[Waverley ARS] Equinoctial opportunities - propagation

[Edwin Lowe]

Here's something from the VKlogger forum written by VK2ZRM about VHF/UHF 
propagation this time of year.  This might be useful.

VKlogger is a great website which users can spot DX openings in VK in 
real time.
Edwin



    Equinoctial opportunities - propagation
    <http://www.vklogger.com/forum/viewtopic.php?fC&t89&start=0&st=0&sk=t&sd=a>


Roger Harrison VK2ZRH
Beyond the morning skeds, the summer season, the winter season and the 
all-too-rare southern hemisphere meteor showers, there are the 
equinoctial seasons for VHF-UHF DX opportunities.

Equinoctial opportunities are like nocturnal opportunities, you might 
get lucky and have some fun!

The equinoxes occur around 20 March and 22 September each year - the 
autumn and spring equinoxes, respectively, for the southern hemisphere.

This year, the spring equinox will be on Sunday 23 September (see 
http://en.wikipedia.org/wiki/Equinox).

There are three modes of ionospheric propagation that offer 
opportunities for VHF-UHF propagation around the equinoxes:

1/ Afternoon-type transequatorial propagation (TEP),
2/ Evening-type transequatorial propagation (TEP), and
3/ Auroral scatter.

So, how to get amongs the opportunities?

1/ Afternoon-type or "super-mode" TEP

(a) propagation involves two F-layer reflections north and south of the 
geomagnetic equator without an intermediate ground reflection ('chordal 
hop');

(b) generally occurs between 1400 and 1900 local mean time (LMT) at the 
point where the path crosses the geomagnetic equator;

(c) path lengths range typically from 4000 km to 10,000+ km, with path 
terminals falling generally in the zone 20 to 40 degrees geomagnetic 
latitude;

(d) maximum observed frequencies (MOFs) typically 40-55 MHz, up to 60-70 
MHz sometimes;

(e) often strong, steady signals with slow fading and some small Doppler 
shift - I've worked it with a 20 W SSB rig;

(f) occurs right through solar minimum years - best around solar maximum 
years.

Check out more details at: 
http://home.iprimus.com.au/toddemslie/aTEP-Harrison.htm


2/ Evening-type of "ducted-mode" TEP

(a) propagation is via ducting or guiding through equatorial plasma 
bubbles -- field-aligned bubbles or tubes of depleted ionisation which 
thread the nighttime equatorial ionosphere, extending symmetrically 
north and south of the geomagnetic equator

(b) has a peak diurnal occurrence between 2000-2300 LMT where the path 
crosses the geomagnetic equator;

(c) path lengths range typically from 3000 to 6000 km (record is 8000 km);

(d) the most regular paths cross the geomagnetic equator within a small 
range of angles close to 90 degrees, and commonly nearly symmetrically 
spaced in a zone between about 10 and 30 degrees geomagnetic latitude;

(e) maximum observed frequencies (MOFs) often extend to 144 MHz -- 
sometimes 432 MHz;

(f) signals range from weak (S1) to strong (S9+), with S5-7 more usual, 
with distinctive rapid 'flutter' fading over long-term peaks and troughs;

(g) Doppler shifts range from +/- 20 Hz up to +50/-350 Hz, sometimes 
with Doppler 'smearing' of 100s of Hz to 1-2 kHz which severely distorts 
signals - even so, I've worked it with a 20 W SSB rig.

Check out more details at: 
http://home.iprimus.com.au/toddemslie/eTEP-Harrison.htm

Good ol' 'auntie IPS' has an informative article at:
http://www.ips.gov.au/Category/Educatio ... torial.pdf


3/ Auroral scatter

(a) both visible and 'radio' aurora most often occur (95% of the time) 
between the geomagnetic latitudes of 70 and 75 degrees, although they 
can occur at geomagnetic latitudes as low as 40 degrees (well north of 
Sydney and Adelaide, but south of Brisbane) and as high as 85 degrees;

(b) auroral ionisation occurs at heights ranging from around 100-110 km 
(mostly) up to 800 km (rarely);

(c) from Australia, the zone where stations will most likely be able to 
communicate is south of 40 degrees geomagnetic latitude (New England 
Tablelands in NSW - Sydney's about 43 degrees S);

(d) distances between stations making contact generally range from 500 
km to 1000 km, up to 2000 km on occasion (eg. ZL3-VK7) - the paths are 
*not* great circle (ie. aim your beam between SE and SW);

(e) signals range from weak to fair (you'll need a bit of grunt), with a 
characteristic "buzz saw" modulation (Doppler smearing and amplitude 
modulation) plus Doppler shifts ranging up to 1500 Hz or more;

(f) 50 MHz, 144 MHz and 432 MHz are all used to exploit auroral scatter;

(g) as solar flares precipitate the conditions that create auroras, keep 
a watch on the IPS indicators (http://www.ips.gov.au).

Posted in the interests of engendering more interest in VHF-UHF.

73, Roger Harrison VK2ZRH

-- 
Edwin Lowe
VK2VEL
Sydney, Australia
ph: 0407416090

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