Right. The conventional wisdom on Tower Talk has been that, barring lossy
coax and/or matching networks, the reflected energy does eventually get
radiated by the antenna, with only minor losses.
For that reason, I wonder why the authors of the study decided to compensate
the gain figures for VSWR losses by using this formula for computing losses
due to a mismatch at the load end of the transmission line (from "Reference
for Radio Engineers"):
Correction = (VSWR+1)^2/(4*VSWR) = 1/(1-Pr/Pf)
Where Pr and Pf are the reflected and forward power, respectively. The
correction was converted to dB before being applied to the gain figures. For
example, the TH-7 had a VSWR of about 3.7:1 at that frequency, while the
converted TH-6 had a VSWR of about 2.6:1 (I'm doing a little eyeball
interpolation here.) 1.74 dB of correction was applied to the gain for the
TH-7, while .95 dB was added to the TH-6 gain figure. The result was about
1.5 dB better gain at 21.025 MHz for the TH-7 than the TH-6. Seems odd that
the same antenna design would show that much difference in gain. Perhaps too
much compensation was added for the TH-7.
I'm probably out of my league saying this, but I wonder if it's correct to
use this particular formula to compensate for the VSWR losses in this
particular test. It seems to me that the *primary* power losses in this test
would have come from the automatic power reduction circuits in the IC-735
kicking in as VSWR increased, and not from any mismatch at the load end of
the transmission line. For example, my solid state rig reduces power by 50%
at a 4:1 VSWR. Wouldn't the power delivered to the antenna at a given
frequency and VSWR be very dependent on the linearity of the power reduction
circuits in the IC-735? For example, it might reduce power by a much greater
factor at 4:1 than at 2:1. Wouldn't the VSWR formula have to take this into
account to be accurate? If VSWR losses from a load mismatch are indeed
negligible, wouldn't it have been better to use a good quality antenna tuner
and not use the compensation formula at all?
73, Dick, WC1M
-----Original Message-----
From: Pat Barthelow <aa6eg@cv.tmx.com>
To: DOUG WALLER <nx4d@sundial.net>
Cc: Jim Reid <jreid@aloha.net>; towertalk@contesting.com
<towertalk@contesting.com>
To: <towertalk@contesting.com>
Date: Tuesday, July 21, 1998 4:15 PM
Subject: Re: [TowerTalk] Steve's antenna study
>
>Yes, Doug, but almost ALL of the reflected energy is reflected back UP
>the coax, to the antenna and radiated, and with each trip down, up, and
>radiated, the vast majority of RF IS
>RADIAATED, even with high SWR, on the HF bands, and with reasonable coax.
>So the "loss" due to reflections at HF is miniscule, and can be ignored,
>except with respect to the impedance presented to a fixed output (as in
>solid state) final. A good antenna tuner will take care of that....
>
>73, DX de Pat, AA6EG/N6IJ
>"The Contest Station from the Government"
>
>Marina Amateur Radio Contest Station; N6IJ
>599 DX Drive
>Marina CA 93933
>
>aa6eg@tmx.com
>
>On Tue, 21 Jul 1998, DOUG WALLER wrote:
>
>>
>> Yes, but remember, impedance mismatches reflect some energy back down the
>> feeder that could have been radiated by the antenna. Doug - Nx4d
>>
>> Jim Reid wrote:
>>
>> > -----Original Message-----
>> > From: George T. Daughters <gdaught6@leland.Stanford.EDU>
To: <towertalk@contesting.com>
>> > Date: Tuesday, July 21, 1998 5:53 AM
>> > Subject: Re: [TowerTalk] Steve's antenna study
>> >
>> > steve wrote...
>> >
>> > <big snip>
>> >
>> > > So you can get high performance (gain) IN SPITE OF high swr.
>> >
>> > this shouldn't be a surprise. but it sure bears repeating!
>> >
>> > Yes, George, What gives an antenna gain has NOTHING to
>> > do with the impedance of the antenna. The impedance
>> > relative to the impedance at the end of the amateurs
>> > transmission line out the the antenna is from where
>> > vswr comes. Gain comes from the geometry of the beast.
>> >
>> > 73, Jim, KH7M
>> >
>> > --
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>>
>>
>>
>>
>> --
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>>
>
>
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>
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