Coax length - important ?
 

I recently moved my shack from an upstairs room to downstairs, much closer
to the garden and antennas. All the antennas previously had a long run of
coax to the old shack. The obvious thing was to have a nice new short run
of coax to my HF vertical which is now only 15' away. So I cut the coax and
since then the antenna is no longer resonant on 40 Metres. Also this week I
was putting up a new HF wire antenna, it was getting dark and raining by the
time I was hoisting it up but so I could just have a listen that night a
grabbed an old (15 years) large coiled up of quantity RG213 coax complete
with rotten oxydized pl259 plugs on each end which had just sat on the
garage wall for years. I just slung the coil down and plugged in at each
end. The plugs looked so rotten it was shameful but it pitch dark by then!
However The SWR was pretty good across the band. Regardless I started my
evening doing a tidy job with some of that nice 5DFB japanese coax all ready
for the following day. Guess what ? I put on the nice new cable and plugs
and the antenna is no longer anywhere near resonant on 80M. So why am I
getting a better result with a long length of still coiled cable sitting on
my patio rather that a much shorter brand new piece. Please could some one
explain to me if the coax length does matter, it has certainly never been a
problem for me in the past on VHF and Six but I am new to HF frequencies.
If you do need to have a certain size run, what can you do with the cable if
you phisically dont need it ?


Many Thanks & 73 for 2004

Keven G7UUD

So why am I
getting a better result with a long length of still coiled cable sitting on
my patio rather that a much shorter brand new piece. Please could some one
explain to me if the coax length does matter, it has certainly never been a
problem for me in the past on VHF and Six but I am new to HF frequencies.
If you do need to have a certain size run, what can you do with the cable if
you phisically dont need it ?


Many Thanks & 73 for 2004

Keven G7UUD

Hi Keven, how long is your antenna? Is it the right length for 40 meters?? What
is the impedance of the coax? Moving the antenna may be enough to change it's
resonance as it may be interacting with it's surroundings...Sometimes a 1/4 wl
75ohm (x velocity factor) length of coax will do the trick...good luck..

Steve
kb8viv

On Sat, 3 Jan 2004 01:35:51 -0000, "Keven Matthews"
wrote:

I recently moved my shack from an upstairs room to downstairs, much closer
to the garden and antennas. All the antennas previously had a long run of
coax to the old shack. The obvious thing was to have a nice new short run
of coax to my HF vertical which is now only 15' away. So I cut the coax and
since then the antenna is no longer resonant on 40 Metres. Also this week I
was putting up a new HF wire antenna, it was getting dark and raining by the
time I was hoisting it up but so I could just have a listen that night a
grabbed an old (15 years) large coiled up of quantity RG213 coax complete
with rotten oxydized pl259 plugs on each end which had just sat on the
garage wall for years. I just slung the coil down and plugged in at each
end. The plugs looked so rotten it was shameful but it pitch dark by then!
However The SWR was pretty good across the band. Regardless I started my
evening doing a tidy job with some of that nice 5DFB japanese coax all ready
for the following day. Guess what ? I put on the nice new cable and plugs
and the antenna is no longer anywhere near resonant on 80M. So why am I
getting a better result with a long length of still coiled cable sitting on
my patio rather that a much shorter brand new piece. Please could some one
explain to me if the coax length does matter, it has certainly never been a
problem for me in the past on VHF and Six but I am new to HF frequencies.
If you do need to have a certain size run, what can you do with the cable if
you phisically dont need it ?


Many Thanks & 73 for 2004

Keven G7UUD


Hi Keven,

It sounds like the coiled excess of the first attempts were serving as
chokes for your antenna. For one, at 15 feet away, that is very close
and certainly puts you in the fields such that you become part of
either the ground, or its loss. As soon as you cut away that excess,
you probably now have (more or less) a straight run. Hence no choking
action and the antenna sees you more clearly now back down the
exterior of the transmission line (classic common mode issues are
revealed by change in SWR attending transmission line length changes).

Try replacing some of that lost length (probably irretrievable now) so
that you can at least build a choke of half a dozen 6 to 8 inch
diameter turns at the feedpoint. OR Add a 1:1 Current Balun at the
feedpoint. I presume you have at least some rudimentary form of
ground (half a dozen radials) to help even out the picture. This last
will stabilize any tune-ups you may need to perform; but once there
should be robust.

73's
Richard Clark, KB7QHC

Keven Matthews wrote:
So why am I
getting a better result with a long length of still coiled cable sitting on
my patio rather that a much shorter brand new piece.

Losses in coax tend to drive the SWR toward 1:1. At 440 MHz, the SWR
on a 200 foot unterminated piece of RG-58 is close to 1:1. If the
SWR goes up when you shorten or upgrade your coax, that's good news.
The bad news is your antenna system needs some tuning. Walter Maxwell
has a chapter on such in "Reflections", titled: "Low SWR for the
Wrong Reasons". One can always accomplish a low SWR with a dummy load.
--
73, Cecil http://www.qsl.net/w5dxp



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Look, Keven,

At 15', you are in the near field of any HF antenna. You do not want your
station to be in the near field of the antenna. All kinds of undesirable
and often unpredictable things happen.
--
Crazy George
Remove N O and S P A M imbedded in return address

Your long piece of crappy old oxidized coax is excessivly (compared to
your shorter fresh stuff) attenuating the signal towards your antenna,
resulting in less power available at the antenna to be reflected back
towards your VSWR meter, & the power that is reflected is being
attenuated on the way back as well, making your VSWR look even better.

If you bend a section of coax & it makes a crackly noise it's only
good for landfill. Doesn't have to be old used stuff either, I've seen
it bad right from the distributor.

Wanta have some fun someday? Measure your forward & reflected @ your
transmitter, then measure it @ the antenna end of the line. Very
Educational.

Howard.


On Sat, 3 Jan 2004 01:35:51 -0000, "Keven Matthews"
wrote:

I recently moved my shack from an upstairs room to downstairs, much closer
to the garden and antennas. All the antennas previously had a long run of
coax to the old shack. The obvious thing was to have a nice new short run
of coax to my HF vertical which is now only 15' away. So I cut the coax and
since then the antenna is no longer resonant on 40 Metres. Also this week I
was putting up a new HF wire antenna, it was getting dark and raining by the
time I was hoisting it up but so I could just have a listen that night a
grabbed an old (15 years) large coiled up of quantity RG213 coax complete
with rotten oxydized pl259 plugs on each end which had just sat on the
garage wall for years. I just slung the coil down and plugged in at each
end. The plugs looked so rotten it was shameful but it pitch dark by then!
However The SWR was pretty good across the band. Regardless I started my
evening doing a tidy job with some of that nice 5DFB japanese coax all ready
for the following day. Guess what ? I put on the nice new cable and plugs
and the antenna is no longer anywhere near resonant on 80M. So why am I
getting a better result with a long length of still coiled cable sitting on
my patio rather that a much shorter brand new piece. Please could some one
explain to me if the coax length does matter, it has certainly never been a
problem for me in the past on VHF and Six but I am new to HF frequencies.
If you do need to have a certain size run, what can you do with the cable if
you phisically dont need it ?


Many Thanks & 73 for 2004

Keven G7UUD

Hi Kevin,

If you have to cut bits off the coax to get the SWR down, your aerial isn't
resonant on the frequency being used. The coax is acting like part of the
aerial.
Get the aerial resonant and it doesn't matter what length of coax you use as
the SWR will not change.

"Keven Matthews" wrote in message
...
I recently moved my shack from an upstairs room to downstairs, much
closer
to the garden and antennas. All the antennas previously had a long run of
coax to the old shack. The obvious thing was to have a nice new short
run
of coax to my HF vertical which is now only 15' away. So I cut the coax
and
since then the antenna is no longer resonant on 40 Metres. Also this week
I
was putting up a new HF wire antenna, it was getting dark and raining by
the
time I was hoisting it up but so I could just have a listen that night a
grabbed an old (15 years) large coiled up of quantity RG213 coax
complete
with rotten oxydized pl259 plugs on each end which had just sat on the
garage wall for years. I just slung the coil down and plugged in at each
end. The plugs looked so rotten it was shameful but it pitch dark by
then!
However The SWR was pretty good across the band. Regardless I started my
evening doing a tidy job with some of that nice 5DFB japanese coax all
ready
for the following day. Guess what ? I put on the nice new cable and
plugs
and the antenna is no longer anywhere near resonant on 80M. So why am I
getting a better result with a long length of still coiled cable sitting
on
my patio rather that a much shorter brand new piece. Please could some
one
explain to me if the coax length does matter, it has certainly never been
a
problem for me in the past on VHF and Six but I am new to HF frequencies.
If you do need to have a certain size run, what can you do with the cable
if
you phisically dont need it ?


Many Thanks & 73 for 2004

Keven G7UUD

Kevin, the resonance of the antenna is determined by the length of the
antenna! It is not related to the VSWR!!!

If you are using VSWR as an indicator of resonance it is meaningless!

Since your VSWR is changing with length of Coax, I suspect you have RF
on the braid of the coax. Install a series of Chokes or ferrites.

W1MCE

Keven Matthews wrote:
SNIP
The SWR was pretty good across the band. Regardless I started my
evening doing a tidy job with some of that nice 5DFB japanese coax all ready
for the following day. Guess what ? I put on the nice new cable and plugs
and the antenna is no longer anywhere near resonant on 80M. So why am I
getting a better result with a long length of still coiled cable sitting on
my patio rather that a much shorter brand new piece.

SNIP

Cecil sed,

"Walter Maxwell has a chapter on such in "Reflections", titled: "Low SWR for
the
Wrong Reasons".

=========================
Should be required reading for all hams.

73 de Jack, K9CUN

Arrrg! Your making my stomach hurt reading this misinformation...

(They told me all I had to do to become a ham was to memorize these
questions.)


"Mike" wrote in message
...
Hi Kevin,

If you have to cut bits off the coax to get the SWR down, your aerial
isn't
resonant on the frequency being used. The coax is acting like part of the
aerial.
Get the aerial resonant and it doesn't matter what length of coax you use
as
the SWR will not change.