Yes, I'm sure. The Zenith picks up nothing as virtual channel 8 even on repeated auto-scans at different antenna positions, or when I try to go to virtual 8 on the remote.
And it also has a menu function where you can go to a real channel number (i.e. - 10) manually. It shows something is there, about like it does on channel 36, but too low a signal strength to bring it in. Turning the antenna doesn't have much effect. Well, there must be something in the way, I guess, but it's still surprising since it's a VHF channel, which should come through pretty readily.
But with a yagi, you have many more dimensions to get right. Driven element, space to next element, first direction, space, second director, blah blah blah. That was my point. The "Loop yagi" doesn't have many things to cut, while a multi-element yagi does.
I prefer SPLAT!, though you need a bit of linux knowledge to run it. Radio Mobile is OK, but you need to set up some options at sufficient tolerance to get a decent profile. The advantage to Radio Mobile is it runs in windows, while SPLAT! needs to be compiled.
Both programs are free.
I've also done LOS analysis in GRASS, but that is really tough to use.
There is a paper by some high school students doing wifi analysis with GRASS and I seriously doubt they did it without some adult help. Just loading GRASS itself is a bear since the order of compilation is critical.
I beg to differ. It's fairly easy to trim the ends of all the yagi elements except perhaps the driven element. You can even round the ends of the yagi elements to add additional bandwidth. With a loop yagi, you have to get the circumference correct the first time, with no easy way to adjust the circumference. I've tried cutting the loops by pretuning with a grid dip meter and couldn't get sufficient precision. It's also easier to build things in 2D than in 3D.
What's nice about the loop yagi and cubical quad antennas is that they have approximately a theoretical 2dB more gain than a yagi with the same boom length and better f/b ratio. Also nice is the ability to build a quad using insulating spreaders and wires instead of tubing. You can also build a loop yagi that works with circular polarization (which is of no benefit for a TV antenna). Quads and Loop Yagis also meet my criteria for the ugliest antenna working better. So why are we not all using quads and loop yagi's for TV antennas? Because they're difficult and more expensive to build than yagis and they don't broadband very nicely. Visualize one of the more elaborate TV yagi antennas with loops replacing all the dipole elements. I don't think it would sell, and would certainly be a mess to build.
Bandwidth for a yagi is also affected by the diameter of the element rods. Large diameter tubing has a wider bandwidth. It's easy to use large diameter tubing (with rounded ends) for a flat yagi, but almost impossible to bend such tubing into a loop or quad arrangement. Typically, loop yagis are made from thin strips of sheet metal with the overlapping ends connected by a pop rivet or spot weld. While this results in an antenna that's a bit more broadband than one made from wire loops, it not even close to a fat element yagi.
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Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
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Paul Hovnanian mailto:Paul@Hovnanian.com
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"Nothing Important Happened Today"
-- King George III, diary entry July 4, 1776
VK3BLN has an interesting and extremely effective DF antenna for 70cm. A DF antenna doesn't necessarily want maximum gain or even F/B ratio, what it wants is to minimize lobing, across the band. Difficult. It's a five-element Yagi that has two short-long pairs. We believe it works like a 3-element yagi but with the pairs coupling to broaden the response. It took a lot of optimising - using a custom build of 4NEC with a multi-dimensional slope optimiser.
Good idea, but probably too critical on the tuning. It certainly would be difficult to preset the circumference to a specific length. For example, at 600MHz, we're looking at a wavelength of: 3*10^8 meters/sec / 600*10^6 cycles/sec = 0.5 meters for a full wave loop. In terms of loop cut length, that's approx: 0.5 meters / 600 MHz = 0.83mm/MHz In other words, if your cut length is 0.83mm off, you're 1 MHz off frequency. Since the channel is 6MHz wide, you can probably be about
+/- 2.5mm off, and still have the peak within the 6MHz channel. You could probably get it that close if you lay the hose clamp on a flat surface, and mark the target value, but I wouldn't want to bet on its accuracy or stability.
Correct. There's a series of articles in the old 73 Magazine in the Homing In column that cover the requirements for rotating antenna computerized direction finding which emphasizes the same point. If you happen to have a perfectly clear signal source, and you spin the antenna, the pattern you'll see on a PPI scope will be the antenna pattern. Add a logarithmic signal strength magnitude indication, it should look exactly like the advertisers data sheet or the NEC2 plot.
However, that's not an important requirement for a TV antenna. Having side lobes are not a good thing due to multipath distortion, frequency selective fading, and digital "ghosting" effects, but can be tolerated.
If it were easy, it would not be fun.
I'll give it a look. Sounds interesting. I've built a few rotating antenna VHF/UHF DF (i.e. T-Hunt) systems using quad antennas. I'm always interested in something better. I found some fox hunting references for VK3BLN but no antenna or NEC deck. Any links?
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Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
You can get hose-clamps with inches of adjustability. I'm certain you could adjust to within 1mm and have it stay there too. The ones I'm thinking of have a screw as a worm-drive on the ribbon.
It does. The BLN foxhunt team has that all in-car with custom SDR with spectrum display (user-settable brick-wall filters to eliminate adjacent-channel interference) and automated lobe analysis that sends bearings to display on the GPS maps. Motorised antenna with motorised polarization adjustment too. They're very hard to beat!
I don't think he's published either the antenna nor the custom mods to NEC, but contact me privately and I can at least send you the dimensions. snipped-for-privacy@gmail.com
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