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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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

Low SWR for
the
Wrong Reasons".

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

Why? ;-)
73 Gary N4AST

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

Crazy George sed:

"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."
---------------------------------

Kind of rules out mobile operation.

Of course the metal body of the vehicle probably shields the station from the
effects of the near field.

73 de Jack K9CUN

I would venture to say we all operate in the near field. How does one avoid
it, particularly on 160 meters?

"JDer8745" wrote in message
...
Crazy George sed:

"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."
---------------------------------

Kind of rules out mobile operation.

Of course the metal body of the vehicle probably shields the station from
the
effects of the near field.

73 de Jack K9CUN

On Tue, 6 Jan 2004 13:14:01 -0500, "w4jle" W4JLE(remove this to
wrote:
I would venture to say we all operate in the near field. How does one avoid
it, particularly on 160 meters?

Hi OM,

The greater part of risk is in the wavelength compared to body height.
Sitting down obviously lowers risk.

Now, for the standing individual of average size, that person is
approaching a quarter wave at 10M (especially if you are a fat
conductor). If you were the standard 1 wavelength away from a 100W
transmission, then the standard 22dB down would be your exposure and
you would experience something less of 1 watt of heating throughout
your body. Touch a christmas tree bulb (7.5W) and ask yourself how
uncomfortable that feels, then average that over your 2M² surface
area.

At 160M, you certainly stand the risk of being much closer than 1
wavelength, but you also stand less risk of being a quarter wave tall
(towering egos do not conduct). In any event, you are probably
sitting down anyway. Your radiation resistance in that band makes you
nearly invisible to the power emitted. Those standing next to VOA
half megawatt towers need to check their insurance clauses covering
acts of incipient stupidity.

73's
Richard Clark, KB7QHC