Zero beat means different things for AM, SSB, and CW. It was easiest for
AM where you turned on your VFO in spot mode and adjusted for minimum
frequency beat note. Remember as you tune the transmitter VFO across a
received signal in AM, the beat note starts out high, goes low actually
through zero frequency and then goes high again. When the bet note is at
zero frequency its called zero beat. A condition that few AM
transmitters maintained perfectly.
On SSB its more of a problem because there's no received carrier to beat
against and the receiver is supplying one of its own. So when you adjust
the transmitter you are adjusting to your own receiver, not the received
signal. It takes some experience or training to tune the SSB receiver
properly. I'll try.
A basic: the injected carrier can be off at least 200 hz from the proper
frequency and the SSB will be understandable.
Another basic: if the receiver is tuned perfectly the harmonics natural
to a voice will be at harmonic frequencies in the receiver audio, not
off. That makes the voice sound most natural. You can test this on AM
broadcast signal in and above the 40 meter band.
If you are off 200 hz, you may understand the other station but he may
not understand you and your response rate will be poor. It simply
requires finding the middle ground between the limits of rough
understandability to find the most optimum point. That point should be
exactly half way between the outer limits of intelligibility.
CW presents a different problem. Your local carrier has to be tuned AWAY
from the signal frequency to create an audible beat note. To compensate
most transceivers such as the Corsair transmit on a different frequency
than the local carrier for receive. One hopes that the side tone is the
same frequency, then you just match the tones. Or that in the Corsair II
the narrow audio filter (left knob) when cranked all the way in is
sharpest at that amount of offset. Some of us prefer a different tone
and tend to tune differently. Its important to correct with the offset
adjustments for TX or RX to make the transmitted frequency match the
received signal to get answer most often. Nearly all ham gear and users
tend to listen pretty close to where they transmitted. And if they are
listing though a 250 hz CW filter you don't have as much tolerance as
you did for intelligibility on SSB. Sometimes the first settings take a
second receiver.
The Corsair II has a spot button whose purpose is supposedly to offset
the receiver and bfo so that the carrier of the received CW signal is
moved into the pass band and you can adjust for a true zero beat (e.g.
zero frequency of the beat note) with the spot button held. Then when
you release it you will transmit exactly matching the received signal.
I've not yet got a key hooked to my Corsair II but will soon.
That's what zero beating is all about. Matching transmitted to received
signal.
If you answer a weak signal signing/qrp, you might not get an answer at
zero beat, some use the same VFO for receive and transmit and need for
you to be off by a beat note to be copied. The more sophisticated QRP
rigs allow for that automatically.
Step by step for CW in the Corsair II.
Find a signal you want to zero beat tuning normally in CW mode.
Push the spot switch.
Tune for minimum frequency (the audio won't pass the DC that resulted in
the detector at zero beat).
Release the spot switch.
Call with key or keyer.
Whether on CW or SSB you will often find that the other station comes
back at a different frequency from where you heard him/her first. That
means he/she prefers a bit of off tuning or you didn't do well. At that
point rather than retune each transmission which can cause a QSO to walk
across the band its better to use the OFFSET adjustments on the CORSAIR
to correct once. Then you won't walk.
Collins S-line and KWM2 in transceive on CW are way out of whack for
reasonable tones... I don't know how the user of a KWM2 survives on CW.
The S-line can be run with separate oscillators and won't have a
problem.
73, Jerry, K0CQ
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