[UK-CONTEST] OT But maybe of interest

[Brian Miller]

Hi Adrian

Thanks for the info about your experiments. We use open wire feeder extensively
at ZL6QH, with some runs being up to 350 metres long.

We use 2.5-3.0 mm diameter solid copper wires spaced for a nominal impedance of
about 600 Ohms. The wires are supported on telephone style poles and insulators,
spaced at intervals of approximately 15-20 metres.

We have also undertaken loss measurements and the results are similar to what
you observed below.

We match our monoband yagi antennas (fixed directions) directly to the open wire
feed lines by using a 3 wire folded dipole driven element. Outside the shack we
use 12:1 baluns to convert between the open wire feeder and RG213 coax which
runs underground into the shack. The result is an SWR of less than 1.5:1 over
most of each band.

See http://www.zl6qh.com/000225.html for more info on the design of our baluns,
and see http://www.zl6qh.com/000332.html for info on our approach to the design
of the yagi antennas.

Note that you can also transform impedances using open wire line by adjusting
the spacing over some distance. In theory, you should be able to transform the
impedance from 600 Ohms to 50 Ohms by simply bringing the two wires together
towards the 50 Ohm termination. For the same reason, the actual spacing along
the line is not critical, as long as you end up with the correct spacing at each
end.

Good luck with your plans.

73

Brian ZL1AZE

----- Original Message ----- 
From: "Adrian Rees (MW1LCR)" <rees.a at btconnect.com>
To: "'UK Contest reflector'" <uk-contest at contesting.com>
Sent: Tuesday, February 06, 2007 9:29 AM
Subject: [UK-CONTEST] OT But maybe of interest


Hi all
As some of you are aware, I have been conductng (pun not intended!) some
experiments using Open Wire feeder systems to counter Transmission line
loss from (what will be) my shack and Monoband Yagi's.

Some of you wanted to know the results of this, and here's some basic
measurements, carried out yesterday, in between chain sawing fallen
trees!

Using a homebrew 28MHz Monobander (3 Elements) with a measured feed
impedance of (R +/- jX Ohms)27.02 + j 0.50, feeding with 347.85 m (70
Half Wavelengths at 28.5MHz x 99.7% Velocity Factor) of 450 Ohm Ladder
line, with 100w in, gave 61 Watts out. A loss of 1.7dB, or thereabouts.

(The feeder was trimmed to present the same impedance  at the aerial
feed point at the shack end. Therefore on the measurement freq, 28.5Mhz,
the actual feeder length was a multiple half wavelength. This was
matched through a 2:1 Balun).

The same Yagi, when fed with a length of RG213 (350m), with 100w in,
resulted in 5.4 watts out. A loss of 12.7 dB, or thereabouts.

I plan on conducting similar experiments with 50MHz and 144MHz Yagi's,
just to gain (pun not intended!) more insight into this feeder
mechanism, as I want to feed all my aerials with Open Wire feeder.

I would appreciate comments on the following:

1. What would be the most efficient method of matching the Open Wire
Feeder, such that the shack end sees 50 Ohms ? (Could be matched at the
aerial or shack end ?).

2. Any ideas how I get open wire feeder around the rotator ?


Thanks
Adrian
MW1LCR



-- 
No virus found in this outgoing message.
Checked by AVG Free Edition.
Version: 7.5.432 / Virus Database: 268.17.24/668 - Release Date:
04/02/2007 01:30





--------------------------------------------------------------------------------


_______________________________________________
UK-Contest mailing list
UK-Contest at contesting.com
http://lists.contesting.com/mailman/listinfo/uk-contest