The AM Forum

Ok, I know this has been batted about before, but I don't think I read a definitive answer on it.

Let's assume the "load" is a dipole, for simplicity... (I guess)

Now, the "usual" is to run up a single coax, with one side of the dipole on the shield and the other on the hot.

We can also run up parallel ladder line, and provide a balanced signal, spread the feed point if you will to get the proper impedance, etc.

What happens when one takes two 50 ohm coax runs in parallel, and feed the center conductors with a balanced signal?

What do we do with the "shield"
What is the characteristic impedance of the paralleled coax lines? 100 ohms? Or??

Since the shield is not connected to the ant (or should it be?) should it be connected to "ground" or a "center tap" of the tuner (like a link coupled affair - an RF xfrmr?).

Shields connected at the top of the run??

How about "active" drive for the shields, as can be done in some other applications...?

Does doing any of this provide any of the benefits of "open wire line"?? Or a waste of perfectly good coax??

Inquiring minds want to know!  ;D ;D

                       _-_-bear

This design is in the Lou McCoy book.  I sent it to Joe (kc6mcw). I dont remember the name of this design off the top of my head. I will call him and ask him to post the details.

C

I found discussion of that in some DX site about 12 or 13 years ago....

The impedance is roughly twice that of the single run.

I usually tie the shields together at both ends, and ground the end at the shack.
But have left the far end open, with no noticeable difference.

Of course there is more loss in this type of feed arrangement than with open wire line, but it has some advantages...

The coax doesn't have to be parallel all the way, since it's shielded, the distance between runs can vary by a lot.

You don't have to worry about running it close to objects. (metal or otherwise)

It is called a Spencer dipole... uses the center conductors of dual coax cables feeding the dipole.

I suppose that would be cost effective if you had enough RG-8 on hand, but it would be a lot easier to use RG-23A for the job. :-\

Dennis

That's the only reson I tried it, was I had just come into possesion of a 1000foot spool of RG8 A/U  ( 9 awg solid center, w foil and braid shield)

Problem there is, it is HEAVY!

Bear,
Cebik and others have written some apps for this arrangement.
Most common is if you have RFI in the shack, run a 10 or 15 ft. line of it
or whatever interior length til out of the shack.  You in effect create shielded balance line.
U want to use coax with a very high characteristic  impedance to make
a better transition to the real ladder line.  It's as Bruce says twice the imp. of the coax.  I think you ground each shield.  Sorry can't remember the RGXX that's ideal for this.  Alsmost tried it couple of years ago. 

Jim

With the shields floating at the feedpoint you have created an electrostatic shield. That is certainly one way to get rid of the feedline radiation problem.

The downside would be the puncture voltage threshold of the coax used.

Carl
KM1H

I have to use the "twin coax" deal when (if) I switch to ladder line. I have a 2" piece of pipe going out the silplate of the house. I figure tie-wrap a 18" section with two pieces of coax side by each. Ladder line to the coax transition, and then ladder line to the antenna feed point. I bet I'll see an impedance bump but nothing to effect me seriously for the bands I use.

Hi Bear,

It seems *much* easier to simply decouple the coax from the antenna using a sufficient number of cores to create enough inductance to do the job.

Either way, with coaxial line (as you know - this is just for a reminder  :D  )  can't tollerate a high mismatch without generating dialectric losses of some amount.

I use coax with the line passing through about 25 type 43 cores at the antenna feedpoint (yes, it is a bit heavy, but the feedpoint is supported on a tower).  Works like a charm !

Regards,

Steve

What about using this twin-coax arraignment for a difficult feed through the house to the attic?  My dipole is directly above the house, and I  have been feeding the twin lead up from the basement shack next to the chimney to the attic and out the ridge vent slot up to the dipole (one long continuous run). Would there be an advantage to using a twin-coax line up to the exit in the roof to keep the line isolated while indoors?   Or should I just run single coax up there and put in a big coax choke to feed the twinlead?

Describe the problem with the current arrangement. What do you hope to improve/fix by going to the dual coax approach?

The shack is located in the basement, the feedline passes straight through the center of the house (single floor ranch) with all the electrical noise in the house I was wondering if the electrostatic shield provided by the dual coax would help keep some of the noise out?  Also, since my antenna system is most likely not balanced I may be radiating inside the house, probably not good.  So the objective would be to keep RF on the feed line and noise off. 

I use a single 160m dipole fed with twin lead and balanced HB tuner to run all bands.  the dipole is zigged all over the yard through the trees. The ends droop considerably so it has a U shape in the vertical plane and a lazy W horizontally. The feed point is at the highest point ~70ft or so right over the house, and the feedline drops vertically for about 55ft or so to the roof where it enters the house.  The lot is small 100x100 city lot.

It's crazy but it works, just wondering if running some new lines would help a bit.

OK makes sense. The coax may help. Or not, if your current balanced line is not really picking up much noise. It would be hard to tell without actually switching.

It would certainly help inside the house; from the description the OW is in no way balanced.  You can also then experiment with ferrite beads to eliminate RFI.

Carl
KM1H