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

[George, W5YR]

The quoted statements at the start of your posting are still, I claim,
correct as stated. Your point that a tuner is advisable is a good one, but
the presence or absence or a tuner does not alter the validity of either
statement. Sound like a lawyer, don't I?

<:}

Actually, Heinz, the power reflected from the mismatched antenna will
encounter another mismatch when it reaches the transmitter output terminals
and be re-reflected and so on. Very little of the reflected power will
actually be conveyed to the final amp, depending upon the reflection
coefficient of the feedline/transmitter mismatch.

A tuner allows the mismatch to be better and more conveniently controlled
and ensures (a) total re-reflection of the reflected wave and (b) a 50 ohm
resistive load for the transmitter. But, even without the tuner,
re-reflection still occurs  . . .

The reason an amp overheats when improperly loaded is because it *is*
improperly loaded, that is it is not seeing its design load resistance,
usually 50 ohms resistive. Thus it cannot put out all of the power it is
being fed, less internal losses.

Under those conditions, unless the drive power or the d-c input power is
reduced, the amplifier is unable to deliver all its available output power
and the difference shows up as a heat rise.

It is an urban myth that "reflected power heats up the final amp." It
"looks" that way, but the actual mechanics are quite different.  The fact
that there *is* reflected power is involved but the heat rise in the amp is
not a result of the actual reflected power appearing at the amplifier
devices.

Good discussion of this in "Reflections II" by Walt Maxwell, W2DU. Also his
QST articles are on the ARRL website.

Thanks for the interest and the comment!

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: "Heinz und Hannelore Breuer" <hbreuer@debitel.net>
To: <tentec@mailman.qth.net>
Sent: Thursday, April 10, 2003 2:04 PM
Subject: Re: [TenTec] Re: Al Gavenas' position on Front-End atenuation


> "George, W5YR" wrote:
> >
> > 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.
>
>
> Hi George,
>
> not exactly. If the SWR is high and you don't use a tuner to match the
> antenna a considerable amount of rf is reflected back to your
> transmitter and might even damage your final.
>
> 73
> Heinz DH2FA, KM5VT
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