Gentlemen:
Ford Peterson Wrote:
-----Original Message-----
From: Ford Peterson <ford@cmgate.com>
To: topband@contesting.com <topband@contesting.com>
To: <topband@contesting.com>
Date: Sunday, November 05, 2000 1:37 AM
Subject: Topband: High Z vs Low Z shunt feed tower
>
>I've got a 68 ft Rohn tiltover with 12' of mast, a tribander, two meter
>beam, six meter dipole on it. I load the whole shootin' match on 160 with
a
>14 awg wire tied to the 67.5' level.
>
>When I designed it, I modeled the tower the best I could using NEC2. Based
>on playing with that model, I chose to place the shunt 1 meter off the
>tower. It shows a 450 +j220 or so (1840) and match it with a doorknob cap
>and 9:1 transmission line transformer. As it turns out, I can get a match
>with the autotuner just about anywhere on the 160 band.
>
>I've been in several private debates over whether the 1 meter distance was
a
>good choice. The large distance gives me a rather high impedance. ON4UN
>indicates that this type of match should be avoided but doesn't explain
why.
>
>Question:
>Is it better to feed this type of tower at a high impedance or low
>impedance?
>
>Ford-N0OQW
>ford@cmgate.com
>
>
I believe the shunt feed system, as long as it uses low loss components,
should be effective, whether it is high Z or low Z at the feed point.
The real issue is having enough radials to reduce ground losses, and well
spliced tower connections, and strap from top of tower to mast to avoid any
RF going through the rotator or thrust bearing. On the last pieces of 25G
we put up, we used zinc filled grease on the splices to avoid rust and
ozidation. Similar greases are available from electrical supply stores for
aluminum connections, like the boom splices, to avoid corrosion.
Depending on how much top loading you use, you should consider whether the
tribander you use is the plumbers delite type where most of the elements are
grounded to the boom like HyGain, some Cushcraft, most Mosley, or whether
you can afford to have the elements all insulated from the boom and,
therefore, less top loading.
Also, don't just start grounding elements on a beam designed with ungrounded
elements. That changes the effective length of the reflector and directors
and can ruin the gain and F/B ratio of the beam.
Good luck 73
George K8GG
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