Keven Matthews wrote:

I recently moved my shack from an upstairs room to downstairs, much closer
to the garden and antennas.

SNIPPED

Kevin, there have been numerous responses to your original post. Let me
be an Elmer for a short moment.

An example of antenna resonance and VSWR follows.

In my mobile I have a resonant 40 meter Hamstick. Resonance means there
is NO Reactance in the antenna impedance. My antenna analyzer indicates
approximately 12 + j0 ohms at 7.225 MHz. This is almost a 5:1 VSWR and
that's what a meter indicates. Now, I added an ICOM AH-4 automatic
antenna tuner at the antenna. The antenna is still 12 + j0 ohms but the
tuner transforms the impedance to 50 + j0 ohms. So, the VSWR from the
antenna/tuner to the 706, approximately 16 feet of coax, is now
approximately 1:1.

Since the length of coax in your installation changes the measured VSWR,
the coax is part of the antenna system and is radiating. So, you need to
isolate the coax from the antenna. There are several ways to accomplish
this. The most direct way is to make a coil of coax about 4 to 6 inches
diameter and having 8 to 10 turns and install it directly at the base of
the antenna. If you have a ground radial system make sure the coax is
underneath [lower] than the radial system. Finally, install some clamp
on ferrites, available from Radio Shack for less than $10, at the 1/4
and 1/2 wavelength on the coax from the antenna feedpoint.

Hopefully this will clean up the RF on the coax.

With a vertical antenna a reasonable VSWR at antenna resonance should be
somewhere between 1.5:1 and 2.0:1.

Deacon Dave, W1MCE

Thanks to all the many respondents to my question! The good news is, that
following the general concensus that the coax length does not matter I
pursued the shorter length again today, resited the wire antenna (G30JV
80plus2) to another location, and the analyer I have now indicates a
distinct improvement. What was throwing me was some stuff I read somewhere
about using odd halfwave length multiples for coax runs - but perhaps this
was another myth!

Thanks Guys

Keven

I would add only one thing to this. The thing used to measure the SWR can
also be responsible for strange results when the SWR is not low. Don't
always assume that the thing used to measure something is exact all the
time.
Steve
K:9:C:I

"Dave Shrader" wrote in message
news:biGJb.213249$8y1.750188@attbi_s52...
Keven Matthews wrote:

I recently moved my shack from an upstairs room to downstairs, much
closer
to the garden and antennas.

SNIPPED

Kevin, there have been numerous responses to your original post. Let me
be an Elmer for a short moment.

An example of antenna resonance and VSWR follows.

In my mobile I have a resonant 40 meter Hamstick. Resonance means there
is NO Reactance in the antenna impedance. My antenna analyzer indicates
approximately 12 + j0 ohms at 7.225 MHz. This is almost a 5:1 VSWR and
that's what a meter indicates. Now, I added an ICOM AH-4 automatic
antenna tuner at the antenna. The antenna is still 12 + j0 ohms but the
tuner transforms the impedance to 50 + j0 ohms. So, the VSWR from the
antenna/tuner to the 706, approximately 16 feet of coax, is now
approximately 1:1.

Since the length of coax in your installation changes the measured VSWR,
the coax is part of the antenna system and is radiating. So, you need to
isolate the coax from the antenna. There are several ways to accomplish
this. The most direct way is to make a coil of coax about 4 to 6 inches
diameter and having 8 to 10 turns and install it directly at the base of
the antenna. If you have a ground radial system make sure the coax is
underneath [lower] than the radial system. Finally, install some clamp
on ferrites, available from Radio Shack for less than $10, at the 1/4
and 1/2 wavelength on the coax from the antenna feedpoint.

Hopefully this will clean up the RF on the coax.

With a vertical antenna a reasonable VSWR at antenna resonance should be
somewhere between 1.5:1 and 2.0:1.

Deacon Dave, W1MCE

For my own take on it I would say

1) You are in the near field of the antenna

2) The fact that the coax is, by own admission "bad"
would indicate there is sumpin' is up as old ratty coax
*can* actually appear to be good by giving a false SWR
indication.

I think this is where CBers get that old "18 feet of coax" thing so many
believe firmly. The thing to do is to use the
shortest run of coax you can, set the antenna by adding or
removing length and keep the thing out of the way of nearby objects (people,
cars, towers, fuel tanks. If you
re-resonate the antenna and re-attach the shorter line, it shouldn't make
any difference.


Jerry
K4KWH
"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

"Keven Matthews" a écrit dans le message de news:
...
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


Hello and happy new year.

It would always be necessary to tune the coaxial feeder
to the frequency of work.


For your problem it is necessary :

To measure the ros in one 1/2 length of cable feeder.
To tune the coaxial fededer with the frequency of work
on multiple of one 1/2 wavelength.
[Certain vertical antenna (as 5/8 lambda) have necessarily
to have 1 wavelength of cable feeder at least].
To control the ros and connect your installation.

Although it is it, value read in one 1/2 longuer of wave is
the same that that read to all the multiple of 1/2 lamda (3 or 10.5 lamda
for example).

Good luck


GG, amateur of radio for 19 years