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

Roy Lewallen wrote:

...
Roy Lewallen, W7EL


YEAH, what he said! LOL

And, I must defer to him, his experience allows nothing less ...

Regards,
JS

"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

Brian Oakley wrote:

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

It's purely because of where the dominant noise comes from, more
specifically whether it gets into the system before or after the
antenna. Atmospheric noise gets greater as you go down in frequency. At
VHF and above, it's less than receiver noise, so receiver noise
dominates and masks whatever atmospheric noise there might be. At HF and
below, it's usually greater than receiver noise, so atmospheric noise
masks the receiver noise. Obviously there's no precise line, so
somewhere typically near the upper end of HF either one might dominate,
depending on conditions, antenna, and receiver.

The noise you get from your VHF radio when you turn down the squelch is
receiver noise. You can prove it by disconnecting the antenna and
noticing that the noise doesn't change. Disconnect the antenna from an
HF receiver and the noise will drop, because it's coming from the other
side of the antenna.

Roy Lewallen, W7EL

On Mon, 29 Dec 2008 20:32:58 -0800, Roy Lewallen
wrote:

It's purely because of where the dominant noise comes from, more
specifically whether it gets into the system before or after the
antenna. Atmospheric noise gets greater as you go down in frequency. At
VHF and above, it's less than receiver noise, so receiver noise
dominates and masks whatever atmospheric noise there might be. At HF and
below, it's usually greater than receiver noise, so atmospheric noise
masks the receiver noise. Obviously there's no precise line, so
somewhere typically near the upper end of HF either one might dominate,
depending on conditions, antenna, and receiver.
(...)
Roy Lewallen, W7EL

This might help:
http://en.wikipedia.org/wiki/Atmospheric_noise

If you extend the red line showing man made noise, at greater than
about 30Mhz, the man made noise (ignition noise, motor noise, etc)
predominates over atmospheric noise.


--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
# http://802.11junk.com
# http://www.LearnByDestroying.com AE6KS

Jeff Liebermann wrote:

This might help:
http://en.wikipedia.org/wiki/Atmospheric_noise

If you extend the red line showing man made noise, at greater than
about 30Mhz, the man made noise (ignition noise, motor noise, etc)
predominates over atmospheric noise.

In my last couple of postings, I was lumping man-made and atmospheric
noise together as "atmospheric noise". Both enter the system between the
transmit and receive antenna, so improving the receive antenna
efficiency won't help the ratio of signal to either atmospheric or man
made noise. The referenced graph doesn't show receiver noise at all,
which dominates at VHF and above.

It can be useful, however, to distinguish between atmospheric noise and
*local* man-made noise, since the latter can sometimes be reduced by
using techniques such as feedline decoupling and using horizontally
polarized antennas.

Roy Lewallen, W7EL