I had time to work on the Gizmotchy antenna project today and I made good progress!
I started by running the numbers of a standard 2 meter 3 element yagi, comparing the driven element length at various frequencies to this Gizmotchy. My hope was that by comparing the element lengths between 142 Mhz and 146 Mhz on a standard yagi, I could understand how much I had to shorten the Gizmotchy elements. When I ran the numbers, I also saw that the Gizmotchy elements were already a little shorter than the standard yagi. Admittedly, I don't completely understand the Gizmotchy, but I assume the shorter element length is due to the existence of the third leg on each element.
Running the numbers helped me understand how much I would need to shorten the elements by, but I still wanted to be cautious and not cut them down too much all at once. As a result, I decided to shorten them by 1/8" at a time and remeasure. Fortunately, I have a decent little bench top disc grinder which made quick work of the aluminum.
The end result was that the elements all needed to be shortened by 1/2" with the new, improved spacing. I wanted the low VSWR right around the 2m FM calling frequency, and here is the result:
The SWR is now under 2:1 for the entire 2 meter band, plus a little on either side 
.
After shortening all of the elements, I double checked the match on both the vertical and horizontal sides and raised the antenna up for some testing. Using the same fiberglass push-up mast, I raised it up to about 12 feet. Keep in mind that I'm on a hill above most of Los Angeles, about 800 feet above the valley below, so I don't need to get the antenna very high for it to be very effective. I hooked up my Yaesu 897D, put it on 2 meter and tested out my work.
The first thing I did was do some simple, typical yagi tests, checking out the side and rear rejection. I found some stations on the other side of the 6000 foot mountain that were barely breaking the squelch. With the side or rear of the antenna, they were unreadable. By pointing the antenna at the mountain, I could actually copy them, even though they were very scratchy. Turning the antenna around on other active frequencies, I was very happy with the side rejection and also found that the rear rejection was consistent with the revised element spacing's front to back ratio figures.
Next, I needed to test the effectiveness of switching between horizontal and vertical polarization, which is the whole point of this unique antenna. As anyone who has messed with VHF or higher bands knows, cross polarization of antennas will severely affect the strength of each stations' signal as you get into the higher frequencies. If the Gizmotchy is truly able to change polarity, it would exhibit these same characteristics.
The first FM station I found to test with was only about 5 miles away. His signal was pretty strong with the beam pointed at him, so I turned down the RF gain while horizontally polarized to get a measurable S meter reading of S9. I then switched the antenna to vertically polarized and he was 40 db over S9.
Next I wanted to test a station farther away. I found a distant repeater that I wasn't receiving nearly as strong and took the following reading on the Yaesu 897d while vertically polarized:
Note the S9 reading. Next I switched the antenna to horizontally polarized and measured again. This picture is of my radio while receiving the same station with the change in polarity:
The station was being received, but not even getting an S meter reading. Needless to stay, I was quite impressed and happy with the initial polarization tests! After the fairly simple mods to the antenna, this antenna is really looking like something very useful!
Next, the antenna will get mounted on the roof with a rotator and remote coax switch to make it easy to change polarization.
Stay tuned...
