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Microwave Propagation in the Upper Troposphere

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Microwave Propagation in the Upper Troposphere Microwave Propagation in the Upper Troposphere Amateur microwave work need not be restricted to operation from hilltop locations. Let’s explore the use of scattering in the upper regions of the troposphere, which offers the possibility of communications over highly obstructed paths. By Bob Larkin, W7PUA; Larry Liljequist, W7SZ, and Ernest P. Manly, W7LHL he selection of propagation ter from raindrops can be effective and This scattering is strongest in the modes for amateur microwave many stations have used this propa- lower portions of the troposphere, Tstations is often progressive. gation mode.2 but can occur throughout the region. Most of us start with simple equip- Better-equipped stations are able to The troposphere starts at the ment and the contacts are usually by work others beyond their horizon by Earth’s surface and includes all por- line-of-sight propagation. Longer dis- use of the propagation mode called tro- tions of the atmosphere where convec- tances are then accomplished by go- pospheric scattering. This mode occurs tion caused by changes in the air ing to high locations. As equipment to some degree at all times. It involves temperature is a major effect. This is reflection in multiple directions (scat- gets better, the use of various objects the region of weather production. The tering) of the microwave signal using for reflections and scattering becomes top of this layer varies between about a possibility. Hills, mountains, water irregularities in the material of the 6 and 15 km (20,000 to 50,000 feet), tanks are attractive, since they are troposphere. Most definitions of generally available at any time. Air- this mode do not make a distinction depending on the region of the world craft are also attractive reflectors of the particular material. This allows and the local weather conditions. The when they are at high altitudes.1 Scat- inclusion of variations in the water best opportunity for tropospheric scat- content, water or ice particles, dust, ter is generally in the lower portion, 1Notes appear on page 11. insects or possibly other materials. where the air density is greatest; how- ever, because of the Earth’s curvature Bob Larkin, W7PUA Ernest P. Manly, W7LHL and local terrain blockage, this region 2982 NW Acacia Pl PO Box 1307 may not be available for propagation. Corvallis, OR 97330 Graham, WA 98338-1307 To extend our range of contacts, we [email protected] [email protected] must use the upper portions of the tro- posphere. In a sense, we are doing the Larry Liljequist, W7SZ reverse of going to the mountaintops 2804 SE 347th Ave to work over the horizon. We stay at Washougal, WA 98671 home and scatter the signals from the [email protected] material high in the troposphere. This Jul/Aug 2003 3 Fig 1—Map of the station locations in Washington and Oregon. Fig 2—Close-in hills at each end obstruct the path between The Cascade Mountains extend north and south through the area W7LHL and W7SZ, setting minimum usable elevation angles of and portions of these are directly north of W7SZ. 9.5° and 4°, as shown. This plot has very different scales for the height above ground and the horizontal distance. This makes the small angles look very large and distorts the scaling between the angles. The commonly viewable volume is at a height of about 8.5 article will discuss our measurements km (28,000 feet). The beamwidths are only about 0.7°, making the of tropospheric scattering from loca- common volume about 1.5 km (5000 feet) in height. The total path tions where the terrain restricts the length is 159 km (99 statute miles). path to the regions above about 4.5 km (15,000 feet). All modes that can scatter or re- flect from high regions are attrac- tive for amateur microwave opera- tion since they can be exploited from home locations. In many parts of the country, this can add six months or more to the microwave season! It also allows many more hours of operation than is practical using portable equipment. This attracted us to this type of operation, as the terrain in the Pacific Northwest is mountainous and thus far from line- of-sight as well as inaccessible for much of the year. We began trying to make contacts between home stations on 10 GHz over some very mountainous paths. This was surprisingly successful, and most often, the propagation mode was de- termined not to be from aircraft or rain scatter. After finding that microwave contacts were possible, we started Fig 3—Cross section of the terrain between W7LHL and W7SZ. It is far from a direct path. making measurements of the signal The tallest peak, shown at about 100 km from the W7LHL end, is the side of Mt St Helens. strengths and signal spectra, trying to The upper trace shows the effect of the Earth’s curvature, including a 4/3 correction for correlate this to the weather condi- refraction. The “Flat-Earth” trace is about 400 meters lower in the center of the path. (Plot by KB1VC; users.rcn.com/acreilly/los_form.html) tions. We also extended the measure- ments to 1296 MHz. We found almost certain indications that the signals Table 1—Path Summary were being scattered by material in the upper troposphere. The nature of Stations Distance Take-off Take-off Common-Volume the material is still uncertain. Neither 1 - 2 (km) Angle 1 (°) Angle 2 (°) Height (km/kft) do we know how well our results W7LHL-W7SZ 159 9.5 4 8.6/28 would apply to other parts of the KD7TS-W7SZ 198 1 4 4.1/13 world. This report is to let other op- W7SZ-W7PUA 138 4 3 4.5/15 erators know about our observations 4 Jul/Aug 2003 Fig 4—The 10-foot parabolic-dish antenna used for both 1296 Fig 5—This photo shows W7LHL’s 10-GHz equipment that is MHz and 10 GHz by W7LHL. This is a surplus TVRO antenna. The mounted at the feed for the parabolic dish shown in Fig 4. A RF equipment is mounted at the center feedpoint. DB6NT transverter, along with an MKU-101B receive preamplifier (0.8-dB noise figure), a driver amplifier and an MKU-102XL output amplifier (all from Kuhne Electronic) are at the feed. A waveguide Table 2—Equipment Used TR switch minimizes losses at the feed horn. A frequency- reference signal comes from the shack on a coaxial cable to Station Dish (m) Power (W) phase-lock the crystal oscillator in the transverter. KD7TS 1.0 1 W7LHL 3.0 5 W7SZ 3.0 10 W7PUA 1.2 9 Fig 6—W7SZ with his and encourage further exploration of dish antenna used on microwave paths. 10 GHz. The micro- Tropospheric-scatter propagation wave equipment is modes have utility because they may mounted at the focus of the antenna. be available when modes like airplane Equipment for reflections, mountain-bounce and rain 10 GHz is similar to scatter are not. In addition, the asso- that used by W7LHL ciated Doppler shift is considerably except that the less than that from either airplane transmitter power reflections or rain scatter, allowing the amplifier uses a TWT (traveling-wave tube). efficient use of narrow-band modula- tion, such as CW or some computer- ized modes such as JT44 or PUA43. The Propagation Paths The work reported here was be- tween home locations as shown in explored to varying degrees, but the locked to the GPS system using W5OJM Fig 1, a general map of the area. The most intensive data collection was for controllers.3 The IF radios used were geometry of the path between W7LHL the path from W7LHL to W7SZ. This DSP-10s that were again phase-locked and W7SZ is shown in Fig 2. The dis- path is interesting since not only are to the frequency standards.4 This sys- tances and take-off angles are in Table the take-off angles poor but almost in tem allows accuracy and resolution to 1. The take-off angle is the lowest the middle is Mt St Helens, with a a few hertz, even at an operating fre- angle that the station can view clear post-eruption height of over 8500 feet. quency of 10 GHz. In turn, it is possible sky, measured from the local horizon. W7LHL and W7SZ are at 650 and 750 to make accurate spectrum measure- This angle is in all cases determined feet, respectively. ments, which imply the movement of by local obstructions, within a few ki- scattering material.5 lometers of the station. The common Equipment Used The only 1296-MHz measurements volume height (lowest altitude that is Table 2 summarizes the equipment described are for the W7LHL/W7SZ mutually visible between the two sta- used on 10 GHz, portions of which can path. On that band, W7SZ used 20 W tions) is set by the take-off angles and be seen in Figs 4, 5 and 6. and a 3.7-meter (12-foot) parabolic the Earth’s curvature. All contacts and measurements de- dish while W7LHL used 40 W and a The heights shown do not include scribed in this article used equipment 3-meter (10-foot) dish. The 1296-MHz atmospheric refraction; this has only capable of very accurate measurements transverters were again phase-locked a small effect. Fig 3 shows more de- of the frequency spectrum. The conver- to the station frequency references. tail of the mountainous path between sion oscillators in the transverters were All stations had sensitive receivers.
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