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

Tue Jun 8 11:57:21 EDT 2004

Steve, what sort of difference did you see in db F/B? Was it 2-3 db or was
it more?

73/Peter SM2CEW

At 00:12 2004-06-08 , you wrote:
>Stew:
>
>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: towertalk-bounces at contesting.com 
>> [mailto:towertalk-bounces at contesting.com] 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:
>> 
>> 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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questions and ask for Sherman, W2FLA.
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