Field Strength Meter Circuit
BBG FM (NBFM ) VHF
Crystal Controlled 1/2 WattTransmitter Circuit - Field Strength Meter Circuit
Field Strength Meter Circuit To Align Multi-Stage Transmitters
is a simple homemade, inexpensive instrument that will put you in the
ballpark If You Don't Have Access To The Regular Instruments That We
Look at the schematic below. Simple but effective. The circuit is
nothing more than a tuned tank with its center frequency set to 160 MHz
- if a fixed C1 capacitor of 10pf is used.
If you can't find a 50uA meter use
a simple analog voltmeter with its microamp setting. Keep the parts
short. You can put the unit right on target if you replace C1 with an adjustable capacitor of between 5 to 20 pf and then take the unit to someone who does TV/Radio repair and have them to calibrate the dial on C1.
With C1 between 5 to 20pf and a calibrated dial you will be able
to tune your transmitters into its actual frequency, rather than one of
You Are An Electronic Engineer, Radio Ham, Pro Or Otherwise You Already
Own Some Form Of Instrument To Align Multistage Transmitters With. Of
course, there are other ways such as the use of a dummy load for it's
antenna, a vacuum tube rf voltmeter or a high impedance voltmeter,
pinning out the collectors of each stage, to name a couple. Chances are
you have found what works for you and you swear by it.
The Chances Are If You Are Not One Of The Above,
you don't have access to a sofisticated alignment instrument. You don't
really need a professional unit for tuneup, if you are not dealing with
a sophisticated governmental agency, and just a simple tool that will
put your multistage transmitter on the mark, or correctly align it's
You Need Is A Simple FSM ( Field Strength Meter ) - But One That Is Within The
Frequency Range Of The Transmitter You Want To Align - And Let Me Clarify That
simple FSM meter is nothing more than a coil that will absorb the
radiation ( Radio Frequency Signal ) that is being radiated (
Transmitted ) by any form of transmitter. Then the AC signal is fed
into a diode that rectifies the AC signal into a dc current to drive a
sensitive DC microamp meter. The Problem Here Is That a
single coil will not only pick up your signal, it will pick up all
signals, including the harmonics of the transmitter you are attempting
to align - so we need to made it frequency specific. In other words, we
only want it to intercept and amplify the RF signal in the specific
frequency range of our transmitter.
Here Is How We Do It.
Rather than building our simple Field Strength Meter Circuit with a single coil that will pick
up and amplify all signals, we place a capacitor across it's coil that
makes the coil and capacitor resonant at the frequency of the output
frequency of the transmitter we want to align.
The more sensitive the meter, the better.
To align your transmitter, simply place the meter about a foot away
from the transmitter and follow the alignment steps. As the needle
starts to climb, move the meter further away. Again as you align each
stage and the needle climbs, move the meter more further away. What you
are doing here by moving the meter is to keep from pegging the needle
and damaging the meter.
You can add a small resistive attenuator inserted between the
antenna and L1/C1 ( The Tank Circuit ) where you don't have to move the
meter - and do a little research on the net for those simple circuits.
I might add also that there are a lot of excellent FSM circuits on the
internet, other than the one I have provided here, some simple - some
complex, so do a little research. I am giving you a very basic design
here that will work for you if you don't have a lot of experience.