[Antennas] resonance

Sat Dec 17 12:06:04 EST 2005

Dear Frank,
> David Robbins K1TTT wrote:
>> Yes, if you are tuned to resonance the impedance at the
>> antenna will be purely resistive. And yes that impedance
>> value can change as you vary the height of an antenna...
>> Yes, I have seen that happen, but does the impedance remain purely 
> resistive as its value changes with height variation? And is a purely 
> resistive impedance the only measure of resonance. (Please no cracks 
> about dummy loads being resonant at ALL frequencies).

Actually, I'm one of those people who would quibble a little about things 
like that. It's because my vision of "resonance" is the more classic vision 
of there being energy stored in one portion of the system (say the 
capacitive reactance) and then seeing that energy decay and build up in 
another part of the system (the inductive reactance). The fact that the 
system can have a substantial amount of energy sloshing back and forth 
between the two reactance components is what we usually think of as 
resonance. In a pure resistor or dummy load there is no energy sloshing back 
and forth between reactance elements. So I do not characterize such things 
as pure resistors or dummy loads as being "resonant".
>> to further confuse things, the change in height can also
>> change the frequency where the antenna is resonant. So
>> if you have an antenna 1/4 wave up tuned to resonance at
>> 7000khz with an impedance of 75 ohms and move it higher
>> the resonant point may move to 7150khz and the impedance
>> at 7150 may be 70 ohms.
>> Have you actually experienced this? I mean acutally seen this happen on 
> an antenna you installed? I dont understand why a height change would 
> have an effect on frequency AND impedance AND resonance. The chart in the 
> handbook shows a definate variation in impedance with height change but no 
> change in frequency.

I won't comment on that issue at this time, but I will say that when 
discussing the real component of an impedance it can be misleading to keep 
referring to it as "resistance". Most people think of resistance as a 
characteristic of a circuit that transforms the electrical engergy directly 
into heat, which is not what antennas are all about.
73 de W9DMK - Bob Lay, Dahlgren, VA
http://zaffora.f2o.org/W9DMK/W9dmk.html
w9dmk at crosslink.net 