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Audible antenna coupler adjustment indicator

Sometimes, when making antenna coupler adjustments, it is more convenient to listen for a change in a sound than to watch the deflection of a meter needle. These videos start by describing a resistive antenna bridge using a meter movement. Then they describe and demonstrate ways to make it audible. First with a piezo transducer an then with a 555 timer voltage controlled audio oscillator. Finally I describe a unit that you can build from scratch without a meter.

Resistive antenna bridge

Watch this if you are not familiar with resistive antenna bridges. They're a bit like a VSWR meter but are easier to build and allow tuning up to be done without radiating a signal. However they need to be removed from the antenna line when transmitting.

 

Making a resistive antenna bridge audible

There's two ways you can do this. The cheap way is to add a piezo transducer (which is basically a sensitive speaker) so that if you transmit SSB you can adjust the antenna coupler until you hear nothing. The second, and better, approach is to add a voltage controlled audio oscillator. This requires an external power source but is more sensitive. Also it works with CW transmitters. You listen for the tone to go lower in frequency, and eventually stop, as you adjust the coupler to optimum setting. Once that's done you disconnect the bridge and can transmit.

 

Self contained audible resistive antenna bridge

This one is a meterless bridge built from scratch. As analogue meters are getting rarer and dearer this is a good choice if you don't have a big junk box.

 

Components

These projects use common electronic parts. Consider the following if you don't already have them.

 

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