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.

w4jle wrote:
Arrrg! Your making my stomach hurt reading this misinformation...

Well, let's add a few words to make his statement correct.

Get the aerial resonant,

with a 50+j0 ohm feedpoint impedance,

and it doesn't matter what length of coax you use
as the SWR will not change.
--
73, Cecil http://www.qsl.net/w5dxp



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Now you are getting technical, beyond the understanding of most of today's
hams.

(I looked everywhere on my SWR's meter and couldn't find a J knob and isn't
a 50 to 0 SWR kinda high)

"Cecil Moore" wrote in message
...
w4jle wrote:
Arrrg! Your making my stomach hurt reading this misinformation...

Well, let's add a few words to make his statement correct.

Get the aerial resonant,

with a 50+j0 ohm feedpoint impedance,

and it doesn't matter what length of coax you use
as the SWR will not change.
--
73, Cecil http://www.qsl.net/w5dxp



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http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----

w4jle wrote:



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



SNIP That's absolutely TRUE if all you want is to "...become a ham..."

However, if you want to install and operate an efficient short wave
radio station you need to learn quite a bit!

DD, W1MCE

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

Well said Sir!

Even the "appliance operators" of old had to know how the appliance worked.

Were I the FCC, your first transmitter and receiver would be built from
scratch and a minimum of 25 CW contacts confirmed before the use of a
purchased rig would be allowed.

Next no one would be permitted to apply for extra class without 5 years
experience and 250 confirmed contacts.Also participation in a public
service capacity, (i.e. traffic net, weather warning net, charity races etc)
and have elemered 5 new hams.

I have a friend that has an extra and has never been on the air. He took the
exams at a hamfest just to see if he could pass them. He said he studied the
question pools for a couple of days on-line. He spent 20 years in the Navy
as a Chief Radioman, so the code was a joke for him.

"Dave Shrader" wrote in message
news:XnFJb.49405$I07.151812@attbi_s53...
w4jle wrote:



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



SNIP That's absolutely TRUE if all you want is to "...become a ham..."

However, if you want to install and operate an efficient short wave
radio station you need to learn quite a bit!

DD, W1MCE

"w4jle" W4JLE(remove this to wrote in message
...
Well said Sir!

Even the "appliance operators" of old had to know how the appliance
worked.

Were I the FCC, your first transmitter and receiver would be built from
scratch and a minimum of 25 CW contacts confirmed before the use of a
purchased rig would be allowed.

Careful, one of the local Ham "Channelmasters" will dub you a
CBPlusser for using the term "contact". It's a pet peeve of his.

Harold Burton
KD5SAK

Just like I had some "newby" attempt to chastise me on a 2 meter repeater
for using "handle". He told me that that word was only used on 11 meters.

I had to admit, I did use it on 11 meters when it was a ham band. But thanks
for the lesson Good Buddy!



"Harold Burton" wrote in message
...

"w4jle" W4JLE(remove this to wrote in message
...
Well said Sir!

Even the "appliance operators" of old had to know how the appliance
worked.

Were I the FCC, your first transmitter and receiver would be built from
scratch and a minimum of 25 CW contacts confirmed before the use of a
purchased rig would be allowed.

Careful, one of the local Ham "Channelmasters" will dub you a
CBPlusser for using the term "contact". It's a pet peeve of his.

Harold Burton
KD5SAK

Note that well-engineered broadcast stations do not accept ANY significant
affect of the length of their transmission line(s) on the SWR/impedance
match of their antenna system, as seen by the transmitter.

Greatest antenna system efficiency and least stress on the transmission
hardware are achieved when transmission line impedance is matched to antenna
input impedance. When that condition exists then the length of the
transmission line is unimportant except for the power lost due to
transmission line attenuation, and the cost to use the length and type of
transmission line selected.

The match of the antenna input to its input transmission line is a function
of their design, AND the installation environment. The physical environment
near the antenna can change its input impedance from the assumed value, and
create an impedance mismatch with a transmission line selected to match the
input impedance assumed for the antenna.

This is usually corrected by an impedance-matching network installed at the
junction of the antenna and its input transmission line. Virtually every
AM, FM and TV broadcast antenna in the US has some means of adjusting the
match between the antenna and its input line to optimise system SWR.

Impedance adjustments can be done at the input end of the transmission line
to the antenna, but will be more narrow-band. Trimming the length of the
main transmission line is another approach, but again, is more
frequency-sensitive than adjusting the match directly at the antenna input
connector.

RF

Visit http://rfry.org for FM broadcast RF system papers.

________________

"Dave Shrader" wrote in message
news:5bzJb.113315$VB2.290693@attbi_s51...
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

"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

Low SWR for
the
Wrong Reasons".

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

Why? ;-)
73 Gary N4AST

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

Harold Burton wrote:

SNIP

Careful, one of the local Ham "Channelmasters" will dub you a
CBPlusser for using the term "contact". It's a pet peeve of his.

Harold Burton
KD5SAK



It may be a 'pet peeve' but what do you call:

"T40E 599 TU de W1MCE"

For me it's a contact! It is not a QSO. It is not a Rag Chew. It's the
minimum requirement for a DXCC type CONTACT.

BTW: is it Harold Burton or HALLIBURTON?? Hmmm ... ???

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

Handle was used on the HF ham bands before most of the readers were born.

Steve

K;9;D;C;I

"w4jle" W4JLE(remove this to wrote in message
...
Just like I had some "newby" attempt to chastise me on a 2 meter repeater
for using "handle". He told me that that word was only used on 11 meters.

I had to admit, I did use it on 11 meters when it was a ham band. But
thanks
for the lesson Good Buddy!

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:D: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

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

Richard Clark wrote:
Now, for the standing individual of average size, that person is
approaching a quarter wave at 10M ...

Unless the individual is grounded at one end, that 1/4WL is
non-resonant. :-) Richard, what is your velocity factor?
--
73, Cecil http://www.qsl.net/w5dxp



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On Tue, 06 Jan 2004 14:28:06 -0600, Cecil Moore
wrote:
Unless the individual is grounded at one end, that 1/4WL is
non-resonant. :-) Richard, what is your velocity factor?

I can see why you ask about velocity if you are not on ground.

Richard Clark wrote:

On Tue, 06 Jan 2004 14:28:06 -0600, Cecil Moore
wrote:

Unless the individual is grounded at one end, that 1/4WL is
non-resonant. :-) Richard, what is your velocity factor?

I can see why you ask about velocity if you are not on ground.

"On ground" and "at ground potential" are very different things.
I wear thick rubber soles on my shoes forcing a current near-minimum
at each of my ends. And no, my feet are not big enough to cause a
microfarad of capacitance through an inch of rubber.
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil Moore wrote:

Richard Clark wrote:

Now, for the standing individual of average size, that person is
approaching a quarter wave at 10M ...


Unless the individual is grounded at one end, that 1/4WL is
non-resonant. :-) Richard, what is your velocity factor?

I don't know about Richard but my velocity factor seriously deteriorated
after age 60 :-)

OOOPPPSSS!! Did I say that :-) Deacon Dave

On Tue, 06 Jan 2004 14:38:16 -0600, Cecil Moore
wrote:
"On ground" and "at ground potential" are very different things.
I wear thick rubber soles on my shoes forcing a current near-minimum
at each of my ends. And no, my feet are not big enough to cause a
microfarad of capacitance through an inch of rubber.

Are you REALLY that concerned about less than 1W? Perhaps you should
wear Doc Martins.

I thought the same thing until I did some field strength measurements. (a
meter/diode/antenna field strength meter - I have an e-field probe now, but
haven't tried it yet)

Get this:
A 2M quarter wave on the roof has much more field strength than a 5/8 on
the trunk! I saw about 20 dB more. I was also surprised that the same
model car with a sun roof had LOWER inside (with the roof 1/4 wave) than
the one without the sun roof. Go figure.
Steve, K;9;D:C:I

"JDer8745" wrote in message
...
Crazy George sed:
"At 15', you are in the near field of any HF antenna. ...
---------------------------------
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 don't understand why you say this. In the near field the E to I ratio
just isn't at 377 ohms yet, so you can have either a higher E or H field.
Steve K;9;D;C;I

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

"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.


This would be true assuming the transmission line is truly 50
ohms. It almost never really is.

I've asked this question before, and i got similar answers. The
point about losses in long lines improving the SWR are true of course.

But my point is that even if the antenna is a true 50 + 0j, if the
coax is not a true 50 ohms, and is more like 55 or 45 ohms, then the
length of the
coax WILL matter, in terms of the measured SWR.


Slick

"Cecil Moore" wrote in message
...
...
...And no, my feet are not big enough to cause a
microfarad of capacitance through an inch of rubber.
-- 73, Cecil http://www.qsl.net/w5dxp


Ahhh! But what about the other end?

The devil made me say this.
Sorry Cecil, just couldn't resist.
Steve K:9:D:C:I

"Dr. Slick" wrote in message
om...
"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.


This would be true assuming the transmission line is truly 50
ohms. It almost never really is.
I've asked this question before, and i got similar answers. The
point about losses in long lines improving the SWR are true of course.
But my point is that even if the antenna is a true 50 + 0j, if the
coax is not a true 50 ohms, and is more like 55 or 45 ohms, then the
length of the
coax WILL matter, in terms of the measured SWR.
Slick

So if I get the set up:
We have a 50 ohm load (antenna) and a non-50 ohm line with a 50 ohm SWR
measuring device.

With this, the "LINE SWR" is still unchanged with line length, but the
impedance seen by the SWR meter will change with coax length. It will vary
around the Zo of the line per the Smith chart. So.... It will show a
varying SWR on this meter. The non-50 ohm like could be considered a
matching transformer, but it won't be matching what we want.

However, with a 55 or 45 ohm line this is all moot - in the noise as far
as being significant to worry about and won't mean anything in practice.


Steve K;9;d;c;i

Where is the transmission line on which the SWR is measured by the meter?

Reg wrote,

Where is the transmission line on which the SWR is measured by the meter?


Allow me to sell you some.