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Author Topic: SMALL 60M MOBILE ANTENNA???  (Read 7786 times)
K0BG
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« Reply #15 on: January 24, 2011, 02:12:23 PM »

One of the ARRL's antenna compendiums contains a article about this very subject, complete with a wire frame vehicle using NEC. From the data, very little radiation actually comes from the vehicle. Before that article appeared, there was a similar one in QEX. I don't have a copy of that one, but the conclusions were very close as I recall.

One of the problems in modeling vehicle is correctly calculating the ground losses. Dr. Belrose, VE2CV, touched on that 3 years ago in a Tech article in QST. I have no argument with that per sé, but the way it is applied in Dave's scenario.

Edit....

After thinking about this for awhile, in order for Dave's scenario to work, the body of the vehicle would have to be resonant. It isn't, and if it were it would be next to impossible to measure. That is not to say, that at some frequency the body would be resonant, and lot of things would effect the actually frequency including the capacitance between the vehicle, and the surface under it.

In the ARRL's Antenna Compendium series, articles by Brown, Belrose, and others, have mentioned secondary resonances. In other words, an HF mobile antenna, resonant on some specific frequency, is swept with a VNA. The print outs show other resonant points other than the one the antenna is tuned for. I have also seen the phenomena. They are always about the same point, even though the antenna resonant point was changed from on band to another. One could assume these were vehicle resonant points, but I don't know of any way to prove that, even by modeling, as one can't predict, or measure the standing waves between the body and the surface it sits on.
« Last Edit: January 25, 2011, 06:23:21 AM by K0BG » Logged

WX7G
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« Reply #16 on: January 25, 2011, 06:23:57 AM »

N3OX you have hit on a good analogy of a ground plane with sloping radials to illustrate how the whip/vehicle form an antenna system. Bumper mount the antenna and the radials are sloping upward. Roof mount the radials and the radials are sloping downward.

In the antenna system described here the radiation resistance of the 4' whip/coil alone is 0.3 ohms. Mounted on the car the system radiation resistance is 1.3 ohms. The current-area of the 4' whip/coil has not changed. Where then does the increased radiation resistance come from? It comes from the vehicle body. And being that the radiation resistance has quadrupled we must conclude that the current area has doubled. The extra current-area comes from the vehicle body. It contributes half the radiation.

Does the vehicle need to be resonant? No it does not. A dipole can be fed anywhere along its length and the current distribution does not change. A common way to build a short, loaded dipole is feed it in the center and place loading inductors on either side of the feedpoint. This is not the only way a short dipole can be resonated and fed. One loading inductor can used and the feed place to one side. This is exactly how this mobile antenna system is built. Hams do this everyday without fully realizing what they are doing. The notion that a vehicle body is a non-radiating "ground" was planted many decades ago in the amateur literature. This error has led thousands of amateurs to incorrectly design mobile antenna systems spending more money and installing larger antennas to achieve less performance.  

Knowing how the vehicle body contributes to radiation allows us to knowingly use it to our advantage rather than unknowingly to our disadvantage.

Modeling accuracy: The modeling accuracy needed depends on the decision or decisions to be made. Precision for the sake of precision can be a trap. If the decision is "antenna A or antenna B" great accuracy may not be required. Loss mechanisms common to antenna A and antenna B, such as ground loss, tend to drop out of the error analysis.

« Last Edit: January 25, 2011, 08:40:22 AM by WX7G » Logged
WX7G
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« Reply #17 on: January 25, 2011, 01:27:42 PM »

As I pointed out before the vehicle-is-a-radiator seems to be the best kept secret in Amateur Radio. I did some googling and am frankly astounded that virtually all amateur authors have missed the fact that the vehicle itself is a radiating half of the antenna.

So far I have found one author who realizes this. Google www.k0bg.com/images/pdf/mobile_vhf

The title is VHF Mobile Antenna Performance
The other half of the story

Read the paragraph titled It's Not A Monopole.
« Last Edit: January 25, 2011, 01:30:57 PM by WX7G » Logged
KC8IUR
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« Reply #18 on: January 25, 2011, 01:40:14 PM »

The author of that webpage is the one you're arguing with.
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WX7G
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« Reply #19 on: January 25, 2011, 01:53:51 PM »

I have not entered into argumentation. I have presented evidence to support my hypothesis. If anyone wants to refute my hypothesis they may do so. I do ask that it be a coherrent argument supported by mathematics.
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K0BG
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« Reply #20 on: January 25, 2011, 02:53:29 PM »

Part of the issue is David, you're trying to apply a VHF concept in this case, with an HF one. In the case of VHF, the vehicle is a fairly good ground plane, and one which doesn't have to rely on the conductivity of the surface under the vehicle. At lease, in any meaningful way. That is not the case for HF.

And, I don't agree with your concept at all (best kept secret), at least not in the context you're applying it. If it was (is?) a best kept secret it has been missed by every other antenna expert out there.

Perhaps you should invite Tom, W8JI, to comment on your speculation.

By the way, the link isn't correct, as you left off a .pdf at the end. And, the article was not authored by me, but is used by permission.
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WX7G
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« Reply #21 on: January 25, 2011, 06:29:17 PM »

Alan, I have presented my evidence. Now it is up to you to refute it with a logical argument. So far you have not done so. Some mathematics and simulations would be nice. Quantitative data, not qualitative data (big, small, good, bad) is needed. If you want W8JI to weigh in you invite him.

You might want to build a NEC model. Or, you can perform actual measurements on an actual vehicle. Or, you can perform measurements on a scale model. A 1/16 model built of cardboard and copper tape is small enough to build quickly yet large enough to contain an MFJ-259 or similar instrument.

I'm saying the vehicle-as-radiator concept appears to have escaped amateur mobile antenna builders. It has not escaped professional mobile antenna builders. They build NEC models that include the vehicle (manpack, ground vehicle, aircraft, naval vessel, satelite, laptop PC, cell phone, and so on). And speaking of cell phones, did you know that on the 850 MHz band the part you hold in your hand is one half of a vertical dipole? The whip or stub at the top is a loaded whip. Think of an HF vehicle/whip as a big cell phone.

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KC8IUR
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« Reply #22 on: January 25, 2011, 07:14:32 PM »

On the 850 MHz band, the part you hold in your hand is an appreciable fraction of the wavelength. My 6' tall truck is not a relevant fraction of even 10m, let alone 80.
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W9PMZ
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« Reply #23 on: January 25, 2011, 11:12:33 PM »

"My 6' tall truck is not a relevant fraction of even 10m"

Now there is a hypothesis to be considered...  6' is only 2' or so from a quarter wave...

Regards,
Carl - W9PMZ
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WX7G
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« Reply #24 on: January 26, 2011, 06:08:36 AM »

It can be hard to let go of long held beliefs. Visualization can often help.

In this specific case it can help to visualize the vehicle/antenna parked as usual over ground. Now visually remove ground. The system is in free space yet the current along the vehicle and whip have not changed markedly. The system resonant frequency has not changed markedly. The presence of ground is not required.

Visualize the vehicle/antenna parked as usual over ground. Now visually invert the vehicle/whip so the whip is nearest ground. If the vehicle was a non-radiating counterpoise before what is it now? Is it still a non-radiating counterpoise, has it become the radiator and the whip the non-radiating counterpoise? Or was it a vertical dipole before and is it a vertical dipole now?

Visualize the RF current. The current returning to the feedpoint from the vehicle MUST equal the whip current at the feedpoint. What path does the vehicle current take? If it takes the path from whip to ground and then to the vehicle it must take a path vertically along the vehicle. Conduction current along a conductor gives us radiation. Conduction current along the vehicle gives us radiation.

Now remove ground from the picture. The current on both sides of the feedpoint must be equal, therefore the vehicle current is equal to the whip current. What path does the vehicle current take? With the whip mounted in the center of the roof some of the current takes a path from the vehicle roof to the whip feedpoint. The magnetic field produced by current taking the path from rear-to-feedpoint cancels the field produced by current taking the path from front-to-rear. The roof becomes largely non-radiating. The rest of the current takes a path along the vehicle sides from bottom to roof. A magnetic field is created and we have radiation. There is no opposing magnetic field to cancel this radiation.

« Last Edit: January 26, 2011, 07:44:46 AM by WX7G » Logged
KI4SDY
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« Reply #25 on: January 27, 2011, 05:58:14 AM »

I would like to thank WX7G for the most informative information I have ever read concerning mobile antennas and how they perform in relation to the vehicle they are mounted on. His clear and concise explanation has debunked the myths and assumed correct information that is found in ham reference material and often repeated here on eHam.net. I wish this was an article on the home page of the site, along with the questions and arguments. It would draw more attention there and have a chance to readily inform new and old hams, so they can design their mobile antenna systems correctly. Grin

In my own study and experience I have come to the conclusion that all antennas are some variation of a dipole.  Wink
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K0BG
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« Reply #26 on: January 27, 2011, 06:40:13 AM »

Visualize it this way.

One leg of the dipole is about 8 inches off the ground, and parallel with it. The other leg is vertical. Yes, the current flow will indeed be equal, and some radiation will indeed come from the leg close to the ground. However, the vast majority will be coupled to the surface below, where it does nothing but heat the surface.
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N4VNV
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« Reply #27 on: January 27, 2011, 10:19:16 AM »

Good Grief, I should have known something like this would start up!  I've been from KY to DE and NC using my hamstick on the rooftop using a 3-magnet mount. It works WONDERFUL! I just wanted to have a 60M mobile antenna I could leave on all the time without hitting tree limbs or moving it lower down on my vehicle. If I wanted to build my own antenna I would not have asked the question. Incidently I have been active on 60M seven years come March. As far as difficulty goes, 80M and 160M antennas are EVEN worse than a 60M mobile antenna, but they do work. I'll keep waiting for someone to manufacture one like I want.
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KI4SDY
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« Reply #28 on: January 28, 2011, 06:12:31 AM »

I don't know which ham stick you are using, but MFJ has just come out with a shorter version of their original ham stick that is only 36 inches tall. They have one for 60 meters, the MFJ 2360T. Early reports are that it performs as well as their longer ham stick. It sells for $19.95, so it would be an inexpensive and easy to switch to try. Personally, I like cheap!  Grin     
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K0OD
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« Reply #29 on: January 28, 2011, 08:28:11 AM »

Did someone mention that max power is defined on 60 meters in terms of ERP? 
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