RadioBanter - View Single Post

July 13th 03, 07:49 PM

In article ,
(Mark Keith) wrote:

Telamon wrote in message

..
Let me explain that I live in town and have
local noise to compete with any signal I pick up. This noise must be
overcome so I only hear the program material of interest. In other words
the volume can be turned up so the program material is very loud without
any background noise or hiss. Antenna efficiency that generates more
signal energy overcomes the local noise sources. You must be unusually
lucky to live in a location where all you pick up is either broadcast
signal or atmospheric noise. I don¹t think most people are as fortunate.

I assume your noise must be shack generated, and is an ingress
problem. I would think anyway. If the noise was local, but picked up
from the antenna itself
along with the desired station, then adding the transformer would not
change the s/n ratio. The noise would increase along with the station
at an equal rate. Everything would "sound" the same. Only the S meter
would read higher.

If you have a noise ingress problem, feedline decoupling is the
answer, not a better impedance match. Also,feedline decoupling, and
impedance matching, or SWR, are totally unrelated. You can have great
decoupling with an 80 to 1 mismatch. Or you can have a perfect 1:1
match with horrible decoupling. They are totally unrelated. I'm not
lucky. I live in the city of Houston amid all kinds of noise
generating crap. But due to decent feedline decoupling, any noise I
hear is picked up from the antenna. And any attempts to achieve a
better match do not increase my s/n ratio, being as I always have
enough signal level to begin with even with no matching.

Most antennas output impedance is nowhere near the typical 50-ohm coax
and a transformation can remedy that.

But it doesn't matter. You don't have enough loss with the mismatch to
worry about with any decent radio. It's just not enough to knock you
out of the water. I did the math on this a few months ago, and posted
here to demonstrate this. This has been debated before many times. I
used coax feed with wild feedpoint impedances just to ensure a worst
case as far as feeder loss. It doesn't amount to enough to hurt you.
If it does, you have a lame radio. If you used a random wire direct
with no feeder, there is even less loss. For receiving, the mismatch
in that case doesn't matter enough to worry about at all.

Well OK I guess my radios are lame or busted. I must be imagining things
when signals go from ³I can just make it out S1² to ³easy to listen to
S3² on the folded dipole with the transformer. My other loop antennas
must not be working right either.

Is the S1 with the folded dipole fed directly without the transformer,
or another antenna? It sounds like you have or had a noise ingress
problem if the noise does not increase at the same level as the signal
when the transformer is added.
If this is the case, again, this would not be a function of impedance
matching, but a function of better feedline decoupling. The decoupling
is improving the s/n ratio, not the impedance transformation. If the
signal was S1, it should have been solid copy, if it is at S3. If it
wasn't, the overriding noise was not picked up by the antenna. It was
picked up on the outer shield of the coax down in the shack, piped up
to the feedpoint, and then piped back down to the radio on the inner
part of the outer shield. "I assume you used coax"..S1 is plenty of
signal level for solid copy if no shack noise is drowning it out.
What's the problem with the loop? Lots of noise also? MK

The antenna is a folded dipole cut for 13 meters connected to the radio
with coax.

I evaluated two stations on this band. One had locally generated noise
interference and the other did not.

I tried a repeat today with switching the matching transformer in and
out of the circuit and compared it to a large ferrite toroid in its
place. The coax made one turn through the toroid. The ferrite worked as
well as the transformer on the station with the local noise on it. No
difference found on the station in the clear. In addition the
transformer did not make a difference in the S meter reading either.

It takes me several minutes to change the transformer in or out and we
had a minor geomagnetic storm yesterday so conditions changing must have
been what I saw as a performance difference.

Today conditions are more stable and I switched the transformer and / or
toroid choke in and out several times averaging the results.

So it looks like the only benefit of the transformer was isolation it
provided on the folded dipole.

--
Telamon
Ventura, California