[TenTec] Re: Al Gavenas' position on Front-End atenuation

[George, W5YR]

For decades there has been the notion that unless a feedline shows as close
to 1:1 SWR as possible, the antenna is no good. Folks have literally spent
months pruning, trimming and trying arcane things trying to reduce an SWR
from 1.2:1 to 1:1. And for no good reason . . .

Of course, it is all nonsense since there is no correlation between feedline
SWR and antenna effectiveness. Any antenna will radiate all the power it
receives in whatever pattern its dimensions and location dictate. IF the
feedline is low-loss, such as ladderline, the SWR can be 10:1 and 90% of the
transmitter power can still reach the antenna and be radiated.

Walt Maxwell W2DU goes into this in great detail in "Reflections II" and in
his original QST series of articles, available on the ARRL website.

As to the second question, let me answer with a question:

What do you do if there is *no* r-f stage to control?

Controls continue to be labeled 'RF Gain" for mostly historical reasons.
Modern receivers seldom have gain-controllable active devices between the
antenna and the first mixer.

The RF Gain control is most receivers actually places a control bias on the
AGC system and indirectly controls IF gain, etc. So, it is hard to control
the gain of a non-existent amplifier stage. Preamps are invariably fixed
gain as are passive attenuators.

But, historically, when receiver design concentrated on sensitivity instead
of strong-signal handling and selectivity, it was necessary to control the
gain of the r-f amplifier stages to prevent overload, cross-mod, etc. My
1946 National HRO-5TA1 had two high-gain r-f stages ahead of the mixer. The
AGC - then called AVC - was poor on AM and non-existent on CW so a manual
control had to be provided and used.

That is where the myth began that operation is improved by running the audio
gain up high and backing down the
r-f gain. In those days, that was the only way to receive CW. Nowadays our
modern receivers have distributed, multi-loop AGC systems that can far
surpass the ability of a human - most of the time! - to control gain in the
receiver.

Allan Kaplan W1AEL at TT recently posted a very good overview on this topic.

Hope this answers your questions, Gene. Thanks for asking!

73/72, George
Amateur Radio W5YR -  the Yellow Rose of Texas
Fairview, TX 30 mi NE of Dallas in Collin county EM13QE
"In the 57th year and it just keeps getting better!"
<mailto:w5yr@att.net>





----- Original Message -----
From: "Gene Buckle" <geneb@deltasoft.com>
To: <tentec@mailman.qth.net>
Sent: Thursday, April 10, 2003 9:06 AM
Subject: Re: [TenTec] Re: Al Gavenas' position on Front-End atenuation


> > fetish similar to the 1:1 SWR craze. There are doubtlessly instances
with
>
> What do yo mean by "1:1 SWR craze"?
>
> If the RF gain control doesn't actually control the r-f stage, why is it
> still labeled as such?