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From: geraldj@ames.net
Subject: Re: Dayton Johnson's 1296 double rhombic
Date: Sat, 30 Nov 1996 17:04:34 -0600
A rhombic is bidirection if and only if its not long and if its not terminated. Most rhombic design theory ignores the fact that the current in the wires decrease due to radiation. It makes computations simple that way. K0MQS found that his long rhombics for 2m (680 feet on a side as I remember) EME had no useable pattern off the back whether terminated or not. At 1296 this Laport rhombic is getting to be long too.

A model in NEC or EZNEC would easily show the effects of such a termination on the pattern because these programs take account of the radiation from the wires. In theory neglecting that radiation the terminated rhombic has no standing waves while the unterminated rhombic has standing waves an so currents in both direction. But if the rhombic is long there isn't much current to be reflected at the unterminated end. Where a rhombic as only a wave or two per side, the uniform current theory works adequately. Where its ten wavelengths on a side its convenient but questionable for predicting performance.

73, K0CQ

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