Today I made and measured a series of 4-bay antennas at a range of sizes based on the CM 4221. The commercial design is an 8" element length and bay spacing. I set up a stand where I could vary the element spacing and length, and tested 7", 8", 9" and 10" antennas. This corresponds to peak gain channels of 88, 66, 51, and 40, respectively.
I tested on my local stations:
The results are about what you'd expect.
- the lowest frequency, ch 16, is also the strongest signal; I had to remove the reflector from the 7" to get any fall-off, despite a large predicted change in gain. You can't beat a good aerial signal!
- channel 28 is noisy, but the trend is to less signal with the shorter antennas
- channel 45 shows almost no change
- the weakest station is ch 58, and its signal gets stronger as the antenna gets smaller, as expected.
- its neighbor, ch 59, shows a similar trend, but with an opposite reaction when the reflector was removed.
I will also note that results with my "good" antenna are consistently better than what's shown here, save for a couple points on 58 and 59 against the 7" antenna. It was an accident, but one I'm happy I made.
Bottom line, you can move the peak gain of an antenna to one end of the UHF band, but only at the expense of gain at the other end. That's no surprise. What's interesting is the ability to tune this type of antenna by a simple scaling of element spacing and length.
Ants_design comparison.zip 2.4990234375k . file