Tricky paths to signal distribution

Getting a signal from the studio to the transmitter is a growing challenge for radio engineers in the Pacific islands.

Until a few years ago, most stations used landlines to feed programming to their medium-wave or short-wave AM transmitters. In an attempt to cut operating costs while improving audio quality, many stations now broadcast on networks of low power FM transmitters.  These transmitters may be on remote mountains or islands, hundreds of kilometers from the studio.

Telephone company landlines often do not extend to remote transmitters. Even if they do, the cost may be more than a broadcaster can afford.

Last year, the telephone company in Papua New Guinea attempted unsuccessfully to take control of the government-owned FM radio network to recover line rental of more than US$1 million.

“Landline costs are quite extraordinary in the islands,” noted Richard Fleming, managing director of Radio Support Services, a Sydney-based company that provides engineering services to a number of stations in the Pacific. 

He says broadcasters are trying to get away from using traditional telephone company lines. 

William Tibben, another Australian engineer who has worked throughout the region, agrees telephone charges are severe. 

“Many telcos are government owned monopolies and are milked for every cent they can produce. In some ways, I see this as being a significant barrier to the introduction of more innovative methods of signal distribution such as ISDN and satellite.”

Satellites

While satellite might seem the perfect method for distributing signals across mountains and oceans to remote transmitters, cost is again a problem.

“Certainly the future will be satellite, but we have a long way to go in negotiating reasonable fees for satellite transponder space and bringing broadcasters together so that a joint uplink facility can be installed,” Fleming said. 

The Pacific Islands Broadcasting Association (PIBA) has investigated establishing a Pacific-wide satellite distribution system for domestic and international use by its members. PIBA’s technical coordinator, Hendrik Kettner, described the possibility as “exciting, at least in technical terms” but said plans have been shelved due to lack of funds.

Some broadcasters may also be nervous about the reliability of satellite distribution. Radio Tuvalu has a new satellite FM network which, at the time of writing, is out of action due to equipment failure at eight isolated downlink sites. 

In between the costly extremes of low tech telco lines and high tech satellite systems, Pacific broadcasters use many different ways to distribute signals. 

Tibben says finding the right approach requires “a bit of theoretical knowledge and a lot of trial and error.”

STLs and repeaters

In Fiji, three national radio stations owned by Communications Fiji Ltd. (CFL) use a hybrid system. From studios in the capital, Suva, signals are fed via Marti STLs to mountain-top FM transmitters which cover the city and a large surrounding area. 

The signals are then picked up “off-air” at two distant repeater sites and rebroadcast. Smaller repeaters pick up the rebroadcast signal and relay it to the most isolated listeners. 

“Complex as it sounds, the system works very well,” said Fleming, whose company provides consulting services to CFL. “The main advantage is cost. The disadvantage is that there is total reliance on the primary transmitters for coverage of the whole country.” 

Fleming said topography does not allow an alternate path to the main repeaters.

The country’s other broadcaster, state-owned Radio Fiji, uses a similar system for its stations. 

Even though there are fewer than ten FM stations in Fiji, each requires several frequencies in the FM band for its transmitters and repeaters. Earlier this year it became necessary for the two broadcasters to devise a plan for sharing the spectrum.

While STLs and repeaters may work well in Fiji, they would be useless in the Cook Islands. 

The Cooks comprise 15 islands with a total land area of only 240 square kilometers, scattered over almost two million square kilometers of ocean.

The old AM transmitter used by the Cook Islands Broadcasting Service (CIBS) cannot cover the entire country but Fleming said listeners on distant islands tune in to “skip” signals after sunset.

“On intermediate islands where one would propose installing an off-air repeater (for FM), there is no power and no population,” Fleming said.

For countries like the Cooks, a satellite FM system such as that installed in Tuvalu seems to offer the most promise.

Lines survive 

In Samoa, state-owned Radio 2AP simplified its distribution system a few years ago when it abandoned a network of STLs and remote AM transmitters, in favor of a central 10 kilowatt AM transmitter for each of its two services. 

Underground landlines run a few hundred meters from studios to transmitters on the seashore near the capital, Apia. 

Tibben, who spent four years as an engineer at 2AP, recognizes that the station is “in some ways bucking current trends which would have replaced those remote AM transmitters with FM,” but feels it was a wise decision, given the Pacific broadcasting environment. 

“Remote transmitters are difficult to maintain when transport, spare parts, information and knowledge are in short supply.”

And when it comes to links, Tibben likes to keep an open mind. 

“The humble old line should not be dismissed too quickly, particularly if it is owned and maintained by the radio station. If telecom companies are involved reliability becomes a significant question. During cyclones lines are well protected which is certainly not the case for aerials exposed to the elements.”

Radio 2AP did not throw away its UHF STL equipment, either. By reversing the signal direction, staff now use it to feed outside broadcast programs back to the studio from around the country.

Sub carriers on satellite

For Fleming, the most complex signal distribution situation encountered so far was in Papua New Guinea. It was also the easiest to solve.

Privately owned sister stations Nau-FM and Yumi-FM wanted national coverage through a network of twelve FM transmitters, but mountain ranges ruled out a private microwave system. The telephone company had an excellent distribution system but the cost of leasing channel space was prohibitive.

The answer was to utilize sub carriers of the privately-owned national television network, EMTV which broadcasts 24 hours a day via satellite. FM broadcast signals are picked up off-air at a satellite earth station on the outskirts of the capital, Port Moresby, and injected into the EMTV uplink. 

“The limitations are that the signals are in mono and the satellite (Rimsat) has an inclined orbit requiring tracking antennae at the downlink sites,” Fleming said. 

EMTV will soon be switching to the Asiasat satellite, however, so tracking antennae will no longer be required. This will reduce maintenance costs and increase reliability. As well, “the signal is changing from PAL encoded to MPEG digital encoded, so quality will improve.”

As in Fiji, however, the network relies on the primary transmitter. Equipment has to be reliable and well-maintained but, given the large area to be covered and the difficult terrain, satellite is probably the only way to distribute FM signals efficiently in PNG.

“The Pacific is a big place,” commented Fleming. “There is no common answer to signal distribution. It comes down to practical engineering to suit the conditions and the client’s budget.”

* First published 24 December 1997 in the International Edition of Radio World