Do Tropo openings occur after sunrise on 50 MHz in VK?

[ZS2FM]

Tropo Ducting appears briefly most mornings on 50 MHz in ZS, an hour or so after sunrise. making country-wide SSB contacts possible. 73 Mike, ZS2FM

[VK2KRR]

Mike

I noticed no one replied to you.

Tropo openings can occur at any time of the day or night depending on the prevailing weather. Usually much better very early mornings to a number of hours past sunrise.

In general from my experience, its rare to get tropo that really ducts 50 MHz well. It does happen, but not often and usually compared to 2m or 70cm, 6m will be fairly poor in comparison.

[VK5ACY]

I have to agree with Leigh...6m does not seem to propagate as well as 2m or 70cm via tropo.

I have worked into VK7 from here at PF95sb on 50MHz at around 900km but signals were fairly weak
compared to tests at the same time on 144MHz where signals were more than 20dB stronger.
At the time of these tests the 'Hepburn Charts' indicated reasonable tropo in the SE direction from here.

Bill VK5ACY
PF95sb

[VK3OE]

It all depends on the depth of the duct. Elevated ducts they are usually quite thin, around 100m, and thus cannot propagate 6m very well.

For ground based inversion layers, the duct height can be around 300 to 400m, and these ground based ducts will propagate 6m quite well, 2m reasonably, but higher frequencies struggle. If any duct is broken by intervening terrain, the duct will stop, and ducted signals cannot progress any further.

For a more extensive discussion of ducts see, in particular figure 10.
http://www.df5ai.net/ArticlesDL/VK3KAQDucts2007V3.5.pdf

Andrew
VK3OE/VK3OER, previously VK3KAQ.

[VK5AKK]

During the early 1990's I had many 6M troppo contacts with Bill VK6AS (SK) at Esperance from my old QTH near the coast south of Adelaide over a distance of 1550 Km.
Bill ran a big antenna and 400W but from what I recall, 6M signals were always down compared to 2M and 70 cm.
I wonder if any 6M troppo was ever worked on the VK to ZL path ?
Cheers
Phil

[ZS2FM]

ZS6XL (Johannesburg) and I (Port Elizabeth) discovered this strange 50 MHz phenomenon during an early SARL VHF contest. There were weak Tropo Scatter signals all the time in the background, when about an hour after sunrise the ZS6XL signal suddenly increased to S9 for several minutes and we established a contact on SSB.

Later I conducted tests with ZS6NK at Polokwame for about a week and the band opened up regularly every day after sunrise. I figured that this was caused by the rising Sun which heated up the upper atmosphere, and created a temperature inversion in the form of a duct that slowly descended as the SUN rises higher in the sky, At a critical point this duct links up with two distant areas to provide strong propagation during that brief period.

I thought that this was only a north/south phenomenon until about a year ago when ZS1IL in Cape Town started to make contact on 50 MHz with ZS5QM in Howick almost every morning. ZS1NAZ also of Cape Town has logged distances of 1000 – 1500 km on this propagation mode. The east/west openings seem to last a great deal longer than the north/south openings.

The 144 MHz band is normally the best Tropo band, but when we compared signals with 50 MHz during these early morning openings, then the latter band is superior, maybe because the duct size favours the 50 MHz wavelength. There also appears to be a skip distance because we have not yet received reports within the 400 km range. We are having a lot of fun on 50 MHz in the early morning. Try it and see if it also works in the VK! – 73 Mike, ZS2FM

[VK3OE]

Hello Mike,

After a bit more thought, the most likely explanation is...

At night a ground based inversion forms that is around 30m in height. This low inversion forms due to radiative cooling of the ground so that the air closest to the ground is cooler that that further up hence the inversion.

When the sun comes up the ground now starts to heat and the inversion layer rises due to convection effects. The inversion can easily be pushed up to aound 300m, where 6m is now viable, over about 1 hour . It will then start to break up so that the 6m propagation will last for around 1-2 hours. 2m could be good at first but as the inversion layer rises 2m will drop off as the temperature gradient required to propagate 2m is not sustainable.

The key ingredient is large flat areas over several hundred km. This effect is probably widespread and common but in VK there are relatively fewer amateurs in the wide open plains than nearer the coast where the terrain is less conducive for these effect to be noticed.

73,
Andrew.
VK3OE/VK3OER, previously VK3KAQ.

[VK7DX]

Hi Mike,,

I get tropo or some sort of inversion layer almost every morning to VK5 and western VK3 with sigs up to 59.
Distance about 1000kms,beyond that there is no ops for a further 2000kms in that direction and wrong time zone for them.
This is from a height of 1050 meters so is not a low level inversion duct.If there was a low level inversion layer it would block sigs to me.
Have fun with it
cheers
Frank

[SM7FJE]

Sorry to say, but most openings claimed to be tropo often are iono scatter or Es. Like with many other mix propagation QSOs it's often extremely difficult to determine the actual propagation mode.
How about SM to LU with 55A report? (3 propagation modes cooperating!)

73
Bo
SM7FJE

[VK3DXE]

In VK3, I've experienced troppo ducting down as low as the 10m and 11m bands.

On cold still winter evenings I used to see enhanced conditions on 27mhz to the west of Melbourne quite regularly, albeit at usually fairly low signal strengths. The best observed was a couple of stations around Warrnambool (approx 225km west) peaking around S9 for about 1/2 an hour around sunset on one or two occasions.

[ZS2FM]

Hi Frank,

It is nice to hear from you again and the confirmation of a similar type of Tropo opening on 50 MHz in your area. I am very familiar with Ionospheric Scatter, Sporadic-E, Tropo Scatter, Aircraft Scatter etc. and believe me this is not any of these propagation modes. This is undoubtedly a high level duct system as ZS is quite rugged in certain areas, for example, signals between ZS1NAZ in the Western Cape and ZS5QM in Natal have to cross the high Drakensberg mountain range.

In the mornings Tropo Scatter on 50 MHz is clearly visible on the waterfall and often audible via SSB too, but at the appropriate time it would quickly increase to 59 for several minutes without any Doppler shift, and then drop down to the Tropo Scatter level again. There appears to be a skip distance, as we have worked stations only from 600 km to 1500 km on this mode. It is definitely Sun related, as the latter rises later in the year so the openings also occur later.

73 Mike, ZS2FM