The Sea To Summit Bike Ride this year provided our ARES/RACES team with an opportunity to test Near Vertical Incident Skywave (NVIS) HF radio communications in mountainous terrain. NVIS provides an alternate means to "line of sight" VHF (2 meter, 147 MHz) and UHF (70cm, 446 MHz) communications, particularly in mountains regions when VHF and UHF repeaters are not available to relay signals.

NVIS operates in the HF 75 meter, 3.9 MHz, and 40 meter, 7.2 MHz, frequency bands. NVIS signals are transmitted straight up into the Earth's atmosphere, reflected by the Ionosphere about 175 miles above the Earth's surface, and returned to Earth within a relatively short distance from the transmitting station. A short distance means anywhere in a radius of about 15 to 200 miles. For distances shorter than about 15 miles in mountainous terrain, the HF radio signals tend to propagate from the sender to the receiver via various modes other than NVIS, including line of sight, ground wave, and ground scatter, as we discovered during this test. By reflecting radio signals off the Ionosphere NVIS has the desirable feature of being able to transmit radio signals from one side of a mountain range to the other side. By using NVIS, one can also transmit radio signals out of deep canyons from which all other modes of communications, including VHF, UHF, and cell phone communications, are impossible. At least that is the theory.

The purpose of this test was to determine if NVIS would indeed provide us with reliable radio communications in mountainous terrain where cell phone, VHF, and UHF communications is not available. If it did, the advantages would be enormous. NVIS could be used by an Emergency Operations Center (EOC), for example at the Ventura County Government Center or East County Sheriff's Station, to communicate with Search and Rescue teams or other emergency personnel in remote areas of the Ventura County Mountains.

We already have good VHF and UHF radio communications for the Sea To Summit Bike Ride which has been perfected over many years of supporting this event. One of the key elements of this communications is a cross band repeater that is set up each year on Reyes Peak in the Pine Mountain area. The repeater relays signals from the first part of the bike ride, from Ojai and the canyon areas along Hwy 33, to net control at Mt. Pinos. So for the bike ride, NVIS would serve as a supplement to our existing communications.

For our HF NVIS test we had radio operators located along the bike ride route at Check Point 2 (Hwy 33 at Rose Valley turn off), Rest Stop B (Hwy 33 at Wolf's Grill), Check Point 3 (Hwy 33 at Pine Mountain turn off), Rest Stop C (Lockwood Valley Rd at Doma Springs turn off), Mt. Pinos, and Reyes Peak (at the end of the Pine Mountain road). We also had a HF mobile radio in the Rover 5 vehicle. In addition to these locations, we had one HF NVIS station located near the Ventura County Government Center operating with the tactical call of Ventura EOC, and a station located at Lake Piru.

HF did turned out to be a good supplement to our Sea To Summit VHF/UHF repeater system as Jose, N6VUY, in Rover 5 demonstrated. Jose's signal from his vehicle's 40 meter vertical antenna was received well on Reyes Peak as he drove the winding route through the canyons along Hwy 33 from Wheeler Gorge to Check Point 3.

From the HF data that we collected on both 40 and 75 meters, it is apparent that we were utilizing multiple HF propagation modes, including:
Line of sight,
Ground wave,
Ground scatter, and
NVIS.

I did not anticipate the significants of the non-NVIS propagation modes.

An important bit of communications theory relevant to NVIS radio signal propagation is the Ionosphere Critical Frequency. Critical Frequency is the maximum radio frequency that can be transmitted straight up and reflected back to Earth by the Ionosphere. If we transmit at a frequency higher than the Critical Frequency, our transmitted signal will pass through the Ionosphere to outer space without being reflected back to Earth. Thus for NVIS to work, we must transmit at a frequency equal to or less then the Critical Frequency. The Critical Frequency is constantly changing. Usually it is highest at noon when we receive the most ultraviolate energy from the Sun and at its lowest levels at night when we receive no energy from the Sun.

The Critical Frequency during the bike ride ranged from about 4.8 MHz to 6.6 MHz. Based on these values for Critical Frequency we should not have been able to communicate on our 40 meter frequency of 7.233 MHz using NVIS. Our 40 meter signals should have passed through the Ionosphere to outer space never to return. On the other hand, NVIS communications on our 75 meter frequency of 3.992 MHz should have been excellent.

However, we had good 40 meter communication between Mt. Pinos, Reyes Peak, Rest Stop C, Check Point 3, Rest Stop B, and Check Point 2. This result indicates that the primary HF propagation modes for communications between these locations must have been via line of sight, ground scatter (our HF signals bouncing off the canyon walls), and perhaps ground wave, since NVIS communications should not have been possible on 40 meters for the Critical Frequencies that we were experiencing during the bike ride. Our HF communications between these points seemed to have nothing to do with the Ionosphere or NVIS communications. Jose in Rover 5 confirmed this result. His 40 meter vertical mobile antenna should have had essentially zero vertical (NVIS) radiation, yet I received Jose very well at Reyes Peak as did others.

This is a good result. It means that we can use HF to supplement VHF/UHF repeaters in very localized areas within mountainous terrain without the need to erect large 75 meter NVIS antennas. It would appear that HF mobile rigs will work out very well is this type of situation. In fact, for line of sight, ground wave, and ground scatter, mobile vertical antennas should be superior to our large NVIS antennas. We need to test out this mobile HF possibility further.

Communications with Lake Piru was a completely different situation. The distances and the severity of the mountain terrain between Bruce (K6ZB) and Steve (WA1ESF) at Lake Piru and the rest of us precluded the use of line of sight, ground wave, and ground scatter as viable HF propagation modes. The only mode left for communicating with Lake Piru was NVIS.

The Ionospheric conditions during our test were very bad. As mentioned earlier, the Critical Frequency was in the 5 MHz range (4.8 to about 5.6 MHz) from 8 AM through noon, the solar x-ray flux was very low (which was actually good for us), and the 10.7 cm solar flux was at 76 (about as low as it ever gets). We are truly at the minimum in the 11 year solar sun spot cycle.

Under these conditions we should have had good NVIS communications with Lake Piru on 75 meters and no communications on 40 meters. That is what we observed. Our 75 meter frequency of 3.992 MHz was considerably below the Critical Frequency indicating that we should have good NVIS communications on that frequency, and we did. Our 40 meter frequency of 7.233 MHz was substantially above the Critical Frequency indicating that we should have no NVIS communications on that band, and that was pretty much what we saw.

While communications with Lake Piru was unusable on 40 meters, most stations could detect the presence of Lake Piru's signal and even detect a word or two occasionally. How do we account for that when we should not have heard Lake Piru at all? One possible explanation is back scatter. Lake Piru's vertical incident 40 meter signal should have been passing through the Ionosphere to outer space without reflection. However, Lake Piru's 40 meter antenna was also radiating energy at lower angles in addition to its straight up 90 degree radiation. The lower angle radiation would have been refracted by the Ionosphere and returned to Earth in central California. Sacramento was probably receiving Lake Piru very well on 40 meters. We see this happen often during the Wednesday morning 10 AM California Emergency Services Net on 7.230 MHz. A small amount of the signal arriving in central California, particularly that reflecting off the Sierra mountains, will be scattered back this direction, taking a second hop through the Ionosphere and arriving at our receivers along the bike ride route. Under the Ionosphere conditions that we were encountering, this back scatter signal would have been very weak. While there are many complex HF propagation modes, back scatter is one of them and perhaps what we were observing with Lake Piru on 40 meters.

The good news is that the 75 meter NVIS communications did work in a situation where that was the only means of communications available.

From a practical perspective, we proved that the 75 meter NVIS communications mode is a reliable means of communications throughout the mountainous regions of ARES/RACES Area 5 and Area 7, even when Ionospheric conditions are very bad. That is important! All of us with our portable NVIS 75 meter antennas have just provided Ventura County ARES/RACES with another tool to use in the ever expanding set of services that we offer to OES and our other ARES/RACES customers.

It was an excellent and valuable experiment!

Ken KJ6RZ Area 2 EC

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Editor: Neil Waybright