"Roy Lewallen" wrote in message
treetonline...
Here's why antenna efficiency is important for transmitting but not for HF
receiving.

First, the definition of efficiency: For a transmitting antenna, it's the
fraction of the power applied which is radiated. The remainder is turned
into heat. For receiving, it's the ratio of the power which is delivered
to the receiver to the power which could be delivered to the receiver if
the antenna had no loss. The efficiency of a given antenna is the same
when transmitting and receiving. Sometimes people use "efficiency" to mean
other things -- this is the meaning of the term in all antenna literature
and texts.

Consider this communications system:

transmitter - antenna - propagation path - antenna - receiver - listener

A receiver unavoidably adds noise to the received signal. So if no noise
is injected in the propagation path, the signal/noise ratio is the ratio
of the signal entering the receiver to the noise created by the receiver's
input circuitry. This is generally the case at VHF and above.

When receiver noise dominates, as above, increasing the receive antenna's
efficiency increases the signal arriving at the receiver, so the
signal/noise ratio improves. This allows you to hear the signal better.
But it only works for VHF and above.

HF is a different story. At HF, there's a lot of atmospheric noise
(injected in the "propagation path" part of the system), and unless the
receive antenna and receiver are exceptionally bad, the atmospheric noise
is much greater than the noise created by the receiver. I mentioned a
simple test in my last posting, to see whether this is the case -- just
disconnect the antenna. If the noise level drops, atmospheric noise
dominates. It's not hard to make a receiver that atmospheric noise will
dominate with a 3 foot whip antenna at HF. So at HF where atmospheric
noise dominates, the signal/noise ratio is the ratio of the signal
entering the receiver to the atmospheric noise entering the receiver.
Compare this to the situation described above for higher frequencies.

Now let's see what happens when we improve the efficiency of an HF
receiving antenna. Because both the signal and the dominant noise come
from locations in front of (that is, on the transmit side of) the antenna,
improving the efficiency of the antenna makes both the signal and noise
greater in the same proportion when they arrive at the receiver. There's
no improvement at all in the signal/noise ratio. The effect is the same as
turning up the receiver volume control. The only way you can improve the
signal/noise ratio is to somehow favor one over the other, such as by
making the antenna directional. And an inefficient, directional antenna
like a Beverage or small loop will nearly always enable you to hear better
in some directions than an efficient, nondirectional antenna because
directionality helps and inefficiency doesn't hurt.

How about transmit antenna efficiency?

The signal strength from the transmit antenna is proportional to the
antenna's efficiency. (It also depends on other things, but I'm just
talking about efficiency here.) So if the efficiency of the transmit
antenna increases from, say, 33% to 66%, the power levels of the signals
at the receive antenna and the receiver double, and there's no change to
the received noise, on either HF or VHF and above. So improving the
transmit antenna efficiency always improves the signal/noise ratio at the
receiver, in this case by 3 dB.

That's why you can hear bunches of HF stations with a very inefficient
antenna, but they won't hear you if you try to transmit using that same
antenna -- it's because the noise is injected into the system between you.
And it's likely that you'll be able to hear stations just as well with the
very inefficient antenna as with a much larger, efficient one.

Roy Lewallen, W7EL

Well said Roy, however, can you explain why this is not so at VHF and above?
I would think that would have to do more with mode than with
antenna/propagation. When I turn down the squelch on my vhf rx i get lots
of noise. Let me know.
TIA,
B