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Author Topic: Shortened mobile monoband whip  (Read 974 times)
AC4RD
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Posts: 1235




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« on: July 03, 2009, 07:24:51 PM »

This is just the first stage of a project, but I thought I'd post a message in case anybody else is thinking about this.

My problem is that since the last time I was active from the car (2003-2004) I've now got a new job and a new parking garage and a new (tiny) car--my Hamsticks on the trunk lid of a sedan would not come close to fitting in the 8' garage roof clearance I've got now.

The project is to cut down a Hamstick style antenna to fit in my parking garage.  Eventually I plan to remove the existing loading coil and replace it with one that will allow operation on 20, and with taps on 17 and 15.

My first step, though, was to remove the 4' whip and replace it with about an 18-20" whip.  With the mounting location I'm using, this is as tall as the antenna can be and still fit in the garage at work.

Then I used a hunting knife to slice off the heatshrink tubing from one-third of the circumference of the loading coil, exposing the coil wire.  Then I used the knife to scrape the enamel insulation off the outer face of the wire in that area.  Finally I added two small (1/2") hose clamps and a 1/8" aluminum rod, making an adjustable jumper to bypass part of the loading coil.

Using an MFJ antenna analyzer, it took only a few minutes to get the jumper arranged such that I had resonance (R = 0) at 14.225mHz.  (I wanted it a bit higher but settled for this, for now.)  Mounted on the car, the resonant point didn't change much, and the antenna displayed around 30ohms of resistance and an SWR a bit under 2:1.  I worked a DL with this antenna about an hour ago, parked in my driveway.

At this point the antenna is ugly and inefficient--but it works to a degree, and it will fit in my work garage.  Next I'll start work on a bigger air-core coil that will give me 20, 17, and 15 as I mentioned above.

I want to thank Alan, K0BG, for his incredibly helpful and informative website about mobile HF antennas.  I learned a lot from his website and it's made it much easier for me to move forward with this.  I also can't imagine I'd have made it this far without the antenna analyzer I bought the other week--WOW, these things are great!  Can't believe I've been playing with HF antennas since 1991 without an antenna analyzer.

The project is ongoing and I'll post a few photos on my website when I get a final version built. :-)
   --ken ac4rd
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K8QV
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Posts: 52




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« Reply #1 on: July 03, 2009, 07:46:55 PM »

Very creative, but why not just get a couple of these:

http://www.universal-radio.com/CATALOG/hamantm/2083.html

These mini hamstick type antennas seem to work okay for what they are - very short antennas! You won't bust any pileups with these either, but they look pretty good and fit most garages. I consistently work 20 and 17, including some DX.

Of course, a serious mobile effort requires a lot more antenna and fiddling about, but it is possible to get on the air this way. Read the reviews on this website.
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K0BG
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« Reply #2 on: July 04, 2009, 04:27:35 AM »

Wouldn't it be nice if the number of DX stations one worked had a direct relationship on how efficient your mobile antenna was? Alas, it means nothing other than QRP is alive and well.

There are other ways to shorten an antenna (within reason), rather than resort to a virtual dummy load. The best one is to use a cap hat. The unfortunate part is, all of the current cap hats don't work very well because the end of their spokes are not electrically connected. If they were, their effective capacitance would more than double!

Of course, you have to put up with the wind load and a few other inconveniences, but the performance hit is up, not down!.

Alan, KØBG
www.k0bg.com

PS: If the first sentence was true, my antenna would be 310% efficient!
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K8QV
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« Reply #3 on: July 04, 2009, 07:37:36 AM »

Jeez, Alan. We know that.

The purpose of putting a station on the air for 99% of the hams is to have QSOs. Making contacts is the primary goal. Very few are solely interested in theoretical numbers. We do the best we can under our circumstances.

In real life, most of us have to make compromises for a variety of reasons. For mobile antennas that usually means cost, aesthetics, clearing obstructions, and possibly ease of removal when necessary. To say that an antenna is no good when it enables plenty of contacts (even DX!) is just absurd. Obtaining the ultimate theoretical efficiency is simply not of interest to most hams as long as their station makes contacts, even though we know what COULD be done to get a better signal report.

Personally, I won't go to the extra expense, trouble, and ugliness to maximize my mobile signal as long as I can consistently work stations around the world with a modest setup. Most operators feel that way. I could do a little better, and I know exactly how to do that, but I concentrate on maximizing efficiency at the home station where it really can count when chasing rare DX. The best mobile station can't compete with that anyway.

The short antennas brought up here DO work. They work Europe, South America, and even once Japan. Doesn't matter in the least if the efficiency of the antenna was 2% or 20%, because the Q was made. It fulfilled its purpose. Making plenty of contacts is enough to satisfy most mobile ops. You simply don't need 15 feet of antenna, 20 hours of labor, a hole in your car, and 100 feet of ground strap to stay busy talking on the radio in your car. Nor do you need an amplifier. A hundred watts (or less) and a simple antenna is obviously not the ultimate setup, but it works just fine for those who don't want to drive an eyesore and just want to talk rather than endlessly tinker and take measurements.

Preaching overkill for the mobile installation and making it unnecessarily complex scares a lot of newbies who might like to try it (I know this from personal experience). The impression is that unless they go to extremes in their mobile station they will never have any contacts. That simply isn't true.
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WW5AA
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Posts: 2086




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« Reply #4 on: July 04, 2009, 07:48:41 AM »

Most of the new hams that I have known were driven away from mobile use because the set up they had didn't work. The main reason is that they were given bad advise. Yes you can make a hamstick almost work, and yes you only loose a little with a mag-mount. Yup, forget the bonding and drilling. Keep up the good work Alan, if one strives for excellence and falls a little short it is better than not having tried at all!

73 de Lindy
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AC4RD
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« Reply #5 on: July 04, 2009, 08:17:20 AM »

"Very creative, but why not just get a couple of these:
http://www.universal-radio.com/CATALOG/hamantm/2083.html"

I did.  And I've posted a review of it in the equipment review section of eham.  I bought the 20-meter version of the Shark, and used it for a week or so.  It does work, no doubt, but by my best guess, it was 2 or 3 S-units down from the Hamsticks I'm used to.  I'm keeping the Shark, it DOES work, but this little construction project is an attempt to get a little more efficiency in a mobile antenna, always with an eye to my very limited vertical space and my very poor mounting options.
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K8QV
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« Reply #6 on: July 04, 2009, 09:12:01 AM »

" I'm keeping the Shark, it DOES work, but this little construction project is an attempt to get a little more efficiency in a mobile antenna, always with an eye to my very limited vertical space and my very poor mounting options."


It can be a fun project, but remember, a short antenna is always a short antenna no matter how the loading is configured. The only way to get significant improvement in efficiency is to go much bigger than you are able to with your height restriction. It's doubtful that you will see much difference between such extremely short antennas. They can't work nearly as well as a large and complex installation, but they may work well enough to satisfy the desire to have a few QSOs behind the wheel (and still fit the car into a garage).

Should your situation change, and you no longer need to use garages, or care about expense and the appearance of your vehicle, you can go all out and vastly improve your antenna efficiency. Still, you can only talk to one person at a time :-)
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K9OF
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« Reply #7 on: July 09, 2009, 06:17:14 PM »

Why not use a 20M Hustler with the fold-over mast? To get in the garage, just fold over the mast. To make it even more efficient on 20, use a 15M resonator (less r in the loading coil) but with a slightly longer stinger.

Good Luck!

John,  K9OF
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KI4SDY
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Posts: 1452




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« Reply #8 on: July 09, 2009, 07:29:29 PM »

Why don't you just bumper or frame mount a 9' whip, with the spring (since these are lower than 11 meter frequencies), and install an auto antenna tuner in the trunk? If you can't afford the auto tuner right now, just install a manual tuner in front and tune while you are waiting for the red lights to change. When you get ready to garage the vehicle, just bend the antenna over and secure the tip with an insulated clip or cord.

This set up has worked great for me. You get more bands, better performance, the ability to run high power mobile and a rugged antenna. It is inexpensive and you don't have to drill any holes in your car!

Sounds like you are doing a lot of work for mediocre results.
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AC4RD
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« Reply #9 on: July 10, 2009, 05:51:15 PM »

"Why don't you just bumper or frame mount a 9' whip,"

Because my parking garage at work has an 8' clearance. :-(   And my bumper is plastic--getting to the frame is tough, though I'll try that if I have to.

And the other suggestion, a Hustler system with a fold-over mast--I just received a mast and 17m Hustler resonator from DX Engineering today.  (The 3' DXE mast doesn't work with the resonator without an adaptor, which their website doesn't mention, but I'll work that out.)  Anyway, a foldover is no good because there's no place for me to pull over and fold over a mast that's out of the way of traffic, and I know perfectly well I'm not going to bother doing that every day anyway.  Soon I'll quit bothering and then I won't be able to work HF from the car.  I need something that does NOT require a lot of fiddling.

My Frankentenna homebrew mobile monobander works, more or less, and I'll give the Hustler a try next--I have also run into what I think is an antenna grounding problem, too, that needs some work.  When the humidity is high or the car is wet, everything works fine, but when the car is dry and humidity is low, pumping more than a tiny bit of SSB through the radio (HF only, not on VHF) makes the radio shut itself off.  I think my problem might be my current antenna mount, which is on the left passenger door, doesn't connect electrically with enough metal on the car to be a good pseudo-ground for the antenna.  I'm fiddling with that now.  I've had some success so far; just don't have things dialed in yet.

Thanks for the suggestions and comments!
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