RadioBanter - impedance mismatch?

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impedance mismatch?

Since I have replaced my feed line with grounded coax I have noticed that
from 30 meters on up is almost completely free of noise. However 120
through 41 meters is still pretty noisy.Could this be due to the impedance
mismatch? I am hoping this will be reconciled when I implement the matching
transformer between antenna and coax. I'm not sure if I used that in the
correct context but I guess you get my meaning.

Brian wrote:

Since I have replaced my feed line with grounded coax I have noticed that
from 30 meters on up is almost completely free of noise. However 120
through 41 meters is still pretty noisy.Could this be due to the impedance
mismatch? I am hoping this will be reconciled when I implement the matching
transformer between antenna and coax. I'm not sure if I used that in the
correct context but I guess you get my meaning.

The transformer (unun) should help to smooth out the peaks in the
antenna system making it more broadband but it's also likely the noise
levels in your area are higher for the low end of the HF spectrum. This
is typically the case for many listening environments.

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Thanks for the reply starman. If you don't mind my asking, what are you
using for an antenna and what is your qth? I'll post my results after adding
the transformer.

Brian
"starman" wrote in message
...
Brian wrote:

Since I have replaced my feed line with grounded coax I have noticed
that
from 30 meters on up is almost completely free of noise. However 120
through 41 meters is still pretty noisy.Could this be due to the
impedance
mismatch? I am hoping this will be reconciled when I implement the
matching
transformer between antenna and coax. I'm not sure if I used that in the
correct context but I guess you get my meaning.

The transformer (unun) should help to smooth out the peaks in the
antenna system making it more broadband but it's also likely the noise
levels in your area are higher for the low end of the HF spectrum. This
is typically the case for many listening environments.

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Brian wrote:

Thanks for the reply starman. If you don't mind my asking, what are you
using for an antenna and what is your qth? I'll post my results after adding
the transformer.

I'm using the so called low noise Doty-L, the plans for which can be
found at the URL below. This antenna system greatly reduced the noise
from appliances in my home environment. As I'm writing now, I have the
R8B on and I'm listening to a station with moderate signal strength but
no noise from the computer. This is also true when the television is on
in the same room. I could not do this when my inverted-L antenna did not
have a properly grounded/decoupled coax feedline system.
My QTH is far upstate NY, not just a few miles north of NYC, as those
urban dwellers like to call 'upstate'. Around here we see any location
south of Albany as 'downstate'. My receiving environment is relatively
quiet compared to more urban locations. I occasionally have to track
down a noisy street light or a bad pole insulator and report it to the
utility company. They've been pretty good with changing them. Here's the
antenna URL:

http://www.anarc.org/naswa/badx/ante...e_antenna.html

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On Sun, 18 Jan 2004 20:28:16 GMT, "Brian"
wrote:

Since I have replaced my feed line with grounded coax I have noticed that
from 30 meters on up is almost completely free of noise. However 120
through 41 meters is still pretty noisy.Could this be due to the impedance
mismatch? I am hoping this will be reconciled when I implement the matching
transformer between antenna and coax. I'm not sure if I used that in the
correct context but I guess you get my meaning.

Not likely. As a practical matter, man made noise is a lot more
trouble on the longer wavelengths. Machinery just tends to produce it
there. The corollary is that in generally the higher frequency, the
more of a problem a good RF ground is. What is a good ground at 60Hz
can be terrible at 1Mzh, and what is good at 1Mhz isn't necessarily
good at 100Mhz.

For example suppose the lumped constant equivalent of your ground lead
is 1 µ H . at 1 Mhz that is 3.77 ohms, so relative to a 75 ohm feed,
that is a pretty good ground. At 100Mhz however it will be 377 ohms,
and relative to a 75 ohm feed, that isn't a ground at all!!! In
addition, the skin depth changes as the square root of frequency, so
at higher frequencies, the surface condition of the conductor becomes
a larger and large issue, further complicating grounding issues.

"Brian" wrote in message link.net...
Since I have replaced my feed line with grounded coax I have noticed that
from 30 meters on up is almost completely free of noise.

I hope the desired signals are still there...Sometimes "no noise" is
not good.
However 120
through 41 meters is still pretty noisy.Could this be due to the impedance
mismatch?

No. If you had a really severe mismatch, you wouldn't hear as much
noise, as the whole noise/signal level would drop.
I am hoping this will be reconciled when I implement the matching
transformer between antenna and coax.

Shouldn't change much. It's normal for those bands to be noisy. I
would be much more worried if they weren't. MK

Brian,

It is common During the DAY for all bands to be Noisier and
especially the lower frequency bands.

READ - Shortwave 101 "How To Listen to World Radio"
http://www.etoncorp.com/listeninggui...gGuide_new.pdf
{ TIPs on Listening to the Shortwave Bands }

Generally the Better Listening Times and Frequencies a

* Day Time above 13 MHz
= = = 31M, 25M, 22M, 19M, 16M, 13M, and 11M

* Night Time below 13 MHz
= = = 120M, 90M, 75M, 60M, 49M, 41M, and 31M

STOP - Before you 'add' a Matching Transformer to your Antenna
and Ground System.

FIRST - Get paper and pencil and record your current reception
for each of the bands.
(Note-Book and Pen - This is Now your "ANTENNAS" Journal-Ledger.)

NOTE: If your are "Organized" you will have a Separate Sheet
of Paper for Each Band. Each Sheet will be divided into two
Columns "Before" and "After".

WHAT TO DO: Select from three to five strong International
Broadcast Stations on Each Band: 120M, 90M, 75M, 60M, 49M,
41M, 31M, 25M, 22M, 19M, 16M, 13M, and 11M.

* Record your S-Meter readings for these Stations and Time of
Day/Night.

* De-Tune by 5-15 kHz to a 'dead-spot' on the band and also record
your Background Noise Level for the Band near each of these Stations.

SECOND - Now 'install' your Matching Transformer between your
Antenna Element and the Coax Cable Lead-in-Line and Ground everything
properly. THEN - 'Repeat' the "What To Do" Process again.

THIRD - Compare your Before and After performance and results
for the Matching Transformer.

TBL: Now You Know and have a Record of the Facts to refer to in
the future when problems occur or when your want to make more
changes for 'improved' Antenna Performance and 'better' Reception.

iane ~ RHF
..
..
= = = "Brian"
= = = wrote in message link.net...

Since I have replaced my feed line with grounded coax I have
noticed that from 30 meters on up is almost completely free of
noise. However 120 through 41 meters is still pretty noisy.

Could this be due to the impedance mismatch?

I am hoping this will be reconciled when I implement the matching
transformer between antenna and coax.

I'm not sure if I used that in the correct context but I guess
you get my meaning.

..