I need your advice, as I have never worked with open wire lines before.
I need to feed many different antennas with open wire line and need to run
the feeline from each about 200 feet from the back pasture to the shack. I
don't want to walk out 200 feet and throw knife switches to chose the
antenna/feedline I want to feed, so I plan to run separate 600 ohm open
feeds for each antenna all the way to the shack.
The problem is I don't know what the effect is or how to handle the
multiple open wire feed lines, as they will be parallel for probably 150-200
feet. I haven't found anything in literature describing this.
For instance, will they interact? how far do you space the feedlines from
one another? If I have 4 feedlines, can I stack them vertically or
horizontally one foot apart from each other? How much is enough separation?
What other precautions do I need? Need they be twisted if they are not near
anything other than the other feed lines?
Any help is appreciated.
73,
Greg, N6GK

On Mon, 1 Aug 2005 17:43:34 -0700, "greg knapp 5"
wrote:

If I have 4 feedlines, can I stack them vertically or
horizontally one foot apart from each other? How much is enough separation?
What other precautions do I need? Need they be twisted if they are not near
anything other than the other feed lines?
Any help is appreciated.

Hi Greg,

Feed lines that are terminated in balanced loads are self isolating.
The balance of the load is with respect to earth and is discussed
under the heading of Common Modality. So your problems are load
defined, not line defined. Naturally there is a continuum of linkage
between lines even in this best of all possible worlds. That
continuum spans from considerable-to-miniscule and it is, as you have
guessed, dependant upon spacing.

What constitutes considerable may be miniscule for another and I will
leave that determination up to you, as you have offered no limits nor
quantitative concerns. If you need a rule of thumb, keep all lines at
least 10 times their own pair-spacing apart. That is, if a twin lead
has 1 inch spacing between the conductors, keep that twin lead 10
inches from a similar feedline.

There are no doubt authoritative reports to offer a justification for
this rule, or any other offered. I would expect this would come from
Laport's work (which several correspondents here have copies of).

73's
Richard Clark, KB7QHC

Wow, then if I had 6" spacing, it would have to be 5 feet from the nearest
other feed line! With the three lines I want to bring in, that is a cross
pole of 10 feet! Wow...I better consider using my old 450 ohm window ladder
line with about 1.x inch spacing so I can get down to a reasonable spread!

This is really opening my eyes up to the logistics of running this type of
feedline...low loss, but definitely more expense, bother, etc. than running
a bunch of coax lines! It almost makes me want to put up resonant antennas
(like parallel dipoles, fed with a balun and coax...so much easier!). I'm
going to have to think this through some more.

Thanks for the input,

Greg, N6GK

"Richard Clark" wrote in message
...
On Mon, 1 Aug 2005 17:43:34 -0700, "greg knapp 5"
wrote:

If I have 4 feedlines, can I stack them vertically or
horizontally one foot apart from each other? How much is enough
separation?
What other precautions do I need? Need they be twisted if they are not
near
anything other than the other feed lines?
Any help is appreciated.

Hi Greg,

Feed lines that are terminated in balanced loads are self isolating.
The balance of the load is with respect to earth and is discussed
under the heading of Common Modality. So your problems are load
defined, not line defined. Naturally there is a continuum of linkage
between lines even in this best of all possible worlds. That
continuum spans from considerable-to-miniscule and it is, as you have
guessed, dependant upon spacing.

What constitutes considerable may be miniscule for another and I will
leave that determination up to you, as you have offered no limits nor
quantitative concerns. If you need a rule of thumb, keep all lines at
least 10 times their own pair-spacing apart. That is, if a twin lead
has 1 inch spacing between the conductors, keep that twin lead 10
inches from a similar feedline.

There are no doubt authoritative reports to offer a justification for
this rule, or any other offered. I would expect this would come from
Laport's work (which several correspondents here have copies of).

73's
Richard Clark, KB7QHC

On Tue, 2 Aug 2005 21:41:42 -0700, "greg knapp 5"
wrote:

Wow, then if I had 6" spacing, it would have to be 5 feet from the nearest
other feed line!

Hi Greg,

Not really, you haven't told us how important the coupling factor is
yet.

73's
Richard Clark, KB7QHC

I really can't answer how important the coupling factor is because I have
never dealt with this factor before.
I assume the only down side of too much coupling is that some small amount
of RF would then be coupled to the wrong antenna and radiated in some
pattern that is not necessarily desirable. So, as I understand it (I sure
could be wrong), if I was running a 1000 watts out, with coupling at -30 db,
1 watt would be coupled to the parallel line which would then be radiated by
the wrong antenna--which, if true, certainly doesn't seem significant. I am
not sure what is a significant level (i.e., significant distortion of net
radiation pattern) in dealing with two simultanteously feed antennas, but I
would think that if it is a ratio of the transmitted power, then 1000 vs 1
watt would seem to cause very minor distortion and would seem to be
acceptable. If my reasoning is OK, then I would think spacing of feedlines
would not need be too great, even if 5-inch spaced 600 ohm was used in place
of 1.x inch spaced 450 ohm line.
Any insight would be appreciated.
73,
Greg, N6GK

"Richard Clark" wrote in message
...
On Tue, 2 Aug 2005 21:41:42 -0700, "greg knapp 5"
wrote:

Wow, then if I had 6" spacing, it would have to be 5 feet from the nearest
other feed line!

Hi Greg,

Not really, you haven't told us how important the coupling factor is
yet.

73's
Richard Clark, KB7QHC

On Wed, 3 Aug 2005 09:18:38 -0700, "greg knapp 5"
wrote:

I really can't answer how important the coupling factor is because I have
never dealt with this factor before.

Hi Greg,

Frank has been carrying a lot of water for you in this regard. I
would point out (and he has stated as much) that his numbers are for a
load of 600 Ohms. Now, any mismatch will not have any impact on the
coupling, but the load's balance to earth will and Frank has done no
study of induced Common Mode currents. Given that there is a world of
possible imbalances possible, it is hard to know where to go from
there.

There is also a world of probable imbalances too. The antennas you
are feeding are far more likely the source of the problem of coupling
between lines than are the lines' proximity. However, proximity only
magnifies the problem. Still and all, even the worst imbalance is
unlikely to pose significant issues. It might be problematic for a
VOA feed of 500KW with leakage of their European service into their
African service, but you are not in that class (and they probably now
how to keep their antennas balanced too).

73's
Richard Clark, KB7QHC

On Tue, 2 Aug 2005 21:41:42 -0700, "greg knapp 5"
wrote:

Wow, then if I had 6" spacing, it would have to be 5 feet from the nearest
other feed line! With the three lines I want to bring in, that is a cross
pole of 10 feet! Wow...I better consider using my old 450 ohm window ladder
line with about 1.x inch spacing so I can get down to a reasonable spread!

This is really opening my eyes up to the logistics of running this type of
feedline...low loss, but definitely more expense, bother, etc. than running
a bunch of coax lines! It almost makes me want to put up resonant antennas
(like parallel dipoles, fed with a balun and coax...so much easier!). I'm
going to have to think this through some more.


I worked in HF radio stations briefly a long time ago, and the
practice there was, IIRC, two wire lines of 3mm HDC with 200mm spacing
so they were around 600 ohms 'ish. There were also some 4 wire lines
with similar spacing (in a box config). These lines were strung out on
pole routes with separation about double the wire spacing. I can't
remember now if there were periodic polarity reversals. These were at
rx only or tx only facilities, so receiver de-sense was not an issue.
Do you need full duplex?

Clearly, the closer the lines the higher crosstalk (coupling from one
line to another), and presumably, twisted lines will have lower
crosstalk (think about the cat5 etc data cables).

Don't forget, that the antennas at the far end are coupled, and
achieving crosstalk of -100dB might not materially improve the
solution!

A pole route carrying a number of lines is a pretty serious project
(installation and maintenance). I would also look around at remote
switching solutions or multiband antennas to see if you can minimise
the number of lines you need to string.


Owen
--

"greg knapp 5" wrote in message
...
I need your advice, as I have never worked with open wire lines before.
I need to feed many different antennas with open wire line and need to run
the feeline from each about 200 feet from the back pasture to the shack. I
don't want to walk out 200 feet and throw knife switches to chose the
antenna/feedline I want to feed, so I plan to run separate 600 ohm open
feeds for each antenna all the way to the shack.
The problem is I don't know what the effect is or how to handle the
multiple open wire feed lines, as they will be parallel for probably
150-200 feet. I haven't found anything in literature describing this.
For instance, will they interact? how far do you space the feedlines from
one another? If I have 4 feedlines, can I stack them vertically or
horizontally one foot apart from each other? How much is enough
separation? What other precautions do I need? Need they be twisted if
they are not near anything other than the other feed lines?
Any help is appreciated.
73,
Greg, N6GK

Hi Greg:

I have just run a free space NEC 2 model of a pair of horizontally
positioned 6" wide, 600 ohm, transmission lines, vertically separated by
12". The model was run from 29 to 31 MHz. On 30 MHz , with both lines
terminated in 600 ohms the isolation was about 39 dB. With both lines
terminated in 6,000 ohms the isolation improved to 66 dB. The actual range
of impedances will be much higher, and complex, but it does give an idea of
the order of magnitude of the isolation. The calculation is based on the
ratio of the peak currents observed.

Hope this helps,

73,

Frank (VE6CB)

Thanks Frank for running the program and for the results. They give me a
good confirmation of what Riachard Clark showed me, I need to get more space
between these feeds than I was planning on. If I had 3 feeds, I guess I
ought to have a cross arm on my wood/PVC poles coming in and spread the
feeds by about two feet to minimize the interaction down to a miniscule
amount. I really do want to feed the antennas separately. Gee, I wonder
what W6AM used to do with his 10 or 12 rhombics fed with open wire all
coming into his home...

73,

Greg, N6GK


"Frank" wrote in message
news:xqNHe.177484$tt5.93925@edtnps90...
"greg knapp 5" wrote in message
...
I need your advice, as I have never worked with open wire lines before.
I need to feed many different antennas with open wire line and need to
run the feeline from each about 200 feet from the back pasture to the
shack. I don't want to walk out 200 feet and throw knife switches to
chose the antenna/feedline I want to feed, so I plan to run separate 600
ohm open feeds for each antenna all the way to the shack.
The problem is I don't know what the effect is or how to handle the
multiple open wire feed lines, as they will be parallel for probably
150-200 feet. I haven't found anything in literature describing this.
For instance, will they interact? how far do you space the feedlines from
one another? If I have 4 feedlines, can I stack them vertically or
horizontally one foot apart from each other? How much is enough
separation? What other precautions do I need? Need they be twisted if
they are not near anything other than the other feed lines?
Any help is appreciated.
73,
Greg, N6GK

Hi Greg:

I have just run a free space NEC 2 model of a pair of horizontally
positioned 6" wide, 600 ohm, transmission lines, vertically separated by
12". The model was run from 29 to 31 MHz. On 30 MHz , with both lines
terminated in 600 ohms the isolation was about 39 dB. With both lines
terminated in 6,000 ohms the isolation improved to 66 dB. The actual
range of impedances will be much higher, and complex, but it does give an
idea of the order of magnitude of the isolation. The calculation is based
on the ratio of the peak currents observed.

Hope this helps,

73,

Frank (VE6CB)

"greg knapp 5" wrote in message
...
Thanks Frank for running the program and for the results. They give me a
good confirmation of what Riachard Clark showed me, I need to get more
space between these feeds than I was planning on. If I had 3 feeds, I
guess I ought to have a cross arm on my wood/PVC poles coming in and
spread the feeds by about two feet to minimize the interaction down to a
miniscule amount. I really do want to feed the antennas separately. Gee,
I wonder what W6AM used to do with his 10 or 12 rhombics fed with open
wire all coming into his home...

73,

Greg, N6GK

No Problem Greg, glad to help. Just for interest I ran the program at
different spacings: at 6" the isolation is 27 dB, and at 2 ft 48 dB. Both
these measurements with 600 ohm terminations.

Balanced transmission lines radiate very little energy, but worst case
occurs at the higher frequency. For example the maximum radiated RF from a
terminated 600 ohm transmission line, at 30 MHz, is about -28 dBi.

I have seen pictures of W6AM's transmission lines, but can no longer find
them on the web. His antenna farm, as I understand, was purchased from a
commercial HF station. Interesting little article on W6AM in
http://www.mvarc.com/images/mvarcJULY05pp9.PDF

73,

Frank