Actually, a Yagi is a fairly narrow band -- they generally won't cover more than one TV channel. Most TV antennas are LPDAs to cover the whole band.

I'll concede that one Ned. I learned about Yagi's in military use. As I remember the training we were given, anything that was Yagi-like was a "modified" Yagi. Having said that, the military often has different terms for items in civilian use. Most TV antennae are modifications of the original design. The Yagi antennae I used a long time ago were adjustable, to account for the narrow bandwidth problem you mentioned. The various elements could be adjusted in length, rather like a trombone, and had an engraved scale on them, to save tiresome measuring each time there was a frequency change.

Another type of antenna we used was a "stacked dipole". There were multiple dipoles arranged in front of a backplane, which acted as a reflector, roughly 1/4 wavelength behind the dipoles. Since an e/m wave undergoes a 1/2 wavelength shift in phase when reflected, this ensured the reflected waves were in phase with those waves which were directly incident on the dipoles. There's more to antenna design than meets the eye! :)

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Back in the period 1969-73 I was using a Yagi antenna that had at least a dozen V-shaped tubules that got progressively shorter in the direction of the radio source. It was called an Allied Colorset and was used to catch popular songs from an FM station over 60 miles away. The antenna was rotatable with a motor and compass arrangement. It was probably 10 feet long by 5 feet (max) wide. (3m x 1.5m).

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Back in the period 1969-73 I was using a Yagi antenna that had at least a dozen V-shaped tubules that got progressively shorter in the direction of the radio source. It was called an Allied Colorset and was used to catch popular songs from an FM station over 60 miles away. The antenna was rotatable with a motor and compass arrangement. It was probably 10 feet long by 5 feet (max) wide. (3m x 1.5m).

If this was a true Yagi, then only one element was driven, and the elements were all within about 5% of the same length. For FM broadcast, I'd expect the last element to be about 4' 9" (150 cm) and the shortest to be close to 4' (120 cm).

If this was a log-periodic dipole array, the shortest element would have been much shorter -- maybe as small as about 2 feet (60 cm).

TV antennas are often hard to figure out because they're really two antennas on one boom.

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Back in the period 1969-73 I was using a Yagi antenna that had at least a dozen V-shaped tubules that got progressively shorter in the direction of the radio source. It was called an Allied Colorset and was used to catch popular songs from an FM station over 60 miles away. The antenna was rotatable with a motor and compass arrangement. It was probably 10 feet long by 5 feet (max) wide. (3m x 1.5m).

If this was a true Yagi, then only one element was driven, and the elements were all within about 5% of the same length. For FM broadcast, I'd expect the last element to be about 4' 9" (150 cm) and the shortest to be close to 4' (120 cm).

If this was a log-periodic dipole array, the shortest element would have been much shorter -- maybe as small as about 2 feet (60 cm).

TV antennas are often hard to figure out because they're really two antennas on one boom.

Only ever saw one example of a log periodic. Again, military use, but long since passed over to the museums. Really handy, as there was no adjusting to do, just tune the transmitter and receiver.

TV antennas are often hard to figure out because they're really two antennas on one boom.

Can you explain that one in a little more detail please, Ned? You've got me puzzled.

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TV antennas are often hard to figure out because they're really two antennas on one boom.

Can you explain that one in a little more detail please, Ned? You've got me puzzled.

In the U.S. (I suspect much of the world is similar, but not the same), channels 2 through 13 are between 55 and 213 MHz. 14 through 69 are between 471 and 801 MHz.

So a typical broadcast TV antenna is usually a broadband VHF antenna, and a broadband UHF antenna fed with the same feedline. Usually the VHF part is at the back, and the UHF part is at the front, but they can be interleaved -- you'd have long elements and short elements all the way down the boom.

The long elements are one antenna (VHF), and the short elements are a different antenna (UHF).

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As far as I remember, my Colorset was an FM antenna designed to catch signals from 88 to 108 MHz.

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TV antennas are often hard to figure out because they're really two antennas on one boom.

Can you explain that one in a little more detail please, Ned? You've got me puzzled.

In the U.S. (I suspect much of the world is similar, but not the same), channels 2 through 13 are between 55 and 213 MHz. 14 through 69 are between 471 and 801 MHz.

So a typical broadcast TV antenna is usually a broadband VHF antenna, and a broadband UHF antenna fed with the same feedline. Usually the VHF part is at the back, and the UHF part is at the front, but they can be interleaved -- you'd have long elements and short elements all the way down the boom.

The long elements are one antenna (VHF), and the short elements are a different antenna (UHF).

I don't think I've seen one like that, Ned. Round here, most of the TV channels are still on VHF, due to the distances between towns/townships, and the remoteness of some populations. UHF is, of course, prevalent in big cities. Often, tall masts are erected in backyards, many with two or more antennae, as the main transmitters are quite distant, and there are a lot of hills in the area.

The channels you mention are common to Europe as well. That much I do know. The main differences between countries are in the gap between audio and video signals. Great Britain has a 6 MHz separation and Europe has 5.5 MHz. Aussie TV sets usually come ready to use with several systems, and nobody I have asked knows the A/V separation, though I suspect they use the 6 MHz system.

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Aussie TV sets usually come ready to use with several systems, and nobody I have asked knows the A/V separation, though I suspect they use the 6 MHz system.

Nope.
Australia is PAL B/G so a 5.5MHz separation between sound & video.

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Grant
Darwin NT

The long elements are one antenna (VHF), and the short elements are a different antenna (UHF).

I don't think I've seen one like that, Ned.

There used to be a lot when there was a combination of UHF & VHF transmitters & they were generally on the same tower. But as more & more channels have gone digital, most of them now use UHF.

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Grant
Darwin NT

Aussie TV sets usually come ready to use with several systems, and nobody I have asked knows the A/V separation, though I suspect they use the 6 MHz system.

Nope.
Australia is PAL B/G so a 5.5MHz separation between sound & video.

Thanks, Grant... you're the first Aussie I've known to have the answer! :)

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Well Japan is using NTSC and so I think they use a 6MHz separation also.

Which, for the record, stands for "Never The Same Color" (or, in Oz, Colour).

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Well Japan is using NTSC and so I think they use a 6MHz separation also.

Which, for the record, stands for "Never The Same Color" (or, in Oz, Colour).

Actually in reality It stands for "National Technical Standards Committee", If You want to get technical that is. ;)

At least that's easier than PCMCIA which of course stands for "People Can't Memorize Computer Industry Acronyms." :-)

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One of the classics in that department is TWAIN (Windows legacy scanning layer), which stands for:

Tool Without An Interesting Name.

As for european TV antennae and frequencies, we use UHF and VHF both, and PAL B/G (along with DVB-T, recently, which requires a different antenna - seems to be vertically polarized with much shorter wavelength).

Hope the staff are closer to figuring out where the interference pattern in Multibeam data is coming from, good luck on that.

Regards,
Simon.

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Hope the staff are closer to figuring out where the interference pattern in Multibeam data is coming from, good luck on that.

I think we went a tad off topic.

OOPS!

Never mind, the subject has been most illuminating. :)

Anyway, There is mention of the RFI matter in HERE

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Hope the staff are closer to figuring out where the interference pattern in Multibeam data is coming from, good luck on that.

I think we went a tad off topic.

OOPS!

Just a little!

Never mind, the subject has been most illuminating. :)

Indeed pretty good.

Two good links are:
TV Systems: A Comparison
PAL

Anyway, There is mention of the RFI matter in HERE

That doesn't say that they traced it to the new digital TV broadcasts does it?!

:-/

Cheers,
Martin

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See new freedom: Mageia Linux
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It shows up in both polarizations (and the polarizations are linear, but it would be possible to derive circular polarizations from the data stream.)

1. When will we search for helical/circular polarizations?
2. Is Enhanced application already sensitive enough? (Sensitive enough for what? I do not know...)
3. Spikes, gaussians, pulses and triplets. What could be the fifth signal type to search for?

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[quote]It shows up in both polarizations (and the polarizations are linear, but it would be possible to derive circular polarizations from the data stream.)

1. When will we search for helical/circular polarizations?

A circularly polarized signal, received by a linear-polarized antenna is only attenuated by about 3db.

A linearly polarized signal, received on a linearly polarized antenna at a 45 degree angle is also attenuated by about 3db.

So assuming that the multi-beam receiver uses linear antennas 90 degrees apart, there is no difference between a circularly polarized signal and the worst case linear polarization.

Unfortunately, if you have a left-hand circularly polarized signal, and a right-hand circularly polarized antenna, the loss approaches infinity.

... and you'd have twice as much recorded data (and a more complex multibeam receiver on the telescope).

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1. When will we search for helical/circular polarizations?

A circularly polarized signal, received by a linear-polarized antenna is only attenuated by about 3db.

A linearly polarized signal, received on a linearly polarized antenna at a 45 degree angle is also attenuated by about 3db.

So assuming that the multi-beam receiver uses linear antennas 90 degrees apart, there is no difference between a circularly polarized signal and the worst case linear polarization...

That's a very good point.

A crafted database search for simultaneous signals for each linear polarisation could infer circular polarisation without the need for clever tricks with the splitters (and all the extra data).

Much more efficient to let the database do some of the work!

Keep searchin',
Martin

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See new freedom: Mageia Linux
Take a look for yourself: Linux Format
The Future is what We all make IT (GPLv3)

1. When will we search for helical/circular polarizations?

A circularly polarized signal, received by a linear-polarized antenna is only attenuated by about 3db.

A linearly polarized signal, received on a linearly polarized antenna at a 45 degree angle is also attenuated by about 3db.

So assuming that the multi-beam receiver uses linear antennas 90 degrees apart, there is no difference between a circularly polarized signal and the worst case linear polarization...

That's a very good point.

A crafted database search for simultaneous signals for each linear polarisation could infer circular polarisation without the need for clever tricks with the splitters (and all the extra data).

Much more efficient to let the database do some of the work!

Keep searchin',
Martin

... a bigger question is: does polarization carry any additional information?

There are practical reasons for choosing a polarization -- for example, circular polarization is really good if you are trying to send (or receive) signals from a spinning satellite (big expensive satellites are stabilized, but inexpensive small satellites are not).

... and generally speaking, for terrestrial paths, horizontal polarization is quieter. Vertical polarization is better for mobile-to-mobile and base-to-mobile for purely practical reasons.

It generally doesn't change once a system is designed and installed.

I don't see how anything like that would tell us any more about ET. Maybe a real ET researcher might have a comment....

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... a bigger question is: does polarization carry any additional information?

There are practical reasons for choosing a polarization -- for example, circular polarization is really good if you are trying to send (or receive) signals from a spinning satellite (big expensive satellites are stabilized, but inexpensive small satellites are not).

And for that reason, if I were to radiate a beacon signal, I would use circular polarisation so that the signal is kept out of any natural sourced noise and to allow reception by a spacecraft antenna at any orientation.

If we took the "Star Trek" view and aligned everything to the galactic plane then linear polarisations could be used. Then again, are there any features of space that rotate/twist the polarisation plane? (I don't know of any other than star or planet magnetic fields, but...)

... and generally speaking, for terrestrial paths, horizontal polarization is quieter. Vertical polarization is better for mobile-to-mobile and base-to-mobile for purely practical reasons.

I wonder how you could have an circular polarised isotropic radiator?... :-(

Or could you have a 'phased array' for circular polarisation. Phew, that would be an interesting headache!

Or?...

...I don't see how anything like that would tell us any more about ET. Maybe a real ET researcher might have a comment....

It would say that this 'ere signal is deliberately artifical and we are trying to maximise how easily it can be received by you...

That extra 3dB over a misaligned linear polarised signal equates to a greatly increased volume of search space! (If we could recieve it with a dedicated circular polarise antenna. Or can the two linear polarised antenna signals be phased/added for zero loss?)


Keep searchin',
Martin

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See new freedom: Mageia Linux
Take a look for yourself: Linux Format
The Future is what We all make IT (GPLv3)