Tom:
I believe you want to say "one-half wavelength" rather than 1/4. If
the transmission line is one-half (electrical) wavelength or a multiple,
then the load's (antenna's) impedance is duplicated at the far end of the
coax. In this way, one could read the antenna's feedpoint impedance at a
remote distance, say, at ground level.
At one-fourth wavelength, the load's impedance is converted to its
complex conjugate. This means, for example, that an open circuit becomes a
short-circuit AT THE 1/4 WAVELENGTH FREQUENCY. That's how 1/4 wave stub
filters work: If you connect a 1/4 wave transmission line across an antenna
or other load (open-circuited at the load) and short the other end, the
short circuit actually gets applied to the load AT THE THE DESIGN FREQUENCY.
An interfering signal can thus be filtered out (by the frequency-selective
short-circuit of the 1/4 wave transmission line) without affecting the
operation of the load at other frequencies.
-Gene Smar AD3F
-----Original Message-----
From: Tom Rauch <10eesfams2mi@mcione.com>
To: towertalk@contesting.com <towertalk@contesting.com>
To: <towertalk@contesting.com>
Date: Sunday, July 05, 1998 6:35 PM
Subject: [TowerTalk] Hairpin
There were some good responses to this question, but I'd like to
add one more thing. This also applies to ANY antenna.
If the feedline is more than a few degrees electrical length between
the antenna and the measurement device, the reactance zero crossing
point might not be the frequency of "antenna resonance".
The conditions of true element resonance (ignoring instrument
tolerances) using a 50 ohm bridge are:
SWR 1:1, Impedance 50 ohms, X=0 with random length 50 ohm lines
-or-
X=0 (any value of R or SWR ) with any impedance line that is an
exact multiple of 1/4 wl long (including zero length).
It is important the line be an exact multiple of 1/4 wl at the
frequency of measurement, not a few percent F lower or higher or even
"pretty close to a multiple of 1/4 wl" !
If a "tuned" or "resonant" feedlines (in terms of electrical 1/4
waves) become more frequency selective when made longer. Small
length errors caused by a change in frequency accumulate in each 1/4
wl section.
(when feeding 50 ohm antennas with very long low loss 75 ohm CATV
cable, the long mismatched feedline will narrow VSWR bandwidth even
if carefully cut to a multiple of 1/2 wl. The same effect occurs when
measuring impedances, a long low loss line adds selectivity to the
system and increases errors if frequency is not exact)
Keep this in mind when using odd impedance lines or measuring things
some distance from the feedpoint!!
73, Tom W8JI
w8ji.tom@MCIONE.com
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|