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Author Topic: 102" vs 108" whip for auto coupler  (Read 4257 times)
K4ED
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Posts: 12




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« on: June 09, 2008, 05:32:46 PM »

As I am shopping for the parts needed for my mobile project, I keep seeing what seem to essentially the same 11 meter 1/4 wave whip antenna listed by some vendors as a "102 inch whip" and a "108 inch whip" by other vendors.

Are there really some whips with six extra inches or is this a measurement thing?  The prices are comparable, and six extra inches of essentially random length wouldn't hurt.

--
de K4ED in Grid FM09
QRA: Mark Sunderlin, QTH: Winchester, Va
"Never forget XYL Z as a factor in antenna efficiency"

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K0BG
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« Reply #1 on: June 09, 2008, 06:00:31 PM »

Once upon a time, you could buy 96, 102, 104, 108, 112, and even 116 inch whips. That is NOT the case today. The nominal length is 102 inches.

Length does matter. For example, a 12 foot whip has four times the radiation resistance of a 6 foot whip. In other words, it is to the square. Obviously, there is a limit to overall length. That said, when you use an auto-coupler with a whip (and you do it correctly which isn't a given by any means), it is essentially a base loaded antenna. If the same length antenna was used, and the coil was in the center of the antenna (rather than at the base), the radiation resistance would be about double, and so would the efficiency.

There is a whole lot more to the formula that what I've presented here, but the bottom line is, just how good of an antenna do you want to rive around with?

Alan, KØBG
www.k0bg.com

PS: You might want to visit my web site.
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K7AAT
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« Reply #2 on: June 09, 2008, 10:15:16 PM »


  K0BG's response was chuck full of good information, but the essence of the poster's question went unanswered.  One must be careful when reading the ad's for 108" whips.  Many vendors include the base spring as part of the dimensions for the whip.

  But the short answer to the question is yes,  there apparently ARE 108" whips still available, such as MFJ's Model 1966.  If you have the choice then the slightly longer would be preferable.  Of course you could go with the outlandishly expensive 12 foot model offered by Array solutions, if you really want more efficiency in a rolling mobile installation.

  Ed   K7AAT
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K0BG
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« Reply #3 on: June 10, 2008, 11:12:18 AM »

Here's a bet. The actual length of the MFJ whip is 102 inches, not 108 inches. They had these at Dayton, and they were 102 inch, just like everyone else's. The other reason I say this is, there is currently only one manufacturer of 17-7 stainless steel whips. The longest supplied length is 102 inches.

If you want to extend the whip longer, go to GLA Systems (texasbugatcher.com), and buy one of their masts. A 4 foot one is under $50 including shipping. If you need a spring, you can buy a big enough one to hold even an 8 foot mast with the whip on top! Problem is, they cost nearly $225!

When you use an auto coupler and whip as a base loaded antenna, the RF voltages can exceed 10 kV even at low power levels. So, what ever mount you use has to be able to withstand a lot of electromotive force. By the way, NO ONE make a ballmount that will stand up to this amount of stress.

Alan, KØBG
www.k0bg.com
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N8EMR
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Posts: 235




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« Reply #4 on: June 10, 2008, 01:05:29 PM »

Array Solutions http://www.arraysolutions.com/Products/mobilewhip.htm has a 12.5 ft whip but its $180, Shakesphere has 16ft and 32ft whip system in there military catalog, AT-120 and in the same ball park price wise as a scredriver antenna.
Moonraker(yes the cb people) make a 9ft military mast.
DX engineering has mast upto 72inches and those can be screwed into each other to build a whip as long as you need.

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W3LK
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« Reply #5 on: June 10, 2008, 03:57:46 PM »

Virtually any of the full sized motorized antennas will out perform a 102" whip and an auto-coupler.

Center loading beats base loading every time.

Of course, if you are after esthetics and not performance ... Smiley

73,

Lon - W3LK
Naugatuck, Connecticut
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A smoking section in a restaurant makes as much sense as a peeing section in a swimming pool.
K6RQR
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Posts: 211




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« Reply #6 on: June 10, 2008, 06:47:47 PM »

Hello -
 Very useful comments. I agree that, if possible, you should try and get a center-loaded antenna. I have tried those and also the bottom-fed whip and I preferred the ones with the coil in or near the center. One approach for a long top whip (for more efficiency) is the collapsible Buddipole "military style" whip antenna with shock cord running through its sections. I have the 7 section antenna that is 12'4" long. It costs $99. I also have numerous other antenna parts like a 2' mast extension from Henry at Texas Bugcatcher. Its excellent. I can add antenna parts as needed. One good thing about the Buddipole is that if it is added to a 54" Hustler base section with a quick disconnect then it is nearly 1/4 wavelength on 20meters and works very well. I use this combination mobile and I have to park because it is so tall but its worth it. I would not recommend it under way though. It is thicker than a standard whip and would have excessive resistance at road speeds. One of Henry's mast extensions and a standard 102" whip would be better. My ideal mobile antenna would be a 5" coil Hi-Q remote tunable antenna with extra long whip and Turbo Tuner. I'm working on a strong mount for that. Good luck with your mobiling.Its great fun!

73,
Bruce  
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AD5X
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« Reply #7 on: June 14, 2008, 03:44:38 AM »

The MFJ whip IS 108" long.  I picked one up yesterday from MFJ at Hamcom here in the Dallas area.  The whip is exactly 108" long.

Phil - AD5X
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NR9R
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Posts: 155




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« Reply #8 on: June 15, 2008, 01:41:47 PM »

To specifically answer your question, my guess is that the difference in length has more to do with cost of materials or cm to in conversion (maybe 275 cm is a standard length of some kind) than performance.  

Logistical concerns, such as availability, mounting, and quality should be more of the deciding factor.

Here is some more unsolicited mobile advice:  

An HF mobile installation has many layers of complexity such as grounding, EMI, DC wiring to name a few.  Worrying about how to optimize your mobile system can come later.  It is best to start with the straightforward approach you have in mind since even a basic installation has many variables to master.  Then you can decide if you want to spend time and money on increasing antenna efficiency, adding an amplifier etc...

This philosophy should be considered by all you gurus on eham. While it is beautiful for so much knowledge to be shared on these forums for FREE, it may be best to cover the basics first.  

Have fun!

Best

Anthony
NR9R
(Guru with training wheels)
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ZR1PJA
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Posts: 49




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« Reply #9 on: June 26, 2008, 05:53:51 AM »

I have found a cheaper and workable solution to the 102' whip. I use the cb fibre glass wound antenna from anttron and instead of the little tunable whip on the top, I have replaced it with a much longer length of stainless steel whip which is coupled to the AH-4. Maybe Alan can comment on the efficiency of such a mix and match.
Brgds
Paul
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