Gang,
Sometime back, in response to a thread about conserving bandwidth,
Dave Mathews asked how one goes about being sure his(her) transmitted
signal is indeed zero beat with the station being answered. There were
several posts, but I suspected the explanations were a little off the
mark. I asked Dave if he'd gotten an answer- - -
On Thu, 20 Feb Dave writes:
>...I didn't really get an answer. ...
I originally posted this reply to Dave alone. But then I
recalled that several people have asked that such answers go on the
list, since there are always "lurkers" who read and learn, and would
like to know this or that, but don't speak up.
Question: Am I clogging up the band (list) with this? Or
should I not post such things to the list. Feedback please.
***Transmitting on a received station's frequency:***
I'll take the dc receiver first, since I think you might have a
misconception there. A dc receiver does indeed have two audio images,
and the bfo will indeed be exactly on the other station's frequency when
you tune to a zero beat- that is, exactly between the two audio images.
But most transceiver designs also incorporate an offset for transmit. A
small additional capacitance is added (or subtracted) from the vfo (it is
called a vfo when used for the transmitter, but the very same oscillator
is called the bfo when it is used for a dc receiver!). The transmit
frequency is shifted by about 700 hz on transmit, so that you can
transmit on the same frequency as the other fellow and still hear him (with a 700 Hz beat note) when you drop back to receive. So the answer is
no, proper tuning is NOT at zero beat with the two audio images equally
spaced from your bfo (unless you have a very primitive transceiver like
the 40-niner which has no transmit offset in the stock configuration.) Proper tuning is when your received beat note is about 700 Hz.
The only trick is, you have to know which way your transceiver
shifts. You can figure it out from the schematic. Is a little more
capacitance added to the oscillator tank during transmit? That will
shift the frequency lower on transmit, so you should be listening to the
other station on the audio beat note that is *below* the zero beat- ie,
the first beat you come to as you tune higher in frequency. That is
how the HW-8 does it, but I added a switch to be able to have the shift
go the other way so's I can listen to the upper sideband note instead
and avoid QRM.
OK, so you knew that already. How do you make sure that your
transmitted signal is really *right on* the other station? Well, with
some rigs there is a switch to disable the frequency offset, so you can
"spot" the other station. Lacking that (and most rigs do, although it is easy to add such a feature), you simply have to match his tone to your
sidetone. If the rig has been designed and aligned properly, that
offset should equal the sidetone frequency. How can you check? The best way is to use another receiver (you can easily hear the bfo of a dc
receiver.) You can also check with a frequency counter, but in that case you have to be aware that the capacitance of the probe will change the
frequency of an unbuffered oscillator. If your oscillator has a buffer
stage, you can use a counter, otherwise not.
The third way to check your transmit offset to ask another ham
who lives nearby to listen to your transmitted signal and tell you when
it is zero beat with HIS receiver. That pre-supposes that he can tell
when a received signal is exactly on his transmitted signal. Alternatively you can both tune in to a SW broadcast station. Transmit briefly, and have the other station tell you when you're right on the SW broadcast
station's carrier freq. (use the {gasp} phone.) Use the lowest power
you can, of course, and try to do this when the band is dead. Then
listen with your set and see what audio pitch you have. That is the
received pitch you need to be hearing in order to be transmitting right
on the other station's frequency. (I am assuming your OHR rig is for
40m.)(no handy SW stations on the other bands.) Of course, you can
use another rf source (instead of a SW carrier) for this check if you
have both rigs in the same room and you've got a grid dip oscillator or
crystal oscillator or rf generator or a third rig or an MFJ-249 antenna
analyzer or... well, even a SW carrier. With a receiver right in
the room with your rig, you can transmit into a dummy load and be sure
not to bother anyone else.
But you have a superhet. Well, that third method will also work
for a superhet. You need a second receiver and a signal source.
Listening with the second receiver, tune your OHR transmitter until
you're right on top of the signal source. Use the lowest possible power and/or a dummy load. (Again, you could have a nearby ham listen as
you zero-beat a SW broadcast station's carrier, using the lowest
possible power when the band is dead.) Then listen to the signal
source with your OHR's receiver and see where you are. That's the tone
(or pitch) you need in order for your transmitted signal to be zero beat
with the other station's transmit frequency. And as I said, the rig
should be constructed so that the sidetone pitch is the same, so that
you can match the received station's pitch with the sidetone pitch.
Even people with "tin ears" can match frequencies fairly closely (with a
few exceptions.)
So the answer is: use the sidetone pitch to guess what the
received station's audio beat note should be. Oh, I forgot, you've got
RIT. TURN IT OFF. Unless you work those stations who "listen up" a
few KHz., the RIT is not terribly useful. And if you *do* want to
work one of those stations, then turn off (or center) your RIT, go up a
few KHz and find a frequency where no one else is transmitting, and then
use the RIT to go back down to the other station. If your RIT lets you
tune both up and down, then set it to its middle position (some rigs use
a pot with a center detent for the RIT control). I really think RIT
has no place in simple gear.
Now, lastly, you should realize that I do VERY little operating.
So other hams may not agree with all I've said. I am mostly a builder
type. I just finished FYBO - I made two (2) contacts in about as many
hours! Well, there were some distractions, but not many. I'm just a
lousy operator. Real operators may feel differently about RIT.
Bedtime for me. Hope that helps.
73, ; D DWink@Juno.com Dan Winkler N7IVR Seattle, WA
----------whom the gods would destroy, they first make proud ----------
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