On Sat, 22 Nov 2003 18:20:46 -0500, Kenneth
%wa2mze%@%bellsouth%.%net% wrote:



Not all fets function well into the vhf range. If you want to build a
solid state dipper using fets make sure you get vhf units, not ones
intended for audio use. There are circuits using fets that simulate
negative resistance (like tunnel diodes) which make for an interesting
oscillator since no coil taps are needed and a single gang cap can be used.

Right. I don't like fets, execpt as switches, RX RF amplifier and in
PA. We had a discussion in a radio club among designers 30 years ago
if it was any reason for using fets in oscillators instead of
bipolars, but couldn't really find any good reason, except they were
fancy.
It is also a circuit using a jfet and pnp-combination which works in a
negative impedance mode, but usually found on lower frequencies. Used
it for some time to check resonnant frequency of unknown tuned
circuits with a counter, but the problem is to control oscillation
level. It worked fine for 50% of the applications while the counter
indicated some higher (or lower) frequency for some circuits, so it
was abandomed. Believe I have seen it mentioned for griddipmeters,
too.
It seems to be a reason why PNP UHF transistors are used in most UHF
tuners found here, usually BF479, BF679, BF979. I've copied some of
the text for DL7QY's dip meter covering up to 1400MHz on
http://home.online.no/~la8ak/m3.htm

-JM


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