How Handle Multiple Open Wire Lines?
 

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

"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)

Why not just run one pair. Then split off at the end with the antennas.
The signal will take the path of least resistance, so the most likely
antenna for the job will be chosen automatically. The ones with high Z will
resist. Resistance is not futile.

My 2¢ (that "¢" key was tough to find)

"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

Thanks for the $.02 worth...it would be a good idea for me, but I will have
several antennas (of various types - dipoles, vee beams, etc.) capable of
being used for the same band, but at different angles from each other. I
hate to "share" my antennas and mess up the patterns I want to use. I
thought about switching way out back, but kind of hard to do A-B testing
when I have a 500 foot run to switch the antennas and get back to the shack!
I'm too old for that!
"Hal Rosser" wrote in message
...
Why not just run one pair. Then split off at the end with the antennas.
The signal will take the path of least resistance, so the most likely
antenna for the job will be chosen automatically. The ones with high Z
will
resist. Resistance is not futile.

My 2¢ (that "¢" key was tough to find)

"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

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)

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

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 wrote:
"I need to feed many different antennas with open wire line and need to
run the feedline from each about 200 feet from the back pasture to the
shack."

This is similar to the problem facing commercial shortwave stations.
They usually have more distance and numbers of antennas and transmitters
to serve several target areas simultaneously.

The preferred solution is a feedline for each antenna. These separate
lines converge on the switching location near the plant. Each
transmitter is equipped with its own feedline too. These converge on the
switching location too.

In the "crossbar" switching arrangement, any transmitter may be
connected to any antenna. Very flexible and it works well.

When separate lines are too extravagant, a single line is switched at
both ends between some transmitters and some antennas. You need at least
as many lines as you have transmitters if all are going to operate at
once and not in parallel.

Most commercial stations have multiband radios but use narrowband
antenna systems. This requires many more antennas than transmitters to
serve many different targets in various bands required on the various
paths around the clock.

If you had only one direction or sense to serve, a single rhombic
connected by a single line to the transmitter might do a compromised
job. The rhombic will take a load at almost any frequency, but its
pattern changes with frequency so its coverage of a target won`t be
ideal at most frequencies.

If you don`t want to walk 200 feet to switch antennas, use relays
controlled from your shack. That`s what professionals do.

Best regards, Richard Harrison, KB5WZI