I've just finished putting up a ~300m NE/SW reversible beverage at G4AQG using
two pairs of paralleled WD1/TT wire for the feeder, spaced 50mm apart.
I used nine 2.7m 3x3 posts with electric fence insulators on top. The posts
are held in the ground with a metal fence post spike.
The wire is secured to insulators at the far end and tensioned with two fencing
wire tensioners at the feed end, using egg insulators just before the
tensioners.
One end post is guyed with fencing wire to a 4ft 5/8" earth stake via a wire
tensioner and the other guyed to a convenient tree.
There are two 4ft earth rods at each end.
I've put about 25lb tension in each line (measured with a travel scale) and
there's hardly any sag in the line and apparently no need for spacers to keep
the lines apart.
Hopefully wind and any ice loading will be within the 200lb breaking strain of
the paralleled wire!
The impedance of the line is about 470 ohms and common mode is around 390 ohms.
Allowing for earth losses in the termination, I reckoned that a 1:1 reflection
transformer was about right.
For 75 ohm coax, I used transformers with 6+6:5 , 7:3 and 3+3 :6 turns ratio
wound on fair-rite binocular cores and direction switched by relay.
I checked line impedance and matching impedance with an Antscope analyser, all
seems OK. The beverage works well up to 30m. There's good f/b from a nearby AM
station on 1485kHz ( at least 15dB) but not had time to measure 160m
performance yet.
Jeremy G3XDK/G4AQG
On 20 Sep 2014, at 15:06, Jim Garland wrote:
> I want to replace the 450 ohm ladder line for my two DXE 720 ft long
> bi-directional beverages with two lengths of WD-1/TT field telephone wire.
> The wires will be secured only at the ends, but will be supported along their
> length by ceramic insulators mounted on 4x4 posts spaced every 60 ft. The
> spacing between the parallel wires will be about six inches. Here are my
> quesetions: First, does anybody know what the likely impedance is of this
> configuration, or if not, how to measure it? Second, if I keep the DxE
> matching trnsformers used with the 450 ohm ladder line, will beverage
> performance be degraded noticeably with the new wures if, say, the impedance
> of the new line turns out to be 600 ohms or so? In other words, as a
> practical matter how important to performance (e.g., F/B ratio and
> low-noise) is it to make sure impedances are matched.
>
>
>
> Finally, a mechanical question. I want to have a pulley at one end of the
> parallel wires to equalize the tension in the wires. I've not been able to
> find a good pulley with a five or six inch diameter, except for nylon
> pulleys used for clotheslines. Do you think one of these could be used.
> There will be no motion of the pulley, since it is only used to equalize
> tension. The field telephone wire is rated at 200 lbs maximum tension, and
> I'm guessing the actual tension will be about half that.
>
>
>
> Thanks!
>
> Jim W8ZR
>
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