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Old 7-Jun-2011, 10:02 PM   #10
GroundUrMast's Avatar
Join Date: Oct 2010
Location: Greater Seattle Area
Posts: 4,681
Originally Posted by ghz24 View Post
... Any idea what kind of gain (if any) this gets at ch 4 frequency?
The older documentation on rhombic antennas often comments on the low SWR over a fairly wide frequency range (2:1 is referred to as I recall), that is not an indication constant forward gain though. The rhombic is a non-resonant antenna, an optimized long-wire design. It's relatively constant input impedance over a wide range of frequencies is the result of it's traveling wave design with resistive termination. The goal of the design is to align the lobes of the individual legs to work together in one direction. The lobes of radiation from a long wire antenna are frequency dependent so it would be logical to assume that as you move away from the design frequency, the forward gain will become 'un-focused'. With good modeling software, exploring the effects of off frequency operation is far easier than it was in the late 40's and early 50's when Laport was number crunching on slide-rule and note pad.

The same shape can be scaled to operate on VHF channel 4 and would have the same gain. The antenna would be about 10 times larger.

Cut for channel 30, the short element would be 71.875" and the long element would be 123.214" which results in the front to rear length a bit more than 181" and the width about 80".

(I have a specific interest in UHF-46...)
If the well is dry and you don't see rain on the horizon, you'll need to dig the hole deeper. (If the antenna can't get the job done, an amp won't fix it.)

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Last edited by GroundUrMast; 9-Jun-2011 at 6:25 AM.
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