BajaNomad - Atmospheric Phenomenon

Atmospheric Phenomenon

MrBillM - 5-30-2006 at 06:42 PM

This last week I encountered a phenomenon that I had forgotten about for many years and I realized I still don't know the scientific reason.

Back in the old days (pre 1985) before my first C-band satellite dish in Baja, we would watch "skip" TV out of Yuma-El Centro. The signal was always better when the wind was out of the South and almost non-existent when the wind was strong from the North. The same was (is) true for the Marine VHF Weather channel from Yuma. I had always wondered what the scientific explanation was.

Back in August of last year, I setup my XM radio to use here in Baja. Since the FM band is mostly clear here, I placed AM/FM receivers on the patio, the casita and the garage. No problem until this last week when we had some significant winds from the SouthEast. Suddenly, the entire FM band in the 88s was filled with broadcasts coming in from the States and creating interference with my XM reception. The wind has now stopped and the stations are gone again. The same phenomenon as before.

Although I have a lot of past experience in various Electronics fields, I have never had occasion to delve much into RF wave propagation. Since Baja is overflowing with experts in all fields of study, what the heck is the answer ?

bajalou - 5-30-2006 at 07:23 PM

I always figured my cell phone conects at Yuma and Black Mtn without roaming is because of Tropospheric Ducting. Works better in the warm months and better in the evening and morning than midday. Wind in San Felipe doesn't seem to bother it but storms/winds between here and Yuma does. Havent had any problems with XM though.

I re-read your post and the problem wasn't with XM reception, but with the rebroadcast to FM receivers.

[Edited on 5-31-2006 by bajalou]

Bajalero - 5-30-2006 at 07:28 PM

Bill , I believe electron density in the upper atmosphere affects radio transmission and thus the "skip" you get . I would imagine a drier north wind would increase the density and a more humid southerly wind would decrease it.

Best ask a amateur radio opperator though , blood and guts is more my forte.

Lero

Bruce R Leech - 5-30-2006 at 07:51 PM

Bill this is kind of a complicated issue but maybe this can simplify it a little . it is a very interesting subject

FM DX

FM DX refer to long-distance reception of

TV and FM radio stations, respectively. The term DX is an old telegraph term meaning "long

distance." These terms refer to the active search for distant radio or television stations

received during unusual tropospheric lower atmospheric weather-related, or E-layer and F2-

layer upper atmospheric ionospheric conditions.

VHF/UHF television and radio signals are normally limited to a maximum "deep fringe"

reception service area of approximately 40-100 miles. However, providing favourable

atmospheric conditions are present, television and radio signals can sometimes be received at

hundreds or even thousands of miles outside their intended coverage area. These signals are

received using a large outdoor antenna system connected to a sensitive TV or FM tuner and/or

receiver.

While only a limited number of local stations can be normally received at satisfactory signal

strengths in any given area, tuning into other channels may reveal weaker signals from

adjacent areas. More consistently strong signals, especially those accentuated by unusual

atmospheric conditions, can be achieved by improving the antenna system. The development

of interest in TV-FM DX as a hobby can arise after more distant signals are either

intentionally or accidentally discovered, leading to a serious interest in improving the aerial

and receiving installation for the purpose of actively seeking long-range television and radio

reception. The TV-FM DX hobby is somewhat similar in scope to other radio/electronic

related hobbies such as amateur radio or short-wave radio, and organizations such as the

Worldwide TV-FM DX Association have developed to coordinate and foster the further

study, and enjoyment of VHF/UHF television and FM broadcast DX.http://anarc.org/wtfda

History

After the introduction of the Alexandra Palace, London 405-line BBC channel B1 TV service

in 1936, it soon became apparent that television reception was also possible well outside the

original intended service area. For example, in February 1938, engineers at the RCA Research

Station, Riverhead, Long Island, New York, accidentally received 3,000-mile trans-Atlantic

F2 reception of the London 45.0 MHz 405-line channel B1 TV service. The flickering black-

and-white footage, which is characteristic of F2 propagation, includes Jasmine Bligh, one of

the original BBC announcers, and a brief shot of Elizabeth Cowell, who also shared

announcing duties with Jasmine, an excerpt from an unknown period costume drama and the

BBC's station identification logo transmitted at the beginning and end of the day's programs.

This reception was coincidentally recorded on 16mm movie film, and is now considered to be

the only surviving example of pre-war live high-definition British television.

http://www.apts.org.uk/recording.htm

The BBC temporarily ceased transmissions on 1 September 1939, as World War II began.

After the BBC channel B1 television service re-commenced in 1946, distant reception reports

were received from various parts of the world, including Italy, South Africa, India, Middle

East, North America and the Caribbean.

In May 1940, the Federal Communications Commission formally allocated the 42-49 MHz

band for FM radio broadcasting. It was soon apparent that distant FM signals from up to

1,400 miles distance would often interfere with local stations during the summer months.

Because the 42-49 MHz FM signals were originally intended to only cover a relatively

confined service area, the sporadic long-distance signal propagation was seen as a nuisance,

especially by station management.

On February 1942, the first known published long-distance reception report of a FM

broadcast station on 41.5 MHz was reported by FM magazine: "Zenith Radio Corporation,

operating W51C, has received a letter from a listener in Monterey, Mexico, telling of daily

reception of this station between 3:00 P. M. and 6:00 P. M. This is the greatest distance, 1,100

airline miles, from which consistent reception of the 50 kw. transmitter has been reported."

http://members.aol.com/jeff560/chronofm.html

In June 1945, the FCC decided that FM would have to move from the established 42-49 MHz

pre-war band to a new band at 88-108 MHz. According to 1945 and 1946 FCC documents,

the three major factors which the commission considered in its decision to place FM in the

88-108 MHz band were sporadic E co-channel interference, F2 layer interference, and extent

of coverage. http://ieee.cincinnati.fuse.net/reiman/09_1994.html During the 1950s to early

1960s, long-distance television reports started to circulate via popular U.S. electronics

hobbyist periodicals such as DXing Horizons, Popular Electronics, Television Horizons,

Radio Horizons, and Radio-Electronics. In January 1960, the TV DX interest was further

promoted via Robert B. Cooper's regular DXing Horizons column.

In 1957, the world record for TV DX was extended to 10,800 miles with the reception of

England's BBC channel 1 in various parts of Australia. Most notably, George Palmer in

Melbourne, Victoria, received viewable pictures from the BBC London channel B1 station.

This BBC F2 reception was recorded on to movie film.

http://www.geocities.com/toddemslie/George_Palmer_TVDX.html

In the early 1960s, the U.K. magazine Practical Television first published a regular TV DX

column edited by Charles Rafarel. By 1970, Charles' DX TV column had attracted

considerable interest from TV DXers worldwide. After Charles' death in 1971, U.K. TV DXer

Roger Bunney continued the monthly long distance television column, which is still in current

publication.

Roger Bunney also published several TV DX books, including Long Distance Television

Reception (TV-DX) for the Enthusiast 1981 ISBN 0900162716, and A TV DXer's Handbook

1986 ISBN 0-85934-150-X.

Tropospheric propagation

The service area from a TV or FM radio transmitter extends to just beyond the optical

horizon, at which point signals start to rapidly reduce in strength. Viewers living in such a

"deep fringe" reception area will notice that during certain conditions weak signals normally

masked by noise, increase in signal strength to allow quality reception. Such conditions are

related to the current state of the troposphere.

Tropospheric propagated signals travel in the part of the atmosphere adjacent to the surface

and extending to some 25,000 feet. Such signals are thus directly affected by weather

conditions extending over some hundreds of miles. During very settled, warm, anti-cyclonic

weather (i.e., high pressure), usually weak snowy TV signals from distant transmitters

improve in signal strength. Another symptom during such conditions may be interference to

the local transmitter, resulting in co-channel interference, which may be in the form of

horizontal lines or an extra floating picture. A settled high-pressure system gives the classic

conditions for enhanced tropospheric propagation, in particular favouring signals which travel

along the prevailing isobar pattern rather than across it. Such weather conditions can occur at

any time, but generally the summer and autumn months are the best periods. In certain

favourable locations, enhanced tropospheric propagation may enable reception of UHF TV

signals up to 1,000 miles or more.

The observable characteristics of such high-pressure systems are usually clear, cloudless days

with little or no wind. At sunset the upper air cools, as does the surface temperature, but at

different rates. This produces a boundary or temperature gradient, which allows an inversion

level to form - a similar effect occurs at sunrise. The inversion is capable of allowing VHF

and UHF signal propagation well beyond the normal radio horizon distance.

The inversion effectively reduces sky wave radiation from a transmitter - normally VHF and

UHF signals travel on into space when they reach the horizon, the refractive index of the

ionosphere preventing signal return. With temperature inversion, however, the signal is to a

large extent refracted over the horizon rather than continuing along a direct path into outer

space.

Fog also produces good tropospheric results, again due to inversion effects. Fog occurs during

high pressure weather, and if such conditions result in a large belt of fog with clear sky above,

there will be heating of the upper fog level and thus an inversion. This situation often arises

towards night fall, continues overnight and clears with the sunrise over a period of around 4-5

hours.

Still Wondering

MrBillM - 5-31-2006 at 08:50 AM

Whether there is a direct correlation between the surface winds and the reception phenomenon. That there are influencing factors produced by atmospheric changes is a given. Those influences will vary depending on the frequency range of the device in question, generally having a greater effect as the frequency increases.

Since we have no effective way to sample the winds aloft and the exact location of High/Low pressure systems over Baja, it is possible that the surface wind direction is merely an incidental indicator.

Still researching.

[Edited on 5-31-2006 by MrBillM]

Roberto - 5-31-2006 at 06:52 PM

Bill, I think you know that TV signals are VHF and UHF. Marine radio is also VHF, as is two meter ham radio. What you describe is a well-known phenomenon known as "Troposhperic Propagation".

I won't go into excruciating detail, but I will point you to a website that has a short explanation: http://www.tpub.com/content/neets/14182/css/14182_96.htm , and let you research the subject yourself if you are interested. I will add this.

From your posts, you seem to have some experience with marine radio. Aside from the (local) range of VHF radio, long-range marine radio is in the HF range. This long-range depends on the ability to bounce radio signals off of different (depending on frequency) layers of the ionosphere. Basically, the signals go out toward space, are reflected by the appropriate ionospheric layer go back to ground, and are sometimes reflected again ... and again ... and again. The number of bounces are dependent on wavelength, ionosperic conditions, solar cycle (sunspots), and many other things. One of the factors making ham radio so interesting. Talking to someone on the other side of the world, once you understand the physics involved, is an amazing phenomenon we have (more or less) managed to harness. UHF/VHF signals are different, and they are not reflected by the ionosphere, and go straight out into space. But, under very specific conditions, a similar effect occurs, involving cloud cover, humidity, wind and other things. I have talked to people on marine VHF that were over 400 miles away many time from my boat - usually in the hours before sunrise and in the spring/early summer. Reports of QSOs between California and Hawaii are not uncommon. Anyway, there is the short version - this subject is as deep as you are interested in exploring.

bajaguy - 5-31-2006 at 06:59 PM

Just to add my 2 cents worth, many years ago my Army unit was co-located with an Army Tropospheric (sp) radio unit. Despite their explainations and drawings, about the only thing I learned was they bounced radio (and other types) of signals off of the troposphere. Can't remember if they were VHF, UHF or HF.

Thanks, Roberto, I'll Check it out.

MrBillM - 5-31-2006 at 07:49 PM

This is one of those esoteric issues that just begs to be answered. I hadn't thought about it for many years since the mid 80s when we were watching a lot of "skip" TV in Baja and checking Yuma Marine Weather. Until, of course, this last weekend when it affected my FM reception from my XM radio receiver. Since the FM radio band, VHF Broadcast TV band and Marine Radio band are in the same general area (55 - 212 MHz ?) I would expect nearly the same result. I simply haven't read anything that specifically addresses wind effect. Midday today I dusted off my Radio Technician textbook (nothing there) and started wading through my ITT "Reference Data for Radio Engineers". So far no luck. I'll check out your link.

The phenomenon is directly related to wind strength and direction. I have laid here many nights in the past watching the reception change as the wind did.

Thanks Again.

OK, I'll Buy That !

MrBillM - 6-1-2006 at 08:38 AM

Tropospheric Scattering.

It explains WHY strong winds blowing TOWARDS the Transmitted signal will improve the reception and wind blowing AWAY from the transmitter has the opposite effect.

Today is already on the plus side of the educational ledger.

Roberto - 6-1-2006 at 09:16 AM

Quote:

Originally posted by MrBillM
Today is already on the plus side of the educational ledger.

Experience would then say "quit while you're ahead" :lol:

, and kick back for the rest of the day.