Hi,

Just getting started with all of this, and want to say a quick thanks to
everyone for all the help.

Another question:

Am I interpreting this more or less correctly:

Looking at all the different types of coax available.
Will be for a receive-only HF antenna.
Antenna will have to be in the backyard about 150 feet or so from house.

The db losses are beginning to add up; at the upper limit of my interest of
30 MHz
we are getting close to around 3.5 db or so.

Is it correct for me to say that the actual losses really aren't all that
significant or meaningful,
and that a good receiver, which I have, can easily make up for them ?

That the only thing of real concern would be the S/N ?

What are the caveats to my statement above ?

Thanks,
Bob

Bob,
You're right, for recieving the losses just aren't going to be that much,
and not
really something to worry a lot about. The average receiver ought to handle
and
make up for those losses. Losses will only be noticable on frequencies that
are
not fairly 'close' to one of the harmonics of the design frequency, and then
not
all that bad.
- 'Doc

Hi Bob

Yes that is correct!

You may however get into larger losses from an "unmatched" antenna. This
depends on how you feed it. If you (for example) ran a dipole type setup
direct connected to the coax end, you will tend to have "notches" as the
feedpoint hits a very high impedance value (eg the wire being multiples
of a full wavelength but centre fed) Whether this will become an issue
or not depends on how much loss occurs. If you (say) cut a dipole for
7MHz there will be this kind of problem at 14 and 28MHz. My gut feel is
that both these freqs will be affected adversely. I havent gone to the
trouble to guess how wide these "notches" will be but the problem will
be worse the higher multiple you go. For transmitting purposes you get
around this by using open wire feeder and a tuner. (And for the purists
reading I have left out the topic of tuned feeders!)

As a general rule the impedance of a dipole dips to a minimum at
resonance. A half wave around 75 ohms and 3/2 wavelengths 150 ohms (I
think) In between there is a reactive component that adds to the overall
figure. Without consulting some books I dont know the average nunbers
off hand but impedances higher than a few thousand ohms are not uncommon.

Now for some quick maths.. If you try to feed an antenna with an input Z
of maybe 2500 ohms with your coax - resulting in a 50:1 VSWR, you will
lose about 3dB from the normal line length, plus another 10dB for the
mismatch.. I's say that will be upsetting maybe from 21MHz up.

This is why its a good reason to use a broadband transformer balun at
the antenna feedpoint. Realising that you are probably going to use
something like that anyhow..

Apologies for the waffle.. You did ask though!

Cheers Bob

Robert11 wrote:

Is it correct for me to say that the actual losses really aren't all that
significant or meaningful,
and that a good receiver, which I have, can easily make up for them ?

That the only thing of real concern would be the S/N ?

What are the caveats to my statement above ?

You have the right idea. Signal/noise ratio is all that counts.

When receiving HF, feedline loss almost never matters. The reason is
that atmospheric noise is strong, and unless a receiver is exceptionally
noisy and/or an antenna is exceptionally lossy, the atmospheric noise
will be much greater than the receiver noise.

The input to the receiver consists of the signal and atmospheric noise,
to which the receiver adds its own noise. As long as the atmospheric
noise is much greater than the receiver noise, you won't hear the
receiver noise. Any attenuation in the antenna system will attenuate
both the signal and the atmospheric noise equally, so the signal/noise
ratio, which determines what you can hear, doesn't change. Any gain
ahead of or within the receiver has the same effect.

Atmospheric noise declines as the frequency increases, so it might be
possible to start hearing some receiver noise over it as you approach 30
MHz, particularly if your receiver is unusually noisy and/or the antenna
system unusually lossy. As soon as receiver noise becomes audible over
the atmospheric noise, the rules change. Then, attenuation ahead of the
receiver will reduce the signal but not the receiver noise -- which is
now the "noise" part of the signal/noise ratio --, so it *will* decrease
the signal/noise ratio.

There's a very simple test to determine whether reducing the loss will
improve your ability to hear signals. Tune your receiver to a spot with
no signals in the frequency band of interest. Turn up the volume so you
can clearly hear the background noise. Then disconnect your antenna(*).
If the noise decreases, it means that atmospheric noise is dominating,
so reducing loss won't help the signal/noise ratio. If the noise doesn't
decrease, you're hearing receiver noise with the antenna connected and
you'd benefit by reducing losses ahead of the receiver. You'll probably
find that the noise will decrease by quite a few dB when you disconnect
the antenna, and this represents the amount of loss you can add before
your ability to hear signals suffers.

When receiving HF, about the only way you can improve your signal/noise
ratio is by using a directional antenna. This will reduce the
atmospheric noise coming from unneeded directions. Particular kinds of
antennas can also improve the signal/noise ratio if the dominant noise
is man made and coming from a nearby source.

(*) In a really marginal case, you might need to replace the antenna
with a dummy load for this test -- an ordinary small 47 or 51 ohm
resistor will suffice -- but it usually isn't necessary.

Roy Lewallen, W7EL

Robert11 wrote:
Hi,

Just getting started with all of this, and want to say a quick thanks to
everyone for all the help.

Another question:

Am I interpreting this more or less correctly:

Looking at all the different types of coax available.
Will be for a receive-only HF antenna.
Antenna will have to be in the backyard about 150 feet or so from house.

The db losses are beginning to add up; at the upper limit of my interest of
30 MHz
we are getting close to around 3.5 db or so.

Is it correct for me to say that the actual losses really aren't all that
significant or meaningful,
and that a good receiver, which I have, can easily make up for them ?

That the only thing of real concern would be the S/N ?

What are the caveats to my statement above ?

Thanks,
Bob

Bob,
If you are comfortable with Lew's explanation, this shouldn't confuse
things, but one minor additional concept.
Regarding your comment:
"Is it correct for me to say that the actual losses really aren't all
that
significant or meaningful, aqnd (sic) that a good receiver, which I
have,
can easily make up for them ?"

While on HF atmospheric noise usually dominates, loss before the
receiver can not always be "made up" by a receiver. If we go up to an
otherwise dead frequency, say, up around 20 or 30 MHz where/when there is no
significant atmospheric noise, we have a slightly different situation.
S/N is still the thing to consider, but the noise of the receiver front
end may now be the dominant noise. Now, consider that the loss of the
feedline causes a reduction of the signal, but, of course, has no affect on
the receiver noise. This feed line loss lowers the signal and degrades the
S/N ratio. The only way to get better is to either reduce the feedline loss
or reduce the receiver noise (noise figure).

73, Steve, K9DCI

"Roy Lewallen" wrote in message
...
You have the right idea. Signal/noise ratio is all that counts.

When receiving HF, feedline loss almost never matters. The reason is
that atmospheric noise is strong, and unless a receiver is exceptionally
noisy and/or an antenna is exceptionally lossy, the atmospheric noise
will be much greater than the receiver noise.

The input to the receiver consists of the signal and atmospheric noise,
to which the receiver adds its own noise. As long as the atmospheric
noise is much greater than the receiver noise, you won't hear the
receiver noise. Any attenuation in the antenna system will attenuate
both the signal and the atmospheric noise equally, so the signal/noise
ratio, which determines what you can hear, doesn't change. Any gain
ahead of or within the receiver has the same effect.

Atmospheric noise declines as the frequency increases, so it might be
possible to start hearing some receiver noise over it as you approach 30
MHz, particularly if your receiver is unusually noisy and/or the antenna
system unusually lossy. As soon as receiver noise becomes audible over
the atmospheric noise, the rules change. Then, attenuation ahead of the
receiver will reduce the signal but not the receiver noise -- which is
now the "noise" part of the signal/noise ratio --, so it *will* decrease
the signal/noise ratio.

There's a very simple test to determine whether reducing the loss will
improve your ability to hear signals. Tune your receiver to a spot with
no signals in the frequency band of interest. Turn up the volume so you
can clearly hear the background noise. Then disconnect your antenna(*).
If the noise decreases, it means that atmospheric noise is dominating,
so reducing loss won't help the signal/noise ratio. If the noise doesn't
decrease, you're hearing receiver noise with the antenna connected and
you'd benefit by reducing losses ahead of the receiver. You'll probably
find that the noise will decrease by quite a few dB when you disconnect
the antenna, and this represents the amount of loss you can add before
your ability to hear signals suffers.

When receiving HF, about the only way you can improve your signal/noise
ratio is by using a directional antenna. This will reduce the
atmospheric noise coming from unneeded directions. Particular kinds of
antennas can also improve the signal/noise ratio if the dominant noise
is man made and coming from a nearby source.

(*) In a really marginal case, you might need to replace the antenna
with a dummy load for this test -- an ordinary small 47 or 51 ohm
resistor will suffice -- but it usually isn't necessary.

Roy Lewallen, W7EL

Robert11 wrote:
Hi,

Just getting started with all of this, and want to say a quick thanks to
everyone for all the help.

Another question:

Am I interpreting this more or less correctly:

Looking at all the different types of coax available.
Will be for a receive-only HF antenna.
Antenna will have to be in the backyard about 150 feet or so from house.

The db losses are beginning to add up; at the upper limit of my interest
of
30 MHz
we are getting close to around 3.5 db or so.

Is it correct for me to say that the actual losses really aren't all
that
significant or meaningful,
and that a good receiver, which I have, can easily make up for them ?

That the only thing of real concern would be the S/N ?

What are the caveats to my statement above ?

Thanks,
Bob