This is a lot mushier for buried radials, but the simple case for elevated
radials is that the standing waves are set by the distance of the end of the
radials from the radial feed, just like it is on a dipole. The end of an
elevated radial MUST be the zero current, high voltage controlling point of
the radial.
I have not found the "ground image" mental simplifying device to be at all
useful. You will find that only a couple certain ideal cases match the
behavior suggested by the image. Personally, I've ditched the ground image
as a useful concept for any situation I can afford on HF and down. If you
can copper-plate a meadow somewhere, it will work for you. If you can swing
that, I have some investment opportunities for you :>)
The real trick with a lot of this is to force these mental simplification
devices to deal accurately with losses that MUST be there. Losses in the
dirt are there. ACCOUNT for them accurately and what is left after the
shakedown starts to make sense.
73, Guy.
2011/9/17 Mike Waters <mikewate@gmail.com>
> Guy,
>
> I'm not saying that I understand this 100%, but I certainly do find it
> fascinating. I have a question, though.
>
> For quite some time, my understanding has been that by making a bottom-fed
> vertical (or inverted-L) longer than 1/4λ --and thereby raising the max
> current point-- that we simply move the point of maximum current farther out
> on the radials. This makes sense to me, if we consider the thought that the
> ground is an image of the antenna, the "missing" portion (for lack of a
> better expression).
>
> Other well-respected hams used to say that this condition significantly
> added to the requirements for the radial system under such a longer vertical
> in such a way that we now need even longer radials. Later, though, one of
> these hams seems to have reversed his beliefs 180°. I don't pretend to know
> the answer. (And at this point, I'm not sure anyone does. :-)
>
> If I use a 5/16λ or 3/8λ inverted-L, how does this change the requirements
> of:
>
> 1. ~60 radials stapled to the surface of the earth ?
> 2. An elevated counterpoise (which would of course require far fewer
> radials) ?
>
> Thanks,
> Mike
> www.w0btu.com
>
> On Fri, Sep 16, 2011 at 11:03 AM, Guy Olinger K2AV
> <olinger@bellsouth.net>wrote:
>
>> This is an answer to an off reflector conversation, relating to a "too
>> long" electrical length over radials reducing performance. I am writing to
>> the [snip]
>>
>
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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