BTL Field Strength Meter
I’ve been wanting to build a field strength meter for some time for use in testing RC transmitters, HTs, and antennas in general. When I found this old CB SWR meter at a HADARC meeting on the ‘backtable sales’ table, I realised that it contained most of the important parts that were required and all in a nice metal box and so I snapped it up.
On telling Tony VK2BTL, about my plan, he mentioned that he had designed a FSM and had published an article about it in the club’s newletter some time ago and kindly provided me with a copy.
The circuit is a simple diode detector connected to an amplifier which drives the meter. D1 rectifies the signal from the antenna, D2 protects the amplifer by limiting the maximum votage at the input. RV1 is used to set the zero point of the meter. Feedback resistor R2 sets the sensitivity.
In his original design, Tony allowed for variable sensitivity by specifing four resistors (1K,10K,100K,1M), one or more of which could be selected via pin-header jumpers. However, he said that in practice he always used the maximum sensitivity (highest resistance) and the 680k was a good compromise value.
The meter is specified as 50uA, but I measured the one I had from the SWR meter at 250uA which works fine as, from what I figure, the amplifier compensates for it.
The power supply is made from two 9 volt batteries in series with ground being the centre connection, so Vcc on the circuit is +9 volts, and Vss is -9 volts. The circuit shows Vss with a triangle symbol (same as ground), it is to the left of the two capacitors at the bottom and a little difficult to see. You can click on the image to get the SVG file, which can be scaled to any size you like.
The DPDT slide switch from the SWR meter was used as an on/off switch, but I usually put the unit away after having forgotten to turn it off and after a month or more the batteries are still fine, so I wonder if the switch is really necessary.
The scrapped SWR meter game me the on/off switch, 10k pot (perfect!) and the 250uA meter and all in a nice black metal case.
The circuit was made deadbug style on a piece of double-sided pcb, it is held in place with two of the original screws which are screwed into nuts made from hot-melt glue. As you can see the batteries only just fit, but some pressure from the front and back panels holds them nicely in place.
The antenna input is a BNC socket, but given that matching impendance isn’t important, a banana or RCA jack or whatever would do just as well.
The front panel artwork was made by scanning the original front panel and modifying it using the GIMP image processing software. It was then printed on the laser printer or normal paper, stuck on and then cut out. I’m quite happy with the look, but next time I try covering it with clear contact plastic adhesive, and/or printing it on glossy paper. I’ll try doing similiarly for the meter scale someday.
3. In Use
The FSM certainly is sensitive; on 70cm, the 1W handheld will peg the needle from across the room. Interestingly, holding the handheld horizontally with the FSM antenna vertical drops the reading down to about 1/4 scale and illustrates the importance of matching the polarities.
On 2m VHF the meter reads about 3/4 scale which I believe is due to the fact that the antenna I am using is approximately a 1/4 wavelength long on UHF, but very short on VHF, but still good enough for relative field strength measurements. In future, I’ll use an extendable antenna salvaged from and old FM radio and adjust the sensitivity by changing the length of the antenna relative to the wavelength being measured.