Hello Doug,
The 50-60% figure depends on your soil conditions, so may vary quite a bit.
With my poor, sandy soil, the Vf is 67.7% with the radials laying on the
ground. When I buried them 6", the Vf was 39.8%. Using these shortened
radials, there wasn't much improvement going beyond 16 radials.
To find out your soil conditions, simply lay a temporary dipole on the
ground and use an analyzer to find it's resonance. Then trim to length. Now
you have your first two radials!
Good luck
Brian K8BHZ
-----Original Message-----
From: Doug Turnbull
Sent: Thursday, December 18, 2014 4:18 PM
To: topband@contesting.com
Subject: Topband: Confusion in ON4UN's Low Band DXing radial
lengthcalculations.
Dear OMs and Yls,
I am replacing raised radials for 160M inverted L with ground mounted
radials mostly because I could not readily get the raised radials up high
enough in my wood and also because of maintenance problems.
This inverted L goes up 100 feet at its top before levelling out for
the final 32' or so. It should I believe have a strong vertical element.
ON4UN's book Low-Band DXing 56th edition is generally excellent but I
do find the coverage of ground radials both confusing and somewhat
contradictory. This surprises me for what is pretty much considered the
bible.
On page 9-14 the text states that the velocity factor falls for
ground mounted radials to the "the order of 50-60%, which means that a
radial that is physically 20 meters long is actually a half-wave long
electrically!" This example is for 80M not 160M. However in the examples
found on page 9-15 the velocity factor change is ignored. I understand
the velocity factor change and have always accepted this. It generally did
not pay to try and cut radials precisely to a given wavelength. I accept
the radial length vs. radial number charts but is this an electrical length
in free space or a length considerably reduced due to velocity factory
change? Example 3 ignores velocity factor correction and from what I can
see this correction is ignore in most of the text concerning ground radials.
What does one do? Who does one believe.
While I am talking about a 160M inverted L; I did reference the
SteppIR BigIR vertical manual, page 18. Lengths should be scalable. I
find no mention of velocity factor and the shortening effect which is
experienced. The recommendations are not very different from those in
ON4UNs book. So does this mean one ignores the change in velocity factor?
I appreciate some guidance with this matter. I would like a
radial field which would take me to within 0.5/1 dB of the maximum
achievable for reducing near field losses.
73 Doug EI2CN
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