[Antennas] twin coax as shielded balanced line

[van lincoln wd8aam]

Question:  If I ran twin coax open wire feeders made from
rg-58/u thru a metal conduit for 2 feet, 

1.  would I suffer a larger loss than in open air?
2.  would I change the impedance of the coax twinlead section?
3.  what kind of loss for two feet are we looking at?
4.  would this be better than plain coax for a two foot run thru metal
conduit?
     (lower loss.?)  Then change to regular open wire line, after the conduit?

thanks
wd8aam




At 11:36 AM 2/3/02 -0600, you wrote:
>Gary, I will try to be as brief as possible.
>
>1. the reason for using a balanced line to an antenna almost always comes
>from the desire or need to operate the antenna on more than one band.
>
>2. tuned feeders usually operate with relatively high SWR - up to 20:1 or
>so.
>
>3. coax tuned feeders - balanced or not - have much higher loss under high
>SWR conditions than does conventional ladderline or oper-wire line.
>
>Bottom line: for multiband use - which means tuned feeders with usually
>fairly high SWR - twin-coax is not the way to go unless (a) the feeders are
>very short and/or (b) you need the shielded coax to make it easier to bring
>the feeders into your shack.
>
>A compromise is to use ladderline from the antenna to the building and then
>use a W2DU 1:1 bead balun on coax to make the transition from balanced line
>to unbalanced line and then run the single coax into the building to your
>tuner, etc. That is how I operate three different antennas here with good
>results.
>
>I hope this is of some help, Gary. Please email directly if you have any
>more questions.
>
>72/73/oo, George W5YR - the Yellow Rose of Texas         
>Fairview, TX 30 mi NE of Dallas in Collin county EM13qe   
>Amateur Radio W5YR, in the 56th year and it just keeps getting better!
>QRP-L 1373 NETXQRP 6 SOC 262 COG 8 FPQRP 404 TEN-X 11771
>Icom IC-756PRO #02121  Kachina #91900556  IC-765 #02437
>
>All outgoing email virus-checked by Norton Anti-Virus 2002
>
>
>Gary Lee wrote:
>> 
>> I had asked a question prevoiusly about the spencer dipole fed with twin
>> rg-11 as a balanced shielded feedline.
>> 
>> I have since read all the responses, and asked everyone I know.  Can't find
>> the referenced cq article from 1989, and haven't found a satisfactory
>> explanation on line.
>> 
>> The consensus seemed to be that this method of feeding could be lossy.
>> Could some kind soul please explain pros and cons without the use of
>> pictures as I am totally blind.
>> 
>> Tried the arrl antenna book, but the pictures were not well described on my
>> cassette copy.  This, it appears, would be a difficult job.
>> 
>> Thanks for any  help.
>
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>
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