(Heathkit) Grid Dip Meter?

[bklein]

Somewhere in my garage is a Heathkit Grid Dip Meter. The minimum frequency range for the standard coils is 2mhz though. Has anyone make their own coils for a lower resonant frequency range? How then did the results compare to the method using a signal generator and scope etc.?

[Overtheedge]

Have a Heath Tunnel Dipper as well as a couple HB dippers. Not sure why I still have them anymore. Dippers had a real use before test equipment got cheap.

Problem with attempting to reduce minimum freq by increasing coil inductance is the vari-cap min/max severely limits the min/max frequency. Just not near enough capacitance range.

Better option is use a test oscillator with an emitter follower. Plug in the test coil and read output freq on a frequency counter. If you make the oscillator with a fixed capacitor (100pf or so) that can be switched in/out, you can determine the coil's parallel capacitance.

I use the LC constant formula
LC=25330/F²
L in µH
C in pF
F in MHz

[bklein]

Thanks, something for me to try.
The Millen and Eico had lower frequency coils/range options that would get down to our area. The Heath didn't seem to. Might not work as you suggest. But sure would make life easier to quickly compare one coil vs another.

[Davor]

So, what is the purpose of lowering the frequency here? Measurements, or pinpointing?
I guess tunnel, or lambda diode, or whatever active element there is will not mind being forced to work at low frequencies, as long as your coil provides enough Q in the tank.

[bklein]

I think the Heath meter's frequency range stops at 2mhz. From what I've read the typical resonant frequency for our coils is more likely to be around 700khz... or at least under 2mhz.

[Overtheedge]

You might get an older copy of the ARRL Radio Amateurs Handbook. I dragged out a 1977 copy and it had a MPF-102 dipper circuit. Pretty sure it could be forced lower by swapping out each of the caps in the tank for 3-4X more capacitance. Of course trying to find a dual 365pf variable is a bit problematic nowadays. Orren Elliot Products makes new ones, but they are expensive. Another option is prowling a Hamfest for variable caps.

No reason you couldn't wind another coil for yours to get it lower in frequency. The problem will be the band's narrow tuning width. Without lots of additional capacitance in the variable, it just won't tune very far up or down. Without fairly wide tuning, it will be difficult to know if you are high or low on the inductance of the coil under test. As I recall, all the HeathKit dippers used a phono-plug for the coil connector.

Looked in the 1995 handbook and no schematic. Reckon it is just old technology. And I was so proud when I got mine back in the early 70's.

You will still need to make the test coil part of a resonant circuit to dip it. Otherwise you will need an inductance bridge or an oscillator circuit and freq counter.

[Davor]

So it is intended for establishing a self-resonance of a search coil. Cool.
I once played with a lambda diode (JFET+BJT combo) dip meter, and it was very sensitive to the changes of Q, hence a dip.
Regarding varicaps, nowadays you'll hardly find a proper high C varicap with favourable voltage to capacity characteristics, but you may assemble a makeshift varicap with a bunch of 1N4001.

[Qiaozhi]

Here's an interesting digital design for a dip meter ->
http://elm-chan.org/works/ddm/report_e.html

and, for reference, here's the Heathkit Grid Dip Meter ->

[bklein]

Wow, thanks much - great link that I didn't come across!

[Overtheedge]

Davor, you are correct. You can reverse bias any semiconductor junction and use it as a variable capacitor with a potentiometer controlling the bias. An often overlooked high capacitance semiconductor junction is the old power transistors that have lots of junction geometry to permit higher current without overheating.

Admittedly, I haven't tried using the power FETs as vari-caps. Might work, but until I've tried it I won't say it would work.

I usually try to avoid diodes as capacitors due to drift problems from junction heating, but for a dip meter, who cares about long-term drift.

There isn't really a problem when the diode is used as part of a PLL controlled oscillator because it is locked/corrected against a reference oscillator that is usually XTAL controlled.

Good catch Davor. One outfit I've bought a lot of parts from is Dan's Small Parts and Kits, but checked his site before posting this and all I saw was low cap tuning diodes.
eric