Thursday, May 30, 2019

An RF current indicator for antenna tests

A handy little instrument for antenna tests is this little RF current indicator here. You can move it near antenna elements, counterpoise elements and open wire feedline to identify points of maximum current. This gives you some reassurance that things are working as the theory books say they should. 

It's so simple that you don't need a circuit. The parts you need are as follows:

* Tinned copper wire - about 30cm long for sense antenna
* Germanium diode as used in crystal sets (eg 1N34A, OA95 etc)
* Disc ceramic capacitor - anywhere from 1nF to 100nF
* Analogue meter movement - anywhere from 50 uA to 1mA full scale deflection

Both the diode and meter are polarised. The cathode of the diode (end nearest the thin stripe) must go to the positive (+) on the meter. Meters are marked with + and - terminals.  I used a small printed circuit board on the back to provide some extra rigidity. Use a hacksaw to saw a slot to separate both connections of the sense loop. 

To use the indicator run its sense wire parallel with the antenna element or feedline to maximise coupling and get an indication on the meter. Move it a little away if the meter reads full scale. Move it up and down the antenna or counterpoise wire to see how the current varies with the meter's position along it. 

PS: Want to read more about antennas? Consider this selection of antenna books. They are affiliate links meaning that I receive a small commission (at no extra cost to you) if you decide to purchase.


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