Field Strength Meter Circuit

Field Strength Meter Circuit

BBG FM (NBFM ) VHF
Crystal Controlled 1/2 WattTransmitter Circuit - Field Strength Meter Circuit
(C) 2017  Bugplan.Com
            A Simple Field Strength Meter Circuit To Align Multi-Stage Transmitters
                                                 Such As The BBG


This 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 Use. 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 it's harmonics.
If 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 stages.


All 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 Here. A 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.
field strength meter circuit
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.
Back To The BBG Main Alignment Page