It is interesting to compare Cebik's calculations for gain versus number
of radials with that which W8JI measured. Cebik calculated that the
gain difference between 4 radials and 64 radials was 1.2 dB. W8JI
measured 5.5 dB difference between 4 and 60 radials.
I am a big fan of modeling but I am very wary of the accuracy of
answers, particularly when modeling verticals. There is no doubt that
experimental data trumps modeling, so there are some significant
discrepancies between the two, remaining to be explained. Other people
have made the same measurements and have gotten slightly different
answers, usually less than W8JI measured, but also more than NEC
predicts. Measure data is taken at close to zero degrees elevation and
calculated data is usually quoted at the take-off angle, but modeling
also says when the difference is due only to ground loss, all elevation
angles get attenuated by the same amount. So even if you did the
analysis and quoted the data for very low angles, the gain difference
numbers would be the same.
I have said for a number of years, I think NEC underestimates near field
ground loss. Unfortunately I don't really have anything to substantiate
that, other than the inability to correlate calculated data with
experimental data.
Most of the data we see published for radials (length, number, and gain)
are based on numbers generated by NEC.
Jerry, K4SAV
donovanf@starpower.net wrote:
>W8JI posted a classic reply to this question nearly ten years ago.
>
>Not coincidentally, sixty 1/4 wavelength radials have since become the defacto
>standard among serious 160 meter DXers.
>
>Here's a brief synopsis, followed by the original text of Tom's original
>e-mail.
>
>All of these measurements are referenced to the performance of sixty 1/4
>wavelength radials. Actual results are highly dependent on your local ground
>conditions.
>
> 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
>
>73
>Frank
>W3LPL
>
>Subject: TopBand: Elevated GP vs. Vertical Antennas - long
>From: w8ji.tom@MCIONE.com (Tom Rauch)
>Date: Tue, 17 Mar 1998 18:30:34 +0000
>
>
>
>
>>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
>
>
>
>
>
>
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