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Author Topic: COPHASING MOBILE 2M  (Read 1325 times)
KD5SJS
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« on: March 23, 2004, 09:14:25 AM »

I was thinking of trying this.  Cophasing two 1/2 wave 2m mobile antenna's.  I would be placing them toward the rear of my suburban on the side glass or luggage rack, possible lip mount the rear door driver and passenger doors in the middle of the vehicle, anyone know of any pro's and con's.  Does anyone have any harness suggestions. thnx 73
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K0BG
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« Reply #1 on: March 23, 2004, 10:35:24 AM »

My first question would be why? It can be done, and it does offer some forward gain if done correctly, but what happens if the station you want to work is in the null? Unless you could rotate it, it's of little use.

There have been several articles in QST about building an array primarily for direction finding. An array could be used with a voting system which would automatically select the correct direction, but the truth is there is little benefit.

Use a good quality antenna in the middle of the roof and you'll always have consistant results.

Alan, KØBG
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WB2WIK
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« Reply #2 on: March 23, 2004, 12:51:04 PM »

It's true that using multiple antennas in this manner can increase gain, but only in selected directions.  What is gained in those directions is lost in others, as we still can't seem to defeat the conservation of energy laws.

If you want *omnidirectional* gain, which as a mobile I think would be more desirable, that is usually achieved by stacking vertical antennas one atop another (not side by side), to compress the vertical angle of radiation -- thus, what is "lost" is radiation at higher angles, which often isn't very useful.

However, a mobile station in motion might find radiation at almost any angle useful at times.  It's hard to predict exactly where you'd like your signal to go, but almost always having a directional array would be undesirable, and that's what you're going to have if you make a gain system out of two vertical whips arranged side by side, spaced at any distance.

As Alan said, a single 5/8-wave whip installed in the center of the vehicle's roof is hard to beat, and the de facto standard by which most all 2m mobile (FM/repeater) antenna installations are judged.  When the 152-174 MHz VHF public service band was in more popular use by government agencies many years ago, in systems where cost was no object and the two-way service providers could spend anything to achieve better performance, what they ended up using was a 5/8-wave whip in the center of the vehicle roof!

WB2WIK/6
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KD5SJS
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« Reply #3 on: March 23, 2004, 06:36:53 PM »

That is great info.  I'm curious though, if a single mount center roofed antenna, on a suburban, would have an eliptical propagation pattern due to the elongated ground plane.  would not a cophased array essentially have the same propagation pattern?  I understand that the cophased array would be front to rear directional, if the array was set up side by side. But given the propagation pattern of a single whip on that shape of a ground plane, it is not essentially quasi-directional.
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K0BG
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« Reply #4 on: March 24, 2004, 10:50:38 AM »

Yes and no. The groundplane in question (the top of the vehicle) is large enough that the skew of the pattern is negligible. Then again, every installation is slightly different. For example, if you used a 1/4 wave as a base antenna and used drooping radials as required for a 50 ohm input, if just one of those radials was 1" lower than the rest, there would be a skewing of the pattern. For the most part the skew is not evident and the difference in signal strength is difficult to measure without complex equipment.

Alan, KØBG
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WB2WIK
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« Reply #5 on: March 24, 2004, 02:18:27 PM »

I'll agree with Alan on this.

At 146 MHz, your radiation pattern will be omnidirectional +/- almost nothing because you only need about 19" of sheet metal all the way around the base of the whip to create an omni pattern.  That much sheet metal is available on the Suburban, and on almost any motor vehicle having a metal roof.

At lower frequencies, other things can happen; but on 2 meters, it's rare for a mobile pattern to be anything but omnidirectional if the whip is mounted in the center of the roof, regardless of the shape of the roof or the vehicle.

WB2WIK/6
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KD5SJS
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« Reply #6 on: March 25, 2004, 12:35:07 PM »

Thanks for the info, I figured out that this dicussion is more samantic than practical, so I've decided that since I use repeaters, it definitely is not worth the effort.  However, if one uses simplex, it might be fun in a fox hunt, but then again, I dont have time for that.  I do read alot of your responses and they're very informative, albeit occasionally condescending, I think people could learn a lot if they just didn't take comments so personal.  I do have a question about a couple of radios, two in particular that Im having a difficult time with deciding between, both are very different.  How do you feel about the Kenwood D700a vs. yaesu ft8900.  I am active with Skywarn in north texas, primarily 2m QSO.  I have not used APRS yet, would that be a beneficial tool in spotting?  I have read all the specs, the reviews in the forum.  Since your mobile equipment is ICOM.  I thought maybe you might have an unbiased opinion.  Thanks again, 73.
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WB2WIK
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« Reply #7 on: March 26, 2004, 01:52:33 PM »

KD5SJS, I don't own either of those rigs, and I'm not even sure I've used them!  So, I have no opinion on them.

You might read the reviews here on eHam.net, and also the more technical reviews that are published in QST and available on line at http://www.arrl.net in the "members only" section -- those are pretty solid and less emotional, based more on lab testing.

My own two meter mobile gear is all Yaesu, actually, and single-band rigs: model FT-3000M, which I think is now discontinued (unfortunately, it was a really good model).  It runs 70W output and covers only two meters, and has a rather "bulletproof" receiver, compared with most I've used.

But I must admit that every single "underdash" mobile FM rig I've ever owned has failed.  Kenwoods, Icoms, Yaesus, Alincos, Standards -- all of them.  It's a matter of time and mileage, but they all bite the dust, and some sooner than I'd expected.  The only mobile FM rigs I've ever owned that did *not* fail in service were old-generation commercial stuff from Motorola, GE, Ericsson, Pye, et al.  Every one of those lasted longer than the vehicles they were in, and I stopped using them only because they're bulky and awkward compared with the much smaller (and much cheaper!) ham gear.

WB2WIK/6
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