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Author Topic: Capacity hats on mobile antennas  (Read 6136 times)
AC4RD
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« on: June 22, 2011, 04:19:22 AM »

A recent thread started by Simon, HS0ZIB, asked about ways to reduce the height of his mobile antenna.   The consensus advice was that he consider using a capacity hat on a shorter whip.

I've had the same situation--I need low clearance to get into my parking garage at work.  My best-performing 20m antenna here has been a 54" DXE mast, a 15m Hustler resonator, and about 38-40inches of whip above the resonator.  That tuned up nicely on 20 and clearly improved performance over the stock 20m resonator with a short (10-16", don't recall exactly) whip.  But the long whip didn't come close to fitting in the garage at work, so I spent a while playing with the long whip with an angle adaptor, making a sort of bent antenna--which worked but looked truly awful.

After people suggested a capacity hat to Simon, I read the information on K0BG's amazingly helpful website, and created a test cap hat on a shorter whip.  I used 3each 12" brass rods, crossed in the center to create 6 spokes, and some copper-clad steel antenna wire for a circle around the spokes.  I soldered the whole mess to the top of a short whip.  ("Mess" is right; this is not a beautiful piece of engineering.)  ;-)

The results were pretty amazing: the same mast/resonator that required 38-40" of plain whip now tunes up beautifully around 14.200 with only 8" of whip topped by the hat.  This fits easily in my parking garage, and 20 has been poor the last couple of days, but I've made a bunch of contacts with this antenna.  No A/B data yet but the short whip with cap hat appears to work just as well as the longer whip without a hat.

So I am exceedingly pleased with my first experiments with a capacity hat.  :-)
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K0BG
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« Reply #1 on: June 22, 2011, 09:15:11 AM »

Just remember, you cant to keep the hat away from the coil. I know that's hard to do with a Hustler, as the coil is mounted close to the top of the antenna. You'd do better using the shorter mast, and a longer top section.

My current cap hat (I'm always looking for more performance), uses a 4 foot mast, post whip, call it what you may, and three loops made from .125 x 72 inch, 6061T aluminum. Just switching from 17-7 stainless steel wire over to aluminum, made about 1 dB of difference on 20 meter, and about double that on 80 meters. Hard drawn brass works almost as good as aluminum.
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AC4RD
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« Reply #2 on: June 22, 2011, 09:39:42 AM »

... You'd do better using the shorter mast, and a longer top section.
... switching from 17-7 stainless steel wire over to aluminum, made about 1 dB of difference on 20 meter, and about double that on 80 meters. Hard drawn brass works almost as good as aluminum.

Once again, I am grateful to K0BG for sharing his wisdom and expertise.   I'll try shorter mast and aluminum in the near future.  Thanks, Alan!
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W9PMZ
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« Reply #3 on: June 22, 2011, 02:33:55 PM »

"made about 1 dB of difference on 20 meter, and about double that on 80 meters."

OK I will bite.....

Always looking to see other techniques in measurement, how did you make this measurement? 

73,

Carl - W9PMZ
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AC4RD
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« Reply #4 on: June 24, 2011, 09:34:02 AM »

... keep the hat away from the coil. I know that's hard to do with a Hustler, as the coil is mounted close to the top of the antenna. You'd do better using the shorter mast, and a longer top section.

I have the day off today, and just put together another cap hat with brass rod, 12" diameter as before, and put this one on a longer whip, as you suggested.   I was hoping to get a Hustler 12m resonator tuned up on 17 meters.  Instead, the new whip tunes up the 12m resonator beautifully on 20m!   I think it may be just a hair too tall for my parking garage, but I think it will be worth trying to work that out.   The *shorter* whip, the first one, inserted all the way in the 12m resonator,  tuned up perfectly in the middle of 17m phone.   I'm looking forward to running some errands tomorrow and trying the antennas out "live" from the car!  :-)
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K0BG
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« Reply #5 on: June 24, 2011, 12:43:13 PM »

I cannot make absolute measurements, but I can measure a difference between two configuration. I use a two port, miniVNA, and the same technique used by Rudy Severns, N6LF. It is explained here: http://rudys.typepad.com/files/qex-ground-systems-part-1.pdf

You can easily see changes in the unmatched input impedance, but you can't be sure the change is positive or negative. For example, you get about the same change in input impedance no matter where you mount the cap hat. However, if too close to the coil, it is due to additional coil losses (end effect), and if it is mounted at the tip of the antenna, you can assume the change is an increase in Rr. Even then is can be questionable if the cap hat's super structure is too close to the body work. The short of it is, it isn't a short process, and one where you cannot assume parameter you cannot measure directly.
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