Now that 10 m is open more often I built a 10 m QRP rig of the Green
Mountain series to try and work HF from home with an antenna on my
apartment balcony (ca. 1,5 x 1,5 m). I don't have any 10 m antenna
yet, so I'll take suggestions on that, too, but first I want to get
this new rig cased and all working.
The 'problem' I have is that the Green Mountain rig has no AGC. There
is an AF gain control in front of the AF amp. Sidetone is derived by
the RX from the TX signal itself.
There is a provision to control the gain of the IF amplifier, an
MC1350, on pin 5. I know how to do this, no problem here. I have a
simple circuit to derive an AGC signal from the audio output, but
using gain control of the IF amp necessitates moving the audio
control pot outside the AGC loop to the output of the AF amp.
Because I have never used a straight key I have added a TiCK keyer. It
has its own sidetone enabled by default. The keyer sidetone can be
switched off, but it needs to be audible for keyer command feedback. I
will mix it with the RX audio before the AF amp. The mixing should not
be a big problem, but two sidetones is too much. I can disable the
rig's sidetone by taking out one resistor, but it looks like this
introduces clicking. Maybe a problem with the RX muting circuit
introduced by my modifications?
I have tentatively set up the rig with sidetone from the keyer. Rig's
sidetone is disabled and audio gain control is done with a 220 Ohm pot
at the audio output. I hear clicking in the speaker and have some
problems with the sidetone level. Maybe I should try the original
setup instead?
What puzzles me is how an RX without RF gain control would behave in
practice? If I eventually would have to add a manual RF gain control
pot, I need space for one more hole in the (not too wide) front than
with AGC or no RF gain control at all. In my experience some form of
gain control ahead of the product detector is needed to prevent
overdriving it and save my ears, too. Do I worry too much?
Did I make this too complicated? I hope to receive your very valued
opinions, in particular from those OM that use a GM-series TRX in
original or modified configuration.
Oh, and if you still remember my antenna dilemma: I thought of buying
an 11 m CB vertical antenna, shorten it for 10 m and stick it to the
railing of the balcony. Would it be any good (at 1 W QRP CW) or should
I rather try something else?
Thanks very much for reading this.
73, OH2ZAZ
Arjen Raateland
--... ...-- -.. . --- .... ..--- --.. .- --..
Finnish Environment Institute, Helsinki, Finland
SAS Support
EMAIL: Arjen.Raateland@vyh.fi
tel. +358 9 4030 0457
fax +358 9 4030 0490
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