Eric's Weekly Post #3 (Warning, science enclosed. Do not fold or bend.)


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Profile littlegreenmanfrommars
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Message 527213 - Posted: 6 Mar 2007, 9:14:49 UTC - in response to Message 526147.

At least in the US, an EPIRB is legal only on a boat, while an ELT (Emergency Landing Transmitter) is on an airplane. You can have a personal locator beacon in your backpack, or in the trunk of your car.


EPIRBs were originally sold here in Oz for use in boats. However, a couple of years ago, someone got into trouble in the Central Australian Desert, and set off his EPIRB. (At the time, it was a first)

Operators at the listening stations that heard and triangulated the EPIRB transmission decided to send an aircraft to investigate, despite the fact that EPIRBs weren't originally intended for land use. They found the occupants of a broken-down vehicle, before they were killed by the heat and lack of water.
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Message 527431 - Posted: 7 Mar 2007, 0:59:28 UTC - in response to Message 527181.



The basic Yagi (pronounced: yargee) array has three elements:

1) The dipole, which is connected to the co-ax, and is thus "driven" as Ned said.

2) The reflector, which is longer than the dipole, and collects radio waves, working rather like a dish does on a parabolic antenna, by reflecting the received waves back to the dipole.

3) The concentrator, which is shorter than the dipole, and has the effect of producing a more directional beam, thus increasing gain, or sensitivity.

Often, there are several concentrators, and sometimes multiple reflectors are employed. I have never seen a Yagi antenna with multiple dipoles.

The result is a directional antenna, which should be pointed in the direction of the transmitter (or receiver, if the antenna is used to transmit). Again, as Ned says, they are the most popular form of TV antenna.

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.
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Message 527579 - Posted: 7 Mar 2007, 11:27:45 UTC - in response to Message 527431.
Last modified: 7 Mar 2007, 11:31:16 UTC

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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Message 527690 - Posted: 7 Mar 2007, 19:32:57 UTC

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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Message 527753 - Posted: 7 Mar 2007, 22:58:22 UTC - in response to Message 527690.

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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Message 527887 - Posted: 8 Mar 2007, 3:45:57 UTC - in response to Message 527753.
Last modified: 8 Mar 2007, 3:47:25 UTC

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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Message 527956 - Posted: 8 Mar 2007, 6:31:25 UTC - in response to Message 527887.


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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Message 528160 - Posted: 8 Mar 2007, 19:37:00 UTC

As far as I remember, my Colorset was an FM antenna designed to catch signals from 88 to 108 MHz.
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Message 528397 - Posted: 9 Mar 2007, 5:54:25 UTC - in response to Message 527956.


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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Message 528434 - Posted: 9 Mar 2007, 9:30:47 UTC - in response to Message 528397.

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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Message 528436 - Posted: 9 Mar 2007, 9:33:45 UTC - in response to Message 528397.

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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Message 528503 - Posted: 9 Mar 2007, 12:32:14 UTC - in response to Message 528434.

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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Message 528673 - Posted: 9 Mar 2007, 20:02:54 UTC

Well Japan is using NTSC and so I think they use a 6MHz separation also.
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Message 528690 - Posted: 9 Mar 2007, 20:35:13 UTC - in response to Message 528673.

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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Message 528799 - Posted: 9 Mar 2007, 23:35:23 UTC - in response to Message 528690.

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. ;)
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Message 528986 - Posted: 10 Mar 2007, 6:40:55 UTC - in response to Message 528799.

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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Message 529019 - Posted: 10 Mar 2007, 9:49:49 UTC

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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Message 529068 - Posted: 10 Mar 2007, 14:04:00 UTC
Last modified: 10 Mar 2007, 14:07:21 UTC

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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Message 530342 - Posted: 12 Mar 2007, 14:20:21 UTC - in response to Message 529068.
Last modified: 12 Mar 2007, 14:21:12 UTC

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,
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Message 530428 - Posted: 12 Mar 2007, 18:18:14 UTC - in response to Message 521648.
Last modified: 12 Mar 2007, 18:19:44 UTC

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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