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Author Topic: Capacity hats on whip antennas  (Read 8954 times)
AC4RD
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« on: August 14, 2011, 11:09:44 AM »

We were talking last week about how to figure how much a capacity hat might change an antenna's resonance; I got a couple of new RM-12 resonators on sale the other day and measured the difference with and without a cap hat, on a simple test rig.  Info and a very bad photo here:

http://www.duke.edu/~kuzen001/caphat1.htm
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K0BG
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« Reply #1 on: August 15, 2011, 05:52:34 AM »

Rather than write an epic here, go to my web site, and read the Cap Hat article.
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AC4RD
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« Reply #2 on: August 15, 2011, 09:19:35 AM »

I've read it, Alan, and about 80% of the rest of your website--there's so much info there I haven't seen it all yet.   But with Simon's question, and also a question on another forum that asked for a description of the cap hats I've built, I thought it was worth posting a link to one small datapoint and a photo.  (I wish I had mentioned on that page that the photo shows the resonator & whip tilted down horizontally to make the photo better, that in ordinary setup and operation they're straight vertical.)

It was reading your website's article on cap hats that made me decide to try them, for more efficiency from a given (and limited) length mobile antenna.   And I'm convinced they make a big difference.   (I agree my construction is lightweight and ugly, but it's all I need.)

What I NEED to re-read on your website are the pages about noise control and bonding; I'm having fun on 15m from home but can't use it from the car because of air-conditioner noise.
« Last Edit: August 15, 2011, 09:24:50 AM by AC4RD » Logged
K0BG
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« Reply #3 on: August 15, 2011, 02:48:26 PM »

Starting about 3 years ago, I built about 20 different versions. The one I settled on, is comprised of three loops. It has the benefit almost as good as four loops, but a lot less wind loading. Due to the weight, it is difficult to support one on a SS whip. But you really don't want to do that anyway.

17-7 stainless steel has a permeability of about 240, compared to 2 for 6061T6 aluminum. You don't notice it much on 20 meters, but down on 80 meters is amounts to almost 3 dB of loss. It is even worse on 160. In fact, you can see the about a dB of difference in field strength between SS loops, and aluminum ones.

Incidentally, photos of a 3 loop design first appeared in the pages of QST in 1961. It was described about 10 years earlier than that. If there is a drawback it is the aluminum wires for the loops. You almost have to use 2045, or 7075 as a second choice, instead of 6061T6, as they last much longer. The 6061 tends to sing in the slip stream, and ends up breaking off. 2045 will last about 4 to 6 weeks, depending on the amount of driving you do. The real problems? A 72 inch, by 1/8 inch piece costs over $60 !
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AC4RD
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« Reply #4 on: August 16, 2011, 04:07:58 AM »

17-7 stainless steel has a permeability of about 240, compared to 2 for 6061T6 aluminum. You don't notice it much on 20 meters, but down on 80 meters is amounts to almost 3 dB of loss. It is even worse on 160. In fact, you can see the about a dB of difference in field strength between SS loops, and aluminum ones.

That's why I've been playing with replacing my SS (Hustler stock) whips with brass rod, hoping for a tiny improvement there.  (You posted a link about the difference between SS and copper a week or two ago.) But right now I'm only active on 20 and 17, where I suppose the difference would be fairly small.

As the days get shorter, I really *should* set up a mobile 40m antenna, I suppose.  On my morning drive, 17 is usually completely dead, and 20 is pretty close to dead, at least from the car.  I sure wish the solar cycle would hurry up and give me some propagation!  :-)
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K0BG
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« Reply #5 on: August 16, 2011, 06:50:09 AM »

That depends on how you define small. The real issue is to put as much SNR in the other guy's front end, that he can hear you Q5. Sometimes, even 1 dB can made a tremendous difference. Comments like, it's only half as S unit, are unfounded.
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W8JX
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« Reply #6 on: August 16, 2011, 12:47:43 PM »

Sometimes, even 1 dB can made a tremendous difference. Comments like, it's only half as S unit, are unfounded.

Maybe is EME work near noise level in UHF but in HF 1 db will never make a "tremendous" difference and is pretty much unnoticeable. Also a half a S-unit is generally 3 db.
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AC4RD
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« Reply #7 on: August 17, 2011, 04:16:49 AM »

A couple of months ago I was on a long trip in my car, and heard a 9M6 working a medium-sized pileup.  That would have been a new one for me.  I tried for 10 or 15 minutes before the path faded, and never worked him.  I also heard a 5BA; I had never worked Cyprus from the car, so I spent 20 minutes waiting and trying when the conditions were best, and finally worked him.  But not the 9M6. 

I can PROMISE you, I'd have been glad to have an extra dB or two in both those situations.  I'm working right now on changing some stainless whips to brass or copper-clad, to try to scrape a bit more performance out of my mobile system.
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K0BG
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« Reply #8 on: August 17, 2011, 06:12:14 AM »

Copper clad wire isn't any better than the base material, unless the cladding is very deep. Most of the time it isn't much more than a coating.

Aluminum is about as good a compromise as you're going to get. The permeability hovers around 2, compares to most stainless steels at around 240. Brass is good too, but isn't as vibration resistant as most aluminum mixes.
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W8JX
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« Reply #9 on: August 17, 2011, 07:42:08 AM »


I can PROMISE you, I'd have been glad to have an extra dB or two in both those situations. 


Been doing HF mobile for over 20 years and I never had a contact that 1 DB or so would have been a deal breaker. (there has been a few times that a S-unit would have) Mobile is a challenging environment at time with QRN and QSB from buildings and power lines. I will not even try to work anyone for a mobile unless they are well above noise floor and I am with them too. I have worked Europe, Russia, Asia and down under many times mobile over the years when band is open. I put this ! db or so concern in same league as those that think you must have LMR400 vs 213 or even 8x for shorter runs for HF. You will never see the difference but some would like to believe otherwise.
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N5MOA
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« Reply #10 on: August 17, 2011, 07:53:54 AM »

Sometimes, even 1 dB can made a tremendous difference. Comments like, it's only half as S unit, are unfounded.

Yep. Every db you can muster counts when the signals are weak.

Half a S unit can, and has, made the difference between a qso or no qso.




I can PROMISE you, I'd have been glad to have an extra dB or two in both those situations.  I'm working right now on changing some stainless whips to brass or copper-clad, to try to scrape a bit more performance out of my mobile system.



Bruce, XW1B last year about this time.  All he could get was the "/m". I wish I could of had "just" another half a S unit going out.




Been doing HF mobile for over 20 years and I never had a contact that 1 DB or so would have been a deal breaker. (there has been a few times that a S-unit would have) Mobile is a challenging environment at time with QRN and QSB from buildings and power lines. I will not even try to work anyone for a mobile unless they are well above noise floor and I am with them too. I have worked Europe, Russia, Asia and down under many times mobile over the years when band is open. I put this ! db or so concern in same league as those that think you must have LMR400 vs 213 or even 8x for shorter runs for HF. You will never see the difference but some would like to believe otherwise.


If all you work is 5/9 stuff, you probably won't notice the difference.

Try working stuff just above (if that) the noise floor. You might change your mind.
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W8JX
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« Reply #11 on: August 17, 2011, 08:44:31 AM »

Try working stuff just above (if that) the noise floor. You might change your mind.

In a mobile again if it is that bad it is going to take more than 1 db. I do not enjoys QSO you have to fight to listen/work and 1 db will not change that. 3 db (1/2 S-unit) might and 6 db would help a lot here. 
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AC4RD
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« Reply #12 on: August 17, 2011, 09:11:02 AM »

Copper clad wire isn't any better than the base material, unless the cladding is very deep. Most of the time it isn't much more than a coating.

Wow--I'm glad you told me that BEFORE I wasted time trying to make a whip out of copperclad antenna wire.  :-)  The brass will probably work OK for me, for now--it's easy to solder, which is helpful, and the vibration isn't much of an issue for me; I've got short whips due to my garage situation.
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K0BG
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« Reply #13 on: August 17, 2011, 11:57:41 AM »

John, I've been operating mobile longer than you've been licensed! If I thought it wasn't worth the effort, I wouldn't have dumped all that money in every mobile I ever had.

I can assure you, that even .5 dB can make a tremendous difference. What you have to start looking at is signal to noise ratio, and not the absolute signal strength. If you understood receiver dynamics, you'd already know that.
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AC4RD
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« Reply #14 on: August 17, 2011, 05:00:28 PM »

I can assure you, that even .5 dB can make a tremendous difference.

I had a bunch of good opportunities to work ST0R from the car on 20m, several afternoons of good reception, but I never did work them from the car.  But  I *did* work them from that lawnmower rig I posted recently, with a 1/4 wave vertical with elevated radiators.

I can guarantee you, I'd have been DELIGHTED to have one more dB in my antenna system, driving along on those afternoons, trying to work ST0R.  :-/ 
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