Your RF Cafe
September 1960 Popular Electronics[Table of Contents]
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Popular Electronics was published from October 1954 through April 1985. All copyrights are hereby acknowledged. See all articles from Popular Electronics.
There is little incentive to build your own field strength meter these days when commercial instruments are readily available and relatively inexpensive. For instance, you can buy an MJF-801 FSM with a 100 kHz to 500 MHz bandwidth for just $30, brand new. More sophisticated, calibrated instruments are available for a lot more, but this basic unit is dirt cheap. However, if you want to read a little about the theory behind a field strength meter and see how one goes together, this article from Popular Electronics provides that opportunity.
Are you curious about the radiation pattern of your CB or ham antenna? Here's a simple field strength meter (FSM) that will give you an indication of relative field strength on either the 6- or 10-meter ham bands or the 11-meter Citizens Band.
Make a short whip antenna, as shown, by soldering a 1' or 2' length of No. 12 or No. 14 busbar to a banana plug. Jack J1 on the FSM is a banana jack and permits the antenna to be unplugged when the FSM is not in use.
Operation. You can use the FSM to check the radiation pattern around your antenna or to see if your transmitter is improperly shielded and radiating r.f. Before these checks can be made, however, the FSM must be tuned to the transmitter. Do this by inserting the FSM's whip antenna into J1 and placing it near the transmitter. Then rig a temporary short-wire antenna to the transmitter, and tune up the transmitter. If yours is a CB rig, just switch to "transmit" and use a clear channel. In any case, keep all experiments down to a minimum so that already burdened Citizens Band and ham frequencies are free of unnecessary interference.
Switch on the FSM and adjust capacitor C1 to the transmitter frequency. The meter will show a sharp rise from the zero mark at the transmitter's frequency. Adjust C1 for a maximum reading on the FSM. If the meter goes off scale, move the FSM further away from the transmitting antenna. At this point, you'll notice that the FSM pickup depends on its polarization with the transmitting antenna: maximum pickup results when the FSM antenna and the transmitting antenna are parallel to each other.
Once the FSM is tuned to the transmitter, disconnect the temporary antenna and connect your regular transmitting antenna. If your transmitter and coaxial transmission line are properly shielded and grounded, you should get no reading on the FSM no matter how close to the transmitter or coax the FSM antenna is placed.
How It Works
Operation of the FSM is similar to that of a receiver using a diode detector followed by a one-transistor amplifier. In this case, the transistor feeds a milliammeter rather than headphones. When r.f. is picked up by the antenna, it is tuned by coil L1 and variable capacitor C1 . Diode D1, connected to a low-impedance tap on L1, rectifies the r.f. appearing across the L1-C1 tuned circuit. The rectified signal is filtered by capacitor C2 and fed to the base of transistor Q1, where it is amplified and fed to meter M1. Serving as a visual indicator, Ml measures the amplitude of the rectified signal, which is proportional to the r.f. field strength. Battery B1 powers Q1 through on-off switch S1.
Posted March 26, 2013