|[AMRadio] Re: Top Band|
k4kyv at charter.net
Tue Sep 4 13:02:18 EDT 2007
I usually operate 1885 or between 1980 and 1990. But often when those
frequencies are occupied (either by SSB or an AM QSO that already has 4 or
more participants, I hear plenty of wide open spaces that are unused,
especially above 1900 kc/s.
I use a converted Gates BC1-T, and it is VFO controlled. I find that it
holds neutralisation perfectly over the entire band, from 1800 to 2000.
Rather than having a crystal ground for the popular AM frequencies, why not
control it with a VFO? It is easy enough to convert the crystal oscillator
stage to VFO control by bypassing the control grid to ground (either
directly or through a capacitor, depending on whether or not any bias is
applied at the control grid. Usually there is an rf choke between the
cathode and ground, with a feedback circuit back to the crystal. Disconnect
the feedback circuit from cathode and apply the VFO signal there. The VFO
signal needs to be low impedance, 50-250 ohms, and about 1/4 watt will
usually do the trick.
This converts the oscillator stage to grounded grid, and with most
oscillator circuits, will be very stable with no tendency to self-oscillate,
and it is very easy to convert back to xtal control.
One VFO source would be to use the output of a solid state transceiver, in
the cw/tune mode, with an attenuator between the transceiver and transmitter
to reduce the power output down to < 1 watt. This will result in a cleaner
signal than simply turning the rf output down to minimum with the carrier
This circuit probably won't work with something like a VF-1 or Johnson 122.
Those VFO's have high output impedance, which makes it difficult to locate
the VFO very far from the transmitter. In this case, the output from the
VFO may be injected directly into the xtal socket, but if there is any
tendency to self oscillate, disconnect the feedback circuit and by-pass the
cathode to ground using a capacitor.
Just as in the case of 75, we should not limit AM operation to a few
specific frequencies on 160, but operate all over the band, wherever there
is a clear spot.
I plan to be back on the band in a few weeks using the Gates transmitter and
quarter wave vertical antenna with radial ground system.
This message was typed using the DVORAK keyboard layout.
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