Monday, December 6, 2010

How to Zero Beat?


By Dick Arnold, AF8X

Not having much success when answering a CQ? It maybe the station calling CQ can’t hear you because you are not zero beat with his frequency. Every CW radio transmits on a different frequency that it receives on. This is called the TX offset.

A receiver tuned to the exact transmit frequency of a signal is said to be nulled. For the tone of the CW signal to be heard, the receiver would have to be tuned off of the center frequency. The most common offset is 600 Hz although as high as 800 hz is still in use in some rigs. In some newer rigs this offset is adjustable.

Many rigs have a spotting tone that can be matched with the incoming signal. When the tones merge, you are zero beat with the incoming signal. Some other radios are adjusted to have matching side tone and offset so that when the incoming signal pitch matches the side tone, you are zero beat.

The less expensive direct conversion rigs have the advantage of being able to hear signals on either side of the carrier frequency. However you must be tuned on the correct side so your offset frequency matches the receiver of your contact. You may use the RIT if available to tune out QRM and listen to the other sideband.

If you have a problem discerning the proper tone to tune to, there are a couple Zero Beat Indicators on the market which use LEDs to show a zero beat condition.

Another method is to tune through a signal from high pitch to null putting your receiver zero beat with the transmitted signal. Note the frequency 1 and then tune from null to that frequency plus 600hz 2. That should put your transmit frequency exactly on the incoming signal transmit frequency, e.g. 1 14.030.10 2 14.030.70.

Normally, unless the other station is using a very narrow filter, you will be heard if properly zero beat, even if the offsets are opposite and of different tones. The pass band allows for some difference in tuning.

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