Topic: Tropospheric ducting

[Richary]

This may be a little off topic for a weather site, but given we look at the weather and it certainly affects this part of the atmosphere, I thought it might be of interest to start a topic.

One of my other hobbies is long distance radio. Certainly shortwave is outside the range of this topic, but FM is another mode of long distance reception. For example I heard some north Queensland sations on the car radio a couple of weeks ago. OK, that is the E layer (Sporadic E propagation) at 100km up so a bit out of our league here.

The other common long distance mode though is tropospheric. This layer occurs between 7 and 20km above the earth so is of interest in the layers we look at for storms. Tonight northern NSW stations to Grafton were coming in quite strong here in Sydney, though I now have a good antenna setup. Normal reception only gets to Taree.

From wikipedia, it talks about tropo like this....

Tropospheric ducting is a type of radio propagation that tends to happen during periods of stable, anticyclonic weather. In this propagation method, when the signal encounters a rise in temperature in the atmosphere instead of the normal decrease (known as a temperature inversion), the higher refractive index of the atmosphere there will cause the signal to be bent. Tropospheric ducting affects all frequencies, and signals enhanced this way tend to travel up to 800 miles (1,300 km) (though some people have received "tropo" beyond 1,000 miles / 1,600 km), while with tropospheric-bending, stable signals with good signal strength from 500+ miles (800+ km) away are not uncommon when the refractive index of the atmosphere is fairly high.


Now given that definition I think the soundings should make it relatively easy to see on the soundings. As conditions are particularly good to the north tonight, I see on the Newcastle sounding for 10am (the latest on BoM) there is a warmer layer at about 8-10km. So this could explain the conditions tonight.

Has anyone had any experience with predicting things like this from the soundings, or any experience with long distance reception?

Incidentally, there is a tropo ducting prediction site at http://www.dxinfocentre.com/tropo_aus.html

Stations from the west (Dubbo/Orange) were also in well, but not much to the south. So it is a "localised" (for want of a better term) phenomenon. Any for those interested, yes if conditions are really good NZ can be heard from Sydney on both Sporadic E and tropo modes if you are lucky.

[Dave Nelson]

Hi Richary and all

   not a bad description from wikipedia,    tho it doesnt give the full story,  It fails to tell you
about the "duct"  which is much like a microwave transmission (metallic) waveguide tube.
The atmospheric duct is literally a duct pipe situated between stable layers of warmer are cooler
air.
  Once the radio wave enters the ducting region it is propagated to the other end with often much
less attenuation that a direct signal would be between the same 2 points.

  The ducting also usually (but not always) occurrs along and parallel to coastal regions where there
the formation of inversions layers common during mornings and evenings of the summer months.
These layers being formed by the onshore and offshore winds around those times of the day.

  I rely on this form of radio propagation to get my VHF  144MHz and higher amateur radio bands
up and down the eastern seaboard.  Yesterday (saturday 12 Jan 08)  was an example of an
excellent ducting day with my 144.1 MHz signals getting as far as Charters Towers and Innisfail in
nthrn QLD.  The signals from some of the stations were outstanding and it was like talking to a local
station.
    Ducting also occurrs between Australia and New Zealand  and altho the formation process is
slightly different the result is much the same.    The ducting, again, usually forms in summer months and
when there is high pressure system in the ~ mid Tasman Sea.  I have observed, over the years, that
the ducting is usually parallel to the atmospheric pressure isobars  between the Australian and NZ
land masses.   And once again VHF radio propagation is possible over several 1000's of km with little
attenuation.

 Richary,  you also mentioned sporadic E  (Es) propagation  that is a different phenomena.  Ionospheric
E layer ionisation and propagation  is a much less understood occurrance when compared to our
understanding of F layer activities which account for most long distance propagation.
 F layer ionisation in general supports frequencies from 3 to 30 MHz and is at its best during Solar Max.
On the other hand there is a lot of "argument" discussion on how much Es is affected by weather and how
much by solar ionisation.
   Es propagation covers generally from ~28 MHz up to around 200MHz.

  there ya go jst some more thoughts

cheers
Dave N

[Richary]

Hi Dave

Yesterday's propagation from Sydney into northern Queensland would appear to have been Sporadic E is the consensus on the radio newsgroups. At the same time accompanying it was propagation from northern NSW and SE Queensland to Adelaide. I was amazed at some of the signal strengths even with the good antenna set up I have, with relatively low power Qld stations overpowering some of the local community stations. I managed to null out New FM Newcastle enough to get a 10 watt ABC station from FNQ. And had a 50 watt country music station from near Emerald blasting in.

It then reintensified again during the evening from about 6:30-10pm with some new stations heard again. I get regular tropo scatter reception to Taree and Grafton isn't unusual either.