Richard, thanks for the useful citations. I've got a couple of Electrical Engineering degrees and think I understand the basics pretty well. What I am looking for is actual manufacturer test data (which used to be easy to find). As you noted regarding the RCA literature, it's like looking for car specs on torque, horsepower, and axle ratios and finding data on genuine leather seats and burled walnut dash inlays.
In looking for a good example to refer you to, I found this website:
http://www.digitalhome.ca/ota/superante ... rmance.htm
The first half of the page shows the benefit to redesigning the antenna without a reception requirement for channels 52-69. You can get more gain (sensitivity) for the lower channels.
The second half of the page shows what I most recently referred to: comparative polar plots. The last set of plots on this page are exemplary. The left side shows a Double-Bay Gray-Hoverman antenna and the right side shows a Channel Master 4228 8-bay bowtie, which is the gold standard, I suppose.
If you look at the lobe width (width of the loop going to the right), the Channel Master is narrower. This means you have to aim it more carefully, and might have to rotate it between stations that are nearby but not collocated. But it also means that you can get the signal and reject the multipath that bounces off some other object and then enters at an angle. This latter problem is the one I am dealing with.
These plots are "normalized." If they were not, you could look at the lobe "height" (distance from plot center to tip) and tell which one had the most gain (sensitivity), but in this case they have been scaled so that the peak is always 0 dB. The scale factor is shown below each plot. We see that the 2-bay Gray-Hoverman has 17.82 dB and the 8-bay Channel Master has 16.93 dB. These are usually given compared to a standard monopole (vertical metal rod). So the Gray-Hoverman is about 1 dB more sensitive, although not as good at rejecting signals arriving off-axis.
Another thing you can see from these Polar Plots is Front-to-Back ratio. It's hard to design an antenna that doesn't have some reception from the back side, due to symmetry considerations. This is a bad deal if you have, say, a local channel due west on channel 40, and a station due east, further away, also broadcasting on channel 40. On a clear night when reception conditions are good, the remote signal enters the antenna's back side and interferes with the desired signal. This is called co-channel interference. You can see that the Gray-Hoverman is better than the Channel Master, especially at 600 MHz (channels 36-51).
There are some polar plots of Yagi antennas at
http://www.astronwireless.com/topic-arc ... tterns.asp as part of a tutorial on polar plots. What I would really like to see os polar plots of a Yagi and 8-bay bowtie done to the same scale and plotting scheme, to "compare apples to apples" so to speak. Haven't found that magic website yet.
Larry
