Actually, the loss is the same as for a single coax. The power is split
between the two and each half power part has a single coax loss. When
you combine them back, it's the same loss as in a single coax. Fox
example, let's say the coax matched loss is 10% and you have 100 W at
the bottom of the line; you will end up with 90 W at the antenna. If
instead your tuner is set for a 100 ohm balanced load and you have 100
ohms at the end of the dual-balanced coax, 50 W will go down each. The
loss in each half of the dual line will still be 10 % and we will end up
with 45 W at the far end of each "half-line". When combined you end up
with the same 90 W as in the single coax case.
The unmatched case may be better or worse than a single coax. Let's say
you are driving a matched 400 ohm line. With a single 50 ohm coax (with
its usual choke or some kind of balun) you will have an SWR of 8:1 and
the additional loss due to the high SWR. If you use the dual coax, each
one will see 200 ohms, and have the loss due to the 4:1 SWR, a lot
less. So far so good, but if the impedance at the bottom of the line is
12.5 ohms, the opposite occurs.
To find out where you are, use EZNEC or an impedance bridge or antenna
analyzer (May 05 QST, p 62) to find the antenna impedance, use TLW
(Transmission Line for Windows, free with the latest ARRL Antenna Book)
to determine the impedance at the bottom of your ladder line. For the
dual coax, divide real and immaginary parts by 2 and run TLW again for
your coax type using the half value as the load on the coax. You will
determine the total loss and half the impedance at the bottom of the
dual coax.
I hope I'm making sense here?
GL & 73, Joel, W1ZR
NJ0IP wrote:
Joel, it is indeed different and I used the word "differently" in my
Subject!
As I see it, your suggestion would have 2x the loss of a single piece of
coax. Since this is primarily my low band antenna, that probably wouldn't
matter too much. I would need about 50 ft. of the twin coax to reach a
point where I could run normal openwire to without obstructing the view of
our house.
Tnx.
73
Rick
-----Original Message-----
From: tentec-bounces@contesting.com [mailto:tentec-bounces@contesting.com]
On Behalf Of joel hallas
Sent: Saturday, October 01, 2005 5:54 PM
To: Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] Openwire fed differently
Well, if one coax is ok, how about 2? You can make a balanced feedline
from two coax cables in parallel, with their shields tied together (I
suggest at both ends, and they may be grounded if you want). With 50 ohm
coax it will be a 100 ohm balanced line if you feed the two center
conductors. I use short lengths of this to bring my ladder line fed
antenna connections through the outer wall of our house. With this you
don't have the problem of being near metal, etc.
OTOH, you have the same loss as with coax. So you want to make this a
short connection between ladder line at each end, or in my case, I go to
a balun with the "balanced coax", as the feed enters the house and then
have a relatively short run of coax to the tuner.
It's not perfect, but it might solve your problem!
73 & GL, Joel, W1ZR
NJ0IP wrote:
I changed to a new thread on this openwire stuff.
Background:
I've moved my shack to the basement of my typical Bavarian house.
The front of our (stucko) house is painted with typical Bavarian artwork
and
my XYL lights up the front side with lights at night.
For some reason she thinks the ladderlines ( I have two, because there are
two antennas) disturb this traditional Bavarian picture and she nixed the
idea of me running the ladderline into the basement window on the front
side
of the house. So now I'm stuck with about $3000 worth of Annecke
symmetrical tuners and no openwire antenna.
Challenge:
Well I'm a cw contester and an SSB rag chewer. I need an antenna to cover
both portions of the band.
I have a solid state Linear Amp (Henry SS750-HF) that needs a relatively
good SWR.
An 80m dipole just won't cut the mustard.
At least here in Europe we don't need to cover such a large frequency
range.
Our 80m band begins at 3500 and ends at 3800.
We don't have any 75m privileges.
Money Money Money:
Here in Europe there are a couple of companies offering remote symmetrical
tuners, but they go for about $2000 a piece.
OUCH.
Well I can run coax out the shack and under ground and place one such a box
in the yard somewhere.
I hate to think of buying even one, and certainly cannot afford two of
these
beasts.
Ideas:
Anyone have any suggestions?
What else is available?
73
Rick
DJ0IP / DJ2T / NJ0IP
Munich, Germany
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