[Fwd: LF: Imaginary resonances]
Thu, 05 Mar 1998 17:54:10 -0500
Peter Martinez wrote:
> >From Peter Martinez G3PLX Kendal Cumbria.
> Andy's observations on misleading resonances in link-fed loading coils
> match my own experience, and I think I understand what's going on.
> If you think of the antenna tuning as equivalent to a small inductor across
> the transmitter and a series tuned circuit across it, the small inductor being
> the link winding, then, neglecting resistance for the moment, this
> arrangment will have the same dual resonance as the textbook description
> of a crystal, with a series resonance (low impedance to the TX) on the
> high-side and a parallel resonance (high impedance to the TX) below it.
> When we introduce the antenna resistance and adjust everything for best
> match with an SWR bridge for example, then swing the tx either side of the
> optimum, the SWR bridge will show that the match gets worse either side,
> but the load to the TX still shows the tendency to drop on the HF side and
> rise on the LF side, so if we put a current meter in series with the TX and a
> voltmeter across it, we see a peak in the current on the HF side of the
> best-match position, and a peak in the voltage on the low side.
> Furthermore, if the TX is a high-power audio amplifier, which is effectively a
> voltage source, then it will deliver MORE POWER to the antenna on the HF
> side of best-match, and a neon lamp near the antenna (or a thermocouple
> meter in series with it) would give a peak on the HF side.
> But on this false HF peak, the transmitter is not only experiencing a lower
> load than it is designed for, but it is off-tune, and the TX can be expected to
> get hot.
> Andy's idea of monitoring the voltage across a 1 ohm resistor is fine for the
> case where the feeder is 50 ohms, but probably not useful for the 4 ohm
> feeder situation. I made a small current transformer on a toroid, with 50
> turns on the secondary and the feeder pushed once through the centre.
> With a 50 ohm resistor across the secondary, it gives me 1 volt per amp. I
> can use a double-beam scope to watch the voltage and current and tune
> the antenna to get them in phase. The ratio of the voltage across the feeder
> to the voltage from the current probe is equal to the load impedance in