Dave,
A couple of winters ago a local group here had access to a tower tall
enough to support an end fed (J-pole style) vertical for the CQ WW160
Tests. We built a wire version including homemade 6 inch spacing ladder
line and ran it during the tests.
On the frequency that we designed the antenna for it was a VERY effective
antenna. Our problems using it were related to the relatively small 2:1
SWR bandwidth it presented and to high voltage effects at the feedpoint
when running 1500W to the antenna system.
The 2:1 SWR bandwidth of the antenna system was only about 30 KHz. We
thought we could get enough additional coverage with a high power T network
tuner to be able to use the system for the contest. And we could _BUT_ the
nature of the shorted 1/4 wave stub feed made the antenna system impedance
appear quite low when operating very far off resonance. Thus, the tuner
was always being operated in its most lossy configuration and had trouble
dealing with 1500W when the SWR was over 3:1. The tuner had problems at
3:1 & 1500W even though it could easily handle SWRs of 8:1 and more with
relatively little additional loss at full output if the load being matched
was above 50 ohms rather than below 50 ohms.
When we could get power to the antenna, it worked quite well until part way
through the first night, the antenna burned itself in half at the
feedpoint. Post mortem revealed that the voltage feed had been producing
corona between the unconnected arm of the stub and the other conductor.
The system tolerated the corona on peaks until N7DD came on duty as
operator. Larry's unique voice characteristics (and radio compression
adjustments) apparently raised the duty cycle to the point where the #12
hard drawn copper wire could no longer dissipate the heat in the cold
winter night air and it melted. As most of you are by now aware, Larry's
voice is all peaks.
Anyhow, the following year, we built a modified delta loop that matches 50
ohms directly and operates at a much lower system Q. It works better as a
TX antenna than the "J", requires only one small support (98 ft at apex),
and is broadband enough that the tuner is not necessary. Since the high
votage ponts are 86 feet apart and not at a wire terminus, even Larry can't
burn it up.
73, Eric N7CL
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Eric Gustafson N7CL | The mountains are high and
6730 S. Old Spanish Trail | the Emperor is far away.
Tucson, AZ 85747-9498 |
| You can't work 'em
INTERNET: n7cl@mmsi.com | if you can't hear 'em.
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