Introduction
Doubling the frequency reduces the probability to one tenth. This means if the band 1 channel has a frequency of 50MHz, then 100MHz will be propagated one tenth of the time period of the band 1 channel coming through, and 200MHz (band 3 television) will be propagated one hundredth of the time. Since most high MUF (Maximum Useable Frequency) propagation paths are multi-cloud, the probability could be higher than these figures.
The MUF of a single cloud can be lower than the frequency propagated by a two-cloud path. In practice, it is difficult to know of a possible propagation path for the highest frequency, because of the geometric restrictions imposed, and unless the DXer and the transmitter are in precise relative positions, the DX station will not be heard.
In the case of Australian channel 6 stations, the probability of receiving them is not very good, since there are relatively few of them, and the propagation paths are very restrictive. FM stations, on the other hand, are broadcasting in more areas, hence sporadic E openings can be observed more often. Because there are several more FM stations (than say, channel 2 television stations), a DXer may hear FM stations during some DX openings, and not see any channel 2 television stations. This does not mean that the band is not open for channel 2, it just means that there are no channel 2 stations available for that particular wave path.
The probability of the MUF reaching channel 5A (138MHz) and the 2 metre band (144MHz) is higher than band 3 TV (175-220MHz). Most DXers will not experience sporadic E on band 3 (Australian channels 6-11), for more than a few minutes a year, on average, and that takes an extraordinary DX opening.
Path distance
The optimum sporadic E propagation path distance is approximately 1,240 miles
(2,000kms),
which is the same for bands 1, 2 & 3 (Australian channels 0-11). The maximum
single hop distance for
Sporadic E DX is about 1,430 miles (2,300kms), a geometric restraint based
on an average height of E-layer
ionization of approximately 65 miles (105kms). The height of E-layer
ionization can sometimes be higher
than normal, thus extending the maximum path distance to approximately 1,520
miles (2,460 kms).
Reports of band 3 SpE in the USA have been have been known to reach up to
approximately 1,600
miles. Long haul SpE openings on two metres (144MHz) between Esperance WA to
Sydney NSW, and
New Zealand to Adelaide, SA have occurred on rare occasions. Curiously
enough, sporadic E paths in the
1,110-1,370 mile range are probably the most common. This is because the
single hop distances, near
the maximum useable frequency (MUF) are also the longest.
In
summary, path lengths of around
1,000-1,300 miles are optimum for SpE DX above 108MHz. However, propagation
is also possible,
though less likely, in the 800-1,000 and 1,300-1,550 mile range.
Polarization discrimination
VHF TV DX signals have a degree of polarization twist, particularly on short and multi-hop paths. However, on single hop paths, it has been observed that receiving antennas with incorrect (orthogonal) polarization, with respect to the transmitting antenna, may result in an average cross-polarization loss of about 5dB. Consequently, TV DXers wishing to receive a particular transmitter, should ensure their antenna has the same polarization to obtain optimum reception.
Practical approach
Since Sporadic E reception, above 138MHz, often involves high path losses,
it is
important that the DXer uses the highest gain and lowest noise receiving
equipment possible, within
his budget. A directional yagi antenna, with at least 8dB of gain, mounted
15-20 feet above ground
level, with low loss 75ohm coax cable, low noise TV tuner and a low noise
MOSFET pre-amplifier are
ideal for receiving weak signals. My own band 3 TVDX equipment,
consists of two 14 element channel
6-11 yagi antennas (one mounted vertically and the other mounted
horizontally) with separate low
loss 75ohm coax cable running to an indoor tunable BF981 MOSFET
pre-amplifier.
Signals are initially
monitored on an ICOM R7000 receiver, using USB mode to detect weak video
carriers. The ICOM will
detect extremely weak video carriers, long before they appear on the screen.
In fact, video carriers
of high powered TV transmitters can be heard daily up to 500 miles via
tropospheric scatter.
Meteor
scatter pings of TV video carriers can be heard daily on channels 0-11.As a
result of this daily
reception, it has been possible to determine the exact video offset of
certain transmitters, and then
keep a record of them, for identification purposes, when a sporadic E opening
occurs.
A TV tuner (eg HS Publications D100) with variable IF (Intermediate Frequency) bandwidth will improve the signal to noise ratio of weak TV signals. Attempts of using band 1 antennas for channels 5A, 6-11 DX reception will only result in disappointment. Since the directional pattern of yagi antennas are sharper at higher frequencies, it is important that any channel 5A, 6-11 yagi antennas are aimed within plus or minus 5 degrees of the direction of the DX transmitter.
Operating techniques and signal identification
Sporadic E openings above 108MHz are generally most
common during the summer months, (December and January in the Southern
Hemisphere.
The alert
VHF DXer will monitor the band 1 and 2 TV/FM DX openings for signs that the
MUF may be
approaching channel 5A and channels 6-11 TV. For example, at my location in
Sydney NSW Australia,
FM signals from Mackay Qld sometimes reach 107.5MHz. When this happens, I
aim my horizontal
channel 6-11 yagi antenna towards Mackay, Qld, and listen for signs of STQ 6
Mackay, Qld TV vision
carrier (175.25MHz), on my ICOM R7000 receiver.
To be prepared in
advance for a DX opening, it
helps to obtain the following information: List of available high powered
(above 10kW ERP) channel
5A-11 TV transmitters approximately 900-1450 miles range from your location,
including video
frequency offset and polarization.
TV transmitter ERP, polarization
and video offset information can
be found in the Department of Transport and Communications Radio and TV
Broadcasting Stations
book.
Most high-powered channel 5A-11 TV transmitters have FM radio
transmitters at the same site.
Unless strong FM DX can be received from the wanted DX transmitter site,
there will be little chance
of receiving SpE signals above 108MHz from the same location. This is of
course providing the FM DX
signals are not blocked by local transmitters.
One problem
for city DXers is local channels
blocking potential DX signals. For example, at my location, FM DX signals
from Townsville and Cairns
in Queensland are strong and relatively frequent during the sporadic E season, due
to their optimum path
distance from Sydney NSW. However, since I have local TV transmitters on
channels 7, 9 and 10,
Townsville and Cairns band 3 reception is impossible.
Another
indicator of possible band 3 SpE is when
short hop SpE signals are coming through. For example, on one occasion, when
4MMM-FM Brisbane
Qld (104.5MHz) was coming through in Sydney, NSW (a distance of 430 miles),
via short hop SpE, the
2 metre 144MHz band was open between Melbourne and Queensland.
One
sign that SpE above
108MHz is possible is when the SpE starts early in the morning and reaches
the FM band by around
8.00-10.00am. Another useful indicator is the amateur 50.11 and 144.1MHz
call channels. By
monitoring these call channels, it is often possible to obtain information
on SpE and tropospheric DX
openings. I have placed most amateur beacons, call channels, and TV
video/audio frequencies, into
the memory channels of the ICOM R7000 to assist in identifying the locations
of DX signals.
Two metre and high band VHF DX via
sporadic E propagation, seems to be more common during periods of low
sunspot activity.
Conclusion
High band TV DX consists of
typically slow fading stable pictures with relatively little ghosting. Some
DXers have received signals up to 216MHz, but this is an extremely rare
occurrence. So I encourage
all TV DXers to try for channels 5A-11 SpE dx, and if you do succeed, try to
videotape or photograph any DX.
Table 1: Channel 5A
and Band 3 DX Reports
Robert Copeman (Sydney NSW): TV2Ch4
Auckland NZ (175.2396MHz Vision, 180.74MHz Audio)
- 1,343 miles Todd Emslie (Sydney NSW): ABTQ 5A
Bowen Qld (980 miles) behind ABWN 5A
Wollongong NSW 144.1MHz VK4FNQ
Townsville Qld (1,050 miles) TV2Ch4
Auckland NZ (175.2396 Vision) - 1343 miles (BFO
level only) 144.1MHz ZL3TY
Greymouth NZ Anthony Mann (Perth WA): RTS 5A
Loxton SA (138.26MHz Vision, 143.76MHz Audio) - 1,448 miles.
Chris Hambly
(Melbourne, Vic, Australia): ch 5a (138.25 MHz), Toowoomba, QLD,
Australia, - 1,000 miles.
Rijn Muntjewerff (Beemster Holland):
USSR ChR6-11 (175-221MHz) RTA ChE5
Algeria 175.25MHz Vision, Geoff Wolfe(Bombala NSW): STQ 6
Mackay Qld (175.25MHz Vision, 180,75MHz Audio)
Table 2: List of
Channel 5A-11 TV Transmitters potentially receivable via SpE
Adelaide SA: ABCW 5A
Northam/York WA (1,300 miles); STQ 6 & ABMQ 8
Mackay Qld (1,100 miles); ABW 8 Wagin WA (1,300 miles) Brisbane Qld: ABWV 5A Hamilton
Vic (950 miles); TVT 6 Hobart Tas (1,100
miles); TV2Ch4 Auckland NZ (1,400 miles); SES 8 Mt Gambier
SA (1,100 miles) Canberra ACT: TV2Ch4 Auckland NZ
(1,500 miles); STQ 6 & ABMQ 8 Mackay
Qld (1,000 miles); TNQ 10 Cairns Qld (1,400 miles) Launceston Tas: ABWQ 6 & STQ 8
Maryborough Qld (1,000 miles); STQ 6 &
ABMQ 8 Mackay Qld (1,300 miles); RTQ 7 Rockhampton Qld
(1,150 miles) Melbourne Vic: ABWQ 5A Nambour Qld
(930 miles); ABTQ 5A Bowen Qld
(1,160 miles); ABWQ 6 & STQ 8 Maryborough Qld (945 miles);
STQ 6 & ABMQ 8 Mackay Qld (1,135 miles) Perth WA: RTS 5A Loxton SA (1448
miles); ABWV 5A Hamilton Vic (1,535
miles); ABNS 6 Cowell SA (1,300 miles); ABMV 6 & STV 8
Mildura Vic (1,540 miles) Sydney NSW: ABMQ 5A Moranbah Qld
(? miles); ABTQ 5A Bowen Qld (980
miles); TNQ 5A Cairns North Qld (1,230 miles), TV2Ch4
Auckland NZ (1,343 miles) New Zealand: Any high powered
channel 5A-11 east coast Australian
transmitter within approximately 1,200-1,450
miles References
1958 QST article on 200 MHz sporadic E. Robert B Cooper.
Copyright © 2003 Todd Emslie