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Author Topic: Screwdriver Problems - No 80m Zero Reactance?  (Read 3573 times)
TERRY_PERRY_EX_W3VR
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Posts: 70




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« Reply #30 on: August 30, 2009, 12:42:54 PM »

Enjoyed the posts, and civil exchange.

I sure hope to see an article.

Enjoyed it.

Thanks.....

W3VR
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K0BG
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Posts: 9839


WWW

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« Reply #31 on: August 30, 2009, 02:28:58 PM »

Larry, I never claimed I was an RF engineer, but you did say you were a master of physics. Assuming you are, I agree with Terry about publishing an article. You can start with the one you proffered in your mobile article earlier this year. That is, the one about the cavity resonance (?) I think it was.

Alan, KØBG
www.k0bg.com
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K5END
Member

Posts: 1309




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« Reply #32 on: August 30, 2009, 06:44:38 PM »

Alan,

I\'m not sure where you got "cavity resonance," (although that is a good topic.) You may be confused.

We discussed the concept of "APERTURES," and how it contradicts your beliefs on mobile HF antenna mounting practices, particularly in the context of the shapes and topology of the metal underneath the antenna.
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K0BG
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« Reply #33 on: August 31, 2009, 05:44:53 AM »

Larry, it is this article (http://www.cummins.com/cmi/content.jsp?siteId=1&langId=1033&menuId=81&overviewId=409&menuIndex=6)

Here is part of an addition you added to your article: In addition to coupling to the vehicle body, it\'s important to consider that the pickup bed and closed tailgate become somewhat of a cavity containing the lower 40 cm or so of the antenna, and the aperture of said cavity--the open top of the bed--is roughly 1.2/20 wavelength for 20 meters, 1.2/40 wavelength for 40 meters and 1.2/80 wavelength for 80 meters.


Alan, KØBG
www.k0bg.com
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K5END
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Posts: 1309




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« Reply #34 on: August 31, 2009, 06:54:12 AM »

yes, you have created a cavity.

I called it a bathtub too.

But "cavity resonance" is another topic entirely.

Do you spend a lot of time arguing with stone walls, fence posts or, yourself?
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K5END
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Posts: 1309




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« Reply #35 on: August 31, 2009, 06:55:56 AM »

And what is with the link to the Cummins fuel/engine page?
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K0BG
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Posts: 9839


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« Reply #36 on: August 31, 2009, 07:17:47 AM »

Sorry about that, I didn\'t catch it. Here\'s the URL: http://www.eham.net/articles/21325

Please explain the cavity scenario.

Alan, KØBG
www.k0bg.com
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K5END
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Posts: 1309




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« Reply #37 on: August 31, 2009, 07:26:13 AM »

You don\'t understand and I\'m tired of arguing about it.

Look it up.

You\'ll remember it better that way.

Besides, I doubt you\'d take my word for it.
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K1KRV
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Posts: 14




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« Reply #38 on: August 31, 2009, 09:39:23 AM »

Hi Larry & Allen,

I used the weekend to install a new tonneau on the pickup bed, so didn\'t do much antenna testing.
 
The new tonneau would be easier to use if I move the antenna from the current stake-pocket mount.  I could mount the antenna on the tonneau -- on a section that bridges the front of the bed.  This is a bit forward of the stake-pocket location, but it may be worth considering.
 
This location has the advantage of bringing the antenna inboard and putting more metal mass under it.  It would be similar to mounting the antenna on the TOP of a bed toolbox, rather than the aft panel of the box.

In my case, the top half of the mast (18") would still be above roofline -- so the coil appears clear.  At roofline the clearance is 8" to the mast.  Halfway up the rear window, 6" to the mast.  Base of the antenna, about 4" behind the cab window.   Has anyone tried a antenna in this location?  Any concerns about capacitance with the roof?

BTW: Nice to see others chiming in!  

Given the \'ground\' covered in this thread (pun intended) I have a few things to work on.  I\'ll report back with new developments.

73,

Kurt
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K0BG
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« Reply #39 on: August 31, 2009, 11:01:52 AM »

Kurt, you might look at Breedlove\'s mount made to clamp over the front rail of a pickup bed. It might be what you\'re looking for.

Alan, KØBG
www.k0bg.com
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K1KRV
Member

Posts: 14




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« Reply #40 on: August 31, 2009, 11:45:32 AM »

Thanks Alan,

Will do!  My stake-pocket mount is Breedlove\'s and the quality is outstanding!  

73,

Kurt
K1KRV
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K1KRV
Member

Posts: 14




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« Reply #41 on: August 31, 2009, 09:40:45 PM »

Allen, Larry & W3VR (if your following along) :-)

After looking at the Breedlove \'front-lip\' mount, I found a way to \'home-brew\' something similar.  Allen, may like the fact that my mount required \'drilling holes\' LOL :-)
 
I heated and bent the ends of a long piece of 3" x 1/4" aluminum flat stock.  It bolts to the truck frame and to the front of the bed.  The top is bent back over the front-lip of the bed, providing a very sturdy 3" x 3" x 1/4" mounting plate.
 
Question: concerning possible replacement of my current \'screwdriver\' antenna:

Would you choose a 36" mast like the Tarheel 100 with 54" whip or the 28" Scorpion "Shorty" with 68" whip?  Here\'s the issue:

The bed-rail height on my truck is 4\' 6" from the pavement, so a 36" \'screwdriver\' with 6\' whip hits the over-hanging trees in my area.  13\' 6" max height is just too tall.  I might \'squeek by\' with 13\' but 12\' 6" would be better.

The Tarheel 100 (or similar) requires 6" underneath for wire connections, therefore (4\'6" + 6" + 36" + 4\' 6" = 12\' 6" total) whip length limit 4\' 6" - 5\'

The Scorpion "Shorty" has side connections, therefore (4\' 6" + 28" + 68" = 12\' 6" total) whip length limit 5\' 8" - 6\'

The mast and the whip are working together, so which combination is likely to provide the best results?  Which end should I sacrifice?  My guess is that \'real world\' performance between these two would be pretty close.  Still, it would be an expensive purchase, so I just thought I\'d ask.

73,

Kurt
K1KRV

P.S. Larry, I did some tests with the Daiwa meter and the radio is putting out.  SWR measurements seem to be in line with the antenna analyzer.  The \'Hamstick\' is working but the \'screwdriver\' is not -- ergo?  

Thanks again for reminding me about W8JI\'s website.  I re-read it and learned alot.  Tom\'s truck is a lot like mine.
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K0BG
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Posts: 9839


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« Reply #42 on: September 01, 2009, 06:42:58 AM »

The "short" Scorpion is as good as most full-sized screwdriver type antennas. The coil, hence the Q, is the same as their standard model. It is just a bit shorter overall. The hit you take depends on the installation.

If there is a drawback to the Scorpion, it is weight. The little one weighs 13 pounds, the big one 18 pounds. Obviously, you need a sturdy mount.

I was unaware the Breedlove required holes, but I'm not surprised. You almost have to to get a strong attachment.

Alan, KØBG
www.k0bg.com
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K1KRV
Member

Posts: 14




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« Reply #43 on: September 01, 2009, 07:21:11 PM »

Hi Allen,

Just for clarification -- Breedlove mounts DO NOT require drilling holes!  

I fabricated a 'home-brew' mount that is LIKE the Breedlove -- MY adaptation bolts through the front of the bed.  

Breedlove makes superb products and offers terrific service.  I recommend Jerry very highly!!

Thanks for your opinion on the Scorpion. The 28" height and side connections work well for a bed-rail mount.  It's a heavy antenna, as you say, but my new mount should be up to the task.

Tarheel and High Sierra don't publish a 'Q' figure and that surprises me.  It would be helpful information to have, even if the figure is a bit lower than Scorpion or Hi-Q.

Hi-Q recommends a 7' 6" whip for their 4/80 model.   Attached to their 3' mast, that would be a tall antenna for truck bed-rail installation! Too tall for me!  

I'll do some 'shopping and post my progress.

73,

Kurt
K1KRV
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KD5AIU
Member

Posts: 12




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« Reply #44 on: September 16, 2009, 10:52:33 AM »

Is this the antenna you have?

http://www.eham.net/reviews/detail/7619

I'd love the experts opinion of this model as
it seems to be the lowest cost HF mobile I've
come across.
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