On 1/11/01 14:33, hasan schiers at schiers@netins.net wrote:
>Now that is a handy observation and helpful formula. It is based on Tom's
>point that the perimeter separation need be no more than .025 to .05
>wavelength for 1/4 wave radials.
>
>Now the question for more compromised situations:
>
>What is the maximum allowed perimeter separation of 1/8 wave radials. In
>other words, how many radials 1/8th wave long would be required to achieve
>"close to" the same performance of 31 radials 1/4 wave long?
That's NOT what Tom said. He was saying that radial spacing closer than
about 0.025 wavelength is virtually indistinguishable from a solid sheet
of metal.
That doesn't meant that short radials can somehow replace longer ones. We
can test this hypothesis by going to extremes -- what if we made a 0.05
wavelength solid disk. Would that replace a 1/4 wavelength radial field
with the same efficiency? No, of course not.
You can use Tom's numbers to figure out how many radials to use (within a
certain length) to be nearly indistinguishable from a solid sheet:
<length of radials> * 2 * pi / (0.025 or 0.050) = <number of radials>
So, if your radials are only 0.22 wavelength long (perhaps because they
are coupled to a very lossy ground), the maximum number you need is
0.22 * 2 * pi / 0.025 = 0.44 * pi / 0.025 = 55.29 or about 55 radials,
evenly spaced.
>1. How many 1/2 wave radials are needed to get to the "flat" part of the
>efficiency curve and how far below 100% efficiency is that.
Tom has already given that answer. If radial spacings below 0.025 are
nearly indistinguishable from a solid sheet, you just have to place your
radials so no wire is more than 0.025 from its neighbor. For 1/2 wave
radials, that works out to about 125 radials.
>2. How many 1/4 wave radials are needed to get to the "flat" part of the
>efficiency curve and how far below 100% efficiency is that.
We use the same equation, and we get about 63 radials. We need fewer
radials, since the size of the radial field is smaller.
>3. How many 1/8 wave radials are needed to .....you get my drift.
31 -- but this radial field is only as effective as a 1/8 wave solid
disk. It won't act the same as a 1/4 radial disk.
>"I only have room for "1/X" wavelength radials. How many do I need before
>I'm wasting copper and effort, and how far down the efficiency curve am I
>going to be?"
I can tell you how many wires to use from Tom's observation, but I can't
tell you offhand what a smallish radial field does to your antenna
efficiency. If it is a lot less than .25 wavelength, your antenna system
is going to change.
>Thanks to Tom and Bill for stimulating my questions on this matter. It is
>not just an academic exercise. I will be putting my full size 1/4w 40m
>vertical back up in the spring and will be ground mounting it. Where I
>choose to mount it is somewhat limited and knowing the compromises involved
>(with an approximate numerical value), will be most helpful.
I'm in a similar boat. I'll be shunt-feeding a tower as soon as I get it
up. From this equation, I can figure out that I need somewhere around
36-45 radials, based on where I have ground to lay them down. Anything
more is probably going to be a waste of effort.
Another consideration is that the last few hundredeths of a wavelength of
radial wire doesn't do as much as the first few, especially when laid on
lossy ground. In some cases, you can foreshorten the radials without too
much ill effect. If you make the radials shorter, you really don't need
quite so many to maintain the same minimum spacing.
If we were laying radial wires made of gold or platinum, we might
carefully optimize where we put metal. But copper wire is cheap. I've got
a 30 lb roll of 14 gauge wire that W7OT gave me for nothing (thanks
Mike!). The hard part is having the space to put radials down, and the
time and effort to get them there. This 0.025 spacing business helps with
the latter. Why expend effort for little improvement?
Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@arrl.net
Quote: "Not within a thousand years will man ever fly!"
-- Wilbur Wright, 1901
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