> 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?
No, and I said as much in my post. The report didn't mention the BN4000
being used on the TH-7, and implied that the Force 12 was used. Like I said,
it's a nit.
> 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.
I don't really think so because so many others have reported similar VSWR
performance to mine on the TH-7. My height and coax length were virtually
identical to the test setup, and I used an unguyed tower as well. I suppose
that ground quality could have some effect on VSWR, but I'm not sure it
would have been as dramatic as the difference between the tested antenna and
Hy-Gain's published specs, especially on 15M and 10M. As for terrain, unless
there are some really radical countours in the near field of an antenna at
50 feet, I doubt that VSWR would be affected so much.
> 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.
Hy-Gain's comments don't address the high VSWR on 20M and 15M. 150 Khz
bandwidth on 20M is half what the converted TH-6 showed (and what my TH-7
and many others show.) VSWR was nearly 4:1 at the bottom of 15M, didn't drop
below 3:1 until 21.125, and only dropped barely under 2:1 between 21.325 and
21.425. 100 Khz bandwidth on 15M is pretty bad, and not normal for a TH-7.
For example, the ARP report says, "the TH7DX from Hygain confirmed its
prevalent notarity (sic) as an extremely broadbanded antenna. Encompassing
the full 2200 kilohertz within its 2.1 tp 2.1 VSWR points and really if
(sic) looked at a chart concerning the TH7DX, you would see that it ran out
of amateur band allocation and was still, in many cases, below the
end-to-end VSWR points." That's what I get on mine, too.
Since the 10M anomaly was uncommon, and you were concerned about it, and the
20M and 15M VSWR were so different from both Hy-Gain's published specs and
the converted TH-6, you should have assumed that the TH-7 was defective in
some way and left it out of the report altogether. Since the anamoly was not
fully explained or addressed in the text, it is not unreasonable to expect
that some readers will look at the bandwidths listed in the tables and
assume that a stock TH-7 has poor VSWR bandwidth.
> 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.
That is interesting, and I'd like to know more about the effects (or lack
thereof) of VSWR variations on gain and F/B performance, especially in the
murkey depths of differences of less than 1 dB explored in the report (any
experts out there care to comment?)
I know that you can't expect your VSWR curves to exactly match the
factory's, but significant deviations are often indicative of some sort of
problem. I suspect that in many cases where a discrepancy exists, a mistake
has been made in assembling the antenna or there is a defective part (a bad
trap, a bad balun, a faulty joint, a bad feedline, etc.) I certainly don't
feel comfortable evaluating the gain and F/B performance of an antenna when
there is clearly something wrong with it. Maybe performance will not be
affected by the high VSWR, maybe it will.
There are reasons besides the effect on gain and F/B why a high VSWR might
be undesirable. Solid state rigs used without tuners are going to waste that
gain by dropping the power output. Extremely sensitive amps like the Alpha
87A are going to refuse to work at all when the VSWR is over 2:1. With the
gain and F/B performance numbers for the big tribanders so close, VSWR
bandwidth considerations may well be the deciding factor between one
tribander and another.
Again, if someone is reading the report with the intent of using the
information as a guide to purchasing a tribander, there is not enough
explanation of the poor VSWR bandwidth reported for the TH-7 to prevent that
person from incorrectly assuming that this is normal for the product. I just
think that the TH-7 results should be omitted to avoid this kind of
misunderstanding. If the TH-7 cannot be adjusted for better VSWR, and a
different one is not available, why not just publish the results for the
converted TH-6 (along with details on any differences between a converted
TH-6 and a stock TH-7)? No useful information was added by including the
TH-7, and a real possibility of misleading the reader was introduced.
Just my humble opinion, of course... :-)
73, Dick, WC1M
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