When I look at actual pictures of the firestik, there are three different levels of winding. The windings at the bottom are spread out much further than the windings in the middle, which are further apart again than the windings at the top of the antenna, which are adjacent. The antennas in this image have five different separations of windings, however, these aren't the seven foot version, however they are the six foot version so they are likely close...
An interesting thing to note, the blue one has a different winding pattern than the others...
Something else that I find interesting is how many windings they actually use on these antennas. Maybe it is just me, but it looks like more than I would have expected they would need at first glance.
Anyway, this makes it harder to make a model of this type of antenna as you need to have at least three different coil layouts throughout the antenna length, and possibly as many as five.
Firestik claims these are top loaded antennas, but the reality is they are actually somewhere between your continuous loaded model and a top loaded antenna. That is just one more thing on the Firestik site that isn't technically correct.
Another thing to consider, at least with 4Nec2, a model like this would require more wires than I can make out of a given element (copper) as I can only put said type of conductor on 30 something wires (I think it was). I would be completely unable to put a rubber covering over said wires, much less be able to factor in the fiberglass in the middle of the coil. These little effects add up, just a thought.
Another concern is the segment size of the individual wires that make up the coil. On an antenna like this these would be much smaller than what we would normally expect to see with most antennas. The more wires used to make up each coil will only make this potential problem worse. How many wires did you use per coil? It is possible you may have to add more segments to the radials you used to compensate.
Not being able to factor in these details, as well as potentially others I have yet to consider (as I have not yet made the model), I fear any gain reports from this type of model will be somewhat optimistic at best.
Eddie, have you by chance checked to see how much of the gain that is shown is horizontally vs how much is vertically polarized with your model? That might be interesting to consider. The coil of wires will act like an inductor, but the wires in your model are also not exactly vertical.
I am looking at making such an antenna model and putting it on my Ford Explorer item I made a while back, but I am concerned about the segment count of the overall model. The ford Explorer item has over 2000 segments to begin with, and takes much longer to run than most models we are used to. This type of antenna will only make that problem worse.
That being said, what I am considering doing is dividing the antenna up into five equal parts, and to start with not using coils, but instead inductor obhects that are also evenly spread out as well. These would be tuned to match the antenna as well as I can on the object. I could then replace each individual inductance section with a coil. It may not match the Firestik design exactly, but it will give an idea of what such an antenna design is capable of comparatively speaking when mounted on a vehicle.
Unfortunately this will take a lot of work, and I am not going to have the time to do anything like this for quite a while.
The DB
