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Re: Topband: My Turn For a Brain Pick - Sanity Check

To: topband@contesting.com
Subject: Re: Topband: My Turn For a Brain Pick - Sanity Check
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Tue, 11 Jun 2013 18:45:58 -0700
List-post: <topband@contesting.com">mailto:topband@contesting.com>
On 6/11/2013 9:24 AM, Jim GM wrote:
So Jim, what are your final conclusions?

I was convinced by K2AV that the Yagi idea (passive reflector) was a bad idea because it is too sensitive to ground characteristics. I went to the Christman phasing method and it plays nicely.

Another mistake I made in NEC was trying to model the radials a fraction of an inch above High Accuracy Ground (I'm using the NEC2 engine), so I was making the elements too long. I switched to a much simpler model with the MiniNEC ground model, the elements grounded with a source at the bottom, and a small resistance load to simulate ground losses. That gave me numbers for element lengths that correlated well with measurement (one element in place at a time).

The physical implementation of this antenna is pretty slick. We're using the military surplus tubular metal mast sections, supported by the standard tripod base fixture, with sections of PVC mast section as each leg of the tripod. There are several vendors who sell these parts mail order or on the net (several are usually at Dayton). Each mast section has an overall length of 4 ft, but you lose 4 1/2 inches when you mate the sections.

One person can raise an element unassisted -- the tripod section is about 5 ft off the ground, so you simply insert sections from below, push them up, add the next. We have also used this mast construction to support small tri-banders and even inverted V dipoles for FD and CQP.

73, Jim K9YC
All good topband ops know how to put up a beverage at night.
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