Michael Drainer said:
I am building a RF Field Strength Meter and need to tune it to the UHF Band.
What size/kind of inductor and transistor should I use for this. All the
plans I have found are for the VHF band.
You can scale things if you make the inductance and its tuning
capacitance smaller by the ratio of the frequencies. In fact, if you
scale ALL dimensions of an inductor, you should end up with one that
behaves just the same at the scaled frequency. (Smaller coils work at
higher frequencies.) If the design's coil is tapped, you should be
able to keep the tap at the same number of turns on the new smaller
coil.
As others pointed out, if the transistor is just a DC amplifier, you
don't need to worry about it. The detector diode (assuming it uses a
diode detector!) likely will work OK, but if you try to go TOO high in
freq, you'll perhaps need a better detector diode. The
dimension-scaling thing works for parasitics too: there is always
extra capacitance and inductance in the wiring. So you'll want to
build things smaller and smaller as you go to higher and higher
frequencies. You eventually hit a practical limit where you will want
to change your whole way of thinking from lumped capacitors and
inductors and resistors to distributed things like cavities and
waveguides and resonators, but with modern components and low power,
you shouldn't hit such limits below a gigahertz, and people work
successfully well above that with lumped components (with care).
As an example, program "Coil" from Saratoga Software tells me that
five turns of 28AWG wire wound on a 4-40 (US thread) machine screw
(then remove the screw) will give you a 35nH coil with a
self-resonance around 2.6GHz and an unloaded Q of about 250 at 500MHz.
That should be useful to over 1GHz.
Cheers,
Tom
