Introduction

DX enthusiasts know that during the summer months, television channels on Band 1 and 2 (45-108MHz) and also FM radio stations (88-108MHz) are regularly propagated long distances via sporadic-E (Es) propagation. However, what chance is there for the optimistic DXer receiving Australian channel 5A (138.25MHz Vision) and band 3 television (175-226MHz) via Sporadic-E?

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 © 2009 Todd Emslie