Monday, April 15, 2019

Dummy antenna - other options

This antenna is 100 meters. The curve would be the same for other lengths but the wave length would be different. At 1/4 wave length the antenna is resistive. Below 1/4 wave length it is capacitive. Above 1/4 wave length it is inductive. If your antenna was 30 meters it would be capacitive over the frequency range. So what happens when you connect the generator, go through the alignment and replace the generator with the antenna?

You setup the circuit with a 50Ohm source and then replace it with a capacitor. This de-tunes the circuit. So what can we do?
You could put a short piece of wire on the generator and radiate a signal. Align the set using the antenna it will be using in service. If you are the end user and have the antenna to hook up.
The manufacturer had to develop a way to align the set that would work with their customers antenna. This is what they developed.
The Hallicrafter version.
The Tandy version.

The dummy antenna I posted earlier was the industry standard.

IEC standard Dummy Antenna

If you don't want to build the dummy you could use a small capacitor (100pfd - 200pfd) to couple the signal.

If your set has an antenna trimmer you are in luck. You set the trimmer capacitor to a high value while aligning the receiver. Then when you connect the antenna you turn the trimmer capacitor down to allow for the antenna capacitance.

If you align a set with a loop antenna you can couple the generator to the loop.

*EDIT: Next post I measure Zin and apply a signal with lower and higher Z to see the difference.

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