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Saturday 15 April 2017

Field strength meter

A field strength meter is a useful tool to have in the shack as it detects the presence of RF, and in the shack that is undesirable and can be a sign of potential feeder radiation, which as is known is often a problem if a balanced antenna is connected to an unbalanced feedline without some kind of balun, RF in the shack can also occur if the antenna is also in the shack (which in my case is more or less unavoidable at present).

A basic field strength meter does not require a battery, it works similar to a crystal set but without the tuned circuit, I had an old unreliable SWR meter for CB and set about salvaging components from it, 2 of these components were germanium diodes (silicon diodes won't work as well if at all), so I found what components I could close to the values specified from a particular circuit (some of these were from the SWR meter I had disassembled) and assembled a crude field strength meter onto a veroboard, in close range of an HT at full power it swings the needle on the meter display fully over to the end stop, a potentiometer reduces sensitivity so I was able to bring the needle back over to the end of the "FS" markings (this SWR meter had readings for field strength but they were not used in the meter as it was a basic CB SWR meter and the majority of CB users usually don't need to know more than if their antenna is working and not going to kill their radio's finals).

I built the meter as a proof of concept and just to practice my soldering skills, as I've not soldered anything in a little while and am a little out of practice, I plan to build a proper one, in a metal box, using exact value components for the circuit, at some point in the future, I could have built one of these for my Intermediate but my instructor had other things he wanted me to do and it didn't really occur to me at the time.

73 de M6RSQ

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