Modeling my Ford Explorer

[The DB]

Today I finished putting together a model of my Ford Explorer. These are the results from said model.

First, this is what the model looks like with a 1/4 wavelength antenna on the middle of the roof.

I have three models based on this object, one with the antenna on the middle of the roof, as pictured above, one with the antenna on the middle back of the vehicle, and one with the antenna about where it is on my vehicle, which is between the other two. I also have a model of a 1/4 wavelength ground plane antenna with horizontal radials at the roof height.

The vertical plot (the pattern as seen from the side of the vehicle), the front of the vehicle is to the right (the 90 degree mark) of this plot.

The horizontal plot (the pattern as seen from above the vehicle), the front of the vehicle is at the top (the 90 degree mark) of this plot.

The antenna mounted on the middle back outperformed the other antennas in the forward direction, including the 1/4 wavelength groundplane reference. My setup is, not surprisingly, between the center and middle back mounted antennas as far as forward radiation goes. As is expected, as you move the antenna towards the back on the vehicle, the pattern shifts more towards the front. An interesting thing to note is the antenna mounted on the middle of the roof actually has more gain to the back of the vehicle than the front, bot not so much that you will notice.

Any other thoughts on these plots?

I do want to add more vehicle models in the future. As it is a lot of work to make one of these models, I'm not sure when that might be...

Here is a link to the .NEC file I used if you want to play with it.


The DB

 

[Marconi]

That is a good looking model, and way more work than I would do. The Explorer really looks long.

Since we were earlier discussing using a much shorter radiator and the match still not being good, how did your match turn out for this model?

I think your small model had a 9' foot radiator, how tall is this one?

I took another route in working with my model. I'm making a shoot out version with two whips and the driven element in the rear. The match on this model is very good, and I can make it resonant and show good resistance if I shorten the radiator to <>99" inches. In my work I see this shortening effect showing up as I get closer to the Earth with the model. This model is simulated to be sitting on 14" wheels and the car is about 7" above the road.

I think I will also make a new thread for it after I check it all out.

I've attached some images below.

DB, could you give me again what the captions for your wire description columns are? I would like to get a model with similar style as you made.

 

[The DB]

I made it using a patch generator, I made several patches for the different sides of the vehicle, stripped out the duplicate elements, and merged the different patches one. It wasn't that bad, the hardest parts were getting the patch generating software to put them where I wanted them and lined up properly. The resonant length differences were much closer to what we would normally expect, 8 foot 6 inches for the antenna being at the back to 8 feet 9 inches for the center mounted antenna. The AGT for all three models is 0.98. The match for all of the antennas I modeled on the vehicle is a little under 2:1.

Lets see... The lines that contain elements look like:

GW    1001    1    0    .0002    .3048    0    .1526    .3048    .0001

The first column, in this case GW is the identifier stating that there is an element.
The second column, in this case, 1001, is the element's tag number.
The third column, in this case a 1, is the segment count. A note here, I use auto segmentation, so I can make single segment elements and 4NEC2 will work around it. I'm not sure if other modeling programs can do that, so you may have some playing around to do with this.
The fourth, fifth and sixth columns are the X, Y, and Z coordinate for the first side of the element, and the seventh, eighth, and ninth columns are the coordinates for the other end.
The last column, here .0001, is the width of the element. All measurements in this model are in meters, partly because I like to use meters, and partly because the patch generator that comes with 4NEC2 only putouts files in meters... I know this is small, but larger diameters for elements in this model were giving me errors, the smaller diameter elements made said errors go away.

Looking at the picture above there are many squares, each of those square are six inches by six inches. This size just happened to be within an inch of all of the measurements for my vehicle. I rounded all of the dimensions, such as where the pillars were, to this convenient figure.

There are nearly 2000 elements, not including the antenna in this model, and even more segments. Do you have a version of EZNec that can handle that? If you are converting by hand, good luck, that will be a lot of work. There has to be a way to import 4NEC2 files in EZNec, that would make the process a lot easier, I know Henry HPSD has said their was...

And your note on the length of the vehicle, I think it is that way in part because I haven't included any of the curves (which would be a serious pain in the ass for I think not very different results) and part that I happen to own the largest version of the vehicle they ever made.


The DB

 

[Marconi]

Thanks for the info maybe I can use part of your idea. My software has a 500 segment limit.

Good luck and good modeling.

 

[Bearcat]

Nice modeling!

 

[The DB]

I did some additional modeling with my Ford Explorer model. Here are models from the back left light mount, and a set of duals mounted from both rear lights. I did the left read because it is a common mounting point for many 2'nd generation Explorers. I did the dual antennas in these locations for fun really.

The pink pattern is a standard 1/4 wavelength ground plane antenna with four horizontal radials mounted at roof height. It is included here as a control. The green pattern is the antenna mounted at the back of the roof from the previous model set, in that set it produced the most forward gain. I included it here for reference. The red pattern is a single 1/4 wavelength antenna mounted on a mount that bolts to the top of the left tail light (about the height of the bottom of the rear window). The blue line is a set of dual antennas mounted at the same height on both sides of the back of the vehicle.

In this particular case, adding the second antenna on the other side of the vehicle pushed the gain in a desirable direction instead of off to the side a bit, this shift in pattern direction will have more of an effect the the slight gain addition from adding the second antenna. On this vehicle, dual antennas in this location will provide an additional 2.55 dB forward gain over a single antenna in the middle of the roof, in the real world this is getting close to almost being a noticeable difference. Mind you, we are still talking less than half an s-unit on a properly calibrated s-meter...

Even more significant, if only slightly more, is the loss of gain to the sides. From the single antenna mounted on the middle of the back of the roof, we get a 3.18 dB loss to stations directly to the sides of the vehicle, which is also in the barely noticeable range.

Finally, to the back of the vehicle the dual antennas provide an additional gain of 2.36 dB, again, near the range of a barely noticeable difference.

So what we have is an expected slight forward/rear gain at the cost of a slight side gain, just like we would expect from a set of dual antennas over a single antenna mounted on the roof between where the dual antennas would be. This bi-directional gain is larger than I would have predicted (and noticeably larger than I would have expected if the antennas were even twice as far apart) based on prior readings and "cophased" modeling experimenting and two antennas of this distance apart. At this point I attribute this unexpected additional change to the shape of the vehicle and where the antennas are mounted on said vehicle more than any other factor.


The DB

 

[Robb]

This would be the right thing for antenna mfrs to use to display mobile antenna applications - so long as a particular model of many different cars can be made. Not an easy to do; but effective nonetheless. Sirio might consider this for the CB ers, as well as those who sell mobile Ham antennas.

This concept could also help people understand just what to expect from mounting antennas in different locations on their vehicles as well.

Nice job.

 

[The DB]

I don't know that I would trust any manufacturer model that was released unless I was able to get close to duplicating those results myself. Unfortunately, for far to many models, there really isn't enough info out there to easily do that. That being said, I tend to be like that with pretty much any model I come across. I do intend to add more vehicle types at some point. My mom's Chevy Impala will likely be next if only for ease of access, and I really want to do a pickup truck as well, and there is a custom antenna someone made on another forum that he is putting on a jeep (no metal roof type) that I really want to model as well..

Many of these models will be "close enough" (yep that is in fact a technical term) to multiple vehicles that you can apply them to. My Ford Explorer, for instance, is the largest version of that vehicle that I think they have ever made, and really isn't that much smaller than my dad's Expedition, and by extension the Chevy Suburban. I, personally, think it is close enough to get a fairly good idea of what a given pattern would look like on such vehicles. It can also be applied to SAV's and the like. Yes, the patterns will be a little different, but they won't be that different...

Also, many cars are about the same size, so a single model object of a car will produce results close to what we would expect from several cars...

I use a tool to make said grids, and the biggest pain in the backside is getting the grids to line up. I made that Explorer object in less than a day, so it really just takes some measurements and time.

Seeing what happens with an antenna on a vehicle instead of a standard ground plane was the whole point. The ability to move the antenna around on said vehicle and see those results is also a huge plus. I have no problem with publishing the results of those experiments as I think it is an area that is lacking when it comes to modeling, and potentially the discussion as a whole. Getting more mobile models out there for reference will only help us as a community, especially as at least on the CB side, I think a vast majority of antennas are mobile antennas.

Currently I am actually working on something else with that vehicle model, essentially comparing base loaded antennas of various lengths and top loaded antennas of similar lengths. It is tedious work, however, so I'll finish it when I finish it...


The DB

 

[Robb]

. . . As opposed to mounting an antenna on a roof, rear trunk lid, or the front hood.
Those examples will have varying and different propagation patterns . . . also the different vehicle lengths and widths should only create minor but measurable differences as well . . .
eg: a 69 Chevy impala vs a Honda civic.

 

[The DB]

I've done some modeling and charting of base and top loaded antennas from the resonant point to about three feet in length on this vehicle object.

The antenna length in feet is across the bottom.

To start with we have efficiency.

This chart charts radiation efficiency in percent over the range of lengths. Even at the three foot length, the difference in efficiency between a base loaded antenna and a top loaded antenna on this model is only 0.6%, less if the antennas are longer. That is the largest difference on this range of lengths.

Next we have the gain data.

Here we see the gain difference between a base loaded and a top loaded antenna on this model. The top loaded antenna will have 1.1 dB additional gain if the antennas are both three feet long, less if the antennas are longer.

This gain info is really only relevant if we are using skywave propagation. Even if another vehicle is within eye sight, the lobe that contains this gain will generally be far above the antenna. So for local data I modeled surface wave data.

This type of modeling measures a received signal at a certain distance and height away from the transmitter. In this case, the measuring point was five miles away and six feet off the ground. This is also not measured in dB like the gain chart above, but uV/M, or microvolts per meter. The transmitting antenna is being fed 100 watts in this simulation.

On a properly calibrated S-Meter...
S8 would be a reading of 25.1 which is essentially at the bottom of the chart.
S9 would be a reading of 50.2 which is much higher than the top of this chart..

The chart only shows about 1/3 of a properly calibrated S-Unit. The full range of data shown would be not much more than a needles width on most S-Meters.

Further, this was modeled on an SUV object, which is larger than most other vehicles out there that an antenna will be run on. On a smaller vehicle, all of these charts would have a smaller difference.

So in conclusion...

Is there a difference between a top loaded antenna and a base loaded antenna? Yes, technically.

Is it enough to notice the difference in the real world? No, not even close.


The DB

 

[Marconi]

. . . As opposed to mounting an antenna on a roof, rear trunk lid, or the front hood.
Those examples will have varying and different propagation patterns . . . also the different vehicle lengths and widths should only create minor but measurable differences as well . . .
eg: a 69 Chevy impala vs a Honda civic.

Robb, in the attachment below I fixed my mobile models to look more like a Bronco than a P/U truck. It is still likely to be smaller in length than DB's models, but I think we are seeing more similar patterns now.

These two models show a much better match, and the gain is about the same, but the radiator lengths are shortened as noted. This shortening is not what I see in my real world setups however.

I am still concerned about my models are not matching like I would expect using a 102" whip on a mobile setup, but I adjusted the radiator length and now I see a nice 1/4 wave match.

Sorry my attached notes are not legible.

 

[The DB]

These two models show a much better match, and the gain is about the same, but the radiator lengths are shortened as noted. This shortening is not what I see in my real world setups however.

I am still concerned about my models are not matching like I would expect using a 102" whip on a mobile setup, but I adjusted the radiator length and now I see a nice 1/4 wave match.

I did have some fairly short antennas after tuning on that initial car model object I talked about and posted pictures from in another thread, that one was provided by 4NEC2, but not in working form, I spent hours making it work. I really haven't done anything with that NEC2 object since.

On the Ford Explorer object I made the center of the roof 1/4 wavelength antenna tunes resonance between 102 and 103 inches in length. The back of the vehicle antenna tunes resonance between 105 and 106 inches in length. With the layout of the models I would expect the results to be fairly similar, I think the biggest differentiator between our vehicle models is my models have interlinked wires to simulate planes for the surfaces of the vehicle while yours has an outline and an occasional cross wire. I think that this causes my models "ground plane" simulation to be seen as more complete, and thus a longer tuned antenna length. Remember that discussion about 1/4 wavelength antennas with four radials not coming out to 36 ohms, and the solution was more ground plane radials? Something like that.

I'm curious, how many segments does your version of EZNec support? My ford explorer model uses 6 inch by 6 inch squares for the planes (the sides, top, bottom, hood, ect) and has over 2000 segments because of it. That, honestly, is likely overkill for the model, but as I have essentially no segment limit, its not a problem for me, and I don't think it will hurt the accuracy any by having a better simulation of the vehicles body. I think you could get results that are nearly as accurate with a larger square area in the planes, and that would lower the segment count as well.

Also, did you ever figure out how to get it to load data from a NEC2 file? I can make it work, but it is kind of a pain to do... You have to modify the file to do it.

Here is the 4NEC2 source file data I used:

EZNec file data after import...

It is a pain to do, but I can do it. As you can see I can even maintain the segment counts...

Hmm, might be interesting to run the exact same model in both programs sometime and compare results... But that will be another day...


The DB

 

[Marconi]

I think the biggest differentiator between our vehicle models is my models have interlinked wires to simulate planes for the surfaces of the vehicle while yours has an outline and an occasional cross wire. I think that this causes my models "ground plane" simulation to be seen as more complete, and thus a longer tuned antenna length.

That is what I saw possible with your model too. I was hoping your vehicle design, using a <>102" inch radiator in the top center, would show a better match than I was getting using only a skeleton of a vehicle with a 102" inch whip.

I did try my P/U version of a Bronco with the bed removed to see how a smaller vehicle worked, and the match got even worse. So, I figured you were right, the complexity and overall size makes a difference, but I don't think that alone fixes the mismatches I see.

I use a 102" whip at 27.205 mhz set close to top dead center in a new version of the Bronco with a top all the way to the back, not a P/U anymore. The match is still over 3.00:1 SWR.

A 102" whip in my real world experience does a much better match than I see in my models. Depending on where I locate the radiator using a 108" whip...the match gets higher still. Henry's model was only 15.5'L x 5.5'H x 6'W, and I think that is smaller than a real Bronco.

Remind me of the matching results on your bigger Explorer with a 102" - 108" radiator set in top dead center?

I'm curious, how many segments does your version of EZNec support? My ford explorer model uses 6 inch by 6 inch squares

I can use up to 500 segments. I set my segments as close to 6" inches for each wire as I can.

I like the other work you posted. I have to dig into that stuff a bit however...it is above my pay grade.

 

[Marconi]

I've done some modeling and charting of base and top loaded antennas from the resonant point to about three feet in length on this vehicle object.

DB, generally speaking, how did you construct these top and base loaded radiators that produced these results?

So in conclusion...

Is there a difference between a top loaded antenna and a base loaded antenna? Yes, technically.

Is it enough to notice the difference in the real world? No, not even close.

Thanks for the details and your report.

For a while now I have made similar claims that theory deals in micro details, facts, and results...while the CB world often takes this same information and makes a mountain out of a mole hill with the results.

Thanks,

 

[The DB]

Remind me of the matching results on your bigger Explorer with a 102" - 108" radiator set in top dead center?

Resonance is between 102 and 103 inches in length. The SWR match is below 2:1, although not really much. This comes pretty close to the antenna tune on my vehicle, which is mounted s little behind that same position.

DB, generally speaking, how did you construct these top and base loaded radiators that produced these results?

The base loaded antennas have the feedpoint on the first segment at the base, and an inductive load at the first segment above the feedpoint. The top loaded antennas have the load on the third segment from the top as most top loaded antennas don't actually have an inductive load at the top. In both cases the antennas were tuned to resonance by adjusting the inductance of said loads. As a note, in general, the top loaded antennas required noticeably more inductance to tune the antenna as the base loaded counterparts, although this is nothing new to me. This number got exponentially higher as the top loaded antennas shrunk below about four feet in length. I'm not sure if this has to do with the length, or if the shape of the model is somehow the cause.

This graph shows the inductance used to tune the antenna in Henry's, the numbers on the left, are in uH.

Some of the antennas sold today are continuous loaded antennas, or in essence, have a load going all the way up the antenna. I wonder how they would compare to base and top loaded antennas. That might be something worth adding to the above data...


The DB