To: <topband@contesting.com>
> Date: Tue, 17 Mar 1998 13:48:28 -0500 (EST)
> From: CQ K8DO <CQK8DO@aol.com>
> buck?"... I asked the same question of Tom recently and while I am still
> attempting to parse the various dB's quoted it appears that somewhere between
> 32 and 64 ground radials is the breakpoint compared to 4 elevated radials of
> 1/4wl...
No, here's how it stacked up in dB. 0 dB is the reference of 60
radials. These are farfield signal levels, accurate to + - .1 dB.
Elevated .03 wl Conventional
four -4.3 dB -5.5 dB
eight -2.4 -2.7
sixteen -.8 -1.3
thirty two -.7 -.8
sixty -.2 reference 0 dB
I selected the 60 radials as the reference antenna, so ALL the
measurements are in reference to the field from that system. In
theory, that system is about 1 dB below perfect.
> It appears to me that 8 elevated radials are closer to reality for the average
> topbander than a ground radial system of more than 64 radials....
That would be OK if you accept being about 3 dB or so down. Myself, I
want that 3 dB since it only takes a few afternoons work and $120 or
so of material. That's about the cheapest 3 dB I can buy, since I
have a 1500 watt PA.
Consider this, going from the 3/8 wl vertical to the four square only
gained me 5 dB! I got almost that just by going from four radials to
60 radials.
It is often claimed 120 1/4 wl radials are ideal. Not according to
extensive tests. With 1/4 wl radials, more than 60 offer very little
advantage. The results were:
30 -1.56 dB
60 -.93 dB
113 -.79 dB
The nearest to ideal measured in the famous L, B, and E RCA report
was 113 .412 wl radials. In that test, the end result was 0.2 dB from
perfect! Going from 113 .412 wl radials to 60 .274 wl radials
will cost you all of .7 dB!
By the way, these tests showed a nine foot on a side ground screen
was meaningless when a large ground system was used, but did make a 3
dB difference when only 15 radials were used. But this was for
a short radiator (1/16th wl tall). With a taller antenna the ground
screen would mean less, of course.
Disclaimer:
Other than my own tests, the data above is available in "Ground
Systems as a Factor in Antenna Efficiency" Brown, Lewis and Epstein,
RCA Manufacturing Co. and was printed in Proceedings of the IRE
Volume 25 number six in June 1937. I converted the results into dB
from the published mV/m, so it is easier to follow.
73, Tom W8JI
w8ji.tom@MCIONE.com
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