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Author Topic: who makes the best small HF mobile antenna  (Read 10593 times)
KB2HUK
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Posts: 164




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« on: June 08, 2014, 05:36:49 PM »

Hi gang!  I have a new car and this will be my 5th mobile install .  My new car is a volkswagen passat TDI and I would like to use HF in the car again with a new smaller antenna so as not to take away from the car ( I know this will be a trade off ) so the question is who make the most efficient smaller antenna to be used at 100 watts . does anyone have 1st hand knowledge of the Alpha MOTO mobile antenna from the Amateur Radio Store  ?  right now I guess I am considering the Alpha moto or the little tar heel .  any experience will  be appreciated . thanks John Molenda kb2huk
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M6GOM
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Posts: 993




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« Reply #1 on: June 09, 2014, 12:26:41 AM »

From the manual:

"Do not allow any coax to
form a loop by crossing
over itself as this may form
a choke that can detune
the antenna.

"

AVOID. RUN AWAY. DO NOT BUY THIS ANTENNA.

Its a very lossy 9:1 UNUN with a 9ft whip and they have the audacity to call it an automatic matching system. You WILL have lots of common mode issues with this antenna as it tries to use the coax as the missing half of the antenna. NEVER EVER buy ANY antenna that tells you you can't choke the coax. I would expect that if you did run power through it you'd see the dash instruments doing funny things and may find the radio resets every time you key due to the massive amounts of common mode.

The Little Tarheel with a 72" whip will be a better performer and if installed properly will have far far less common mode. I use one of those installed as per the advice on Alan K0BG's website and I'm regularly not believed to be mobile when talking to the USA from here in England. VK, 12000 miles away, on 20m is my furthest contact /M which was done driving down the road at 70MPH. It works well on 20-10m, OK-ish on 40 however 80m proves QRP works if you make a contact as its about 1% efficient on that band but even that will be better than this Alpha thing. If 40/80m are your main bands of interest, realistically you're looking at something like a Scorpion SA680 but even that is only 3% efficient or so on 80m so 100W in gives an ERP of 3W.

If you are hell bent on this type of antenna, buy a 7ft-9ft CB whip and a mount, get a cheap 9:1 UNUN off Ebay or make your own and you've got this antenna for 1/10th the price.
« Last Edit: June 09, 2014, 12:33:18 AM by M6GOM » Logged
KB4QAA
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Posts: 2450




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« Reply #2 on: June 09, 2014, 08:29:43 AM »

For screwdrivers Scorpion and Tar Heel get good marks.  Hustler whip/resonators are long time favorites.  Single band whips "ham sticks" are effective at 20m and above.

rule of thumb:
-The skinnier the coils, the less efficient.
-Working 80m with anything slender and short will be very difficult, 40m fairly poor.  Efficiencies on these bands can be down in the single digits.

-The ATAS120 is noted as a very poor antenna.

See Alan's website  
www.k0bg.com
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K0BG
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« Reply #3 on: June 09, 2014, 10:53:13 AM »

You can have efficiency and/or bandwidth and/or small size. You can pick one, perhaps two, but not all three!
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N0TES
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Posts: 6




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« Reply #4 on: June 30, 2014, 12:09:18 PM »

You can have efficiency and/or bandwidth and/or small size. You can pick one, perhaps two, but not all three!

Listen to what K0BG has posted here as well as what others who share what he has at http://www.k0bg.com/, and not the misinformation, because the Alpha MOTO is neither a 9 foot antenna nor does it use a 9:1 UNUN; although there is a different mobile antenna that does fit this description perfectly. The MOTO antenna is two of the three things that Alan lists, which is Small and (surprisingly) Efficient. The Alpha MOTO is not very broadbanded. Also, please remember that you will need an antenna tuner to use the MOTO.

The coax is always part of any antenna system, but rather than form a coaxial choke coil to hide the problem some other antennas experience with common mode current, Alpha Antenna designed the Alpha Match to use the metal of the vehicle to properly balance the antenna.

Just remember to keep listening to Alan, and whatever antenna you install, follow what Alan says, "it is the metal mass directly under the antenna, not what's along side that counts!"

Feel free to contact me directly if you have any more questions, as I am not on these boards very often, but do what I can with the limited people I have here at my small business! You might also contact any of these people who use the MOTO http://www.eham.net/reviews/detail/10340.

73s,

Steven Deines
N0TES
Owner
Alpha Antenna
https://www.AmateurRadioStore.com
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M6GOM
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Posts: 993




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« Reply #5 on: July 01, 2014, 03:34:29 PM »


The coax is always part of any antenna system,

Yes it is part of the antenna system but not one you want RF travelling on the outside of once its entered the vehicle.

Quote
but rather than form a coaxial choke coil to hide the problem some other antennas experience with common mode current, Alpha Antenna designed the Alpha Match to use the metal of the vehicle to properly balance the antenna.

So you'd be OK with someone wrapping several turns of coax around something like a Mix 31 toroid or using some clip on ferrites as close to the antenna feedpoint as possible?
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K5LXP
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« Reply #6 on: July 02, 2014, 06:40:15 AM »

Startin' to sound a lot like an Isotron for the roof of your car...

Mark K5LXP
Albuquerque, NM
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N0TES
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Posts: 6




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« Reply #7 on: July 02, 2014, 09:38:56 AM »

If you find that there is a need for additional protection from common mode than is already present in the Alpha Match of the MOTO antenna, then M6GOM is correct about the technique that one should use to create a choke, as Alan has documented here: http://www.k0bg.com/common.html. Just remember what K0BG says, the "resulting choke exhibits an impedance of about 2.2 kΩ at 10 MHz".

Thank you,

Steve/N0TES
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K0BG
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« Reply #8 on: July 02, 2014, 03:27:18 PM »

Since my name and site has surfaced here, excuse me if I chime in.

Every single HF mobile antenna installation, no matter what it is, or how it is mounted, will have some level of common mode. The reason is simply because there is ground losses associated with every HF mobile installation! If you do not choke the common mode, then the coax will be part of the antenna. Therefore, RF will escape from the coax which can cause RFI egress. That is to say, cause issues with the vehicle's electronics. And, the coax can act as a receive antenna as well. Thus, the vehicle'e electronics will interfere with you—RFI ingress.

I don't know what is in the base of the Alpha HF mobile antenna. If I had one, I've be inclined to find out, if for no other reason than there is no free lunch. In other words, there is always a tradeoff with every installation.

All this said, there are a few specific design attributes everyone has to continued with. Effective electrical length matters, whether this is done by overall physical length, or electrically with the addition of a cap hat, raising the current node as high in the antenna as possible is the holy grail. Anything less, it just that—less!

As alluded to, it is the metal mass directly under the antenna that counts—not what is along side. Bumper mounting, mag mounts, clip mounts, lip mounts, or any other abbreviated scheme, is not the breakfast of champions!

Very large coils—over about 3.5 inches in diameter,have excessive distributed capacitance, hence low Q. Base loading is approximately 50% as efficiency as center loading, and there is no way around the issue! Shying away from 17-7 stainless steel whips is a step in the right direction. Proper matching is easy to accomplish using a shunt coil. Tuners, couplers, and ununs are unwarranted, and almost never needed. The higher up on the vehicle, the better. And the one most important attribute of all....

Anything less than drilling a hole is a step backward!
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M6GOM
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Posts: 993




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« Reply #9 on: July 02, 2014, 04:00:21 PM »

If you find that there is a need for additional protection from common mode than is already present in the Alpha Match of the MOTO antenna, then M6GOM is correct about the technique that one should use to create a choke, as Alan has documented here: http://www.k0bg.com/common.html. Just remember what K0BG says, the "resulting choke exhibits an impedance of about 2.2 kΩ at 10 MHz".

Thank you,

Steve/N0TES

Puts my mind at rest Steve. Its just the way the manual is written. It sounded like many of those questionable "no ground plane/counterpoise required" magic vertical base antennas that claim to defy the laws of physics when the truth is that they use the coax instead and choking kills the performance/tuning.
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N0TES
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Posts: 6




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« Reply #10 on: July 02, 2014, 08:24:04 PM »

The Alpha MOTO is simply nothing more than a HF mobile antenna that is very stealthy, is easy to use (you need a tuner), and works pretty good when it is properly installed...it's not a miracle or the holy grail (yet). You have prompted me to consider how I might update the MOTO's manual to include information on how a person could create a proper choke when common mode is experienced, or at least reference sites that show the proper way to build one.

Thanks again to everyone who provides feedback, which helps me improve the User Guides and that makes all Alpha Antenna systems better with each upgrade that we release!

73s,

Steve/N0TES
http://AlphaAntenna.com
https://AmateurRadioStore.com
« Last Edit: July 02, 2014, 08:49:08 PM by N0TES » Logged
HS0ZIB
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Posts: 434




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« Reply #11 on: July 15, 2014, 02:48:53 AM »

I have a Shorty I antenna from KJ7U (Larry's Antennas) mounted on my pick-up.  It is (without the whip), just 21 inches when retracted.

(I bought it because it seemed to be the only screwdriver that could be shipped to my QTH in Thailand - all other screwdrivers were too long for the USPS postal services).

In use, I have had QSOs up to 10,000 miles via greyline and 5,000 miles is a daily experience.
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N6AJR
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« Reply #12 on: July 18, 2014, 09:11:40 AM »

For various reasons I tend to go with the Yaesu ATAS 120
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W2EM
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Posts: 28




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« Reply #13 on: July 20, 2014, 04:30:33 PM »

I bought an Alpha Moto antenna at Dayton Hamvention this year and mounted it on my truck it in a less than desirable location on a Back Rack (along with a Larson VHF/UHF antenna).  I bonded everything on the truck with ground strapping and have the radio body and tuner behind the back seat. Cables are running out of the cab back vent.  I have been pleasantly surprised with its performance during the short time I’ve had the antenna. I’d get it again due to its price and low visibility.  There in not a lot to go wrong with it. I needed a bed cover so used the Back Rack for antennas.  I would have liked to have a Scorpion screwdriver but didn’t want to deal with the mounting issues at this time.
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