[TowerTalk] Flaky dipole

[Jim Brown]

On Tue,8/2/2016 9:31 AM, Jim Thomson wrote:
> Yes, it is low loss when suspended in free space in a vacuum.  If it is
> wet or dirty or in contact with most anything the loss goes WAY up.
> seehttp://www.k6mhe.com/n7ws/Ladder_Line.pdf    and
> http://www.karinya.net/g3txq/wet_ll/
>
N7WS was the first to publish on this, at least 20 years ago. I found it 
in one of the ARRL Antenna Compendiums, which have a LOT of great stuff 
in them. Wes set up carefully controlled experiments in which he wetted 
ladder line and compared loss wet and dry. He showed that wet line was 
lossier than coax.

> ## can u get the  300/450 ohm line..without the windows ??
> Seems to me that 300 ohm TV type line back in the 60s /70s
> had no windows, I remember it well.  It was also available
> in a windowed variety.

Ladder line can be built from wire and suitable spacers. There's a guy 
who exhibits each year at Dayton who makes what look like nice spacers 
for ladder line.

> ##  along the same subject, back in the 70s, solid twinlead
> was available in 75 ohm.  Came in 2 x formats, small and large.
> The large stuff would easily handle high power.   Back then,
> the kw rated 75 ohm twinlead was fed  directly to the dipole,
> then the other end had a PL-259 installed on it.   Dunno if its
> still available or not.

At least 25 years ago, I bought a "new old stock" 250 ft spool of Belden 
"75 ohm kilowatt twin lead," not having a use for it at the time, but 
figuring that someday I might. 10-15 years later, I used it to feed a 
dipole in Chicago. I still have it. It's a pair of stranded copperweld, 
#12 or #10 (I can look it up) completely encased in a heavy dielectric 
and jacket. I also have an old Belden catalog that published loss data 
for it. You can see it in a graph of loss data for various coaxes on 
page 2 of k9yc.com/Coax-Stubs.pdf  Note that nearly all of this is mfr's 
published data, not measured.

About seven years ago, I helped N6RZ's widow dispose of his stuff. In 
it, I found a few lengths of Amphenol's 75 ohm twin lead.
>     With no windows in the line, water wont collect.

Yes, but it could certainly collect on the jacket of that 75 ohm twinlead.

> ##  On a similar note, a buddy used paralleled 50 ohm coax,
> to make  100 ohm, shield, balanced line for his quad project.
> Shield tied together at each end.  100 ohm Z between the
> 2 x center conductors.   Made from 2 x 213-U  taped together
> every few inchs.  Can also be done with 2 x RG11  to make
> 150 ohm, balanced, shielded line.

Yes, except that the runs you described have the lines 
"series-connected" to yield twice the impedance. They could also be 
parallel-connected to yield 25 ohms for two 50 ohm lines or 37.5 ohm for 
two 75 ohm lines.

And as you probably know, the loss in dB in parallel runs of coax, 
whether connected in series or parallel, is the same as in a single run 
of the same coax.

73, Jim K9YC