I've been experimenting with a 2-element sniffing antenna, and have had some good results, so thought I'd pass along what I've done so far. I started by using an antenna-modelling program to see what spacing and element-lengths would give me the best Front-to-Back ratio. For this antenna, overall gain is not a factor since when you are close enough to be sniffing, the signal from the fox is going to be very strong.
The results of the modelling were pretty surprising, in fact, I didn't believe it, and re-ran the program several times, using different starting element lengths and spacings, but letting the program adjust both factors for greatest Front-to-Back. Using a frequency of 146.565 Mhz, and optimizing for horizontal polarization, the program always came back with these results:
Spacing: 4.027inches (102.3mm)
Reflector: 19.836 inches (503.8mm)
Driven: 18.938 inches (481.0mm)
Gain: 4.63 dBd
F-B: 12.32 dB
Impedance: 7.0 + j3.3 Ohms
Horiz. Beamwidth: approx. 70 degrees
Vert. Beamwidth: approx. 120 degrees
It was the close spacing that seemed odd. By the way, the element lengths shown and used hereafter are only half the actual length. My modelling program likes that format, and I just got used to it.
Well, the next step was to try it out. It happened that I had an old Cushcraft 4-element beam not being used at the moment, and as luck would have it, the boom already had two holes spaced 4.0 inches (100mm apart) down at the end for a mounting bracket. Also, the existing Reflector and Driven element lengths were 19.875 inches and 18.938 inches. All close enough for this experiment.
The existing driven element was removed and replaced at the hole nearest the end of the boom. Then, the existing reflector was placed it in the hole 4-inches (100mm) away. The other two existing elements were removed and set aside. The boom was left otherwise intact, making a nice long handle, to allow one to get the elements away from the body when in use. I made no attempt to play with the feed. The existing Beta Match was left as-is, and might as well be considered un-tuned.
OK, so how well does it perform? So far, I've used it on about a half-dozen hunts, some in open areas, some in areas where reflections were obvious (such as under and near a bridge spanning the Rio Grande river). The first thing I noticed was that the Front-to-Back seems quite good, but it is also very broad, which agrees with the model. Once you get use to that, it works very well. The antenna is very manageable and easy to use. You can switch polarity with a flick of the wrist, and sometimes that is useful, even in the near field, for pinning down the exact location of the fox. The elements, although full size, can be maneuvered through trees and brush without a lot of difficulty, and the weight is almost negligible.
I guess, given that the impedance is so low, that I need to play with the tuning of the Beta Match, or perhaps try another matching system. If I do make any changes that improve this antenna, I'll post them here.