Dip meters

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A dip meter from Art, VE1EP


I had a home-brew dip meter (see photo at right) that had been given to me some time ago by Art, VE1EP, but it had no coils and used an obsolete 15v battery. I worked out the circuit diagram and tried to find out where the design came from but extensive Web searching failed to find a source. If you know where it came from, I’d be pleased to know. Maybe Art designed the circuit himself. Anyway, I decided that getting Art’s dip meter running and wiring coils for it was more work than I wanted.


Heathkit dip meters


At the end of May I attended the Downeast Ham Radio Fleamarket in Halifax, Nova Scotia. This was my first attendance at a ham radio fleamarket for some years. Amongst other things, I went there to see if I could pick up a dip meter. In walking around the fleamarket, I soon found a Heathkit GD-1B dip meter but it had no coils, had a broken switch and had a mains cord which told me that it was an old tube design. However, since it was free I was glad to take it home to see what I could do with it.


Looking round some more, I found an HM-10A Heathkit Tunnel Dipper. I hadn’t heard of a tunnel dipper before but I was assured by the seller that it provided the same function as a dip meter. It was Heathkit’s solid-state replacement of the GD-1B, using a tunnel diode in its oscillator circuit instead of a tube. (Later, I looked up Wikipedia’s explanation of tunnel diodes to see how they were used in oscillators).


At first I thought that the Tunnel Dipper, too, was missing its coils but the seller demonstrated how they were hidden behind some foam rubber in the lid! I was pleased to buy the tunnel dipper for $20.


The Heathkit HM-10A Tunnel Dipper (1962-1971)


Back home, I inspected the Heathkit Tunnel Dipper (see photo at left). It runs from a single AA battery and so I was apprehensive that the battery, probably long unused, would have corroded the insides. I was pleasantly surprised to find that this was not the case, maybe because it was not an alkaline battery. I plugged in the appropriate coil, turned on the tunnel dipper and tried, unsuccessfully, to find a dip at the resonant frequency of my 28.15 MHz trap. To find out why, I took the cover off and found that where the coils plug in there is an extra contact. Four of the coils have a long centre-pin on their RCA connector which connects to the extra contact to bring extra capacitance into the oscillator circuit. On my tunnel dipper, this contact had moved so that it no longer contacted the pin. I moved it back in place and tried again. Still no dip. By now I am becoming frustrated with dip meters!


I was able to tune my Elecraft K2 transceiver to the frequency in question and found that the dipper was oscillating at the correct frequency. I tried measuring the trap resonance again and did, this time, find a very small dip when placing the trap almost touching the tunnel dipper’s coil. This lack of sensitivity is perhaps connected to the fact that, with the sensitivity control full on, even with a new battery, the meter does not go full scale. There is a three-transistor buffer and amplifier driving the meter and I need to check that it is functioning correctly. Update: see Fixing the Heathkit HM-10A Tunnel Dipper.


The Heathkit GD-1B Dip Meter (1954-1960)



Next I turned my attention to the Heathkit GD-1B dip meter (see photo at right). The slide switch on the back, which powers the oscillator, thus switching the meter between a field strength meter and a dip meter, was broken. I took the case apart and found a dressmaking pin inside the switch which maybe the previous owner had inserted to short-out the broken switch. The switch was not repairable and so I replaced it with a “new” one. In taking the dip meter apart, I found that one of the wires to the milliammeter had become detached although I’m not sure if that occurred during the disassembly. Anyway, I reattached it. The rubber grommet through which the power cord runs had disintegrated so I replaced that, too. Finally, this being a kit construction, there were a couple of areas where bare wires might contact the case and so I added some electrical tape to prevent it.


I powered up the dip meter and tested the power supply circuit. According to the circuit diagram that I found on the Web, the power supply generates an appropriate voltage for the tube’s filament and also 90 volts DC for the B+. I checked these and they were correct. I could see the glow of the filament and the milliammeter moved to the left of zero.


On the Web I found some information on winding coils for the GD-1B. The frequency bands mentioned did not correspond to those on the dial of my GD-1B but it gave me ballpark specifications. I constructed a coil on a piece of 1/2” CPVC pipe with turned-down 1/8” brazing rod for the plug-in legs. By trial and error I managed to create a coil with the correct windings for the band that covered my 28.15 MHz trap. I plugged it in and tried for a dip at the trap’s resonant frequency. Boy, did I get a dip! I was able to use a loose coupling between the dip meter and the trap and still get a useful dip. This is how dip meters should work! Not bad for an old tube dip meter that’s maybe fifty years old!


I subsequently wound coils for two more bands. For more information, see details of my home-brewed Heathkit GD-1B coils.


The need for a reduction drive


The coax traps used in my trap dipole have a high Q-factor which means that the dip occurs in a very narrow band of frequencies. For the dip meter to be successful, it needs to have a reduction drive on the tuning capacitor. This was another nail in the coffin of the dip meter I received from Art but Heathkit’s dip meter and its tunnel dipper fulfill the requirement with a 2:1 drive: a 360 degree turn of the dial translates into a 180 degree turn of the capacitor shaft.


The need for a digital readout


A second requirement is the need for a digital readout. An analogue dial is O.K. for some things but tuning a trap to 28.15 MHz is not one of them. To bleed off a signal to drive a digital counter, I wanted to draw a current light enough to leave the oscillator unaffected while picking up a strong enough signal to drive a counter without amplification. Looking at some circuits, I found that a digital frequency meter was often fed by connecting via a 10 pF capacitor.


I tried this with the tunnel dipper but it runs from a 1.5 volt battery and feeds it’s meter via a 3.3 pF capacitor: loading the oscillator circuit by taking a signal through a 10 pF capacitor kills it. Conversely, the 90 volts of the GD-1B dip meter proved to easily handle the situation. There is no place to easily add a connector to the GD-1B and so I soldered a 10 pF capacitor to the anode at pin 1 and ran a shielded cable from the capacitor and a nearby ground and out through the grommet that contains the power cord. I connected it to a Micronta digital frequency counter and it operated flawlessly. On the left is a photo of the resulting setup including three coils that I built for the dip meter.

 

Dip Meters

In 2012 I needed a dip meter to measure the resonant frequency of the traps I was building for my indoor trap dipole.

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