7/30/2019 Garry Nichols 2010 Entry
2/4
Well this year I decided to try 40 M CW for the HomeBrew Radio
Contest. The circuit originally
came from Owen Pool's web
site:http://bellsouthpwp2.net/w/u/wuggy/fetgen.htm.
It worked surprisingly well on the BCB so I took it up to 80 M
CW and then 40 M CW. I also
made a few changes. I use only 1 turn on the tickler coil. I
also moved the audio pickup point tothe drain lead to make things
louder. I did not notice any significant degradation in
performance
when I did this, but I am not expert at evaluating this.
That change required adding a .001 pF to ground from the drain.
Without that I got nothing in the
phones. I guess the RF needed to circulate to ground from the
drain and was blocked by the high
impedance of the phones.
Since the phones served also as source resistance in the
original circuit, I added a pot in the
source lead. The best operating point seemed to be at about 2k,
Rs, which is what my oldBrandes phones measure in DC resistance.
Less Rs causes the gain to roll off rather rapidly and
more causes it to roll off slowly, based on listening tests.
I tried an RFC between the .001 and phones, but it seemed to add
no benefit so I removed it.
http://bellsouthpwp2.net/w/u/wuggy/fetgen.htmhttp://bellsouthpwp2.net/w/u/wuggy/fetgen.htmhttp://bellsouthpwp2.net/w/u/wuggy/fetgen.htmhttp://bellsouthpwp2.net/w/u/wuggy/fetgen.htm
7/30/2019 Garry Nichols 2010 Entry
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Body capacity effects are not too bad.
I added a screwdriver set bandset variable and a bandspread
variable made up of a 10 pF ceramicin series with a 149 pF
variable. Once I get things set to 40 M the bandset is left alone.
The large
jar lid stuck to a regular knob gives sufficient vernier action
and is comfortable to the fingers.
The tickler coil is swung near the cold end of the tank coil by
the wooden lever, which is pivoted
on a screw counter sunk under the vertical wood post holding the
tickler. It is not difficult to
operate at all.
This circuit has never presented me with any unpleasant sounds
on any of the three bands thatI've tried it out on. So I decided to
stay with the Brandes and not use a Bogen transformer and
SPP and risk getting some sort of blast of sound because of this
change. Sensitivity is pretty good
with the Brandes.
I think that a stage of audio amplification and a bit of backoff
on the gain of the MPF-102 might
make this set an even better performer.
My antenna system for 40 M makes use of a two draw steel filing
cabinet as ground, a 5 foot run
to the window of my second floor room and then 24 feet sloping
to an insulator and a brick on
the lawn. (This is surprisingly good on the BCB, also!) It is
resonant just above the 40 M band,according to my dip
oscillator.
Antenna coil coupling used while listening is a bit closer than
depicted in the photo. When I was
down on the BCB I was experimenting with the antenna coil inside
the tank coil, so that is why it
is mounted on an arm. Notice the big brass nut stuck to the
opposite side of the wood block?
Keeps the assembly from tipping down with the weight of the
antenna coil!
word of thanks to Scotch/3M for their "permanent" foam mounting
squares, from which I seem
to be lashing up everything lately. It is to me what Duct Tape
is to Red Green!
