A New First for 47GHz - VUCC #1 and 114km N.A. Distance Record
by: Brian, WA1ZMS
The North American distance record for the 47GHz amateur band has been broken, if by only by a short distance. Brian Justin, WA1ZMS/4 and Doug Sharp K2AD/4 set the new record on April 5th, 1998 at 22:05z. Brian was located just off the Blue Ridge Parkway at Apple Orchard Mtn. in Bedford, VA while Doug (with the help of Geep, WA4RTS) was located on Leigh Mtn just outside Farmville, VA.
The QSO was made using wide band FM over a distance of slightly more than 107km on a line of site path. Signals on both ends were initially 559 with modulated CW being used to exchange callsigns, grid squares, and signal reports. The dish antennas on both ends were then peaked for maximum signal and then the QSO was continued using FM voice for over 30 minutes. The former tropo record of 105km, held by WA3RMX/7 and K7AUO/7, had remained steadfast since 1988.
Both stations are of the home-brew variety using both surplus commercial parts and home made sub-assemblies. The heart of both stations are 100mW Gunn oscillators that were found at the Dayton Hamfest in 1996. The Gunn sources were retuned to just inside the ham band and spaced 70MHz from each other to allow for a 70MHz IF to be used in a Gunnplexer style configuration. The Gunnplexer mode of operation allows for full duplex voice to be used on each end.
Brian's station used a 2 foot dish, while Doug's used a 1 footer. Both dishes used a splash or "penny" feed based on the HB9MIN design. Each feed was made out of a piece WR-22 waveguide which also incorporates a three screw tuner to match the feed and allow adjustment of the receiver mixer current. Located between the Gunn source and the dish feed of each station is a waveguide circulator with a diode detector mount attached to the third port of the circulator.
The output of the detector is feed to a home-brew 70MHz IF amplifier. The amplifier provides the proper RF and DC impedance matches for the diode. It also acts as a convenient location to monitor the DC diode current. This current is controlled by the amount of Gunn oscillator power reflected from the feed. This reflected power acts as the receiver local oscillator and mixes with the incoming signal to produce a signal at the IF frequency. If the station's Gunn sources are 70MHz apart in frequency, then that becomes the IF frequency. This IF is tuned and demodulated using an ICOM R-7000 receiver at each end.
Both stations are modulated using power supply 'pushing'. This is achieved by using an adjustable voltage regulator, such as an LM317, as the Gunn's voltage source and applying microphone audio to the 'adjust' pin of the regulator. Only a few microvolts of power supply variation is needed to result in several kilohertz of deviation due to the high tuning sensitivity of the Gunn diode. If the Gunn sources had varactor diodes mounted inside the waveguide cavity, the varactor could be modulated with a separate supply voltage. However, these sources used did not come with such a convenient option and so the alternative method of modulation was adopted. If the microphone of each station is substituted with an audio oscillator, then F2 or modulated CW can be used. If the audio tone is keyed continuously, it can be used as a tuning aid and makes the signal sometimes easier to find than an unmodulated carrier.
The Gunn sources are very sensitive to temperature changes and will drift several megahertz in just a few minutes. This fact drove the station design to use a tunable IF. Once both stations are powered up and pointed at each other (see below) the IF receivers are tuned until the signal is found. If one station finds the IF frequency before the other, that frequency can be given to the other station via a liaison channel. Both stations can then tune and track the IF as it drifts due to the free running Gunn oscillators.
To aid in the tuning of the stations on long haul paths where the signals are likely to be weak, each station has with it an X-band phase locked crystal source, good to within 1ppm, and a waveguide diode multiplier. The fourth harmonic of the 11.775GHz signal falls within the ham band and can be tuned in on the IF receiver to help determine the frequency of that station. Once the frequency of each station is known, the difference between the two will be the QSO IF. Although the stations will still drift in frequency, much of the tuning ambiguity can be eliminated using this frequency marker approach.
Before any QSO can take place on such a high frequency, the issue of antenna pointing must be addressed. The 3dB beamwidth of a 1 foot dish at 47GHz is on the order of 1 degree in both azimuth and elevation. Both stations must point their antennas to within that tolerance or the QSO most likely won't happen. Each dish is equipped with a rifle scope that was sighted in on a short line-of-site path. Since the promise of an optical path over distances greater than 80km cannot be made, other methods of dish pointing must be used to ensure the correct pointing angle.
Maps and GPS receivers can be used to determine the exact location of each station. Simple computer programs can then be used to determine the true heading between stations. These true headings can be converted to magnetic headings using the current magnetic declination angle for the location of each station. A surveyor's compass can then be used to point the dish to the correct heading.
If the heading to a known object in the foreground of each station can be calculated, then that object can be used as a pointing reference. The heading to the other station is just an angular offset from that object. Using this approach, pointing angles accurate to less than one half a degree can be achieved and this is sufficient for the beamwidth of these stations.
Contacts, A New First, and Another Record
Not being satisfied with just holding a new record that broke the old one by a mere 2km, our sites were set on obtaining the very first ARRL VUCC award for this band. It is unknown if anyone else has made such an attempt, but we felt we could give it a try since we had over 15dB of signal margin on the Apple Orchard to Leigh Mountain path.
Since the combination of oxygen and water absorption on this band have a great effect on the path loss, the weather must be watched for upcoming good 'millimeter-wave days'. The loss due to oxygen is close to .15dB per mile and remains constant except for very high altitudes. The loss due to water is a function of the absolute humidity on any given day. Colder air can hold less water than warm air. Therefore, the colder the day, the less the water losses are on a given path.
In early May of 1998, an attempt was made to try to work the required 5 grid squares from one fixed location. The callsign of W2SZ/4 was used for the fixed station while WA1ZMS/4 was used by the rover station. Since our club, W2SZ, holds other VUCC firsts on 3, 5, and 24 GHz we felt it would be fitting to have the callsign be given VUCC #1 on 47GHz as well. Many folks within the club have helped out by locating parts and test equipment.
It was decided that the summit of Apple Orchard Mountain, due to its height, would be the best location for the fixed station, while the rover station would drive to 5 grid squares and QSO with the fixed station. Prior to the drive to any of the grids, Doug was given possible sites within the grid that would yield a line of site path to the summit of Apple Orchard without other hills blocking the path. But at the distances involved, we were not guaranteed an optical path due to haze and fog. Doug drove to each of the suspect hill tops only to find in most cases that trees and other local obstructions were in the way. Careful observation on his part allowed him to find other hill tops and scenic vistas that could be used. The location inside a particular grid was determined by the use of a GPS receiver. Once Doug's location was radioed to me, I calculated the pointing angles for both stations and he then verified that his location was not blocked by foreground clutter. At this point the stations were set up and the frequency of each verified by the crystal sources. We found that in less than a few minutes, signals were heard on both ends and the QSO was made. This was repeated 4 other times from 4 other grid squares. The fixed station was located in grid FM07fm, while QSOs were made with the following grids: EM97, FM07, FM06, EM96, and FM08. The path distances for each QSO were: 68km, 59km, 60km, 114km and 60km respectively. Observant readers will note that the QSO to EM96 broke our own record and set a new North American distance record. It is interesting to note that the QSO was made in fog and visibility from Apple Orchard was only about 8km.
This QSO demonstrates the fact that millimeter wave contacts are possible over non-optical line of site paths. No attempts have been made yet over physically blocked paths but that is best saved for next winter when the humidity is low.
The next task is to see if it is possible to injection lock the Gunn sources and reduce the receiver bandwidth to yield better, more 'sensitive' stations. The current world record is 184km held by HB9MIN and DK4GD using SSB. Based on our measured margins, it is POSSIBLE to challenge that record, but a very cold dry day and a long line of site path would be needed. This means that our work is not over yet.
My next attempt will be on 76GHz, but I'm still in the parts collection stage. I don't expect anything noteworthy to happen since it took two years of collecting 47GHz parts to make VUCC #1 and two new North American distance records.