Field Strength Meter

[Scott Mann]

How does the circuit at
http://www.mitedu.freeserve.co.uk/Circuits/RF/sfsm.htm
work and what principles does it take advantage of.

In particular what does the capactior do in the circuit?

I am using in in conjunction with the circuit located at
http://www.pc.clara.net/radio/index2.htm

['Doc]

It by-passes AC around the VOM.
'Doc

[Richard Clark]

On Tue, 29 Aug 2000 18:42:31 +1000, "Scott Mann" <ma...@uq.net.au>
wrote:

>How does the circuit at
>http://www.mitedu.freeserve.co.uk/Circuits/RF/sfsm.htm
>work and what principles does it take advantage of.
>
>In particular what does the capactior do in the circuit?
>
>I am using in in conjunction with the circuit located at
>http://www.pc.clara.net/radio/index2.htm
>

Hi Scott,

The capacitor is the heart of the indicator circuit. It holds the
rectified over the air signal while the multi-meter presents the
reading of voltage as an indication of the field strength.

I presume you mean to use the FSM to measure the effective output of
the AM transmitter. You should only need to increase the inductance
of the inductor L1 in the FSM to roughly the same value as the
component T1 in the transmitter. This is not a strict requirement
however as the current FSM design may have enough sensitivity to work
for you anyway (but it would work a whole lot better with more
windings in L1). The reason for this difference is that the two
circuits operate at vastly different frequencies. Even so, at close
proximity (within 20 feet), this shouldn't matter.

73's
Richard Clark, KB7QHC

[Richard Harrison]

Scott Mann wrote:
"How does this circuit work?"

The publisher of the circuit could not supply it when I addressed the
URL.

All field strength meters receive the signal and display a strength
indication.

They are receivers of some calibrated sensitivity, large or small,
depending on the particular use. To give good results they must be used
with a particular antenna and operated under defined conditions. They
often contain a calibration oscillator to set the sensitivity for
accurate measurement.

Don`t have the circuit, so the capacitor function is unknown. I`ll guess
that it was some variable type either used to set the frequency or
sensitivity of the meter.

Best regards, Richard Harrison, KB6WZI

[Richard Harrison]

Scott Mann wrote:
"How does this circuit work?"

The circuit of your FSM appears this morning. It is a 1/2-wave
rectifier, the equivalent of an untuned crystal set to be used with an
external d-c meter to indicate r-f voltage from its antenna.

The capacitor`s function is to charge to the peak value of the r-f
waveform to increase output to the d-c meter.

Best regards, Richard Harrison, KB5WZI