[Antennas] RE:Vertical Antennas

2002年9月28日 23:07:18 -0500

Before the "obfuscating factors" get too bad, are we talking about 1/2-wave 
antennas that are
*physically* a half-wave tall, or an *"electrical* half-wave "long" ?
My CushyKrap.... Er, Custcraft.... RV-7 is advertised as an "electrical 
half-wave antenna", but it's only 24 feet long.... Not quite a *physical* 
quarter-wave on 40M.
I've used it at nearly ground mounted (about three feet above the dirt) and 
on a piece of chain-link "Top Rail" tubing at about 20 feet above the 
ground. I *still* don't understand how it works, and even the "Cushcraft 
engineers" can't (or won't?) explain it to me when I confront them at ham 
conventions! I gotta' admit, however, that for an air-cooled dummy load it 
works pretty well on all the bands it covers.
Karl K - W8TIF
McKinney, Texas
-----Original Message-----
From:	Billy Cox [SMTP:[email protected]]
Sent:	Saturday, September 28, 2002 10:17 PM
To:	[email protected]
Subject:	Re: [Antennas] Vertical
>I agree. Purchase the study by K7LXC
>(Champion Radio http://www.championradio.com/forsale/).
>>Excellent piece of field work but, as I recall, it did not investigate the
>'elevation' question. If I recall correctly, the study reports on ground
>mounting of those antennas whose manufacturers required radials and
>elevating (to 30 ft?) those antennas that had their own 'short' radials
>(e.g. the CC R-7). Sort of an apples and oranges comparison but quite
>useful nonetheless.

Yes, indeed. You are correct, they didn't address the elevation question.
>Not sure that changing the feedpoint position on a halfwave length of
>radiator (vertical or horizontal) changes the radiation pattern. Yes, I
>would expect it to have a different SWR due to impedance change, but not
>sure that means the radiation patterns change. Did I misinterpret the
>point? Or do I have a technically incorrect understanding of what is
>happening?

Let me use a better example here.
A multiband 1/2w type vertical antenna, with the base at 33' up.
When we use that antenna on 20m, the base is roughly 1/2w up.
But when we use that same antenna on 10m, the base is now 1.0w up.
That same antenna, at the same height, is NOT going to have the
same pattern on those two different bands as to wave angle.
The elevation patterns are going to be different due to the difference
in height as to wavelength. QSL?
>Seems like the concept behind so called 'short radial' verticals, is that
>they are a variation of half wave length (like a vertical dipole) radiator
>whose feed point has been physically lowered. Changes the impedance but
>don't believe it changes the radiation pattern all that much. Just like a
>half wavelength horizontal radiator can be fed 'off-center' and still
>achieve the same radiation pattern but present a different matching
>requirement. Maybe I have it all wrong.

There seems to be several ideas ... the GAP vertical is more of the
1/2w vertical dipole fed in the semi-center, while the R5/etc series
is more of a 1/2w vertical fed at the base. As you say, the difference
lies as to the matching. Now, there may be some major efficiency
issues as to ground loss still, 1/2 or 1/4w.
As to the role of the 'short radials', I ponder if their real role is more
like what we see on the Ringo Ranger II or the Isopole design, as
they are trying to keep the RF off the outside of the coax shield?
Back to my original point ... 1/4w verticals ARE more ground or
groundplane dependent ... such as the HF2V, 14AVQ, etc.
And both 1/4 and 1/2w *may* benefit from ground screens. If you go
high enough (multiple wavelengths) then only 4 radials may be OK
for the 1/4w ... think of a simple 1/4w 2m ground plane as an example.
The common 5/8w VHF antennas adds more to the discussion, as it
DOES need a ground plane also.
>The WWVH point is interesting. My first reaction was that it is
>essentially a 'ground mounted' vertical dipole

Yes, probably due to mechanical/structural reasons there ...
>(a very inefficient antenna for long hauls)

Depends ... on what height we put it at ... look at your models again.
I also wonder WHAT ANGLE did/does WWVH want to target?
>and that the radials are used to try and achieve an enhanced 'salt
>water' like effect of lowering the TO for the primary lobe. Could be
>wrong as I have no first hand knowledge.

I'd say you are on the right track here ... Or just to raise the system
efficiency as high as possible.
>Big structure (to handle high power? to provide broadbandedness?)

Probably due to the RF power levels, they don't seem to QSY much. :-)
>but it does not really appear elevated much. So, all the more reason to
>add radials even if they aren't physically attached to the radiator.
>My guess is the extra radials help lower the takeoff angle of the primary
>lobe but could be wrong.

See above ... I think it may be several points.
>Wonder if they elevated that antenna to 200' if the radials would still
>be helpful (probably be real difficult to fed it though).

I think both of your above thoughts are true. My guess is that physical
limitations may be main reason, then electrical.
>Perhaps someone has firsthand knowledge and can clear this up rather
>than looking at a picture and having to guess.

Yes, I'd love to know why/how they designed that also ...
>Thought the latest version of EZNEC (3.0) from W7EL was considered
>pretty accurate now. Would appreciate hearing from anyone who knows
>otherwise as I use it quite a bit to model. Didn't think it had a 6DBi
>error like earlier versions of NEC, but wouldn't be the first time I 
didn't
>get the word!

I use it quite a bit also. But read the notes as to ground planes, and
radials. Especially close to ground. Watch out for some Beverage
models also.
Two of us built 80m4x arrays after original QST articles. One is still 
using
an elevated radial system (me), the other has gone back to a ground
mounted system ... and proves to be roughly 6 dB better than the elevated
array (which is mine). When I rebuild the array ... it will go back to 
being
a ground mounted array also. Here's part of why we may miss the "6 dB".
My array has a pattern ... you can hear a G3 station with it pointed NE,
switch to the SW and NOT hear the G3 station, but now hear a ZL ... so
you think WOW, with that great F/B, it's got to be a killer antenna.
Reality, as compared to the other 80m4x, now using the ground mounted
radials, it's roughly 6 dB down on transmit. Still it plays much better 
than
the normal 80/75m low dipole, but it's not all that it could be and the
plans here are for the bases to go from being 8' up to ground level.
My observations agrees with what W8JI/ON4UN/N7CL measured also.
ON4UN has a 80m4x in his book ... that uses 1 radial per vertical, and
the feedpoint is elevated. Aha you say ... Well, let's look at the whole
picture on this, as John and I have discussed this when he was over
here a few years ago. Look at what is BELOW his 80m array, which
is strung around ... his 160m tower vertical, he has MANY radials on
the ground ... (shades of WWVH?)
A most interesting discussion!
73 Billy AA4NU
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