OT: RF propogation in cold weather

It's too cold, fewer birds in the sky at this time to refract and absorb that precious RF. That global warming really puts a pinch on air travel.

Jamie

A bus is a vehicle that runs twice as fast when you are after it as when you are in it.

Could the transmitter antenna be exhibiting a better match and therefore a better radiance? You know, due to some thermal expansion of a balun spacing or such other element. I know systems normally account for thermals, but you are seeing a difference. Are you using an indoor or outdoor antenna? Because it could be a better 'tune' on your 'fly catcher' too.

This month's Popular Mechanics has a home made indoor HDTV antenna project in it in the DIY section.

I have a nice brand name, boosted indoor antenna, but the only thing about it the really works well is the amp. I should couple that with this antenna and a nice back shield.

He put it on a lazy Susan! I guess it would need some thick grease or a stop brake on the rim. Pretty cool though... it has the right dimensions for the elements and their spacings.

I was unable to find a link to an online article.

o

If you mean frequenies above ca. 90MHz - then it is most likly tropospheric ducting caused by meteorological inversion between you and the transmitter(s):

Citat: "...Temperature inversion can notoriously result in freezing rain in cold climates..."

Sign of inversions - first upward then downward chimney smoke:

Citat: "...English: Ice fog over Fairbanks, Alaska in winter 2005. Temperature approximately minus 30F. Joseph N. Hall Note the mirage at the base of the Alaska Range..."

My money is on the increased density of the air and a simultaneous inversion ducting.

It was 18F on the back deck this morning 60 miles north of Sacramento.

Jim

Probably moisture. Water vapor absorbs RF, especially in a few frequency windows. Just a few dB can make a big difference to DTV.

It was 3F here this morning. Crunchy.

The same low humidity, with clear skies, enhances radiation cooling of the surface to space.

John

I don't know where you are in the PNW, but all my life living near PDX region it's always been the case that reception has been much better during periods like this.

It may have to do with the saturation mixing ratio for water vapor, which very rapidly decreases with lower temperatures (the curve looks exponentially rising.) Maybe also with turbulent dynamics and turbulence cell size.

Interesting question. Must be a paper on it, somewhere.

Jon

"John Larkin" wrote in message news: snipped-for-privacy@4ax.com...

I second the moisture. I see signal loss ( skips, blips and blocks ) during fog, rain, snow and other wet conditions with OTA ATSC signals. I'm close to fringe are reception too, ~30 miles

Cheers

Well, you are at about 6500MSL and we are at 3000 exactly. With a standard lapse rate of 3.5F per 1000', that would be a drop of about

12.3F. If my thermometer is correct at 18F, then you should have been at 5.7; I'd say that is within experimental error.

Jim

I'm beginning to think that this is the reason. Its just as cold now as it was when I first observed the phenomenon, but the distant TV station has gone back to crap. The inversion layer has probably gone away as the temperature gradient changed.

--
Paul Hovnanian  paul@hovnanian.com
----------------------------------------------------------------------
Have gnu, will travel.

I've done the gradient measurement in my head a few times, just to de-bore the drive, using the altitude signs between Auburn (1000 ft) and the summit (7230 or some such) and the car thermometer, usually under an hour's driving time. I figured 3.0 +-0.5 maybe per thousand feet, so the 3.5 value probably works up there.

John

(Hey, Paul, you might want to run a new channel scan, 22-3 is running The Three Stooges right now. (the new Antenna-TV program channel)).

The signals I get direct are improved, but the stuff that I receive from a bounce off a bluff on the opposite side of the bay are giving lower signal quality numbers, down below the usable point on some channels, so it's probably both. (I'm a lot closer in to the Seattle transmitter farms in the city center than he is).

Mark Zenier snipped-for-privacy@eskimo.com Googleproofaddress(account:mzenier provider:eskimo domain:com)

We teach this stuff when we do pilot ground schools. To one decimal place, the standard adabiatic lapse rate is 2C/3.5F per thousand feet up to around 18k'.

Jim

30 years back when I took ground school, that's what I was taught, as well. (18k is where you have to go IFR.)

I've looked at temp vs altitude curves (just now) and the general curves seem to suggest that the 2C/3.5F adiabatic rate works to perhaps 34k' -- just eyeballing them. Never flew that high as a pilot, though. I'm single engine VFR.

Jon

The gotcha I was thinking about is that an airplane goes thousands of feet above the surface, which might be different from the surface going up thousands of feet. Seems pretty close.

John

PV=nRT whether you're close to the ground or not.

Tropospheric bending; same phenomenon as mirages. ;-)

Cheers! Rich

Me, three. ;-)

Cheers! Rich

Yup, It works for skydiving too. ;-)

Cheers! Rich

I've probably got a couple of thousand hours at 500' AGL, mostly in the midwest going to and from Oshkosh.