[TenTec] Velocity factor

[Dr. Gerald N. Johnson]

On Sat, 2007-01-27 at 15:27 +0000, JACrux wrote:
> I thought about 75 ohm twin. I also thought about making my own open wire 
> line. But instead I use the heaviest grade of ladder line, 14g (US) stranded 
> copper plated steel. Sadly not sold in UK (as far as I know) but light enough 
> to carry home from Dayton. Apart from being incredibly strong and flexible, 
> the characteristic impedance is apparently a bit less than other grades, 
> somewhere between 350 and 400 ohms. 
> I run the TenTec auto ATU into about 15 feet of RG-8 coax to a balun hung 
> outside the shack, then around 80 feet of ladder line to the top of the pole. 
> The only downside is the change in SWR with rain/snow/humidity. But re-tuning 
> is a small price to pay for multi-band convenience without switches.   
>  
In a tune feeder application, the higher impedance ladder line often
makes for more convenient tuning. The worst case for a tuner with a coax
or 72 ohm twinlead feeder is where the dipole is one wave long and the
feed line is an odd multiple of 1/4 wave long. Then the 1000 ohm feed
point transforms to 5 ohms and tends to be out of the range of the
tuner. But with a 300 ohm line that transforms to 90 ohms and the
opposite 50 ohm dipole transforms to 1800 ohms, both generally easier to
tune than 5 and 72 ohms from the 72 ohm feeder.

I've run into problems tuning through a toroidal balun. High impedances
at the balun can lead to core saturation and sometimes I've tuned the
balun and dumped all the RF into the balun, the match didn't change with
the antenna disconnected.

Having said all that, I have an 80 meter inverted V fed with 52 ohm coax
(RG-142). Using a single ended tuner with a dual 140 pf across the coil
and the input to the center of the capacitor and a 140 pf in series with
the antenna connector, I've tuned it from 80 through 10 meters,
including the WARC bands. The coil isn't big enough to get to 160
meters.
-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer