Modeling the Buddipole Antenna
The Buddipole is an antenna system for ham radio that's ripe for experimentation. In the pictures it's usually a dipole, but in practice it's more like a lego antenna kit. Unfortunately, it's a lot easier to make an antenna configuration that doesn't work than one that does, so people often turn to modeling software to do their initial experiments.
The Buddipole, however, is something of a challenge to model. Budd himself said to me: "EZNEC modeling for this antenna has proved to be a burden on many knowing folks. Don't spend a lot of time on it." Well, saying that to a programmer is equivalent to issuing a challenge.
First I think it's worth discussing why you'd want to model an antenna. With the Buddipole, consider:
Finding the optimal adjustments for the coils can be tricky. It varies not just with your operating frequency, but also with antenna configuration, height above ground, and other factors. Wouldn't it be nice to use a model to get your adjustments in the right ballpark before you start?
With the Rotating Arm Kit (RAK) you can adjust each arm's angle independently. How does this change your radiation pattern? A model can show you where your signal is going. (This part, in particular, was the source of some surprises for me.)
If you want to explore alternate configurations, e.g. a vertical with counterpoise whip vs. counterpoise wire, which configuration is going to work better?
Keep in mind that modeling is not a panacea. Reality is always more messy than the computer can simulate. However, it can get you in the right ballpark and let you visualize things that are very difficult to measure in real life, like radiation patterns.
There's a couple tricks, however, to modeling the Buddipole:
You need to translate the user-visible adjustment on the coils to an impedance. There's a spreadsheet on the Buddipole User's Group that I found to be inaccurate, which caused me considerable trouble early on.
The Buddipole has multiple materials in its construction, mostly aluminum and copper, and the model needs to account for the different conductances of these materials.
When using the Rotating Arm Kit (RAK), the antenna arms do not form a straight line, they're offset from each other by the width of the center VersaTee.
Also with the RAK, you can have fun with the angles of each arm, so the model should accommodate various arm angles.
Modeling With cocoaNEC
Fortunately these problems are easy to solve with a tool called cocoaNEC by Kok Chen, W7AY. cocoaNEC includes its own mini-programming language called NC, which lets you build the model in code, rather than entering a bunch of elements by hand. This lets you do some math in the model itself, for example the coil impedance calculations and the trigonometry around arm angles. (Note: cocoaNEC is Mac-only, sorry Windows guys.)
In my model I use global variables for things the user can readily change:
Location of coil taps (in turns)
Angle of each arm
VersaTee height above ground (i.e., mast height)
You'll notice in the model that the calculations are set up for the Buddipole Deluxe kit with long whips and a balun. My kit is from 2007; other vintages may vary slightly. You can easily change the whip length if you have standard or shock-cord whips. I always run with whips fully extended, but you could break these out into global variables if you want. The position of each element in space is determined with simple trigonometry within the model.
For determining coil inductance, I used the "short air-core cylindrical coil" formula from Wikipedia, which gives results that match my real-world testing (more on that below).
For conductance, I again used values from Wikipedia: 59.6e6 S/m for copper and 37.8e6 S/m for aluminum.
While NC's approach to modeling is already a tremendous improvement over tools like EZNEC, its super-weapon is optimization. You can run multiple values for variables and watch their effects in realtime. For example, this is a view of a vertical with an angled counterpoise, as the counterpoise goes from -90 degrees to +35 degrees (QuickTime required):
Keep in mind the scale on each radiation pattern, in particular the elevation one. You can clearly see the sweet spot where you've got a low takeoff angle and some front-to-back directivity.
Another important use for optimization is finding the right settings for your coils. For a given antenna configuration, you can let cocoaNEC try each coil setting and tell you which one has the lowest SWR.
The first surprise was modeling the Buddipole in its traditional dipole configuration. Using the 16 foot mast, here's the radiation pattern I see at 14.080MHz (20m band):
Most of the signal is going straight up! Indeed this makes a decent NVIS setup, but that's probably not what you were going for.
Compare this to putting the red arm vertical and the black arm at -20 degrees from horizontal. Red is the new pattern, dashed black line is the dipole for reference:
The maximum gain isn't as high, but its takeoff angle is a lot better: there's a lobe around 20 degrees which is going to get your signal out instead of up.
Model vs. Reality
Again, the model is never perfect. My results, however, match surprisingly well with reality. Joel W4LL and I used these models before setting up our 2008 ARRL Field Day station with two Buddipole kits, and they proved invaluable.
Since we had two antennas, we could do some A/B tests. The dipole vs. vertical patterns shown above played out exactly as the model predicted: we could hit local stations (NVIS propagation) better on the dipole but our reach was much further with the vertical.
In addition, the coil settings were very close—the model would either get us dead on or within one turn.
Sure enough, math isn't pure fantasy. While I agree with Budd that modeling in EZNEC is an exercise in frustration, using a better tool like cocoaNEC with NC actually works quite well. I've found the results both surprising and tremendously useful, especially as I explored vertical and angled dipole configurations.
While NC may look more daunting than the traditional spreadsheet-style NEC interface, it adds tremendous flexibility to your models. For example, the ability to do trigonometry in your model allows you to think in terms of angles rather than (x,y,z) points in free space. This makes your model easier to build and less prone to bad data entry. Hopefully someone will partner with Chen W7AY to port NC to Windows and Linux platforms.
Finally, I'm short on the tools required to properly analyze physical antennas (e.g. analyzers and field strength meters) but the computer model gives me tools, for free, that let me diverge from the traditional dipole configuration. The Buddipole should be viewed as a lego antenna kit, and modeling lets me explore the opportunities it presents.