Just dont place me in the Who Cares...." crowd Paul. I dont do anything
without careful consideration and knowing exactly what to expect. Plus
all my antennas are monobanders so its a simple issue. As mentioned
before Ive modified the LK-500ZC and MLA-2500 pi-networks a bit on
80/160 to facilite complete loading and power transfer.
On 40M I tolerate a 2.5:1 on the 2/2 on the high end and about the same
at the very high ends of 20/15/10 where I seldom operate anymore anyway.
The amps always loaded well and the antennas performed during contest
days. Those are 26 year old W2PV style antennas that have served me well
until the 12/5/08 ice storm made a mess out of 10/15. Time to fire up YO
and redo to a more modern design.I suppose.
All feedlines from 160M to 1296 mHz are 3/4, 7/8, and 1" CATV hardline.
No worry about jacket deterioation or water getting into connectors.
Carl
KM1H
----- Original Message -----
From: "Paul Christensen" <w9ac@arrl.net>
To: <amps@contesting.com>
Sent: Thursday, April 23, 2009 12:44 PM
Subject: Re: [Amps] High SWR
>> Short answer is the only thing SWR is good for is determining at what
>> point the RF voltages induced by the mismatch will damage your
>> transmitter system. This is why modern rigs fold back transmitter
>> power when excessive SWR is detected, This keeps the voltage
>> developed
>> at a low enough level to prevent damaging the finals.
>
> I think the importance VSWR in HF multiband operation lies somewhere
> in
> between extreme views. I need to know three things before I become
> worried about VSWR: (1) line characteristic Z; (2); the matched line
> loss
> spec in dB/ft.; and (3) antenna input terminal Z range. If I know
> these
> answers, I can then compute VSWR and determine systematic loss due to
> a
> combination of VSWR and the manufactures specified matched line loss
> in
> dB/ft. Also, when I know these answers, I can compute an optimum line
> length for maximum power transfer from the transmitter into the line
> when
> the line characteristic Z is not equal to the antenna input Z.
>
> What bothers me about the "who cares about VSWR" crowd is that they
> are
> often ignoring total system losses as a result of unknown VSWR,
> unknown
> matched line loss, and transfer loss between the Tx and line. Even if
> my
> line had zero loss, the resulting Z seen at the TX end of the line can
> make
> power transfer inefficient from the rig into the line.
>
> It doesn't take much to get close to reasonable values of line VSWR
> (and
> only then say "who cares about VSWR") and input Z at the Tx side of
> the line
> to make the system reasonably efficient. But I would not want to hide
> my
> head in the sand and use some random length of line, and some random
> length
> of antenna -- and expect my transmatch to efficiently create a
> conjugate
> match. The system may be tuned and resonated for reactance
> cancellation,
> but wouldn't it also be nice to know how much was lost in the process?
> If
> the answer is yes, then you need to know more about the antenna system
> especially the line Z to antenna Z mis(match) and specified matched
> line
> loss.
>
> Paul, W9AC
>
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