Tom Rauch wrote:
>
> 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
>
Hi Tom and all
Maxwell-Reflections Quote Page 2-7
1. The SWR in a feed line cannot be changed, adjusted, or controlled
in any practical manner by varying the line length.
2. If SWR readings change significantly when moving the SWR bridge a few
feet one way or the other in the line, it indicates either "antenna"
current flowing on the OUTSIDE of the shield, or else an unreliable
instrument, or both..............Some writers insist the bridge must
be place a 1/2 wl intervals from the load to obtain a correct reading.
This is INCORRECT...
3. Any reactance added to an already resonant (RESISTIVE) load of any
value for the purpose of compensation to reduce the reflection on the
line feeding the load will, instead, only increase or worsen the
reflection. It is for this reason, although contrary to the teachings
of several writers, that the lowest feed-line SWR occurs at the self-
resonant frequency of the radiating element it feed, COMPLETELY
INDEPENDENT of feed line length. Any measurements which contradict this
indicate that either the measuring equipment, or technique ( or both )
are in error.
Capitals are to point out emphasis in the written text.
That about says it all.
73
Ed
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