DFD4 Frequency Counter
A frequency counter is a handy tool for any ham radio shack. It is useful for homebrewers to determine the frequency of oscillators or transmitters. They also come in handy to ensure a transmitter or transceiver is exactly on the frequency it should be, and can read out the frequency of an older analog-dial transceiver or a small homebrew transceiver with no frequency readout.
Scanner enthusiasts use frequency counters (usually handheld) to find new, interesting frequencies to listen to. A frequency counter can find the frequency of a transmitter nearly instantaneously, while a scanner has to search through a band of frequencies. If the radio transmissions are brief and infrequent, then the scanner may never find them. But a frequency counter can very quickly. Of course the downside is that you must be close to the transmitting antenna in order to read the frequency with a counter, or else use a high-gain directional antenna.
Another good use of a frequency counter is in a DF fox hunt. Whether in a contest or real-life transmitter hunt, a frequency counter can be used when the approximate location of the transmitter is known. Searching around the area until the counter reads the frequency of the transmitter will allow transmitter hunters to find the exact location of the transmitter. Since most frequency counters require a strong signal in order to read the frequency, one must get fairly close to the transmitter (depending on what kind of antenna used on the counter) to read the frequency on the counter. Getting close to the transmitter is the goal in fox hunting.
The frequency counter I built is the DFD4A kit available from Almost All Digital Electronics. It is a simple but effective frequency counter kit that covers 0-3 GHZ. The DFD4A is one of a family of digital frequency displays designed to be used in equipment, however the DFD4A is designed to be used as a benchtop frequency counter which is what I was looking for. The counter reads from 0-30 MHZ in HF mode, and from 10 MHZ - 3 GHZ in UHF mode. It can also read out the frequency to 1 Hz in HF mode, or 100 Hz in UHF mode.
I received the DFD4A kit and quickly assembled it. I attached a 9 Volt battery to it and was pleased to see that it worked fine. However, the DFD4A kit does not come with an enclosure or hardware, so I was hard-pressed to find a decent enclosure for the kit. It would not have to be large, since the kit consists of two small PC boards, just a little bit larger than the digital display. However, since I didn't have anything suitable on hand, I ended up ordering a project enclosure from an electronics supply wharehouse. I was at first thinking of building the frequency counter as a handheld unit, however I decided to make it a bench top counter since the best enclosure I could find was that style.
The enclosure I got for the counter consists of two plastic halves for top & bottom, plus aluminum front & back panels. I would need three switches for the counter, one for a power switch, one for an HF/V-UHF band switch, and one for a fast/slow resolution switch. I found some toggle switches I salvaged out of an old CB. There were three of them mounted on a small piece of PC board, and they were all DPDT. Since only one of the switches needed to be DPDT, they would work fine.
At a discount store, I found old Motorola cell phone battery packs for sale cheap. I was hesitant at first to buy one since I couldn't find any kind of voltage or mAH rating, but I went ahead and bought one just in case it might come in handy for some project. The rechargable nicads inside the battery back did come in handy for this project, since they provide 6V and have a capacity of 700 mAH. The frequency counter itself uses a 78L05 voltage regulator, which provides the counter with 5V. The 6V battery pack would work fine, and since they were nicads I would be able to recharge them.
I installed two BNC jacks in the rear panel for the antenna connections. I also installed a 2.1mm coaxial power jack to connect a 6V external power supply. I also installed an LED in the rear panel to indicate when external power is connected and the batteries are charging. I attached double-sided foam tape the the bottom of the battery pack and battery-charging PC board to hold them to the bottom half of the case.
I screwed up drilling the holes in the aluminum front panel for the switches, but since I had a bunch of scrap PC board on hand, I made a second front panel from that. It worked quite a bit better since it was easier to work with, and I was able to make a decent front panel. Once I had all the holes cut & drilled in the front & rear panels, I spray painted them with white paint. I then labeled the panels using rub-on lettering, which I oversprayed with clear paint to protect the lettering.
The hardest part of this project was trying to find a way to mount the frequency counter boards to the front panel. I had cut a rectangle out of the front panel and knew I'd use either glue or epoxy to attach the window to the panel. However it was more of a challenge to attach the actual counter boards. I had to line the display up with the window and somehow fasten it to the front panel. I decided to use some spare computer standoffs used to mount PC boards into computers and epoxy them to the front panel. After several tedious tries, I finally managed to get the display lined up and let it set long enough for the epoxy to set. When it hardened, it held the PC boards onto the front panel quite well.
The three switches were easy to mount since they were still soldered to a piece of PC board. After drilling the three large holes for the switches, I held it in place and made a pencil mark through holes that already existed on the switches' bodies. I then drilled four small holes through the front panel and then drilled the holes larger on the two outer switches, then ran screws through the holes and fastened them down with nuts on the back side of the panel. I only needed to do two of the switches since all three of them were held fast by the PC board.
The antenna I use is a telescopic whip on a BNC connector that came with a scanner I bought. Since I only use external antennas with my scanners, the whip was laying around in a box. Now, it works quite well attached to either BNC jack on the frequency counter. I actually have two of the whips, and if I want I can use both of them attached to the counter at the same time.
Overall I'm very happy with the way this project turned out. I was wanting & needing a good frequency counter for a long time, and now I'm happy to have a good one that I put together myself.