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Author Topic: Question about Capacitance Hat  (Read 253 times)
W1CEW
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« on: December 15, 2006, 03:04:17 AM »

http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=11462

So I've been looking into building this very simple/cheap 80 meter vertical, but am not clear on one thing:  Do capacitance hats electrically connect to the main radiator?  What keeps it from just looking just like the radiator but lengthened slightly rather than some kind of capacitance value?

Thanks and 73,

Chuck
W1CEW
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AA4PB
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« Reply #1 on: December 15, 2006, 04:04:04 AM »

Yes, it connects to the main radiator. It provides additional capacitance between the top of the radiator and Earth ground. That permits you to tune a physically short radiator to resonance with less series inductance and therefore less I*R loss.
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W5DXP
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« Reply #2 on: December 15, 2006, 06:14:40 AM »

There's a small amount of confusion about what a top hat really is and really does. Some hams consider the top horizontal portion of an inverted-L to be a top hat. Technically, it is not a top hat and is instead simply part of the radiating antenna. An ideal top hat doesn't radiate in the far field. Well designed real world top hats don't have a whole lot of far field radiation because they are symmetrical to the vertical radiator and provide much more capacitance than radiation resistance.

The simplest top hat possible is probably a horizontal wire connected at the center to the top of the radiating element. When the current in the vertical radiating wire gets to the horizontal top hat wire, it divides equally and goes in opposite directions. Equal currents in opposite directions are 180 degrees out of phase and thus tend to cancel far field radiation through destructive interference. This effect can be observed using antenna modeling programs like EZNEC, a free demo copy of which is available at: http://www.eznec.com
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73, Cecil, w5dxp.com
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
NG0K
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« Reply #3 on: December 15, 2006, 06:50:09 AM »

I use a cap-hat on my 40m short vert and it really works well.  Normally cap-hats should be symmetrical as previously stated.  But I've modeled a few antennas with cap-hats that weren't and I've found that symmetry is not that critical.  So if you can't get the top-hat wires totally opposite of each other, it probably won't be that noticable.

73, Doug
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73, Doug - NG0K
W1CEW
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« Reply #4 on: December 15, 2006, 07:07:36 AM »

It just seems amazing to me that you could make an 75/80 meter vertical that was only 40 ft high, so I'm thinking about doing it.  My 160 Meter horiz loop is up around 40 feet, and isn't really cutting it for long haul DX, so I was hoping something like this would be better.

Hmmm, I wonder if a half dozen or so 35-40 ft non reasonant radials along the ground would be sufficient?

-Chuck

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N3OX
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« Reply #5 on: December 15, 2006, 07:29:05 AM »

"Hmmm, I wonder if a half dozen or so 35-40 ft non reasonant radials along the ground would be sufficient? "

No.  Make them 0.05 wavelength apart at the tips, minimum.  This means you need at least 20 of them if that's the longest you can get.  

The thing about a shortened vertical is that it's got a low resistance at resonance, and hence, once you match it, more current flowing in the ground system.

Forty feet is fairly modest shortening, but 6 radials isn't enough.  I use a forty-foot base loaded vertical with 27 radials between 20 and 40 feet long, but it's at best -3dBi... I make lots of contacts, though.  If you put down 20 40 footers and top load, you'll do better than me, for sure.

Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
W1CEW
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« Reply #6 on: December 15, 2006, 08:10:37 AM »

Hi Dan,

Just curious if your radials are buried, and is the vertical ground mounted?

Thanks so much for the tips -- I assume I'd be in the exact same boat if I were doing this home brew vertical versus any of the typical commercial varieties, because (Butternut, Hygain, MFJ) tend to be shortnened as well.

73,

-Chuck
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W7ETA
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« Reply #7 on: December 15, 2006, 12:37:06 PM »

W4RNL has a lot of info on line centering on your questions.
73
Bob
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N3OX
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« Reply #8 on: December 15, 2006, 01:37:16 PM »

Vertical is ground mounted, and the radials are just lain on the grass and stapled down with galvanized wires.

I marked off 27 radial lines on a piece of paper, spiked it down in the middle of the yard, and attached the free  end of a spool of wire to the spike, and walked back until I was lined up with each line and had hit the edge of the lawn.

Took three-quarters of a day of hard work, but now they're down and I can experiment.  I am thinking of adding a few extra long ones in the directions I can (routing them around the house) to maybe pick up a little extra gain on the low bands... symmetry isn't as important as total amount of ground covered by radials.

Pictures of my vertical are at www.n3ox.net/projects/lowbandvert

I'm thinking of wire inverted-L or top loaded with wires for 160 though.  I base loaded mine on 160 and it's only 20kHz between the 2:1 SWR points and it shifts when it gets dew on the coil ;-)

73,
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
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