Optimizing my antenna for resonance

[rkiskan]

Hello and happy new year to all.

I have been looking at resonance vs. swr arguments in the past few days and have some basic questions.

When looking at the resonance plot on an AIM4170, the resonance as I understood it, was those points where the Theta changes from + to - or vice-versa. In some cases this can be at peaks, dips or even somewhere inbetween.

Should I only focus on those resonant points where Zmag is lowest (in a dip)?

Should I be trying to get the SWR to bottom out on the resonant value (where Zmag dips) or very close to it?

What should I be aiming for wrt Zmag - 50 Ohms or 0 Ohms? I am actually getting quite good Tx results at frequencies when the SWR is lowest but Zmag is abt. 40-60 Ohms and not at resonance.

What is the implication if a resonant point with a high Zmag is close to a lowest SWR value? Do this mean that the antenna is not radiating at that frequency?

thanks
Robin

[Bob]

Jan 1, 2010 8:26:36 GMT -5 rkiskan said:

Hello and happy new year to all.

I have been looking at resonance vs. swr arguments in the past few days and have some basic questions.

When looking at the resonance plot on an AIM4170, the resonance as I understood it, was those points where the Theta changes from + to - or vice-versa. In some cases this can be at peaks, dips or even somewhere in between.

Yes, the resonance points are where theta passes through zero. This is the point where the equivalent reactance changes from inductive to capacitive (or vice versa).

Should I only focus on those resonant points where Zmag is lowest (in a dip)?

It depends on the matching network you're using. Generally you don't want Zmag to be too large or too small. At extreme values, the matching network may have more loss, even though it theoretically does transform the impedance to 50 ohms.

Should I be trying to get the SWR to bottom out on the resonant value (where Zmag dips) or very close to it?
What should I be aiming for wrt Zmag - 50 Ohms or 0 Ohms?

You would like for Zmag to be close to 50 ohms with a low reactance. The tuner can take care of moderate departures from 50 ohms with good efficiency. If Rs is near zero (even if Zmag is large), the loss will be greater.

I am actually getting quite good Tx results at frequencies when the SWR is lowest but Zmag is abt. 40-60 Ohms and not at resonance.
What is the implication if a resonant point with a high Zmag is close to a lowest SWR value? Do this mean that the antenna is not radiating at that frequency?

Even when the calculated value of SWR is low, the loss in the antenna may still dissipate a lot of the power and it may not radiate as well as another antenna which appears to have a higher SWR. A high SWR may not be so bad if the whole antenna system has low loss. The transmission line is especially important. For a vertical, the grounding system is very important.

--73/ Bob