OT - RG6 dual coax as feedline?

[WB9GZL]

Hello from K2 #4206 (Mike WB9GZL) with an OT
question for the sat/cable/over-the-air-TV gurus. Springtime in Wisconsin for me involves trying new antennas
for my K2. This past winter I was given a new
500-foot reel of RG6 dual coax w/messenger.


 


Impedance:
75+/-3 Ohm


18 ga solid
cu centers


aluminum foil
+ 60% al braid


 


My
extensive searching has not uncovered any transmitting applications for this
coax. Would you have an opinion if it could be used (with an appropriate balun
at the TX, for example) as "twin-lead" feedline for balanced antennas such as an all-band doublet or
perhaps an antenna with more variable characteristics such as a large loop?


 


Thanks for
any help you can provide, and for making this reflector an outstanding source
of collective wisdom. Best regards from Mike WB9GZL.




     
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[n7ws]

It can certainly be used as a balanced 150-ohm transmission line, but I would stay away from any applications where it operated at appreciable SWR.

In other words, both of the applications you propose.

Wes  N7WS



> My
> extensive searching has not uncovered any transmitting
> applications for this
> coax. Would you have an opinion if it could be used (with
> an appropriate balun
> at the TX, for example) as "twin-lead" feedline
> for balanced antennas such as an all-band doublet or
> perhaps an antenna with more variable characteristics such
> as a large loop?



     
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[Jim Brown-10]

On Sat, 25 Apr 2009 09:05:54 -0700 (PDT), Mike C wrote:

> Would you have an opinion if it could be used (with an appropriate balun
>at the TX, for example) as "twin-lead" feedline for balanced antennas such
>as an all-band doublet or perhaps an antenna with more variable
>characteristics such as a large loop?

This configuration is a very good way to use coax if the impedance of the
feedline (150 ohms) is a good match to the antenna. I would bond the coax
shields to ground for lightning protection.

73,

Jim Brown K9YC


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[Phil Kane-2]

On 4/25/2009 9:05 AM, Mike C wrote:

> This past winter I was given a new 500-foot reel of RG6 dual coax
> w/messenger.

> My extensive searching has not uncovered any transmitting
> applications for this coax.

  One of our local gurus, the late Judge Greg Milnes, W7OZ, fed
  the HF antennas on his three 100-foot-plus towers with 75 ohm
  "hardline" that he, probably like yourself, got from a local
  cable company that was surplussing it.  It survived his
  "loafing along" 1500-watt amplifier output with no problems.  The
  1.5:1 SWR was easily tuned out.

--  73 de K2ASP - Phil Kane
    Elecraft K2/100   s/n 5402

    From a Clearing in the Silicon Forest
    Beaverton (Washington County)  Oregon

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[Don Wilhelm-4]

Mike,

I know you have it in hand, but I would suggest using that cable as a
parallel transmission line only for the lengths that must run indoors.  
Use normal ladder line or open wire transmission line for most of your
transmission line run.

My reason:  The paralleled line certainly will work, but the loss is the
same as a single coax of the same material, and in a multiband
situation, it certainly will be run at a high SWR.  However, a shielded
parallel line can be handled just like coax and run near surfaces where
normal parallel line should not be located.  As such, it is a good
solution for the shack entry cable.
Do tie the shields together and ground them at the entry point and in
the shack.

You can use the remainder of it to feed resonant antennas.  If it is
anything like normal RG-6 it will easily handle up to 500 watts even
with a 1.5 SWR and will do it with less loss than RG-8X.  Because of the
aluminum shield, use normal crimp type F connectors, or if you can
locate them, there are BNC connectors for RG-6 coax with aluminum shield.

73,
Don W3FPR

Mike C wrote:

> Hello from K2 #4206 (Mike WB9GZL) with an OT
> question for the sat/cable/over-the-air-TV gurus. Springtime in Wisconsin for me involves trying new antennas
> for my K2. This past winter I was given a new
> 500-foot reel of RG6 dual coax w/messenger.
>
> Impedance:
> 75+/-3 Ohm
>
> 18 ga solid
> cu centers
>
> aluminum foil
> + 60% al braid
>
> My
> extensive searching has not uncovered any transmitting applications for this
> coax. Would you have an opinion if it could be used (with an appropriate balun
> at the TX, for example) as "twin-lead" feedline for balanced antennas such as an all-band doublet or
> perhaps an antenna with more variable characteristics such as a large loop?
>
> Thanks for
> any help you can provide, and for making this reflector an outstanding source
> of collective wisdom. Best regards from Mike WB9GZL.
>  
>

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[N5GE]

On Sat, 25 Apr 2009 10:58:52 -0700, Phil Kane <[hidden email]>
wrote:

>On 4/25/2009 9:05 AM, Mike C wrote:
>
>> This past winter I was given a new 500-foot reel of RG6 dual coax
>> w/messenger.
>
>> My extensive searching has not uncovered any transmitting
>> applications for this coax.
>
>  One of our local gurus, the late Judge Greg Milnes, W7OZ, fed
>  the HF antennas on his three 100-foot-plus towers with 75 ohm
>  "hardline" that he, probably like yourself, got from a local
>  cable company that was surplussing it.  It survived his
>  "loafing along" 1500-watt amplifier output with no problems.  The
>  1.5:1 SWR was easily tuned out.
>

I also used the cable co. hardline in the 80's, but I built a
transformer for each end of the coax.  It worked very well.

The transformers were in a QST article, because their was so much of
that type of coax available back then.  Check the back issues of QST
for "CTV Coax" and you'll probably find a lot of articles regarding
the use of that coax.

73,

Tom, N5GE
K3 #806, K3 #1055
XV144, XV432
W1 and other small kits.
http://www.n5ge.com

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[Jim Brown-10]

On Sat, 25 Apr 2009 17:29:15 -0400, Don Wilhelm wrote:

>My reason:  The paralleled line certainly will work, but the loss is the
>same as a single coax of the same material,

I don't agree, Don. Loss in coax at HF is all I squared R. If you match to
twice the impedance, you'll have half the current. This means that the
fraction of the loss in the center conductor will be 6 dB lower at the
higher impedance. On the other hand, the loss in most coax shields is lower
than the loss in the center conductor, so I would expect losses to be about
3 dB lower, not 6 dB.

>and in a multiband
>situation, it certainly will be run at a high SWR.

Agreed. This is not a good solution for antennas of random impedance. But it
IS a good solution for antennas that match to about 2.5:1 or better.

73,

Jim K9YC





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[Alan Bloom]

On Sun, 2009-04-26 at 10:33 -0700, Jim Brown wrote:
> On Sat, 25 Apr 2009 17:29:15 -0400, Don Wilhelm wrote:
>
> >My reason:  The paralleled line certainly will work, but the loss is the
> >same as a single coax of the same material,
>
> I don't agree, Don. Loss in coax at HF is all I squared R. If you match to
> twice the impedance, you'll have half the current.

Actually you'll have 1/sqrt(2) or 0.707 the current.  So you'll have 1/2
the power loss in each coax, but there are two coaxes so the total power
loss is exactly the same as with a single coax.

As long as everything is matched it doesn't matter how many coaxes you
use.  The loss is always the same.

> This means that the
> fraction of the loss in the center conductor will be 6 dB lower at the
> higher impedance. On the other hand, the loss in most coax shields is lower
> than the loss in the center conductor, so I would expect losses to be about
> 3 dB lower, not 6 dB.
>
> >and in a multiband
> >situation, it certainly will be run at a high SWR.
>
> Agreed. This is not a good solution for antennas of random impedance. But it
> IS a good solution for antennas that match to about 2.5:1 or better.

The same could be said about using a single coax as well.  One advantage
to using two coaxes is that each carries only half the power (0.707 the
voltage), so you get twice the power-handling capability.

One issue with making parallel-conductor line with two coaxes is that
they must have the same electrical length.  If they are cut from the
same roll of high-quality coax, then identical physical length should
equate to identical electrical length, but it still might be worth
checking to be sure.

> 73,
>
> Jim K9YC

Al N1AL


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