[TowerTalk] Re: Radials over salt water
Thu, 11 Jan 2001 13:17:14 -0500
> Hmm. 1/4 wave radials means a circle of 1/2 diameter. That's a perimeter
> of pi/2 wavelength, which divided by .05 yeilds about 31 radials.
> Similarly, .025 spacing at the ends is about 63 radials.
> If >63 radials is like a solid screen, I wonder why there's so much in the
> amatuer literature about installations with 120 or so radials.
> Bill Coleman, AA4LR,
Good observation Bill.
You hear that because people are parroting the FCC requirement,
which is based on "twice as much as near-perfect is good enough".
The FCC not only wants a system to work, they want it to have
"headroom" if something happens over time and it deteriorates. If
you read the RCA study that the FCC based its decision on, it
confirms the rule of thumb I offered.
As another point, BC stations almost always use ground screens
near the base of the tower. Yet it does absolutely nothing for the
signal, as long as the radials are sufficient in number. You can't
make people not use ground screens, no matter how much proof
you offer. It just "looks good".
Another wive's tale is four elevated radials are noticeably more
efficient than 120 radials on the ground. How can anything be
"noticeably more efficient" than something that is already almost
If they know that much about it, why don't they know 60 radials
are an immeasurable amount less than 120 when the radials are
1/4 wl long? The truth is 30 radials on the ground are barely
noticeable as being worse than perfect, and a lot less headache for
RFI and lightning damage than any sparse counterpoise system.
If you follow the general rule I outlined, you will be way out on the
flat part of the efficiency curve. You would only use 120 radials if
they were about 1/2 wl long, and you chose the narrowest end
Use more than that, and you are wasting copper. I know a couple
people with 360 1/4 wl radials! They could have used 100 1/2 wl
radials, and actually have picked up a couple tenths of a dB or so
and used less wire (and spent less time installing the ground).
I don't tell them that, cause it's a done deal. I listen to them telling
people that, and hear the admiration in the other stations voice
when they reply "no wonder you're so strong".
Maybe we'd better keep this a secret Bill.
73, Tom W8JI
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