The still propellers of the Alouette helicopters hung limp. All around Ondangwa's makeshift military airport terminal, which doubles at night as a movie theater for South Africa soldiers, it was quiet and oppressively hot. Across the runway three huge radar arcs swirled continuously, scanning the sky for enemy aircraft.

Earlier in the day, a Lockheed C130 from Pretoria had eased down at Ondangwa and a contingent of brown-uniformed recruits, along with a handful of officers' wives and children, emerged from the huge plane.

Most of the new arrivals would spread out during the next few days over the hot, dusty flatlands of a country not their own, to fight South Africa's war against the guerrilla forces of the Southwest Africa Peoples Organization (SWAPO). Some of them would travel north just 40 miles in landmine-safe vehicles to the half-mile deep "no man's land" where a 10-foot fence stretches for much of the 900-mile border separating Namibia from Angola.

Day and night, South African soldiers patrol this "no-go" area on foot, trying to prevent infiltration by members of the Peoples' Liberation Army of Namibia, the armed wing of SWAPO.

Stationed across that fence is what the white-minority government regards as its ultimate enemy -- one of the largest Communist-bloc military presences in sub-Saharan Africa, including Cuban, Soviet and East German personnel. In addition to assisting the Angolan government in its own antiguerrilla war, the Communist forces work closely with the estimated 5,000 SWAPO guerrillas at bases in Angola.

In a low-level hit-and-run bush war over the past 12 years, SWAPO has failed to dislodge the South African troops, whose formidable presence includes a huge air base at Grootfontein, complete with underground installations.

Yet, "there's a feeling that the good old days are over," said one informed observer. "The guerrillas no longer stop just long enough to discard their shoes and guns before they run back over the border. They have improved and their morale and determination are much better."

South African military officials admit that SWAPO is better trained and equipped than a year ago. Recently, there have been indications of an increased commitment by its Marxist backers, with reports of a buildup of Cuban and East German forces in southern Angola.

With these developments on the SWAPO side, this war, long ignored by the international community, is now at the center of a U.N. effort to keep it from developing into another Rhodesian conflict.

The U.N. plan for a negotiated settlement, drawn up by the United States and four allies, could, however, be totally disrupted by a major escalation in the war. There has been speculation recently that South Africa is preparing a raid on SWAPO bases in Angola to prevent infiltration of the guerrilla force before the elections South Africa is holding in Namibia starting today.

These fears were promoted by South African military claims that SWAPO activity, after falling off markedly since a similar raid last May, has reached the level of intensity it had before that attack. The commander of South African forces in Namibia, Gen. Jannie Geldenhuys, said recently that the number of incidents involving SWAPO guerrillas in October was the highest since last May and one of the highest since April 1966.

Reports of a South African military buildup in Namibia could not be confirmed here, but military observers say such a buildup is not needed for an airborne strike at SWAPO bases in Angola.

The SWAPO guerrillas who do manage to pass through the "no-man's-land" at the border -- South Africa estimates that there are 100 to 200 at any one time inside the country -- concentrate on laying land mines and sabotaging water and communication lines. Geldenhuys said. Twice this year, explosions sabotaged transportation lines in southern Namibia, more than 60 miles from the "operational area" along the northern border. The explosions were a psychological blow to South African attempts to keep the war away from the towns where most of the territory's 100,000 whites live.

The suspicion that explosives were being transported to the south led to the imposition last May of a dusk-to-dawn ban on driving in Ovamboland, one of the principal centers of the war.

Increasingly, the SWAPO guerrillas are choosing rival political organizers as targets, Geldenhuys said. Since May, SWAPO is alleged to have killed five party workers of the Democratic Turnhalle Alliance, a South African-backed coalition of conservative parties that has emerged as SWAPO's most effective competition.

Most Turnhalle leaders are constantly accompanied by bodyguards. According to the South Africans, the local population also is beginning to take measures to resist SWAPO activity. Turnhalle "organizers now address rallies with a pistol on their hip," said Geldenhuys. "That's a new kind of spirit. They are taking their own countermeasures against terrorism."

While this may be an encouraging sign to the South African military, it is a sign of future civil war to others. It is promoting the rapid creation of a Namibian army to take over when the country becomes independent. "We are substituting South African officers with locals as fast as possible," Geldenhuys said.

Since the SWAPO guerrillas depend on the civilian population, especially the 395,000-strong Ovambo tribe, for food, shelter and information, the South African security police are involved almost as much as the army in this war. Reports of civilians being summarily beaten, harassed, jailed and even shot on the mere suspicion of helping or of being a "terrorist" are too frequent to be dismissed.

According to South African figures, about 50 civilians have been victims of guerrilla activity so far this year, most of them from land mines.There are no figures for civilian deaths as a result of South African activity, and as far as guerrilla deaths are concerned. South Africans just say "very many" have died

So far this year, 32 South African soldiers have died, military authorities say, slightly fewer than the 35 who died all of 1977.

Despite this relatively small toll, there is some questioning among South African soldiers about their presence in a country soon to become independent, according to several reports. "These guys who go to the border, they don't want to be there and they ask why are we fighting in a country that isn't ours?" said one young white South African who had served there.

At the same time, some observers believe the South African government welcomes the low-level bush war against SWAPO to train its own forces for what may eventually be a guerrilla war on its own borders.