How Power and Signal to Noise Works on the Low Bands

Below is from the Top Band Mailing list and was sent to me by Alan Swinger, K9MBQ

Hi topband lovers,

The basic concept that explains how power and signal to noise ratio works on
low band is very simple. On high bands we need the signal to bounce back to
the Earth. Without ionization, the signal does not reflect back and  power
does not matter, 1w 100 w or 10 Kw won’t make the signal to bounce back to
the Earth.

In last month’s large contest, most stations worked more QSOs on 160m than
10m. The MUF was not high enough to bounce the signal back.

Contrary to high bands ( 20 to 10 m ), on low bands the signal always
bounces back to the Earth because the MUF is always above 3 MHz, even on the
solar maximum the MUF is above 160 or 80m most of the time. Now that we are
just entering solar minimum we can see that happening every day.

If the signal on low bands always bounces  back to the Earth we can say that
we have propagation all the time on 160m, or most of the time on 80m, the
issue here is just attenuation.

Attenuation is the name of the game, and it plays on both sides, A to B and
B to A  location, Initial power  minus  the attenuation is responsible for
whether the arrive signal is heard or unheard, local noise at the receiver
side will compare to the arrival signal to make a QSO possible or not, if
the signal is 3 db above noise level QSO is possible on CW, 8db above
possible on SSB. The bandwidth is very important. On digital modes the real
BW is the calculation inside the software that can result in less than 1 Hz
of bandwidth. JT modes can hear below  noise floor (500 Hz)  because of
that, the real BW is just few Hertz.

If the signal always arrives at B, any one db power increase at A will
increase the signal to noise ratio at B by one db. If your signal is at
noise level at B location and you increase 3 db of power, 100w to 200w the
operator at B will be able to copy you. If your signal is already 10 db
above noise, increasing the power will just actuate the radio AGC and there
won’t be any real gain besides comfort.

The local noise can be very different at A or B, A could be in a city lot
and B could be in a very quiet Island with no manmade noise, B can use a
narrow RX antenna with high RDF and the noise floor can be 10 or even 20
better than A. Most people call this one way propagation, but in reality it
is just power budget difference at each location, local noise on the
receiver side plays as much as power used on the transmit side.

My take is that we can work on both aspects , reduce local noise with good
directivity on  the RX antenna, but power is limited by law, and ethic!

A low band station with a RX near 12b RDF, 80 to 70 degrees front lobe and a
KW can work 100 countries on 160m during any two month period of any year,
and can work 150 or more countries during a 12 month period any year of the
solar cycle on 160m. During solar minimum it is  common for a good contest
station on 160m to work 100 countries during a contest weekend.

Understanding this basic concept can help any low band operator improve his
or her performance.

My two cents