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Author Topic: Open Wire "Coax"  (Read 5746 times)
AA5WG
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Posts: 497




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« on: February 26, 2013, 06:39:03 AM »

Hi to all,

If two coax cables were tie wrapped together to form a balanced transmission line what would be the correct method to calculate this balanced line impedance, i.e. two 50 ohm cables tied together? 

Should both ends of this balanced system have their ground/shields tied together?

Or, should just one end have their ground/shield tied together?

Would spacing the two cables further apart have and effect on the lines impedance?

Thank you,

Chuck 

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AA4PB
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Posts: 12856




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« Reply #1 on: February 26, 2013, 08:13:08 AM »

The impedance would be 100 Ohms.
The shields should be connected at both ends.
Changing the distance would not change the impedance. The impedance is determined by the center conductor to shield separation.
The loss per foot will be very high when compared with real open-wire line or ladder line.
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KE3WD
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« Reply #2 on: February 26, 2013, 01:49:44 PM »

And the correct terminology for such is, "shielded balanced line"...
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AC5UP
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Posts: 3875




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« Reply #3 on: February 26, 2013, 04:21:03 PM »

The loss per foot will be very high when compared with real open-wire line or ladder line.

Not sure I'd say "very", but the shielded balanced line will have approximately the same characteristics as the coax it's made from in terms of loss and voltage ratings. Should be somewhat more forgiving of a mismatch as well. And don't forget that you can do this with RG-59 or RG-6 for a 150 Ohm balanced line. Given the cost and availability of RG-6 this might be worth considering. Especially if the line needs to run underground or in a setting that would be difficult for ladder line. The shields should be tied together at both ends and some references will tell you they should be grounded at both ends. I prefer grounding only one end as that's an old audio trick for avoiding a ground loop, which could be completely irrelevant to this concept....

BTW:  Now that I think about it, "very" high loss if we're talking about RG-174 in parallel.   Grin
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KD0REQ
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Posts: 950




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« Reply #4 on: February 26, 2013, 07:13:09 PM »

sounds like the IBM Token-Ring coax of a couple decades ago.  it was two wires TP inside the coax shield.  don't remember if it was air core or not, like the old ugly Motorola car antenna coax stuff.  iirc from some studies in a college 500 class, that was 125 ohm impedance and I would not want to run more than QRP down that stuff.

so it's been done, kinda looked like a two-pin N connector to join it.  you would be much happier with LMR and a balun.

now, IBM found the limitations early, including serviceability, and they moved to twin RG-58s welded together at the outer insulation, and used BNCs for connectors.  you get the same effect tying all your lines together going up a tower so the same woodpecker gets them all at the same time Wink
« Last Edit: February 26, 2013, 07:15:12 PM by KD0REQ » Logged
AA4PB
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« Reply #5 on: February 27, 2013, 05:10:32 AM »

Define "very":

At 30 MHz, 100-feet of matched Wireman 551 ladder line will have 0.35 dB of loss.
At 30 MHz, 100-feet of matched RG-8X will have 1.82 dB of loss.

Given that the parallel line senario usually is operated at quite high SWR, the total loss in coax could be significantly more than ladder line. The typical use of parallel coax lines is a short run (2-feet perhaps) to get through a window, wall, etc. and then a transition to ladder line. With a short run the loss in the coax is much more reasonable.

I wouldn't recommend using a long run of parallel coax to feed a doublet on all bands.
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KE3WD
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« Reply #6 on: February 27, 2013, 07:10:02 AM »

My experience has been that insertion loss is quite minimal, certainly not something to fret about here. 


73
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AA4HA
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Posts: 1451




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« Reply #7 on: February 27, 2013, 07:40:28 AM »

sounds like the IBM Token-Ring coax of a couple decades ago.  it was two wires TP inside the coax shield.  don't remember if it was air core or not, like the old ugly Motorola car antenna coax stuff.  iirc from some studies in a college 500 class, that was 125 ohm impedance and I would not want to run more than QRP down that stuff.

so it's been done, kinda looked like a two-pin N connector to join it.  you would be much happier with LMR and a balun.

now, IBM found the limitations early, including serviceability, and they moved to twin RG-58s welded together at the outer insulation, and used BNCs for connectors.  you get the same effect tying all your lines together going up a tower so the same woodpecker gets them all at the same time Wink

You see the same coax and connectors used on receivers like the R-390, R-390/A and on some SP-600's (boat anchor receivers). It is also called twinax and is in that 100-125 ohm impedance range. Usually we run off of the "C" connector that is also on the back of the R-390, R-390A that is around 50 ohms impedance or we make or buy a small balun to adapt the 125 ohm impedance to 50 ohms and run with regular coax cable.

The 125 ohm impedance used to be used to connect the receiver to a doublet antenna.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
WB6BYU
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Posts: 13288




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« Reply #8 on: February 27, 2013, 08:45:35 AM »

Amp Supply used to sell 150 ohm shielded, balanced line, similar to a
pair of RG-59 coaxes but with a single shield around both conductors.
It's great for loop antennas where the feedpoint impedance is in the
100 - 400 ohm range.  But I haven't found any other good uses for
it, so it has been sitting in my garage for the last 15 years.

The losses are the same as for a single length of coax with half the
load impedance, so the same SWR.  Maybe that would have been a
good use for some of the RG-17 I just got rid of...
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AA5WG
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Posts: 497




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« Reply #9 on: March 04, 2013, 05:48:03 AM »

Thank you to all for your inputs.

Chuck
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PA1ZP
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Posts: 239




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« Reply #10 on: March 04, 2013, 11:15:26 AM »

Hi All

Just wanted to make a remark about open line losses.
Open line losses are very low as long as the antenna connected to them is high enough in impedance.

I do work with a very short aluminium rotary dipole of 2 x 7 mtrs and 11 mtrs of open line.
This homebrew 11 mtr long abt 500 ohm line is much thicker as wireman and still has about 4 dB loss in only 11 mtrs of transmission line.
Then I do have about 4 dB tuner loss.

So open line losses can be very low but that also depends on the antenna connected.
Ofcourse this is a antenna that is way to short for 80 mtrs but because I use bulky open wires and an even bulkier tuner I still can use this antenna on 80 mtrs CW and even SSB and even QRO is not a problem on 80mtrs.

Do not try this with wireman lines these will absolutely be fried in a few seconds.
Do not try this with a MFJ tuner like the MFJ949 or so this also will give smoke signals very fast with only 100 watts.

On bands 40 - 10 mtrs no smolke with wireman or MFJ even with 100 watts.

For the "coaxial" open wires it is a fine method to get out of the shack with a short piece of these wires.
It is also a method to use just the center conductors and insulation of RG213 (RG8) ty-rapped together.
Tha same construction can be used as a hanging loop around the mast in case you want to use a open wire fed rotary dipole.
« Last Edit: March 04, 2013, 11:19:48 AM by PA1ZP » Logged
VE2JFK
Member

Posts: 1




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« Reply #11 on: March 12, 2013, 06:16:46 PM »

The method is useful to go through a wall, or window.
Rupture of impedance would not be very important to a balanced feedline.

Balanced feedlines are very efficient.

I use a multiband Spiro antenna (10 to 80 meters) that is fed with a 450 balanced line.
Tuning 40m and 80m is easy and the matching very close in capacitance. Smiley
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marcram
AB2YC
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Posts: 53




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« Reply #12 on: March 18, 2013, 09:19:45 AM »


The method is useful to go through a wall, or window.
......


Thank you, I am moving over to balanced line and have been
looking in to methods of bringing it through my metal siding.
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KE3WD
Member

Posts: 5689




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« Reply #13 on: March 18, 2013, 01:42:00 PM »


The method is useful to go through a wall, or window.
......


Thank you, I am moving over to balanced line and have been
looking in to methods of bringing it through my metal siding.

You can run unshielded balanced line straight through the siding, perpendicular to the metal siding without problem.  Just don't let any of the balanced line run parallel to that siding and it is a good idea to use a piece of PVC pipe as insulator for the hole. 

Balanced line should not be run parallel to any metal.  Perpendicular for a short run such as a penetration really doesn't bother it. 


73
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W9GB
Member

Posts: 2623




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« Reply #14 on: March 18, 2013, 02:11:41 PM »

Quote from: KD0REQ
sounds like the IBM Token-Ring coax of a couple decades ago.  it was two wires TP inside the coax shield.  don't remember if it was air core or not, like the old ugly Motorola car antenna coax stuff.  iirc from some studies in a college 500 class, that was 125 ohm impedance and I would not want to run more than QRP down that stuff.
so it's been done, kinda looked like a two-pin N connector to join it.  you would be much happier with LMR and a balun.
Scott --

You are mixing 2 different cable technologies, used by IBM at different times.
Let's clarify this -- so readers are not confused.

TWINAX : A coaxial cable with TWO internal parallel conductors inside a common shield.
It was created by US military as RG-108/U, after World War 2 (MIL-C-17G, MIL-C-3655).
http://www.amphenolrf.com/products/twinax.asp?N=0&sid=5146590047F1E17F&

TWINAX, as sold today, can have a characteristic impedance (Z) of 78, 95, 100, 124, 150 ohms depending on conductor size and spacing of internal conductors.
http://sacramentomountainsradioclub.org/images/06Coaxial_Cables.pdf
IF you look at the back of a Collins R-390A HF receiver built for US Intelligence and Military in 1950 and 1960s, a TWINAX connector used as the Antenna connector.  

IBM Rochester and IBM Austin used this coaxial cable for the IBM 5250 and System 3 (later 34, 36, 38, and AS/400) computer systems.  They used 100 ohms for their installations, as I remember.  You still find it in older System 3 installations, but it was phased out in 1990s along with IBM 370 mainframe SNA coaxial cabling (RG-62/U, 93 ohm).
====
The IBM Cabling standards were introduced in early 1980s.  
IBM Type I cable was a large conductor 2-Pair Shielded Twisted cable for Local Area Network (LAN) usage.  
IBM Token-Ring used this cabling.  The IBM cabling standard faded by mid-1990s, with adaptation of EIA/TIA 568 cabling standard for network and premise cabling.
http://en.wikipedia.org/wiki/TIA/EIA-568

====
German Radio Amateurs were experimenting with SIAMESE Coaxial Cables (2 separate coaxial cables with a bonding of outer jackets) about 15 years ago to create a Shielded Balanced transmission line.
This Coaxial cable was very expensive to manufacture (no military/commercial usage), so it did not "catch-on" for widespread usage.
« Last Edit: March 18, 2013, 02:50:13 PM by W9GB » Logged
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