Jim,
the answer is already in your post. If a load is not purely resistive and
same value than source and load, the line act as an impedance transformer
and moving along the line an SWR meter the read value has to change. This is
maximally valid on relatively small SWR values and on 10 meters where short
cables are not so short in term of wavelenght and SWR meters (better not to
speak about some amateur ones) are not enough precise to assure a true 1:1
reading when they do.
If the LPF is placed on line between PA output and tuner, it's not
impossible the slight SWR, as seen by the bird coupler, is also due to an
unwanted impedance transformation caused by the filter.
I wouldn't mind if 50w (0.147 dB) or twice is the reflected over a 1500 of
forward power but I would instead mind to avoid from using bad designed
antennas or mind to adjust the others (known as matchable) able to give
proper impedance and reasonable bandwidth without tuners.
If true that a yagi dipole lenght doesn't affect so much the gain, SWR can
be caused also by wrong parasitics lenght or spacing and this may affect
substantially (some dB/s) the gain and can't be overcomed by any tuner.
It's probably wise to remember that reflected power is only a conventional
unit used to point out a mismatch and some SWR is anyway not the hell.
73,
Mauri I4JMY
-----Messaggio Originale-----
Da: Jim Reid <jreid@aloha.net>
A: <amps@contesting.com>
Data invio: martedì 28 settembre 1999 1.53
Oggetto: [AMPS] Where to read Reflected Power
> What I noticed is that when I have the antenna tuner set for
> almost perfect match using the 259, then switch to the
> rig set up, I read reflected power at the Bird coupler on the
> back of the RF deck, but none on the antenna tuner meter. Or,
> I can adjust the antenna tuner so that there is no reflected
> RF power read on the back of the RF deck after the LPF,
> but read around 50 watts reflected now at the antenna tuner
> meter?
>
> This is on 10 meters, and I also found that I could not get the
> antenna tuner to match to my C4 beam until I had added a
> 3' length of 213 to the feedline coax coming from the antenna
> to the back of the tuner! This tells me that on 10 meters one
> is liable to read about anything anywhere in the set up when the
> antenna does not match the feed system exactly
--
FAQ on WWW: http://www.contesting.com/ampsfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|