[NLRS] 2M and 450 ohm ladder line

[Dr. Gerald N. Johnson]

On Mon, 2007-02-26 at 22:53 -0600, Cathy James wrote:
> 
> 
> Is 450 ohm ladder line practical for 2M feedline?
> 
> The loss charts in the handbook and antenna handbook imply that loss 
> would be very low.  However, I've heard other sources say that the 
> spacing between conductors is too large a fraction of a wavelength for 
> proper cancellation to occur.
> 
> It would solve some issues I have with getting feedlines out of the 
> house if I could use an existing 450 ohm ladder line as my 2M feed.
> 
> Alternately, is 450 ohm ladder line acceptable for 6M?
> 
> Cathy
> N5WVR
> 
I've used an 80 meter double extended zepp fed with various feed lines
including 450 ohm line of 1" spaced #18 on 6 and 2m. In its best
directions I barely work further with the 5 wl yagi.

It works best with minimum bends and when spaced well away from other
conductors. Yet it probably does radiate noticeable. Years ago W0PFP
tried it on 6m but found his tropo range was greater with good coax. I
used his scraps for years.

And when it radiates it also receives things like computer and TV hash
that the coax may keep out.

Ladder line is fairly intolerant of ice plus wind.

Probably 300 ohm TV line has less radiation loss at 2m, but the trash on
the market these days barely has copper in the plastic and the form
flies in the wind to snap it off even without ice.

One time many years ago, I was on a kick of testing transmission line
losses by looking at the input SWR of a section that was shorted at the
other end. With my home made bridge (wire inserted under the braid
style) the difference in forward vs reflected power tended to come close
to reference book values for 8214 and RG-58, so I built a half wave
balun. With a short on it there was a bit more loss than the run of coax
to that balun. Then I put on several feet of heavy duty 200 ohm balanced
line with a short at the other end. SWR measured practically 1:1. It was
matched. When I moved a sheet of copper up to the short, the swr went up
to where it showed the loss I had expected. Which hints that maybe most
of the field was directed by the line, not carried in the conductors.
Like a two wire G-line.

There have been occasional reports of running very long (500 feet +)
open wire feedlines at 432 that made communications from a deep valley
possible by putting the antenna on top the bluff.

If you have an HF antenna fed with ladder line, it may well get out
better in its optimum directions (and at VHF it will have many lobes
because its many wavelengths long) than that 3 element quad. 25 years
ago I figured 169 contacts in 119 grids on 6m in the June contest wasn't
too bad with that 80m double extended zepp. At least I wasn't bothered
by needing to rotate the antenna.

I may try some open wire line for phasing stacked antennas in the future
but it will be 200 ohm line made of probably 12 gauge wire spaced not
very much. 2.74 wire diameters center to center or 1.74 wire diameters
air gap. I think that will work at 1296. The handbook formula 276 log
(b/a) is incorrect below about 300 ohms. It makes the conductor touch
above 100 ohms. The correct formula is 120 inv cosh b/a... So long as b
is greater than a, the impedance is above zero and the wires don't
touch.

You will need an antenna tuner for 6m and for 2m. I've built several,
generally a variable link or a fixed link with a series variable
capacitor and a balanced capacitor on the antenna coil with taps on the
coil for the feed line. Probably 6 or 8 turns 1 to 1-1/4" diameter for
the antenna coil tuned with a dual or butterfly 20 pf at 6m and 4 or 6
turns 3/8" diameter with a dual 10 pf at 2m. Variable link slipped
between extra wide spaced turns at the center.

Today, I prefer 3/4" surplus CATV cable, it has low loss and is often
free, though connectors can be a bother, but I make my own.

-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer