Tom (and James),
I am well aware that my comments concerning the 5/8ths wave was based upon
subjective/anecdotal evidence. I am in a science (Astrophysics) by
profession..... I do know the difference. HOWEVER, I cannot completely throw
out the simple fact that when I altered my 20 meter omni antenna for Winlink to
the 5/8ths, that I received UNSOLICITED comments from the system users stating
(100 percent of them) that my signal was much improved into the areas they
happened to be sailing. None of those people, not a single one, knew that I
was changing my antenna. The purpose being just that..... to see if anyone
complained or said anything else concerning performance from THEIR point of
view. In reality, THAT is the point when supplying a service..... What do the
USERS think of the performance, not what I think or what a FS meter says.
Even if I had the equipment to measure the performance and the equipment says
that my signal should be improved, but the Winlink user comes online and
universally states my signal sucks, I will REMOVE that antenna. I know that
isn't likely, but if the modelling software states the signals should be worse
and they are, 100 percent, reported as improved, then there is something at
play that the modelling software isn't taking into account........
Not arguing, just answering that initial question that from my EXPERIENCE with
that antenna, it works better than the quarter wave at the same locations I
happen to be...... AND ONLY ON 20 meters. I won't speak to any other band,
although I would think it would work there, too, because I have not put one up
for those other bands.
As an aside, alot of folks are using that so-called non-resonant vertical
antenna that is roughly 43 feet tall....... They seem to be having some success
with them on the bands. Physically, they are pretty convenient..... and on 20
meters, they happen to perform pretty well, judging from user comments,
anyway..... and at that band, it is roughly 5/8 wave in electrical height. So,
again, I find it interesting that actual experience argues with the modelling
software (in MY particular instance). But, again, in my case IN HAWAII, I had
an outstanding location that, for some reason I don't know, happened to favor
the 5/8 by alot more than even the theoretical gain would indicate. NOBODY
would comment, much less 100 percent of commentors, on a slight gain of 2 db.
Heck, that wouldn't be worth the effort. However, I think you are quite right,
Tom..... something else is at play.... ground clutter (I had some.... loads of
tropical trees and plants in the area), some significant
ly tall sailboat masts RIGHT ACROSS THE STREET from my house (very close),
etc, etc. Then you consider the seawater ground that was so close by......
There are many factors to take into account, not much of which does a modelling
software take into account. Undoubtedly the answer is there and not directly
related to antenna gain. I did try elevating it on top of a 40 foot pipe mast
and using 8 resonant radials....... it made no difference except to increase
the chances that high winds would knock it down. In terms of SUBJECTIVE
performance and comments, no difference. So, I put it back on the ground and
carried on.
Remembering what the antenna was for, and where it was located, putting up a
horizontal 80 or 90 feet in height simply wasn't possible. The SERVICE I was
providing required OMNI directional capability, so even a dipole would be unsat
in at least 2 directions...... those being the directions an emergency call
would not be heard...... Not a good situation for the given purpose, right?
I will say that I didn't appreciate the comment concerning how I came to my
conclusions about antenna performance. Insults only prove that one has run out
of reasonable arguments..... and that is ALL it proves. Given that, this will
be my last post here and I am likely removing myself from the list. Insults
are NEVER science...... not now, not ever!
Mike AB7ZU
Kuhi no ka lima, hele no ka maka
On Sep 7, 2013, at 18:59, "Tom W8JI" <w8ji@w8ji.com> wrote:
>> The above modeling results just don't support that contention/posit so I'm
>> wondering what else comes in to play that could lead folks to love the 5/8
>> wavelength vertical over a shorter version, regardless of frequency? I
>> don't see one performance comparison that supports that claim. I'm not
>> saying the "claiming person" isn't correct but....I don't see how!
>> Help - what am I missing here?
>> 72, Jim Rodenkirch K9JWV
>
> The 5/8th wave obtains the small amount of gain it has through effects of
> ground reflection. The current maximum is elevated, and that elevation causes
> additional phase shift with the illumination of earth out some distance from
> the antenna. The re-radiation of earth is sometimes explained by an "image
> antenna". The image antenna is a fictitious antenna directly below the real
> antenna, and this "image antenna" simply represents what the earth at a
> distance does from the illumination from the main antenna.
>
> The 5/8th wave moves the current maximum slightly higher than a quarter wave
> above earth, so the fictitious image moves slightly lower than a 1/4 wave
> below surface.
>
> If you move the 5/8th wave above earth, such as in a groundplane well above
> the earth, the extra length no longer provides gain. Instead, it actually
> reduces gain at low angles.
>
> Another effect is the extra height above ground of the high current area can
> help get the antenna's main radiating area a little bit higher above ground
> clutter.
>
> This would also apply to an extended double zepp, where the second antenna
> half makes the image unnecessary. Each half of the double zepp is the image
> of the other side, so we don't need earth.
>
> So it is a very specific benefit from the 5/8th wave caused by moving the
> current height up above a reflecting surface, or in the case of a double zepp
> moving current away from a second identical element while still having a
> common center feedpoint.
>
> One of the biggest antenna hoaxes played on people was the 5/8th wave CB
> groundplane antenna. Two meter 5/8th wave groundplanes are the same. The work
> on the broadcast band to increase groundwave signal because soil is often
> reasonably low loss on the AM BCB. If the soil out some distance from the
> antenna is lossy, or if it does not exist, the 5/8th length causes increased
> low angle loss.
>
> This is why when you look at models, outside of specific cases like some
> cases of low broadcast band use, we have a tough time seeing the gain
> imagined or claimed. A field strength meter has an equally difficult time. :)
>
> 73 Tom
> _________________
> Topband Reflector
_________________
Topband Reflector
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