Thursday, March 15, 2012

Zero Beat and Offset in CW

How to Zero Beat Another Station
by Jack Wagoner WB8FSV

CW stations should always try to zero beat each other. That means to adjust your rig's transmit frequency to exactly match the transmit frequency of the other ham you would like to talk to. Hearing two CW stations conduct a conversation a few hundred cycles apart is a waste of frequency space, and is inviting QRM. How does one zero beat another station? Easy to do on phone or SSB, just tune so that the other fellow's voice sounds normal. But trickier on CW because when you put your receiver exactly on a CW station's transmit frequency, you hear nothing, zero. In modern transceivers, in the CW mode, the receiver's BFO is offset from the displayed, transmit frequency in order to produce an audible tone. In other words, the transmit and receive frequencies are far enough apart for you to hear a pleasantly pitched tone when your transmitter frequency is tuned to exactly that of the ham you are listening to. This frequency offset is frequently about 600 Hz or Hertz.
Here is how I zero beat another CW station with my own rig, a Kenwood TS 450. I tune into, or sweep through, the other CW signal, the pitch going from high to low, until the other ham's CW signal disappears. Now my receiver is zero beat with the other ham's transmit frequency. But I want my transmit frequency to be zero beat with the other ham's transmit frequency. So then I tune again, with the other ham's pitch going from low to high, until I am 600 Hz away. For example, if the other ham's transmit frequency is 7137.90 kHz, I would tune my transceiver to 7137.30 (7137.90 minus .60 equals 7137.30.) to transmit exactly on his transmit frequency. The direction you tune or sweep, the pitch going either from high to low or going from low to high, is rig dependent. On Kenwood ham radios you would tune the pitch from high to low as you tune higher in frequency, to reach the 600 Hz offset and be zero beat with the others ham's transmit frequency.
I wrote the above paragraph several years ago, and currently I zero beat using a different method. I still have my Kenwood TS-450, but now as I tune around looking for a station to contact, I leave my RIT(receiver incremental tuning) turned on. Leaving your RIT on while tuning goes against convential wisdom, but I find it works for me. I leave my RIT on about 500 to 600 Hz up. When I discover another station I wish to zero beat, I tune by ear so that their CW tone drops down in tone to almost nothing, meaning that my transmit frequency is now approximately zero beat with theirs. Then I reset my RIT back up a few Hertz so that I can hear the other station. Takes me one or two seconds. Tuning by ear for an approximately 600 Hz tone just comes with experience. I have found that this method of zero beating works best for me. Recently I have become a DXing nut, and I find this new method faster for me. There is no one best method for zero beating. Whatever works best for you and for your rig.
This zero beat frequency stuff is pretty weird, it confuses me at times, and I hope I explained it correctly. The frequency offset for CW in most transceivers explains why when you are listening to a CW signal in the tranceiver's "CW" mode, and you switch to phone, to "LSB" or "USB," you loose the CW signal and have to go search a bit for it again.