According to the field strength graph in N6PL's vertical antenna
book, 15 1/4 wave radials under a 1/4 wave radiator would yield
an efficiency ~70% of that which would be acheived using 113
radials (e.g. - 1.5dB). This data appears (although it doesn't say
explicity) to correspond to average soil (~5ms/m). The book
references G.H. Brown's work at RCA as the source of the
data.
YMMV.
73 de Mike, W4EF..........
----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: <towertalk@contesting.com>; "jljarvis" <jljarvis@abs.adelphia.net>
Sent: Saturday, March 09, 2002 4:22 PM
Subject: Re: [Towertalk] grounding an elevated vertical
> > There was significant quantitative work done on 1/4 wave verticals
> > with elevated radials, in the late 70's and early 80's. This was
> > intended for AM broadcast use, but the experimental work was carried
> > out at 1.8 MHz by amateurs who are also consulting engineers.
>
> Actually there was no quantitative work done at all, other than
> models. The only data from measurements used standard FCC
> slope-of-ground-conductivity measurements, and they can easily
> be all over the place for accuracy.
>
> When i looked at data that was taken from these measurements,
> they didn't even use the mean FS value where the data had
> "flyers". (Flyers are where the FS goes through abnormal changes
> in level with distance.)
>
> The only accurate way to measure an efficiency change would be
> to only change the radials, and measure at the same points with
> ONLY a radial change. To the best of my knowledge that was only
> done a few times, and the change in FS was significant in favor of
> the conventional system.
>
> > To be sure, copper plating earth will improve the performance of a
> > vertical. It's a question of marginal improvement in efficiency
> > versus expense. 120 buried radials are the broadcast norm. But 16
> > radials will get you to 90% efficiency. And 3 or 4 elevated radials
> > will do the same.
>
> 16 radials does not normally get you to 90% efficiency, and neither
> does 3 or 4 elevated radials unless they are 1/4 wl high or higher
> above earth.
>
> > Empirical data presented to the FCC showed that 4 radials, elevated @
> > 10' were the equivalent of 120 buried radials, in terms of field
> > strength. If I recall the experiment, though, there were REMAINS of
> > those 120 radials still in the earth, and the ground conductivity was
> > exceptionally good.
>
> The only system like that I recall was in the Carolinas, were soil is
> not particularly good. That paper had almost zero technical merit
> because:
>
> 1.) The measurement was based on using slope of attenuation with
> distance to calculate expected FS, and relatively little data was
> taken. You can get almost any result you like within ~3dB using
> that system, or more if the engineers aren't very careful or totally
> unbiased.
>
> 2.) An unknown amount of original radials remained.
>
> 3.) The station where the measurements were made reported poor
> coverage to me on the telephone, but they said they really didn't
> care because they only needed to cover a few miles. They claimed
> a proof was never done, and never needed, because they were a
> 250 watt non-directional (running at reduced power).
>
> Elevated radials have largely fallen from use now at BC stations,
> just as they did in the 1930's when the original RCA study was
> made.
>
> If we think back, virtually all original ground systems were small
> resonant counterpoise systems. Then we got smart, did tests, and
> started using the better system. Later, we tried going back to the
> system that was abandoned in the 30's!!!
>
> Two steps forward, one step back?
>
> Bottom line is use as many radials as you can as long as you can,
> and if you can only use a few radials by all means elevate them. If
> you have 8 radials at 1/4 wl height, it is indistinguishable from 60
> radials or 120 radials on the earth (based on IEEE papers detailing
> measurements comparing one system both ways).
>
> If you have 4 radials, try to keep them 1/2 wl above earth.
>
> In all cases with small elevated radial systems you should use a
> feedline choke at the antenna, and keep the radials isolated from
> earth paths at the operating frequency.
>
> Even a 1/4 wl groundplane with four radials has considerable
> feedline "earth currents" unless it is choked at the feedpoint.
> 73, Tom W8JI
> W8JI@contesting.com
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