[TowerTalk] Mosley antenna problems

[Guy Olinger, K2AV]

Yes. Questions for all you I-love-low-SWR-best aficionados:

--------------------

1) Two identical 50 ohm trap tribander installations in identical
condition, except one has somewhat lossier traps. Which will have the
lower swr figures all across the bands?

 A) The tribander with the LESS lossy traps.

 B) The tribander with the MORE lossy traps.

2) Which of the above antennas will have the better performance?

 A) The tribander with the LOWER swr figures.

 B) The tribander with the HIGHER swr figures.

3) Two identical 50 ohm tribander installations in identical
condition, including identical traps, except one is fed with 100' of
RG8/U coax, the other with 200' of identical RG8/U. Which installation
will have the lower swr across the bands at the end of the feedline?

 A) The tribander installation with the 100' foot feedline.

 B) The tribander installation with the 200' foot feedline.

----------------------

This concept should be in the element 2 question pool. It is amazing
that this old wive's tale persists: "lowest swr is always the ultimate
measure of performance".

Answers: 

1) B) Some energy that would have been returned as reflected energy in
A) will instead be dissipated as heat due to the additional loss,
resulting in a lower measured swr. The greater the loss, the lower the
swr.

2) B) On transmit, some of the mismatched power of A) may be returned
to the antenna and largely reradiated, particularly if a tuner is
used. On receive, signals passing through the lossier traps will
simply be lower in level at the receiver.

3) B) The additional loss of the 200' feedline will reduce the
reflected energy, resulting in a lower swr reading.

SWR is a tricky concept (if you don't believe that, search towertalk
archives on "swr" and jump out of the way). SWR needs to be
interpreted with care. I put my callsign cap in my pocket and ask the
above questions casually to strangers at hamfests. I mostly get
surprised looks and wrong answers.

Sigh...

(You can read between the lines about why this comes up in a thread on
Mosley antenna problems.)

On Wed, 19 Jan 2000 13:17:29 -0800, you wrote:

>
>Several posters have offered low swr as an indication that
>a particular beam is good.  A 50 ohm resistor would be even
>better?
>
>k7puc

--.  .-..
73, Guy

Guy Olinger, K2AV
k2av@contesting.com
Apex, NC, USA

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