[TenTec] OT 75 Ohm twin feeder

[Dr. Gerald N. Johnson]

On Mon, 2008-06-16 at 23:18 -0500, Stuart Rohre wrote:
> I would take that ARRL handbook write up with a grain of salt.  In the day 
> they wrote that, zip cord had rubber; or other colored insulations that 
> could contain metallic dyes that would increase loss.

The write up in my ARRL antenna book, 1994 edition pages 15-2 and 15-3
was based on tests made in 1979 with PLASTIC, not rubber cord, type
SPT-1 from GC. Losses were high. Losses with rubber were higher in the
older days.

The concept was revisited in QST in the last couple of years. Same
result, but maybe more recent materials and tests.

Because of the close spacing, the electric field is concentrated in the
plastic which then makes the plastic contribute more to the loss. The
PVC molecule is large and there may be some molecular resonances at low
VHF frequencies to add to that loss, to say nothing about the pigments.
> 
> Today's clear vinyl might not be as lossy.  PVC is higher loss than 
> polystyrene plastics, but might be tolerable in zip cord.  There are many 
> colors of zip cord, and you would have to strip off some insulation and test 
> insulation alone, (no wire!) , in a microwave with a cup of water.  If it 
> does not heat up by the time the water boils, the loss is very low in that 
> insulation.

At 2.45 GHz. If molecular resonances are happening at lower frequencies,
its not absolutely sure that the losses will be high a microwave,
however PVCs sometimes used for loading coil forms for HF mobile
antennas that melted in the HF application also showed high losses in
the microwave test. The test I've used is to make a two plate air
capacitor that will fit closely to the dielectric sample while mounted
on my Q or RX meter and note the change in measured Q when the
dielectric is inserted into the air capacitor. That way I can measure at
the operating frequency (up to 250 MHz).
> 
> -Stuart Rohre
> K5KVH 
> 
73, Jerry, K0CQ