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Author Topic: Improving RF current flow in the 'other half'  (Read 418 times)
K1DRW
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« on: January 28, 2007, 12:57:11 PM »

Ignoring other considerations for the moment (ground loss, vertical placement, etc), to focus on rf current flow in the 'other half':  As an alternative to the standard configuration of using the vehicle body as the 'other half' with its issues and inefficiencies, why can't some instantiation of a dipole configuration be employed to achieve a better return path.  Why couldn't another 1/4 wave mast/coil/whip 'compromised' configuration be used for the other half?  At first thought it would seem to present more of an opportunity to achieve a more efficient RF current flow than the vehicle body presents.  Could the mounting issues of this other 1/4 wave component be addressed with the understanding that it is not radiation we are after but just a more efficient, controlled return path (maybe enclosing it) ?  Just thinking, but not knowing the answers.
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N3OX
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« Reply #1 on: January 28, 2007, 02:31:51 PM »

Trying to come up with an efficient "isolated" tuned other half of a mobile antenna to avoid the grunt work of bonding the vehicle together properly is pretty much the same as fiddling around with elevated counterpoises on lower HF verticals when you have a cornfield to lay radials in.

In the end, the guy who puts down the 120 quarter wave radials in his cornfield is going to whoop your you-know-what on 160m unless your elevated counterpoise is as extensive as their on-ground one.

It's the same with the mobile installation.  In the 160m vertical case, the earth is a good place for RF currents to go, you just need to help them out where they're concentrated at the base of the antenna. Lay down the radials.  Forget about it.

In the mobile installation, you've got a number of quasi-independent chunks of metal possibly seperated by semiconducting rust.  Bond it all together.  Forget about it.

And this coming from a dude who will do most anything to experiment with antennas.  I just think a good RF ground should be established first if you're going to experiment.  Experiment with the antennas themselves, not the untuned ground, which should just be as large and high-quality as possible.

Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
K0BG
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« Reply #2 on: January 28, 2007, 03:38:55 PM »

Once again, Dan's comments are dead on! Here is another thing to think about.

Assuming you decide to use a "dipole configuration" (instantiation Huh), the other side of the dipole will be under the same constraints as a single vertical element working against the body of the vehicle in question. Therefore, it will have as much loss. One could possible argue that a second radiator is less lossy than the body of the vehicle itself. And perhaps be correct. However, the body is still there, and the mass of metal effects not just one of the radiators, but both! It can be likened to placing a dipole too low to the ground surface. In other words, the losses will end up being more than just a single radiator mounted atop the body of the vehicle in question.

This argument, is an old one, and has been hashed almost to death in several of the amateur radio related publications over the years.

Again, as Dan referenced, the best you can do is bond the body of the vehicle as well as you can, and hope for the best. In any case, about the best you can achieve varies between about 2% on 160 meters, to about 80% on 10 meters, and even this level of efficiency takes a fair amount of work. It is not, a simple week-end, throw-it-in-the-car type of installation.

If you wish to learn more, visit my web site.

Alan, KØBG
www.k0bg.com
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K1DRW
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« Reply #3 on: January 28, 2007, 03:30:32 PM »

Thanks for the response, Dan.  Perhaps I could have been more descriptive in my posted query.  I asked the questions not for the purpose of avoiding the grunt work.  I have spent considerable time already doing that grunt work (much bonding, optimizing antenna placement, mast length, coil size, etc.)  And, also aware that it may well be a work in progress forever (as I learn/understand more, continue making improvements).   My thoughts/questions are coming on the tail end of the process as wondering how one particular aspect of the system/configuration (maximizing RF return current) could be approached differently or unconventionally.  
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N3OX
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« Reply #4 on: January 28, 2007, 08:38:21 PM »

"I asked the questions not for the purpose of avoiding the grunt work. I have spent considerable time already doing that grunt work"

Gotcha.  Sorry I assumed you were looking for a magic bullet :-)

I think my point, unfortunately, still stands.  You've got a couple of possibilities  for a "vehicle independent" antenna. (As Alan points out, though, no such thing, but let's press on...)   A vertical dipole is one of them.  Unfortunately, if you want to do a vertical dipole you have to take the height you have available for radiator and cut it in half and your radiation resistance drops.  Add this to the fact that the vertical dipole would have the same coupling issues to the vehicle as the antenna fed against the car; worse if you consider feedline decoupling issues.

If you try to put the other half of the antenna in some sort of shield will, at best, put a capacitor between the antenna base and the vehicle, effectively lowering the antenna base capacitance to ground.

A bigger vehicle would increase coupling to the earth, but that's kind of an expensive solution ;-)

I hate to say it, but you just can't get around the basic issues with short antennas like mobile ones.  Alan's explanations are very good, and the link from his antenna efficiency page to W8JI's article on short antennas should be followed.

Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
K6HWH
Member

Posts: 18




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« Reply #5 on: January 28, 2007, 10:12:46 PM »

The Hi-Q antennas are sold in matched pairs, either horizontal or in a v-shape, for use, among other things, as a dipole on a mast on a vehicle, with substantial improvement over a standard mobile antenna:

"It is based on the Hi-Q-3/80 XTL (Extra Light) HF mobile antenna. A pair, back to back, assembles in minutes. The Hi-Q loading coil (Q=360) handles SSB legal limit and will tune CONTINUOSLY from 10-80 M including the NEW 60 M band. Works great on a Tri-Pod or a mast from your vehicle, no radials needed and uses a 52-ohm cable without any matching device.

It has 2.2 dB gain in comparison to the standard HF mobile antenna."

Not sure how high above the car the mast would need to be--might only be practical for stationary use.  Check it out at http://www.hiqantennas.com/

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N3OX
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« Reply #6 on: January 28, 2007, 10:31:38 PM »

"It has 2.2 dB gain in comparison to the standard HF mobile antenna."

Of the same design as one of the halves.

It wouldn't have 2.2dB gain compared to an antenna twice the height of one of the halves similarly loaded and fed against the vehicle body.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
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