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[TowerTalk] Re: Topband: Tophat? Does it have to be at the top?

To: towertalk reflector <>
Subject: [TowerTalk] Re: Topband: Tophat? Does it have to be at the top?
Date: Mon, 7 Jun 2004 13:13:57 -0700 (PDT)
List-post: <>
Makes me wonder if anyone has removed the cap hats on
the 40-2CD and slightly extended the element tips. I
know that VE6WZ built some replacement high-Q loading
coils for the 40-2CD but it's unclear if he uses the
old hats with the new loading coils. See:

73, Stew K3ND

> If they just needed to tweak the tuning very
> slightly and
> found that adding a hat was cheaper than retooling a
> coil
> form, then the hat is probably OK. If they added the
> hats to
> decrease loss or increase efficiency or if the
> capacitance
> of the hat is large compared to the antenna
> impedance
> extended beyond the hat, then they made a really bad
> decision.
> A small-sized hat, when placed near the coil,
> increases
> current in the coil **without** really decreasing
> current in
> the area of antenna extended out beyond the hat. In
> a large
> antenna with a small coil, the increased coil
> current makes
> little difference. In a large coil like a mobile
> antenna on
> low bands, it can noticeably degrade coil ESR. The
> increase
> in effective series resistance of the coil *can be*
> as
> severe as proportional to the reduction in
> inductance
> required by using the hat. In other words if coil
> reactance
> is 500 ohms and a hat makes the system only require
> 250 (by
> adding stray C across the coil), it COULD as much as
> ~double
> the effective series resistance of the coil!
> The really wild thing is this occurs without an
> increase in
> bandwidth, so you may think the efficiency is still
> good. In
> general any capacitance across the coil is bad news,
> but
> ONLY when the coil is somewhat large in reactance.
> If it's a
> low reactance coil it has so little loss even
> doubling loss
> won't hurt.
> If the hat is large and has a low impedance compared
> to what
> is above the hat, then you might as well throw away
> whatever
> is above (or beyond) the hat.
> I've started to put some stuff on my web page about
> the
> effects of current distribution on radiation
> resistance and
> about capacitance hats. It was also the subject of
> my talk
> at Dayton, that should be on
> 73 Tom

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