Antenna Application Note 47899.12. de W3IY 07/15/02
6ft Rover Yagis Closely-Stacked.
As I mentioned in another application note on this site, many rumors and misconceptions exist in the amateur radio world about stacking VHF/UHF antennas on the same mast. One of these is that you have to space yagis far apart to make the work well. This just aint so! I have been using a closely stacked array of 6-foot rover-yagis on a rotator, mounted to my front bumper for a couple of years, now. It has worked well, but I have always wanted do some NEC2 modeling on it, to see if it was supposed to be working. (Notice, in the true rover-tradition, I tried it first, and as a last resort, I'm doing some analysis...hihi). Well, as I expected, the gain of the antennas is not appreciably degraded. In actuality, this array is stacked below a 6 meter halo, which in included in the model. I was more concerned about the interaction between the yagis, since they are stacked only 1 foot apart on the rotatable mast. I plan to model the 6 meter antenna soon, but I already know that it doesn't hurt the closest yagi (2 meters) much, as I have had great results on this band. To see the wire model of my rotatable rover stack, click here. Image
The configuration is the 2 meter yagi on top (this is the high-priority band), 432 in the middle, and 222 on the bottom. All antennas exhibit acceptable VSWR (approximately 1.5 to 1, worst case).
Here is a summary of the findings (using 11 segments-per-element).
Antenna Free-Space Gain Stacked-Gain Stacking Loss
144 MHz 11.17 10.71 0.46dB
222 MHz 12.39 12.12 0.27dB
432 MHz 14.66 13.52 1.14dB
Here is a pattern of the 2 meter yagi, in free space. Yagi Alone
Here is the pattern of the 2 meter yagi mounted in the rover stack. 2m Yagi in Stack
Here is a pattern of the 222 MHz yagi, in free space. Yagi Alone
Here is the pattern of the 222 MHz yagi mounted in the rover stack. 222 Yagi in Stack
Here is a pattern of the 432 MHz yagi, in free space. Yagi Alone
Here is the pattern of the 2 meter yagi mounted in the rover stack. 432 Yagi in Stack
As you can plainly see, the degradation of gain is quite small, and acceptable. Don't let the paranoia of others (who have not tried stuff) keep you from fielding a nifty yagi array, that gives you what you want. I have tuned up my yagis by themselves, and have not needed to tweak anything after placing them in the stack. They really seem to work well. I do notice that 432 seems a little down, but I think this is because I am used to a full-sized RIW-19, when I am parked, and because at 432, signals just don't seem to poke through all the foliage I always seem to be driving through. It's really nice to be able to operate in 6m through 432 while "in-flight". The 6-foot yagis are short enough to be safely rotatable while driving, and they give me a significant performance advantage over halos. It can be a problem, driving through a cloverleaf interchange, trying to keep the beam on someone, however...hi. Always be safe when you are driving. Bring along a rover-partner to share the radio fun, so someone can concentrate on driving.
In the near future, I plan to build a wire model of my rover van, and include it in the modeling exercise. I suspect it will mostly affect the performance of the 6-meter halo.
Go get 'em, guys! There's lots of stations waiting to work you out there. Give out a few grids. Have fun.