Why do you tune for max noise when manually tuning an antenna?

Tuning for the maximum noise on a frequency you intend to use for transmitting as well as receiving on your antenna system is a technique good operators use to get close to the best signal transfer match without interference to other stations using the frequency, or adding QRM to the band.

When other good received signals are present, they will be above the received noise, and you can reduce the gain or amplification of your receiver for a better signal to noise ratio as you listen to the other stations.

On transmitting, you will find you are very close to the best match of your transmitter to your antenna system, if you have pretuned for best signal and noise, and only a minor adjustment is usually needed to bring your system to the best match for minimum VSWR on transmit.

Try it, it works as good as your ears and experience allow.

KC6VVT

Though it might sound wrong at first to maximize noise, when what you really want to maximize is signal-to-noise ratio, you have to realize that the source of this noise is not the antenna, it is the atmosphere. Maximizing noise in this case is maximizing a signal being received. This only works if the atmospheric noise is higher than your receiver noise, which on HF it usually is, but not VHF. On VHF or higher, people aim their antenna at the sun to get enough noise to use this trick, and that only works with very good receivers (low noise figure). If you had a signal to use instead, that would be better, but it has to be weak enough that you can see a difference when you tune. Beacons are good for this on VHF or 10 meters, or it's pretty easy to make your own small signal source and put it out far away from your antenna (at least 10 wavelengths in the direction your antenna is aimed)

The nice thing is that there is a law of reciprocity, that says if it works good on receive it will work good on transmit. But this law only applies if your receive impedance is the same as your transmit impedance, which in most cases it is, but if you have a home brew receiver, or you're using a shortwave only receiver, it might be made for a wire antenna and therefore have a high input impedance. Putting an antenna tuner in line and trying to use the same tune for transmitting probably won't work.

BTW, this is not the same as tuning for maximum Q, Q is related to bandwidth, not center frequency. And reducing the gain of your receiver does not improve signal-to-noise, it reduces it typically (if you do it by adding front-end attenuation), but for strong signals this reduces distortion.

-Dan, KW2T