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Author Topic: Real mobile ant measurements  (Read 4964 times)
WX7G
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« Reply #15 on: July 29, 2010, 07:10:13 AM »

My antenna installation is described in my measurements and analysis. The method I provide can be conducted elsewhere where the ground loss resistance is different. It is a universal method.

When we measure inductor parameters on the test bench the inductor is installed in a test "fixture." The test fixture is part of the measurement and the inductor under test becomes "embedded" in the fixture. The fixture alters the measured values. So, we must de-embed the inductor from the fixture. To do this the fixture must be characterized and an electrical model produced. From this we take the measured parameters and via an algorithm or a substitution method we arrive at values for the "pure" inductor. An example of this is determining IC input RLC values. One program, Eagleware, uses the substitution method. The measured data is fed into it. It has an electrical model provided by the engineer. These models can be fairly complex. The knowns are in the model. These are the fixture RLC parameters. The program then puts in values of RLC for the IC until the measured and simulated data overlay.

So, to measure the inductor on the bench we have a test fixture even if it's just two wires from the RLC meter to the inductor. We measure and then de-embed the inductor from the fixture. The RLC meter measures voltage and current amplitude and phase. The installed antenna can be thought of as an inductor test fixture. The antenna analyzer measures voltage and current amplitude and phase. We de-embed the inductor from the "fixture", and we have the inductor parameters of RLC.

This is how it is done everyday in industry and science and it works the same for antenna design, measurement, and analysis. Complex ICs having millions of transistor, power supplies, they are all designed, built, and work the first time. A circuit (antenna) consisting of a bit of wire is certainly much simpler than these circuits.

« Last Edit: July 29, 2010, 03:11:51 PM by DAVE CUTHBERT » Logged
WX7G
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« Reply #16 on: July 29, 2010, 09:08:07 AM »

Ultimately it comes down to decision making. Do I buy and install antenna A or antenna B? Where it becomes apples and oranges is when antenna A and antenna B are installed differently.

Let's say I want to work 40 meters from a van. I've narrowed my choices to two: A screwdriver antenna mounted on the left rear bumper or a Huster RM40 coil and whip mounted on the roof without a mast. Both antennas are the same height above the street. One costs $30 the other $400.

We build a van model in NEC. We then build the two antenna models in NEC using the models provided. Place the van over "average" ground and compare them. What if we don't trust the "average" ground model? Place the van in free space. Place it over "perfect" ground. What if the screwdriver is not the Tarheel 200A provided a model of but is a Hi-Q? Vary the loading coil Q, probably from the 240 of the Tarheel to the Q of 400 that the Hi-Q might exhibit. Compare at a Q of 240 and a Q of 400.

We now have numbers comparing the modeled performance of antenna A and antenna B. We know the prices. We can now make an informed buy decision. No guessing or defaulting to the pat answer that the Hustler is inferior. Perhaps the Hustler is better given the mounting configuration. This could save us from spending $400 on an antenna that is outperformed by a $30 antenna.

Alan, which do you think provides the best signal on 40 meters?
A) 2010 Ford E-series van, Hustler RM40 coil/whip mounted in the center of the roof
B) 2010 Ford E-series van, Tarheel 200A HP mounted on the right rear bumper with a whip having the same height above the street as A

« Last Edit: July 29, 2010, 11:07:00 AM by DAVE CUTHBERT » Logged
WX7G
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« Reply #17 on: July 29, 2010, 01:19:28 PM »

Alan, I ran simulations of the van and the results are fascinating. They are a bit different than I anticipated. I figured the center of the roof would be the winner.

The center of the roof turns out not to be the best spot for this short whip antenna. Placed in the center of the roof, over half of the displacement current is intercepted by the horizontal vehicle roof thereby reducing the current along the vertical portions of the vehicle. Vertical vehicle RF current contributes to radiation. The antenna consists of the vehicle and the whip.

Moving the short whip to a corner really boosts the radiation resistance and the gain. Although the pattern is now skewed there is more signal even in the null.

« Last Edit: July 29, 2010, 03:13:02 PM by DAVE CUTHBERT » Logged
K0BG
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« Reply #18 on: July 30, 2010, 07:06:25 AM »

Dave, about three years or so ago, I went through the iterations to build a stick figure vehicle in EZNEC. It took nearly 200 segments to build a 3 box vehicle. I learned more than I anticipated.

First, the program does a lousy job, calculating changes in ground loss. Since ground loss is the biggest single factor in determining the energy radiated at low angles (<25°), it's very difficult to qualify the patterns when you know the ground loss figure is questionable. You can't measure it directly, and you have to be very careful using the input impedance as a yard stick as it'll lie to you often as not. Remember, the ground losses are there as a result of standing waves between the body, and the surface the vehicle is resting on. And just about every nuance you can think of effects those standing waves, even temperature.

This leave measuring the field strength as the only viable tool in determining efficiency, and to a lessor extent, ground loss. Problem is, you can't use just one TOA, and try to apply it to all angles. Very few amateur have the equipment, or a site, to do field strength measurements with any degree of accuracy. As a result, we rely on EZNEC, or NEC 4 or 5, and when we do that, we don't do much more than guess.

There is about 10 years of historical data on the annual 3905 Group's Antenna Shootouts. They're inherently inaccurate as a whole, because the measurements are done at just one angle. But, if you accept some level of accuracy, they're as good, or better in most cases, than relying on EZNEC or other numerical processor. So, to answer you question about which is better, one has to know more than just the same vehicle, with a different antenna installed. Based on your supplied data, it's a toss up, because you haven't said where the measurement was made, in relation to the vehicle in question, or do you know the ground loss, and a few other factors. In other words, there is no answer. And, as Tom pointed out in the Quote I posted, you can't measure one parameter, and assume another one you didn't (or can't) measure.

Then, there is the own it factor. Everyone thinks his install is the best. The fact is, none of them are. Every single HF mobile installation is a compromise. It is just that some are a bigger compromise than others. So, if there is a best way, it's mounting an antenna as clear, and free of surrounding objects as local conditions allow. The other stuff (coil Q, and ground loss) are secondary. That said, Q losses are important, but once the Q exceeds about 250, very little is gained by increasing it. While a Hustler works, as does a Hamstick, they're not the stuff of champions, all else being equal.
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WX7G
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« Reply #19 on: July 30, 2010, 08:21:49 AM »

I am not using the simulation for absolute gain; I am using it for relative gain. This is like the difference between an absolute measurement and a relative measurement. They are different animals and have different purposes. If we want to know what is better, antenna A or antenna B, a relative measurement is all that is needed.

Using simulation to find the relative difference between two mobile antennas, the "winner" over one ground type will be the winner over a different ground type. Ground largely falls out of the analysis. That is why I said to model it over average GND, in free space, and over perfect GND. The superior antenna is superior over each GND type or in free space.

My van NEC model has 1500 segments. The "goodness" of a model can be checked by setting all losses to zero and seeing how close to 1 the Average Gain number is. This number compares the power into the antenna to the radiated power.

Based on my simulation I see that a Hustler RM40 mounted on the roof at the left rear of the van is superior to the Tarheel 200A HP mounted on the left rear bumper (1' from the vehicle). Better performance, same height above the street, and $30 rather than $400 spent.

I also see that mounting the RM40 in the center of the roof is not good. Simulation shows that it is not the best location and shows why this is so. This is what I like about simulation. What-ifs can be quickly tried to gain a better understanding of what matters and why. In the case of the bumper mounted antenna the van works against us. It increases the loading coil current (and losses) while not contributing to radiation. In the case of the roof mounted antenna the van becomes the dominant antenna; it contributes to radiation.
« Last Edit: July 30, 2010, 09:09:23 AM by DAVE CUTHBERT » Logged
K0BG
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« Reply #20 on: July 30, 2010, 09:50:17 AM »

Quote
Using simulation to find the relative difference between two mobile antennas, the "winner" over one ground type will be the winner over a different ground type. Ground largely falls out of the analysis. That is why I said to model it over average GND, in free space, and over perfect GND. The superior antenna is superior over each GND type or in free space.

This is not true. The optimal position of the loading coil is highly dependent on the ground losses encountered. As the ground losses increase, the optimal position rises toward the top of the antenna. The center position is perhaps a compromise, but even that assumption may be incorrect as we cannot know, for certain, the ground losses present. Keep in mind, we're not working in free space!

If you have access to the original articles by Jack Belrose, VE2CV (QST, September 1953, page 30), you might want to refer to them.
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