In message Tue, 23 Sep 97 10:30:38 CDT,
"RICHARD ANDREN" <andrenr@ul.com> writes:
> Now I'm getting confused..... Doesn't an antenna act and resonate
> differently depending on it's height (I think).
Well sort of Richard. If you have an ARRL Antenna Book read the section on
antenna fundamentals and you will get a better understanding than what I
will say. Resonance is the lack of reactance, in otherwords a pure
resistance. This occurs on an antenna at only one frequency. When the
reactance is tuned out the circuit or antenna is at resonance. Now when we
have an antenna that is not resonant connected to a feedline and then to an
antenna tuner and we tune the tuner such that when looking into the tuner we
see a pure resistance then the SYSTEM including all of the parts is at
resonance BUT the antenna is still non-resonant. The tuner just made the
antenna APPEAR resonant and the system as a whole IS resonant. Now it's
true that the antenna impedance does change with height but that may or may
not change the resonance of the antenna. The antenna may be resonant at say
10ft with an impedance of 20 ohms and at 100ft may be resonant with an
impedance of 60 ohms--the antenna is still resonant but the impedance has
changed. The antenna tuner will tune out the reactance to make the system
resonant AND also match the impedance on the output to look like 50 ohms at
the input.
> I've tuned antenna's 10 feet of the ground, and had to retune when they
> were 60 feet off the ground.
As I said above the impedance will change with height but resonance may/or
may not change.If you adjust the antenna length to stay resonant the
impedance will be different due to height change.
>
> Also, I thought a tuner would correct the L and C to obtain resonance
> (within reason) so you could resonate any object... screen door,
> gutters, long wire, etc.
It does to the system as a whole including the tuner but doesn't change the
TUNING (resonance) of the antenna itself.
In the end does it matter where and how we achieve resonance? The most
efficient power transfer from a transmitter to an antenna is when we have
the least components between the two, with those that we MUST have, have the
lowest loss. Therefore, for a given frequency a resonant antenna, with the
lowest loss feedline and MAYBE NO tuner will be the most efficient. However,
this world is not perfect and we usually must compromise at times. So the
BEST that we can practically have is a resonant antenna with ladder line and
an efficient tuner (I like the Johnson Matchbox) and when system conditions
change we can make slight tuning adjustments which will not be noticed on
the other end.
I've gone on much too long as some/most will agree.
73 Jim K4CGY
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