The answer is YES....I have removed the capacity hats from the yagi and
extended the element tips to compensate by adding some small AL tubing
tips. At first I left the hats on, but early this past winter season I
removed them.....the benefit of standing on my roof and working on the
antenna : ) (the photos on my webpage are old).
Ever wonder why the bandwidth on these beams is so broad, and why it is
a perfect match to 50 ohms ???
I assumed is was mostly due to the lossy coils, but discovered it is
mostly the result of clever (but compromise) tuning.
When the Yagi is tuned to the Cushcraft dimensions, I discovered by
doing field testing on the beam that the max. F/B was set much below the
design resonant frequency. For example, if the Yagi is tuned for 7065
min. SWR, then the max F/B seemed to be way down around 7005 or so. By
field testing the beam ( making polar plots of the pattern) I re-tuned
the reflector (shortened it) to bring the max. F/B up to around 7060.
Doing this lowers the feedpoint impedance because of increased mutual
coupling, so I needed to add a hairpin coil to get a 50 ohm match.
Also, the bandwidth went way down to around 150 KHz, which is not a
problem for me. Modeling also confirms that the re-tuning improves the
F/B somewhat and provides a bit more forward gain.
I wouldn't necessarily recommend this mod, since the easy match and big
bandwidth is good to have. If you're a purist looking for that last dB,
then you may want to think about it.
de steve ve6wz
> -----Original Message-----
> From: email@example.com
> [mailto:firstname.lastname@example.org] On Behalf Of GALE STEWARD
> Sent: Monday, June 07, 2004 2:14 PM
> To: towertalk reflector
> Subject: [TowerTalk] Re: Topband: Tophat? Does it have to be
> at the top?
> 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 K3LR.com
> > 73 Tom
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See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
TowerTalk mailing list