On 13 Oct 2006 05:34:13 -0700, "
wrote:

Dan, locating the tuner at the feedpoint is certainly a method of
reducing feedline loss. You will need very long arms or a remote
automatic tuner. I have been surprised by how slow the market has been
to match / excite the potential demand for a remote auto-tuner with
balanced output (I think SGC does one, but I have no idea of how it
performs).

Otherwise, the feedline is critically important in antenna system
performance, and you can only determine that by analysing the entire
system as a system.

....

rhetorically. A lot of folks will say their antenna works even if their
antenna is a 40 foot dipole fed with 50 feet of RG-58 on 40m.

They can hear people with it, their rig's autotuner loads it up just
fine on 40m, and they can "make contacts" but their effective radiated
power is going to be 1.6 watts... and feedline loss is an
environmentally-unfriendly way to be a QRP operator.

Yes, but note that 20dB of feedline loss on 40m probably does not
degrade the receive signal to noise ratio because even when external
noise is decreased by 20dB, it still swamps the receivers internal
noise, and signal to noise ratio is dominated by the (external) signal
and external noise (both attenuated).

For example, if you look at Fig 2 at
http://www.vk1od.net/fsm/FSAmbientNoise.htm you will see that expected
ambient noise power in 2kHz bandwidth on 7MHz is somewhere in the
range -87dBm to -108dBm, with somewhere above -103dBm (~S4) being
typical in a good location. Attenuating noise at -103dBm by 20dB gives
-123dBm, still way above the noise floor of modern receivers at about
-135dBm. So from a rx perspective, it "works", demonstrated by copying
other stations ok, just the S-meter reading is low.

One of our new Foundation Licencees asked me a question through a site
feedback form some months age. It is relevant to this issue in a near
QRP context, FLs are limited to 10W PEP on SSB, 3W carrier on other
modes. Foundation Licencees qualify with about 6 hours of training,
mostly on operational practice and very little technical content. The
question and my response are below:

Question:

Why can't I get contacts on 80m. I don't have room for an 80m dipole,
so I am using my 40m dipole that works just great on 40m. Of course,
it would not be impedance matched on 80m, so I have used an ATU (which
I was taught matches the antenna and line to the transmitter) and can
easily get SWR of 1:1 (excellent match!). My antenna is fed with 25m
of RG58C/U as an expert told me that it has a loss of only 0.6dB and I
wouldn't notice that.

Answer:

It is true that the loss on 25m of RG58C/U on 80m under matched line
conditions is about 0.6dB and for most purposes that might be quite
acceptable. For reasons beyond your current understanding, your line
is not matched, and the loss on your coax is much more than 0.6dB
(about 15%), the losses are around 21dB (about 99.3%) and around 0.7%
of your 10W output is reaching the antenna feed point. The good news
is that the gain of your shortened antenna is still quite good, and
your ERP (you learned about ERP for your licence) is probably around
0.1W (EIRP) broadside to your shortened dipole.

If for instance you were trying to work another station that is using
a 100W transmitter and an antenna system efficiency of better than 75%
(realistic), their ERP is probably around 120W (EIRP). To an
independent observer who hears you both above the noise and has
equally good propagation from both of you, your signal is 5 S-units
lower than the other station. That disparity will make it very hard
for you to work all but the very strongest of stations because of the
ambient noise level, and even then they will be straining to hear you
in the noise.

Your antenna efficiency will not as seriously impact your receive
performance, because both the external noise and the desired signals
are attenuated by the same amount, and are still well above the
receiver's internal noise. The signal / noise ratio will be similar to
using an efficient antenna, just the S-meter reading will be lower.

When you run less power than the other stations that you want to work,
the performance of your antenna becomes even more important.

Owen
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