[TowerTalk] Flaky dipole

[Grant Saviers]

First, I have to admit to a strongly held opinion that "ladder line" is 
one of the worst feedlines one can chose for getting the rf to the 
antenna.  By "ladder line" I mean the windowed polyethylene insulated 
"twin lead". The "low loss" mystique is misleading.

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.
see  http://www.k6mhe.com/n7ws/Ladder_Line.pdf   and 
http://www.karinya.net/g3txq/wet_ll/

Then there is the matter of mechanical fragility of the solid conductor 
CCS version, which unless carefully purchased probably has a copper 
flash thinner than skin depth over steel.  So, steel is partly 
conducting the rf, the worst choice of conductor possible. Even then, 
ultimately the CCS will fatigue or rust.

In the old days, hams made their own "open wire feeders", which with 
larger gauge #12, 10 or even 8, and widely spaced porcelain insulators 
really was low loss, but one could still see the small swr affect of 
moisture/rain on the characteristic Z.   If one can find the insulators, 
it is a PITA to make.

OTOH, 73cnc.com (enet review http://www.eham.net/reviews/detail/9318) 
has some nifty injection molded insulators sized just right for snapping 
in parallel 14 ga THHN insulated wires at 3.5" apart to make 600ohm Z 
"open wire feeders".  That is great, but one problem with THHN is the 
0.004" thick nylon outer sheath will rot away from sunlight UV exposure 
and that looks ugly and will change the Z a slight amount with just the 
0.015" thick PVC left.  I've seen one post that uses the weighted 
average of nylon and pvc as the resultant permittivity of the layered 
insulation and use that number when I model wire antennas made with THHN 
(insulation makes a difference).

With the nominal od of 0.105" with PVC and nylon present for 14ga THHN 
the nearest solid copper is #10 at 0.102" diameter, so that would make a 
terrific low loss "open wire feeder" with a nominal Z of 510 ohms.  10 
ga solid copper snaps in easily to the 73cnc insulators.  The larger 
wire diameter and no insulation should make a 10ga version lower loss 
than the 14 ga THHN version, but really only suitable for permanent 
installations.

My 2c, end of rant.

Grant KZ1W

"a few tenths of a db here and there and pretty soon there is 3db loss"  
(or 8db for ladder line)

On 8/1/2016 17:38 PM, Tom Osborne wrote:
> Hi Tin
>
> Wonder what kind of ladder line you are using.
>
> Some of that stuff with the solid core wire corrodes pretty bad.  It is
> copperweld and if you scrape off the copper, the steel under will rust.  I
> had that problem here and the problem was that where the ladder line
> connected to the antenna, it got really rusty and ugly.  Had to take it all
> apart and clean it up good.  73 and GL
>
> Tom W7WHY
>
>
>
>> Hi folks. Sending this to multiple groups to gain expertise.
>>
>> My primary dipole is acting strange.  Its 135 feet of wire up 40 feet.  Fed
>> with ladderline. As I am transmitting I noticed the swr going from low to
>> high suddenly for a few seconds and then back.  Initially I thought it was
>> my auto tuner showing damage, but I put an MFJ manual tuner in place and I
>> can clearly see it happening on the meter.  I dont think it is wind moving
>> the wire, as it was calm when it was happening.  The feed goes from the
>> rig, through the antenna switch to the tuner, then out the window via a
>> bulkhead connector and to the 4:1 balun via a 2 foot coax, and then to the
>> ladderline.
>>
>> Any ideas on where to look for issues, and or testing methods?
>>
>> Thanks
>> TIM
>> W8TAH
>>
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