________________________
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>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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