DTV delay - Does it matter?

:doh:

Guess what you're not getting tonight?

:hug:

:kiss:

 

I can hear the pots and pans flying across the room from here

 

We probably haven't heard too much about hillbillies complaining as you have to know how to read to be able to call a number

The "all or nothing" signal reminds me of when cell phones first switched from AMPS to TDMA. Yes, I'm showing my age here. Needed a lot more tower sites to restore coverage

 

Hi Jayman,

Its well known that ducts will form along weather fronts, usually a few hours before the rain and heavy storms start. Specific occurences have been documented by hams over the years, and matched up to the oncoming weather front with data from the National Weather Service.

The mechanism is pretty straight forward. The atmospheric humidity developes a layering, which effects the refaction index versus altitude. This happens all the time, and its not a duct. Its what hams call tropo-speric enhancment. On calm nights in the summer it happens almost every evening. This is called local tropo enhancment, and extends out about 50 miles. There is a normal diurnal inversion that forms about 4 hours after local sundown.

When a really strong inversion forms one can see 200 miles ranges on VHF. You can almost always know if its going to rain the next day in the summer, by what you can hear on VHF radio the night before.

In a duct, a continous front up to 1000 miles long will make a single continuous inversion layer of strong intenstity of the refraction index change. Its all at the same air-pressure altitude but its not very wide, say 50 miles. A station under the duct can launch a signal into which will be guided along not unlike optical fiber. Even dual inversions at different altitudes have been noted, which keeps the wave refracting above the ground. In some of these occurences, stations up high can access the duct, and those only a few hundred feet lower cannot or visa versa. A real interesting situation occurs over water. With reports on a calm pacific of surface level ducts forming from beach (and no higher) locations between Califorina and Hawaii.

The reason UHF has a greater occurence of these phenonenah, is the vertical height of the inversion layer. The higher the frequency, the shorter the vertical height needs to be to guide the wave. The reason people do not notice the UHF activity as much, is due to the lower noise performance of receivers and houshold antenna installations, and in mobile radio, the smaller capture area of common simple antennas used (1/4 wave vertical) and with hams, the sparseness and sharp beamwidths of high performance stations at 432 or higher MHz. My professor said they corrected for this in their research and found the propagation at UHF was indeed stronger and more common. At VHF, however, if a duct or milder inversion layer terminates into a hillside, the signal can more readily difract over it. At UHF, pine trees, or other such plants can quickly attenuate the signal.

The older microwave links were effected by this too. There are even times when a link would go down because the normal vertical beamwidth of the antenna would be steered into the ground short of the terminus by the weather caused change in refraction index, and then reflected over the terminus. Without a method to tilt the antenna up a few degress, the links were dead. These links are designed with what is called Fresnel Zone clearance. To avoid midpoint ground reflections from combining with the direct signal strong enough, and in the wrong phase to fade out the path.

 

Good and concise explanation, thanks. Never was into ham, and never will be.

The older troposcatter system used by the military, such as the White Alice system, needed quad diversity to deal with the effects you noted. With the quad diversity and a real, live crew manning each site - an obviously huge investment in manpower and money - the links proved fairly reliable

With cost-cutting, a lot of the remote sites were turned into unmanned stations. I don't think they saved that much money. Although you can design a system to work around most atmospheric effects, it's very difficult to design for the effects of CME, GIC, and strong auroral events

What would typically happen is you would have to dispatch a helio from one of the few manned Northern sites, land at the remote site - sometimes 2 hours away, right at the edge of being Bingo on fuel - power cycle everything, to restore the link

Another huge disadvantage to unmanned remote sites, is s minor problem can quickly escalate to a total loss and major fuster cluck. Say a generator begins to overheat.

A lot of sensor information is carried out-of-band and somebody 2-6 hours away receives a warning of high generator temp. Some of the sites even have a few video feeds, and you see a small fire suddenly spring to life in the generator

Ok, then what?

You try to do a remote EPO and hope the fire magically self-extinguishes? No, what usually happens is this

PIN-3

http://www.lswilson.ca/firepin-3-2.jpg

http://www.lswilson.ca/firepin-3-7.jpg

http://www.lswilson.ca/firepin-3-8.jpg

If a manned crew had been there, most likely they could have saved the site "train" from having more than minor smoke damage

 

Absolutely.Nothing gets your acting faster than losing your analog TV signal. Here in SA we're also beginning a switchover (first in Africa). We face a number of problems. Many people are too poor to afford the new set-top boxes.

 

It seems odd to me that people should have to pay so much to be bombarded by advertising.