[TowerTalk] Steve's antenna study

[K7LXC@aol.com]

In a message dated 98-07-19 18:50:48 EDT, dick.green@valley.net writes:

> 2. A small nit about the actual test procedures: The protocol calls for
>  assembling the antennas according to the manufacturers' specifications, but
>  evidently a Force-12 B-1 "bead" balun was used with the TA-33, Skyhawk,
>  TH-7DX and TH-7DX (6). Although it shouldn't make any difference at all, I
>  would have preferred to see the tests conducted with the antennas
configured
>  exactly as supplied (or recommended) by the manufacturers. For example, I
>  don't think a balun comes with the TA-33, and the TH-7DX comes with a BN-26
>  or can be used with a BN-4000B (like I do.) I think it's unlikely that the
>  average TH-7 owner is going to use a Force-12 balun.

      As you'll see on page 5, the KLM 4:1 balun was used on the KT34XA, the
BN4000 was used on the TH-11 and the Mosley PRO77 used the factory specified
'no-balun'. I think a BN4000 was also used on the TH7 (I'm doing this from
memory) since we didn't have and wouldn't have used a BN-86. The Force 12
balun was used in the absence of a specified balun and in reality was only
used on the TA-33 and Bencher Skyhawk. Do you really think a different balun
would have changed the results?
>  
>  4a. The reported VSWR bandwidth results for the TH-7 are troubling. Judging
>  from the shape of the reported curves, the antenna was tuned for the
>  standard settings, which are optimized for phone. If you compare the
>  reported curves with Hy-Gain's published curves, the curves reported for
20M
>  and 10M look pretty much like the corresponding published curves, except
>  that they are raised considerably higher. The 15M curve looks correct for
>  about 3/4 of the band, but should start to tilt upward towards the high end
>  of the band. It, too, is raised considerably higher than Hy-Gain's version.
> 
      While these designs, dimensions and performance are pretty repeatable,
your mileage may vary. It could be due to YOUR antenna height, coax length,
ground terrain, etc.
 
>  4b. I think the average reader might incorrectly conclude that Hy-Gain has
>  misrepresented the VSRW bandwidths for the TH-7, or at least that one
cannot
>  expect broadband VSWR performance. I'm sure that the author's didn't intend
>  this, but they never really comment on what might have caused the VSWR to
>  deviate from the published specs. They simply comment that the VSWR was
>  higher than they expected on 10M, and that a TH-6 that had been converted
to
>  a TH-7 had lower VSWR on all the bands than the stock TH-7 (but still
higher
>  than Hy-Gain's specs for the TH-7.) The way it's written, one might
conclude
>  that all TH-6's that have been converted to TH-7's will have better VSWR
>  bandwidth performance than stock TH-7's, but surely that's not correct! I
>  don't know the details on the TH-6 conversion and whether the resulting
>  antenna is identical to a TH-7, but I would have felt more comfortable
>  reading about either a properly performing stock TH-7 and converted TH-6,
>  instead of a defective TH-7 and a converted TH-6. Was the latter offered
>  because the former was broken? They don't say. The report says that VSWR
>  tests were used to make sure that nothing was wrong with each antenna, but
>  the VSWR curves for the TH-7 clearly do not come close to those specified
by
>  the manufacturer. Why wasn't the discrepancy explained? Why wasn't this
>  antenna either fixed, eliminated or replaced?

     Hy-Gain comments that our 10M swr condition had been encountered before
but was not common. Rearranging the feedpoint mechanical parts had been
successful in lowering the 10M swr on other antennas. The comment was left out
of the report.

     We were concerned about the swr and that's why we tested 2 antennas. 
>  
>  4d. Should we also be suspicious about the gain results for the TH-7,
>  especially on 10M? Intuitively, I think the extra element should boost the
>  gain at least a little bit over 20M and 15M. Yes, the traps and coax could
>  be lossier at the higher frequency, but it still doesn't seem right. I know
>  that a VSWR compensation figure was used, but the high VSWR on all bands,
>  especially 10M, still makes me suspicious about all of the results obtained
>  for this antenna. I think that's the downside of not explaining what was
>  wrong with the test TH-7's VSWR. In commenting on the high VSWR on the TH-7
>  (and the more normal curve for the converted TH-6), the report recommends
>  that adjustments be made to lower the VSWR, but this will have no effect on
>  performance. A few pages back, the report says that VSWR behavior, gain,
and
>  F/B are closely related. I know this has to do with design tradeoffs, but
if
>  the VSWR curves aren't following the manufacturer's curves, isn't it
>  possible that the gain and F/B won't either?

      Why get hung up on verifying that YOUR swr is/is not identical to the
factory specs? Personally I thought that the gain of the TH7 on the bottom
half of 15M (over 5 dBd!) being at the place where the swr was over 2:1 was an
extremely interesting result. So you can get high performance (gain) IN SPITE
OF high swr. 

Cheers,  Steve  K7LXC

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