I'm working on building an elevated radial vertical. After installing the
radials, I have a question about what I'm finding.
Using a small toroid that I split in two, I wound 9 turns on each half, glued
the halves to the inside of a plastic clothes pin, and installed a full wave
bridge using 1N914s and a 0.01uF cap. The small clip attached to an elevated
radial provides a relative voltage indication, which provides a current sense.
With 100W into the antenna, a reading of 4vdc to 25vdc is obtained from an
individual radial. The readings a quite repeatable using a digital VOM.
There are three elevated radials, each about 1/4wL and each about 120 degrees
apart. At resonance, I find that the current was not well balanced between the
radials. So I started trimming. It turns out that by trimming the highest
current radial, the system came into relative balance.
By trimming the excess length off a couple of radials, I found that I could
balance the relative current reading pretty good. At resonance, this is what I
obtained.
trimmed pre-trim
North radial = 10.0 vdc 21.8 vdc
SW radial = 11.0 vdc 10.5 vdc
SE radial = 10.5 vdc 5.6 vdc
Here is what surprised me.
A 1.4% change (up) in frequency moved the trimmed values to:
North radial = 12.7 vdc
SW radial = 11.3 vdc
SE radial = 8.0 vdc
A 4.2% change (up) in frequency moved the trimmed values to:
North radial = 17.2 vdc
SW radial = 6.2 vdc
SE radial = 4.5 vdc
Now admittedly, this is somewhat kludge in that the 706 I was using to drive
the antenna was shutting down due to a bad load, but it is the relative reading
that I believe to be important. I was expecting to see the values remain
somewhat constant over frequency after trimming. Instead, I find a nearly 4:1
difference between them.
The goal of the process was to minimize pattern skew. I'm thinking it might be
quieter if I achieve a good null at zenith. Perhaps this is a twisted notion
to begin with. I find that the position of the radials is very important. The
SE radial needs to snake its way through some bushes and repositioning that
radial has a significant impact (negative) on the current carried by that
radial.
Is this normal for an elevated vertical? I know I am splitting hairs on this,
but I'm trying to understand better what is happening. Modeling using NEC2
does not seem to predict these behaviors.
Any empirical evidence from others who have experimented with this would be
appreciated.
73
Ford-N0FP
ford@cmgate.com
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