i am in the process of homebrewing a 7/14 mhz transciever. i am in
india, we dont get toroids easily.i tried an double diode mixer built
using tv baluns double barrels, but they were lossy (about 15db loss)
and the third harmonic of the vfo created so many birdies that i
couldnt tune anything beyond 14200. (the if is 10mhz and the vfo goes
4 t 4.4mhz on 14mhz operations). so, i switched over to a single FET
mixer.

i measured Idss as 6ma, pinch-off at 2.4volts on the BFW11 that i have
in the junkbox. i baised the BFW11 for 1.2 volts, 3 ma current. the
input to the BFW11 is a double tuned band pass that i tune up for max
signal. (a 14.318mhz computer crystal in a battery operated oscillator
is my weak source). the antenna coupling is via a 2:15 link over the
first coil of the bandpass. it has excellent signal handling and
pretty low noise levels.


i have wired up a 300 ohms folded dipole using tv twin flex. i connect
the 300 ohms line directly to the receiver (the two turn link over the
first coild of the DTC bandpass filter). the antenna is about 6 feet
off the terrace in a gangly inverted V.

i have noticed a strange behaviour. if i connect just one lead of the
antenna to receiver input (leaving the grounding unconnected), i get
pretty good signals. when i connect the ground input too (to the other
end of the 300 ohm twin lead), then the noise and the signal goes up
by 2-3 S points. however, the noise increase is nearly 10 db over and
above the signal increase. esp., the shot noise. the weaker DX signals
are nearly lost if both the leads are connected.

to resolve this problem, i have tried the following:
1) i inserted a 470 ohms pot in the antenna input, and tried
attenuating the signal, the signal and noise go up and down
proportionately. the increase in noise is the same at all attenuation
levels as long as the attentuation keeps the atmospheric noise above
the receiver noise level. so, attenuation is does not make the noise
go away.
2) i tried putting a variable broadcast capacitor of 365 pf in series
between the hot end of the first coil of the DTC and the hot end of
the antenna and tuned it across the range. NO change, the signals and
the noise increase and decrease proportionately upto 50 pf and no
further change after that.
3) i tried detuning the FET drain tank. NO change. it behaved the same
as above two. the signal and noise levels change together.

these days, we do have very high atmospheric noise on 14mhz in south
india. but i guess that in anycase, connecting the antenna tigher
should increase the signal to noise ratio, not decrease it.

any ideas?