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The perfect length for a 75M double extended Zepp




 
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Author Topic: The perfect length for a 75M double extended Zepp  (Read 8301 times)
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K1JJ
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"Let's go sailing, Tommy!" - Yaz


« on: September 28, 2005, 12:01:48 AM »

One of the guys has a new double extended Zepp on 75M. It appears to be quite a great performer at 90' high. He axed me to model it and let him know about the pattern.  Presently it is about 320' long, center fed with open wire at 90' high..

I did the modeling and found that a 320' center fed dipole on 75M has a very narrow bi-directional pattern of only about 34 degrees wide. Plus, the side lobes are quite big, down only -10db. A lot of power was being wasted in these side lobes as well as the front lobe being too narrow.

I found that by DECREASING the length 20'-25' to about 295'-300' produces a MUCH cleaner pattern with the side lobes now down about -20db and the front lobe wider at about 40 degrees. This is what you want for this antenna since it is fixed in one direction.

The resulting forward gain drops only 0.4db with this power now added to the broader frontal lobe. The sidelobes also put their power into the broader frontal.

A big improvement on this antennas is to add a pair of reflectors, end-to-end behind the 295' center fed. About 40' behind is fine and a length of 129' per reflector. From a bird's eye view it looks like two, 2el Yagis side by side, with similar gain. The front to back is at least 20db. 

A trick to get the best pattern is to pull the reflectors out so that the end insulators of the center-fed and relectors line up. You will need a 30' rope connected to the inside reflector insulators to hold them apart. 

This system has about the most gain you can get on 75M using simple wire techniques and supports. With about 7-8db gain over a standard dipole, it'll make anyone a channelmaster. Plus with the front to back, you will knock down all the atmospheric noise off the back and hear much better. If it is up high, add another 10db for low angle advantage over a low dipole over a 2,000+ mile path..

73,
Tom, K1JJ

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