I don't necessarily recommend "two-coax balanced line", although in fact
I do use a short piece going through a wall (yes. feedthroughs are a
good sol'n, too) for one end of my rhombic (QST, Nov 2004).
The answer to Sinisa's question is generally "no". Lets say you have a
single coax and your loss is 10%. FOR THE SAME MISMATCH, and a balanced
situation, 50% of your power is in each coax. Each coax loses 10% of its
applied power, but each has 50% of the total power, so each loses 10% of
50% or 5%. The net loss is 5+5% or the same as the loss of a single
cable (but at twice the cost!).
The "same mismatch" caveat is interesting, because you won't generally
have the same mismatch. For example two 50 ohm cables in balanced mode
have an inpedance of 50+50 or 100 ohms. If you have a 50 ohm antenna, it
will provide a 2:1 SWR to the balanced coax and higher loss than a
single 50 ohm system. The opposite case also works, so the mismatch
loss will depend on the actual antenna impedance, but it won't be the
same for both cases unless you use a matching network at the antenna to
match the antenna to whichever arrangement you use.
Seems a lot of trouble and expense, except in unusual, and short,
arrangements.
73, Joel
Joel Hallas, W1ZR
NJ0IP wrote:
>Guys, this may be a stupid question, but if you use two parallel pieces of
>(long) coax, wouldn't you have twice the loss. On the other hand, resistors
>in parallel would mean half the loss. In any case, coax is lossy, heavy,
>and expensive, so why would you use a long run of two coax cables?
>
>Using short lengths to going through the wall is a good idea, but then why
>would you need to tie the shields together at all and why ground them?
>
>Sinisa?
>
>Cheers,
>Rick
>
>-----Original Message-----
>From: tentec-bounces@contesting.com [mailto:tentec-bounces@contesting.com]
>On Behalf Of Darwin, Keith
>Sent: Thursday, February 02, 2006 9:05 AM
>To: Discussion of Ten-Tec Equipment
>Subject: Re: [TenTec] openwire feed OT
>
>Ken (and others)
>
>Oh, man, true confession time here :-). I've only done what I described
>one time, years ago. I built a coax parallel feed line for a loop
>antenna. The coax went from the back of the tuner, through the window
>and all the way to the antenna. I connected the shields together at the
>tuner end of the line and tied it to tuner ground. It worked fine. I
>figured the same technique would be great for constructing a parallel
>balanced line for use inside and near things that would upset the field.
>Truth is I'm still not sure what to do with the shields :-)
>
>I believe that the 2 pieces of coax can go their separate ways as long
>as they start together and end together. The shield isolates the center
>conductor preventing it from being affected by nearby stuff including
>the other half of the parallel feed line.
>
>I got the idea from an article in QST but don't really have the
>engineering or physics data to truly explain how it all works.
>
>In my personal case I feed RG-8x coax from the rig to the tuner which is
>mounted 50 feet away. From the tuner I run RG-213 coax 30 feet to the
>base of my 28' vert antenna. Doesn't work well on 80 but seems to work
>well on 40, 30, 20, 17 & 15.
>
>- Keith KD1E -
>
>-----Original Message-----
>From: tentec-bounces@contesting.com
>[mailto:tentec-bounces@contesting.com] On Behalf Of K. Indart
>Sent: Thursday, February 02, 2006 11:31 AM
>To: Discussion of Ten-Tec Equipment
>Subject: Re: [TenTec] openwire feed OT
>
>Keith,
> I have two questions: Do you keep the two coax pieces separated by
>the same spacing as your feed line,...or can you make ONE hole in the
>wall/window and feed both coax through one hole ?
>
> Does it matter which end of the shields get tied together; inside the
>wall or outside the wall ?
>
> Do you make the lengths of the coax JUST long enough to protrude
>outside and inside of the wall or how long ?
>
>Thanks, and it sounds like a very good idea.
>
>73, Ken WA4RPH
>
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