Modeling my Ford Explorer

[Marconi]

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.

My 27.205 mhz model's radiator in the top center is resonant at a length of 88.9" inches, so I guess that my skeleton like model is as you suggest.

The back of the vehicle antenna tunes resonance between 105 and 106 inches in length.

My model with the radiator in the center rear of the Bronco needs to be about 94.4" inches, so it is a bit longer...but not near what you see.

 

[Marconi]

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.



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

I was just curious if the top load was a physical design using a coil or if they were both using a matching function in 4Nec2.

In my thinking about your idea here...I was considering using a top hat to load the top of the antenna similar to the old GR45 by True CB Quad. http://www.lightningantennas.com/wp-content/uploads/2011/04/GR45GIF.gif

 

[The DB]

I was just curious if the top load was a physical design using a coil or if they were both using a matching function in 4Nec2.

In my thinking about your idea here...I was considering using a top hat to load the top of the antenna similar to the old GR45 by True CB Quad. http://www.lightningantennas.com/wp-content/uploads/2011/04/GR45GIF.gif

4NEC2 allows me to place an inductive load on the antenna itself. I can have a series or parallel RLC circuit. I am using series, and the R (resistance) and C (capacitance) components are set to 0, and the L component is inductance which varies based on position and antenna length. I can actually change only the inductance and watch the feed point impedance and current distribution change of the antenna change.

I've done some models where the antenna is a continuous loaded antenna. In that case, the entire length of the whip is an inductor... It is more efficient than either the top or base loaded models I have yet compared it to, however, its performance seems to be right about in between the two. In any case, as they are so close in performance anyway, you won't notice the difference. That being said, I have only done one of the six feet worth of comparison so far so I guess it is possible that things may change at some point...


The DB

 

[The DB]

Here I've taken the five foot lengths form above and directly compared the top and base loaded versions of the antenna. This is to give an idea of how such patterns change by the position of the load.

Lets start with far field gain, in this case, the front of the vehicle is to the right of the plot.

Now from another view, the top looking down. The front of the vehicle is towards the top of this plot.

Here we can see slight differences in the patterns. The difference in forward gain on this vehicle is 0.06 dB, while the difference in gain to the back of the vehicle is 0.3 dB. Also, in the vertical pattern, while the maximum gain points are at the same angle, the top loaded antenna tends to have a slightly stronger signal below that point, and the base loaded antenna tends to have a slightly stringer signal strength above that angle.

Now for the local plot data, to start with this is as viewed from the top.

As we can see, there is a difference as to how the general patterns we see differ from a surface wave pattern. While the patterns here do look similar, it is important to note the differences. Where as the gain pattern, which really only tells us about skip, has a larger difference in gain to the rear of the vehicle than the front, when working with local signals, i.e. surface waves, we see that this difference is much more consistent around the vehicle.

Now we will look at the vertical pattern, this will look different than most modeling outputs people are used to seeing. In this case the vertical part of this plot is showing the height above ground, in this case, zero to two meters (or close to 6 feet). The horizontal axis is showing the signal strength, with stronger signal strengths being further to the right. As a note, the measurement point is five miles from the antenna.

Here we see the top loaded antenna has slightly more surface wave signal strength, and unlike the gain pattern shown above, this gain is very consistent in all directions from the antenna. It is important to note, again, that these differences seen here are so small that you will never notice them. We are talking something like 1/20 of an s-unit...


The DB

 

[StrangeBrew]

You guys are tempting me to get some modeling software. Just what I need, more stuff that I don't have time for.


The results from the dual antenna model does kind of confirm what I've seen with relatively closely spaced antennas, there always seemed that there might be front/back gain at the expense of performance to the sides but it was always so close that I couldn't really be sure. It's definitely a far cry from the commonly held wisdom that antennas closer than nine feet apart will cause a meteor to drop from the sky and crush your car.

 

[The DB]

Is that what causes meteor's?

I think the thing that helped that "cophased" antenna model is the shape of the vehicle, and where the antennas were on the vehicle. It did have more of an effect than I expected... I had a different model I posted in another thread of a smaller hatchback, and there was no difference between the antenna in the center of the roof and the "cophased" antennas on either side of the middle of the roof. That model in the other thread wasn't mine, however, it was modified from an object that came with 4NEC2 to actually work without errors.

I'm curious to see where this line of experimentation will lead me, but for that I need to make more vehicle models...


The DB

 

[Marconi]

I think the thing that helped that "cophased" antenna model is the shape of the vehicle, and where the antennas were on the vehicle. It did have more of an effect than I expected

DB I tend to agree with the points you make here.

Right now I'm trying to modify the basic vehicle...similar to you're suggestions to make models bigger and smaller. My plan was to report a few observations, even if I might have to eat my words later.

DB, I've modified my Bronco model, that was made to look like a P/U truck, into a model that has a cover on the back similar to your Ford Explorer. I did not use your design technique, however.The model is still small, so I also made a model bigger in length and height as well. The model still requires a radiator shorter than 102" - 108" inches...in order to show resonance with a decent match for CB.

Considering my modeling such results seem strange to me, because my real world experiences never indicated the radiator needed to be shortened to match.

I had a different model I posted in another thread of a smaller hatchback, and there was no difference between the antenna in the center of the roof and the "cophased" antennas on either side of the middle of the roof.

Your report, that nothing changes here surprises me. However, I haven't worked much with my smaller model yet, more work to come.

I'm curious to see where this line of experimentation will lead me, but for that I need to make more vehicle models...

I am curious too. Problem for me is my visions isn't too good, and I'm seeing stars just looking at all the wires and the data...and trying to reconfigure my mobile models into different configurations. This amount of detail may be why very few if any manufactures do modeling work to support their promotional ads.

Models later.

Good luck and keep us posted.

 

[The DB]

Your report, that nothing changes here surprises me. However, I haven't worked much with my smaller model yet, more work to come.

They weren't exactly the same, but the results were so close that no one would be able to tell the difference... The front and back were essentially the same, however the sides of the vehicle the "cophased" antennas as a 0.1 dB loss (I know, the picture says collinear, but that was a mistake)...

The red and blue lines on this plot...

41

I likely will not use that vehicle object again, it was not made by me, I only modified to function. I think it could have been made better. It is also a very small vehicle, like Geo Metro small. I don't see many Geo Metro's around and even when I do, I don't see any antennas on them making me think this modeling object is more or less obsolete.


The DB

 

[Marconi]

I'm not going to do much with a smaller vehicle either.

 

[Marconi]

http://hittman.us/pictures/11-17-15/1.jpg

DB, on your models above I realize we are working with very low gain values for the models just because they are low to the Earth. On the image above I see the models briefly identified on the far left near the bottom, and I see 2 values to the right near the bottom that should be gain values for the antennas on the left.

Why are there two values noted for the gain and what should we take two values to mean?

 

[The DB]

There are three values noted for gain. The green plot is a 1/4 wavelength antenna mounted at the roof height, it is there for reference. The red plot is a single antenna in the middle of the roof of that model, and the blue plot is a set of "cophased" antennas, one at each side of the roof. It is hard to see the separate patterns between the red and blue plots, but there is a difference, however slight it is.


The DB

 

[Marconi]

I don't question there are three models.

My question was, I see two values for the gain on your 1-4.out model, -0.5 < dbi < -0.4. To be sure, are these two values for gain noted here...indicating the maximum and the minimum gain for 1-4.out pattern?

 

[The DB]

Yes, that is exactly right.


The DB

 

[Marconi]

I'm not going to do much with a smaller vehicle either.

Attached below are my two models of a smaller vehicle that I reported showed a match that was worse using a 102" whip on my original Bronco model. I did not try to tune these two because both models require the radiators to be made much shorter.

The patterns for the co-phase and the center mounted vertical radiator are similar...Just like you suggest. I also see the typical increase in gain with the co-phase setup over the single antenna. I don't think there is much to gain making the vehicles smaller.

I think the thing that helped that "cophased" antenna model is the shape of the vehicle, and where the antennas were on the vehicle. It did have more of an effect than I expected... I had a different model I posted in another thread of a smaller hatchback, and there was no difference between the antenna in the center of the roof and the "cophased" antennas on either side of the middle of the roof.

I agree the patterns for both our small vehicle models look to show strangely different patterns, and I didn't expect that either.

I also added an overlay of these two models to show how similar they are.

 

[Marconi]

Yes, that is exactly right.

Thanks.

I don't remember noticing your models gain being shown this way before, so I wasn't sure what the two values for gain meant.

That is a nice feature to have if working with a co-phase or a beam setup. I've done something similar before with Eznec, showing the max/min difference for gain, but I have to produce two images to show such results, and I was never sure how the viewer took the info to mean. 4Nec2 is slick.

Is this a select-able feature with 4Nec2?