NE GA ARES IDEAS

NVIS ANTENNAS

 

There has been what can be called more than somewhat mild excitement in Northern California emergency communications circles over a form of high frequency radio propagation. It's not new, but I venture to say that very few have used and understood it. "It" is called NVIS -- Near Vertical Incident Skywave.Patricia Gibbons, WA6UBE, presented a paper on NVIS at the 1990 Pacific Division ARRL convention in San Jose. It caused quite a stir. She quickly ran out of handouts and has since received dozens of requests for more. The handouts included reprints of articles from military communications magazines reporting the results of many tests.

Near vertical incident skywave means forcing your radio signals to travel straight up (i.e., 80-90 degrees) and back down. This achieves radio coverage in circle having a radius of 300 miles and more. Stop and think about that for a moment. Complete coverage within such a circle on frequencies between 2 and 10 or 12 Megahertz. Some readers may wonder what's so good about this. So now is a good spot to say that if only DX (long distance) is your thing, skip on and read one of the other fine articles in this publication. We are talking about dependable local area high frequency communications -- the type we need for tactical public safety communications in the Radio Amateur Civil Emergency Service, the Civil Air Patrol, SECURE, search and rescue, forestry, pipeline and similar services. In tactical communications we don't want DX.

How frustrating it was in years gone by to drive away from, say, a 4585 KHz base station, only to lose a good 400 watt signal a mile from the transmitter! All the while receiving, loud and clear, a 50 watt transmitter some 200 miles away. Very frustrating. We really didn't know why. When VHF-FM radios and repeaters came along, most of us retired HF mobile radios for tactical communications.

The reasons we haven't enjoyed good HF tactical communications, whether AM or SSB, have been the base and mobile antennas. The classic dipoles, a quarter to a half wave up in the air. The mobile antennas, designed for use by Amateur Radio operators, have the same general propagation characteristics -- low take off angle for DX. Virtually every Amateur Radio mobile HF antenna is unsuitable for day-to-day tactical communications. They are variously bulky, mechanically weak, won't survive continual whacks from limbs and low overheads, look like Neptune's trident or a misshapen coat rack. They may be fine for hobby communications but not for tactical public safety use. In that type of service we want one, simple antenna that is permanently installed and we don't have to think about or fuss with again.

So how do we achieve NVIS? By getting those sky hooks down near the ground. Let's start with the base station antenna. Horizontal, of course. Dipole or long wire. Place the antenna as low as two feet above the ground but no higher than about thirty feet without a counterpoise. Use an appropriate and sturdy antenna tuner; you will use the one antenna for all frequencies between 2-12 MHz.

A longwire antenna is suitable in field setups but not recommended on office buildings or other urban environments. The reason is that unbalanced antennas frequently create interference problems with telephones and other communications and electronic equipment. These problems are substantially reduced or eliminated with a balanced antenna system.

The antenna tuner of preference is one that is automatic. Such tuners are available now that do not require any control cables; they require only the coaxial transmission line from the transceiver and a 12 volt DC cable. The tuner is placed at the far end of the coaxial cable. There are then two basic options: a longwire or a balanced (dipole) antenna. The longwire can be any length -- the longer the better to approach the lowest operating frequency. A very good ground connection is necessary and often quite difficult to obtain on a rooftop. (When we are talking about running ground connections we mean the shortest possible runs of 2 to 3 inch copper strap -- never wire or braid.) For a balanced antenna, you can place a 4:1 balun on the output of the antenna tuner, thence to a 450 ohm feedline to the dipole antenna. Any NVIS antenna can be enhanced with a ground along the surface that is 5% longer than the antenna and separated by .15 wavelength at the lowest frequency to be used.

For the HF-SSB mobile radio, a sixteen-foot whip is probably the best. Such a whip may be both costly and difficult to find. For NVIS, the antenna is used folded down, both in motion and at rest. That's right, it is not released to go vertical. Most us use the heavy duty ball joint mount, heavy duty spring, and readily available 106 inch whip. To further improve the NVIS propagation at rest, the mobile whip is adjusted to go parallel to the ground and away from the vehicle.

A further enhancement is to remove the whip and run out a longwire 30, 50, 100 feet long. Patricia Gibbons carries orange traffic cones, about 18 inches tall, and notched at the top to lay the antenna wire away from the vehicle.

The Russian military have been using NVIS antennas on their vehicles for quite some time. They appear to be about 4 meters long and about six inches above the top surface of the vehicle. At least one American manufacturer makes an NVIS antenna for both military and civilian vehicles. On a van it looks no more obtrusive than a luggage rack.

The automatic antenna tuner is located in the rear of the vehicle and as close as possible to the mobile antenna feedpoint. An HF-SSB mobile radio was recently installed in one of our State Office of Emergency Services trucks. The installer and the vehicle were 80 airline miles away and the time was about 2 p.m. In the State SECURE (State Emergency Capability Using Radio Effectively) system this calls for using a 7 MHz channel. We established contact; the mobile signal was received here in Sacramento at about S5 to S6. I then asked him to loosen the ball mount, flop the antenna down horizontal and away from the truck. I could tell by the pause and tone of his voice that he thought I had lost it. When he returned to the air his signal jumped to S9. By the same token he thought I had cut in a linear amplifier because of the improvement to my signal. I assured him that the improvement was due solely to his flopping his antenna horizontal.

You need not be concerned over the orientation of an NVIS antenna; it is omnidirectional.

Every Monday night from 7-8 p.m. we conduct a State RACES net on 3545.5 kHz using AMTOR. One night the net was concluded and secured. While the hams were cleaning up one of them noticed that we were being called; there it was on the screen. But it belied the loudspeaker; there were no discernible AMTOR signals -- only a high noise level. Yet, there was that station, WA6UBE, calling us at W6HIR. Yes, it was Patricia Gibbons proving an NVIS point again! She was transmitting to us from 82 miles away with an antenna lying on the ground along her driveway and using 3 watts of power!

On another statewide evening RACES net, our Monday night 8 p.m. 3952 kHz voice net, Bill Pennington (WA6SLA) compared two antennas. One was a vertical and the other was a horizontal quite close to the ground. His observations were interesting and typical of NVIS propagation. Almost all of the signals received on the vertical were higher in voltage than the NVIS antenna but, be that as it may, the signal to noise ratio is superior with the NVIS antenna. The noise floor is measurably lower on the lower antenna, thereby providing better overall communications. I heard more than one Amateur say, after listening to Gibbons' NVIS presentation and subsequent demonstrations, decide to jump back into HF-SSB mobile radio again. These people, like I, are interested primarily in the mobile tactical public safety communication applications.

There is an easy method to improve the NVIS radiation of your dipole antenna. Let the feedpoint sag five to ten feet below the horizontal. This will alter the radiation to improve the vertical angle to achieve an approximate 2 dB improvement at no cost. Many are excited about an old but little understood and practiced means of HF radiation. If you need it, try it. You'll like it. RB152-156

ANTENNA SAFETY
Use extreme care when raising or elevating antennas for the first time. Be sure you can see any overhead or nearby power lines that are within the fall radius of your antenna. Last month a television station technician raised his telescopic 40-ft mobile antenna into unseen overhead powerlines. The resulting short circuit threw him clear and set his van of fire. He regained consciousness, attempted to get the fire extinguisher from the burning van, and was electrocuted. RB 86.

LOG PERIODIC AT OES
A high frequency log periodic antenna was installed at OES headquarters in Sacramento by the efforts of 11 hams, two commercial sky hook crane operators, three Sacramento firefighters with a 150-foot snorkel and 6 State employees. The hams assembled the KLM 40-through-10 meter log periodic on the ground and awaited less windy weather before it was hoisted atop a 75-foot tower. A special thanks to these hams for a job well done: Les Ballinger WA6EQQ, Herb Bennett KA6VHF, Jim Buckman N6HOS, Bob Cloud W6CFQ, Keith Crandall K6QIF, Marion Henson W6NKR, Herb Hildebrand W6UBI, Chris Huber N6ICW, Norm Nelson KA6YRC, and Jim Pratt N6IG. The antenna is connected to a Kenwood 930S HF radio at the State OES Headquarters RACES radio station W6HIR. Sgd/Stan Harter, KH6GBX, State RACES Coordinator 87-5