[TowerTalk] Two "identical" antennas AREN'T

[Bill Hider, N3RR]

Dave,

Several things:

As the VSWR rises when the antenna is raised, what happens to the resonant
freq?  Does it rise as well?

What's wrong with a 1.4:1 VSWR??? - Nothing - I submit, if it holds at the top
of the tower.

Another way to test this antenna is to elevate the antenna vertically, with
the reflector down and the directors up, boom vertical, held away from metal
objscts (like the tower) with the reflector at least 4 feet off the ground. In
that position, the VSWR, resonant freq  measured will be identical to that
measured when the antenna is horizontal at 1/2 or higher wavelength above the
ground.I have made this measurement many many times.  It works just fine.
That's what I'd do in your circumstances since you say it's an antenna of your
own design.

I don't have the following info, which might alter my recommendation if I knew
them:

What is the ultimate height of the antenna(s).  Will they be stacked?  Are
there other antennas on the tower?  Either near the 20 ft test height or
higher/lower?  You refer to "imperical recomendations of the manufacturer",
yet you say it's your own design.  What do you mean?

Bill, N3RR

D. Rodman, MD wrote:

> I am looking for troubleshooting suggestions.
>
> I just assembled two new 15 meter antennas for my station.  These are new
> replacements.  They are my own electrical design.  They are 6 elements on
> 36' boom.  The antennas are fed with a hairpin match (0.375 OD/4.125"
> spacing) and a commercial coaxial balun.  The design resonant frequency
> was 21.2 MHz with a YO 1.0 VSWR match at 20".  We assumed a 5" lead length
> based on imperical recomendations of the manufacturer.
>
> The antennas have been up and down the tower numerous times to about 20'.
> The antennas have been carefully measured and elements are within 1/8"
> between the two antennas.  Both have new 7/4 wavelength flexible hardline
> coax feeds (approx 65').
>
> One antenna has a VSWR dip at 21.440 and the other is about 120 kHz lower.
> Both come no where near the predicted 21.2 match freq.  The driven
> elements required lengthening about 3/4"-7/8" and rather than a flat VSWR
> predicted by YO, the antennas can not get the VSWR below 1.4 at the bottom
> of the band.  The matches are 26" long on the 21.440 antenna and 28" on
> the 21.320 antenna.  I have tried as many manipulations as I can to alter
> the match and DE length, but there is no obvious way to lower the antenna
> resonance I can see.
>
> I am looking for suggestions on what I might do next.  Both antennas are
> on the ground.  I notice the VSWR tends to RISE as the antenna goes up.  I
> am reluctant to get the antenna at the top of the tower, only to find the
> VSWR is not suitable.  Can anyone offer suggestion on why this phenomenon
> is occuring and/or what I can do next??
>
> Thanks,
>
> Dave
> (KN2M)
>
> David J. Rodman, MD
> Assistant Clinical Professor
> Department of Ophthalmology
> State University of New York at Buffalo
>
> FAX 716-859-4565
> Office 716-854-1137
>
> e-mail: rodman@acsu.buffalo.edu
>
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