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