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

[k2qmf at juno.com]

Hi All,

If you check the picture at the end of VE6WZ web site
you will see that he still uses the Top Hats on the 40-2CD.

73,  Ted  K2QMF

On Mon, 7 Jun 2004 13:13:57 -0700 (PDT) GALE STEWARD <k3nd at yahoo.com>
writes:
> 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:
> 
> http://www.qsl.net/ve6wz/CC_coil.html
> 
> 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 K3LR.com
> > 
> > 73 Tom
> > 
> 
> 
> 
>         
>                 
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