This article is a summary of the author's practical observations with long distance (DX) reception of overseas 88-108 MHz FM broadcast radio stations.
Most "deep fringe" type 88-108 MHz high gain directional antennas feature eight to ten elements. These antennas are normally used by rural listeners for the purpose of hearing city FM stations up to 70-150 miles distance. Serious DXers, on the other hand, are interested in hearing FM signals from hundreds or even thousands of miles distance.
Outdoor "deep fringe" type multi-element FM (88-108 MHz) yagi antennas are needed to compensate for weak long-haul Es or TEP signals. For example, 88-108 MHz FM signals via double-hop sporadic E are generally weaker compared to single-hop signals. This is because of extra path attenuation caused by losses from reflections off the second Es cloud.
The author uses two Triax FM-8 eight element FM yagi antennas, which have approximately 6 dBd gain. Both yagi antennas are mounted horizontally and vertically on separate masts at opposite sides of the house. The use of opposite (orthogonal) antenna polarization is useful because signals propagated via double-hop sporadic E generally have random polarization. Single-hop Es signals generally tend to preserve their original transmitted polarization.
Since long-haul FM DX signals generally arrive at low angles over the horizon, it is ideal to mount yagi antennas at the maximum practical height possible. A DXer who has a high elevation and good take off to the horizon has an advantage for long haul DX. This is called height advantage. For example, if a receiving location is located on a mountain, one km above sea level, expected distances are proportionally increased for single and double-hop Es.
In city areas, overload will be experienced if the FM tuner's RF front-end is a Bipolar type, with an associated wide RF bandwidth. It is recommended that a tuner featuring a Mosfet or GaAsFET varactor tuned RF front-end is used to minimize cross-modulation and overload problems. The Onkyo Integra series, Kenwood KT-6040, Yamaha T-85, and Sony STSA5ES FM tuners are all excellent in terms of weak signal sensitivity, including image and overload rejection.
Australian FM stations currently use 200 KHz channel spacing. However, since several overseas countries use 100 KHz spacing, Australian FM DXers can use this to beneficial advantage. The author's Onkyo T-9090 II FM tuner has been modified with 110 KHz Murata ceramic filters, hence it is often possible to receive certain stations only 100 KHz away from local signals. Some examples of adjacent channel signal separation is 96.0 MHz Fiji next to local 96.1, 93.0 MHz RFI New Caledonia next to local 92.9, and 90.0 MHz RFO Noumea next to 90.1.
When attempting to receive a DX signal that is only 100 KHz away from a semi-local signal, first determine if the semi-local signal is weaker on horizontal or vertical polarization. It is then possible to reduce the strength of the semi-local, which will enable clearer reception of the adjacent DX signal. Then switch in 110 KHz IF filters, and if required, de-tune 25 KHz away from the semi-local in order to minimize adjacent channel splatter. For example, tuning to 95.975 MHz next to a local 96.1 MHz signal.
The Onkyo T-9090 II FM tuner has five 150 KHz MURATA ceramic IF filters (SFE10.7MZ2-A), which are used for reception of DX signals +/- 200 KHz adjacent to locals. For reception of DX signals +/- 100 KHz away from local signals, four Murata 110 KHz filters (SFE10.7MHY-A) are used. Even more impressive adjacent selectivity selectivity performance is obtained when using the Sony XDR-F1HD DSP FM tuner.
For very good 100 KHz adjacent channel selectivity performance, try using a Kenwood KT-6040 AM/FM tuner (modified with four Murata SFE10.7MT 80 KHz filters, and four Murata SFE10.7MHY 110 KHz filters).
The signal strengths received with long-haul FM DX signals are often at or just above the tuner's threshold sensitivity. Providing external man-made and terrestrial noise is sufficiently low, the use of a low noise pre-amplifier improve the signal to noise ratio of weak signals. Home constructed BF981 MOSFET tunable dual gate RF pre amplifiers are currently used for weak signals. The author's BF981 RF FM pre-amp has a noise figure of approximately 1.5 dB. The input and output coils are tuned by varicap diodes. The -3dB bandwidth is approximately 2 MHz when peaked, thus overload problems are minimal.
The use of wide-band VHF non-Mosfet pre-amps is not recommended because of their poor signal handling specifications. Overload and images will be received across the dial. In these instances, a pre-amplifier should not be used.
Most FM tuners that feature Mosfet RF front-end technology will often not greatly benefit from the addition of a preamplifier. This is because the noise figure of the tuner's first RF stage is often below the ambient external noise levels. This is especially true for city receiving locations.
A second-hand Kenwood KT-6040 FM tuner is more than adequate for serious FM DX. The KT-6040 features high RF sensitivity, via the internal Toshiba 3SK121 GaAsFET first RF amp. Most commercial RF preamplifiers will have a noise figure higher than the KT-6040's first RF amp, hence should be avoided.
VHF DX Indicators
50-52 MHz amateur beacons, 45-108 MHz TV video carriers, and six metre ham signals can be useful as early warning propagation indicators for potential 88-108 MHz FM DX openings. A receiver such as the Icom R7000/7100/8500 or AOR AR-5000 can be programmed with all the available 45-108 MHz indicators. For example, KVZK chA2 American Samoa 55.25 MHz video carrier is a useful early warning indicator for Fiji, Norfolk Island, Tonga, and American Samoan FM. Another useful indicator is the 50.029 MHz P29 Port Moresby beacon (now unfortunately QRT). The P29 beacon alerted the presence of 93.1 MHz Yumi FM Port Moresby, Papua New Guniea.
For reference purposes, keep a record of any 6 metre beacons, amateur, or TV video and audio transmitters that are located within 6,000 miles.
Practical Reception Examples
Indonesian 88-108 MHz FM has been received via sporadic E in QLD, WA, and NT. I need to determine which Indonesian FM transmitters are in double-hop sporadic E range, i.e., approximately 2,800 miles. To determine distances from my QTH, I use a great circle distance and bearing calculator.
During December and January, the following VHF indicators for are monitored for possible signs of Indonesian FM:
50.057 VK8VF beacon.
88-108 MHz FM from Darwin.
chE4 62.24/24/26 Indonesian and Malaysian chE2/E3/E4 video carriers.
When some of these indicators are present, FM signals are monitored on the low end of the 88-108 MHz dial. For example, transmitters with a power of at least 1kW, which are below 95 MHz.
When attempting 2Es reception of Papua New Guinea FM, I listen for signs of the 50.029 MHz, P29 Port Moresby beacon (now unfortunately QRT). Once the P29 beacon is received, all the available PNG FM frequencies are closely monitored, which have been programmed into the Onkyo T-9090 II tuner. I use the same approach when trying for reception of FM from the Pacific Islands. For example, when KVZK TV-2 American Samoa is received with clear 59.75 MHz TV audio, I listen for signs of KSBS FM, American Samoa on 92.1 MHz. As a general rule, If KVZK TV-2 audio is received, I check for signs of certain Pacific Islands FM within a maximum radius of about 3,000 miles.
To help us understand the possible distances for FM stations we can receive via sporadic-E, I have included Emil Pocock's article on the Doughnut Effect.
The Doughnut Effect by Emil Pocock, W3EP
"Sporadic-E paths between 1490 and 1740 miles are more difficult to complete than longer and shorter paths. The maximum single-hop distance for sporadic-E contacts is about 1430 miles, a geometric restraint based on an average height of E-layer ionization of 65 miles or so. Curiously enough, sporadic-E paths in the 1120-1360-mile range are probably the most common. This is because the single-hop distances near the maximum useable frequency (MUF) are also the longest. As the MUF rises above 50 MHz, the paths shorten up."
"It may be possible that some sporadic-E paths at 1490 miles or even longer are also completed by unusually long single hops, perhaps from patches of E-layer ionization that are somewhat higher than the average 65 miles. Even so, it is more likely that sporadic-E paths longer than 1490 miles are via multiple hops. If that is indeed the case, then a 1490-mile path must involve two hops with an average of 740 miles each (the hops do not have to be of equal length, so long as they total 1490 miles). The problem is that 740-miles paths are unusual at 50 MHz, because the required MUF to create such short hops is high, perhaps in the 100 MHz range. Thus in order to complete a 1490-mile path at 50 MHz, two separate sporadic-E centers with MUFs of 100 MHz and spaced 740 miles apart are needed. That is a big requirement!"
"As the path lengthens from 1490 miles, the required MUF for the two sporadic-E centers drop, thus making it more likely that the required geometry will be achieved. In theory, this suggests that as the distance approaches 2850 miles, there should be a greater incidence of double-hop sporadic-E."
"When the probability of sporadic contacts are graphed in two-dimensional space, a sort of doughnut shape emerges. Sporadic-E contacts are rarely shorter than 250 miles. That is the hole. As the distance lengthens from 250 miles, the occurrence of sporadic-E contacts increases until 1420 miles is reached. That is the main part of the doughnut. There is a sharp drop-off at 1420 miles amounting to a sharp boundary until around 1740 miles or so, then contacts become more and more likely until 2850 miles, when the second, but less sharply defined boundary is reached."
"At 2850 miles and longer, there are many possible configurations of hops that make the 2850 to 3200-mile void less clearly defined. A 3000-mile path could be completed by three 1000-mile hops, for example. The MUF requirements for 1000-mile hops are not as high as for 750-miles, although finding three sporadic-E centers lined up optimally is not common either. You can make your own calculations and discover the various possibilities for difficult distances."
"This line of logic suggest that there may be some prime distances for multi-hop sporadic E. If the most common single-hop contacts near the MUF fall into the 1000 to 1350 mile range, then the most common multi-hop paths might be expected at 2200-2700, 3350-4100 miles, and so forth".
After looking at Emil's article, we can conclude that FM DX signals will be most commonly received in the 900-1,300 and 2,200-2,700 mile range, etc.
To determine probable reception for Es, 2Es, and TEP FM DX from your QTH, use a great circle distance and bearing calculator. Look for stations located between 500-1,400 miles (Es), 1,900-2,800 miles (2ES), and 1,900-3,700 miles (Evening TEP).
FM Station Reference list
The author often refers to Australian and Pacific Ocean area FM station reference lists, which are researched by two Australian DXers. Exotic weak signals via multi-hop sporadic-E or evening TEP are not usually accidentally received. By using reference lists, I am prepared in advance for possible DX openings. The most important information is the transmitter power, frequency, polarisation, and location.
Optimum Receiving Locations
Generally speaking, the northern areas of Australia are best for overseas TV FM DX. In locations such as Melbourne VIC, and Launceston TAS, it is relatively rare to receive overseas FM DX. Queensland is ideal for the Pacific Islands. The Northern Territory, Northern Western Australia, and Northern Queensland are optimum areas for sporadic E and transequatorial DX from Asia.
Based on observations of multi-Es openings from Australia, it seems that a larger percent of openings occur over water paths, for example, Fiji, Samoa, Futuna Island, Tonga etc. I think this is mainly due to ocean salt water, which has greater conductivity. This conductive medium is more favorable for reflections back into the E layer.
Multi-hop Es is more common in certain areas of Australia, and the world. This is because the geographic distribution of Es varies considerably from one area to another (E.K.Smith, 1978).
The foEs (the maximum frequency capable of reflection, at vertical incidence from the sporadic E region of the ionosphere) world map (E.K.Smith, 1978) showed localization of the foEs. In this paper, the foEs averages are quite high around Darwin, Japan, South east Asia, and the Philippines. For this reason, Japanese DXers rarely experience a poor Es season!
Evening Transequatorial Propagation
Reception of Asian and Pacific Islands 88-108 MHz FM is possible in Australia during periods of moderate to high solar activity. This includes locations between Darwin (12 S) and Alice Springs (24 S). Evening TEP results are generally much better for receiving locations above (15 S).
At Darwin, Northern territory, reception of Japanese, Malaysian, Brunei, Korean, Indonesian and other Asian FM tx's are possible during moderate to high levels of sunspot activity. This mode of DX is via Evening TEP, or spread F propagation.
Reception of 88-108 MHz FM from Northern Queensland and Northern Territory has been received in southern Japan, via evening TEP. During the present sunspot cycle, Japanese DXers report that evening TEP MUFs, to areas of north Queensland, are usually no higher than 87.27 MHz ABTQ3 Townsville.
Evening TEP MUFs are highest for signal paths crossing the geomagnetic equator at right angles. For Australia/Indonesia this means a bearing of about 180 degrees from Japan. The results are optimum if signals travel perpendicular to the geomagnetic equator and stations are located equidistant from it. The corollary is that a receiving station in Cairns (16.51s/145.43e) is well placed for reception of eastern coastal areas of northern Japan. A receiving station in Broome (17.58s/122.15e) is well placed for reception of Chinese FM.
Although evening-type TEP is much more tolerant of path obliquity than afternoon-type TEP, the path angle is limited to ~ 5-10 degrees for MUFs greater than 88 MHz. Hence, Korea and Japan are the most common countries received above 88 MHz in Darwin.
During our recent Darwin DXpedition we noted Malaysian, Singapore, Brunei, and Indonesian 88-108 MHz FM. These signals were received via sideways scatter from the equatorial ionization!
Lower frequencies, i.e., 45-70 MHz have considerable path obliquity. For example, 55.25962 MHz KHON TV, Hawaii is often received in Darwin.
At FM band frequencies the DX signal is reflected (more correctly, refracted) back to earth at the path midpoint by intense ("spread F") ionization close to the geomagnetic equator. A 3100-mile path has a midpoint at 10 degrees north while a 2500 mile-path has a midpoint at 14 degrees north. The geomagnetic equator at these longitudes is halfway between these two, i.e. at 12 degrees north. A simple calculation, like one does for sporadic-E skip distances, shows that reducing the distance from 3100 miles to 2500 miles only drops the MUF by about 2 MHz at 100 MHz. This is negligible when the MUF is well over 100 MHz.
Caribbean 88-108 MHz FM has been received in Southern Brazil via Evening TEP. The distances range from approximately 2,800-3,400 miles.
Indonesian, Darwin, Australia, and Philippines FM has been received in Southern Japan, via evening TEP during the months of March-May and September-November.
The optimum times for Trans Equatorial Skip(TEP) are during the evening hours between 2000 and 2300 local time. The Equinoxes (March/April and, September/October), are generally the most favorable periods.
The data collected shows that distances of 2800-3400 miles are possible at the 88-108 MHz FM band via evening TEP. Other signals are been noted at about 3,000-6,000 km via class 2 TEP.
Optimum Diurnal Times for Sporadic E
Most multi-hop sporadic E openings occur during the Australian summer months of December and January. At my location, January is optimum for reception of Pacific Islands FM. Daylight hours between approximately 1000-1800 is best. Multi-hop sporadic E openings on FM tend to be more common during periods of low sunspot activity.
88-108 MHz FM received at Sydney
New Zealand 1,400 miles.
Norfolk Island 1,000 miles.
New Caledonia 1,200 miles.
Papua New Guinea 1,710 miles.
Tonga 2,000 miles.
Fiji 2,000 miles.
Futuna Island 2,346 miles.
American Samoa 2,739 miles.
Author's current list of FM and TV signals received via long haul sporadic E at: Ref 1.
The author's most distant FM DX via double-hop sporadic E is:
Sydney - KSBS-FM 92.1 MHz, 15 kw, American Samoa: 2,739 miles, or 4,409 Km.
The current Australian FM DX distance record is David Headland's reception of KSBS FM 92.1 MHz, Pago Pago, American Samoa, received at Marlo beach, Victoria, Australia, at a distance of 2,972 miles or 4,783 km.
Band 2 Multihop Es Distance Record
On 31 May, 2010, 88.7 MHz La Voz de la Luz, a religious station from Salvaléon de Higüey, Dominican Republic was received via 4302 mile (6925 Km) Es by Mike Fallon, East Sussex, UK. Receiving equipment used consisted of an Triax FM-8 FM yagi, and Sony XDR-F1HD tuner.
Fig 1: Equal area map of Australia and Pacific Islands.
The Final Test
The real test for determining the efficiency of FM DX receiving installations is the range of distances observed for daily 88-108 MHz tropospheric scatter. The "Big Guns", i.e. DXers who are adequately equipped for exotic long-haul Es DX, are able to hear tropospheric scatter FM signals out to about 250 miles (400 km) on a daily basis. Aircraft scatter is also possible at distance ranging from ~ 350-450 miles. The equipment needed for these results are typically:
- A single wideband 88-108 MHz "deep fringe" type FM yagi (~ 5-10 dBd gain) mounted outside at least 20 feet above ground.
- Low loss RG6 Quad, Hills DSC2.1, or RG11 75 ohm coax cable.
- A quality 88-108 MHz FM tuner with (preferably) Mosfet or GaAsFET RF track- tuned front-end, variable IF bandwidth, and digital frequency display. With narrow IF bandwidth selected, the weak signal sensitivity should be around 0.6 uV (mono). The Sony XDR-F1HD and XDR-S3HD both work very well with tuned RF LNAs.
By using the techniques outlined in this article, 88-108 MHz FM reception of 16 overseas countries have been received in Australia by the author. It is hoped that other enthusiasts can use this material to further extend the boundaries of VHF propagation.
Darwin DXpedition. Full report on Tony Mann and Todd Emslie's recent trip.
EA6VQ's TEP page: compilation of 144 MHz TEP information.
EA6VQ's TEP page. Japan to Australia 144 MHz TEP information.
Copyright © 2009 Todd Emslie