[TenTec] Openwire fed differently

Sun Oct 2 13:58:23 EDT 2005

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 at contesting.com [mailto:tentec-bounces at 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
>>
>>
>>
>>_______________________________________________
>>TenTec mailing list
>>TenTec at contesting.com
>>http://lists.contesting.com/mailman/listinfo/tentec
>>
>> 
>>
>>    
>>
>
>
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>