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| Sometime ago, I set a goal to establish a 900MHz spread spectrum radio link tailored for Ham Radio activity. An interesting concept, but how would it work and what features should be included? I found many differing opinions while "reading the mail" so the ideas presented here are but suggestions. I am for a stand alone voice, video, and data radio, not controlled by a computer, which will be compatible with simple digital technology. The testing on this experimental link follows that theme and has shown that it is technically possible if interest from the Ham community can be encouraged. |
| Butte, Montana is located in a bowl shaped valley, with part of the town situated on "high ground"---as much as 500 feet above the valley floor. The Rocky Mountain Continental Divide is east of town approximately 8 miles with an accessible repeater site atop of the ridge at about 2,000 feet above the valley floor making it a great location for testing low power VHF/UHF equipment. |
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The SS radios used with this test are 1 Watt, DSSS, commercial transceivers manufactured in Belmont, California. The radioa are "textbook" design using discrete parts and the radio could be "homebrewed" with a bit of work and some changes. These changes aren't complicated or difficult. Because the radios are data radios with a voice channel, few modifications are needed for Ham use. However, I designed a CODEC to plug into the data port for total digital operation. |
| The video for this test is slow scan TV using an I-C designed by PMC, a southern California company. The three radio antennas are a 6dBd Sinclair panel and two 10dBi Antenna Specialists Yagis. To simplify the testing operation, I made an interface unit to activate the transmitting station's push-to-talk line to send my call sign, with the TV image, every 10 minutes. At the mobile receiving end, there is a transceiver, a SSTV monitor, a hand held data terminal and a handset (below left). |
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The 6dBd vertically polarized, panel, base station antenna used for the first phase of testing was located indoors on a quad stand and pointing out of a window of the uptown transmit site. A continuing image, data and voice message was sent as the mobile unit moved to various places in the valley, checking for the signal strength, QRM, picture and data quality. (Base setup) |
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| Each of the valley test sites received strong signals which were close to the free space calculations. For example, at 4 miles from the base station, the level was better than 20dB/S9 and the voice transmission came through even with severe interference. Because of the interference, errors were experienced with the data and TV at some locations. No attempt was made at filtering or changing the antenna configurations, which can greatly improve the reception and overcome the interference problems. By increasing the transmitter power to 10 or 20 Watts, there should be no difficulty in communicating throughout the valley. |
| Next, a 900MHz, narrow band test for DX was conducted using a 1 milliwatt f-m transmitter and the 6dBd antenna. It was heard at a surprising 3 miles from the base! Then I added a 10 Watt final amplifier which yielded excellent signals to 9 miles. Both slow scan TV and 9600 Baud data were transmitted during this phase of testing, cutting thru the interference very well. Although this is not SS, a comparison of NB to SS is interesting. It also proves that conventional Ham Radio can work on this "noisy" band. | 
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| Due to the approaching winter, a serious distance test via the mountain top repeater site had to be postponed. Meanwhile, equipment improvements were made and packet DSSS radios were configured for the next try. |
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Summer arrives! With the snow gone it is time for the repeater test. The site is located high atop the Continental Divide at an elevation of over 8.000' asl---about 2,200' above the valley floor. From this location one "can see forever". The goal is to test the SS signals to the ex-smelter city of Anaconda, Montana---west of Butte---a distance of about 25 miles. |
| An Antenna Specialists 10dBi Yagi, pointing west, with a 5 degree down tilt, was installed on the antenna tower at the repeater site. The coaxial cable required but 20' to enter the building offering minimal cable loss. The DSSS packet radio was configured as a store and forward repeater at a 9600 Baud rate. Because these radios have a voice option, it is easy to add a call sign identifier and the SSTV signals to the input. The DB9S data port can also be used as an additional node in a multistationed network. |
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All of the stations in this system are compact and simple. The mountain top repeater consists of a radio, the antenna with misc. hardware, a power supply and the SSTV image generator---including a callsign identifier. The base station, located in town, is essentially the same except for a "quick fox" data generator at the DB9S data port. |
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| After the radios were installed and the system checked, the DX test was on. With the base station transmitting a continuing "quick fox" message, which was forwarded by the repeater, I headed toward Anaconda, Montana. Seven stops were made along the way to check and record the received signals. Only one location---Gregson Hot Springs---was a problem because of the shadow of the surrounding hills. All of the other locations had strong signals very close to the calculated signal strengths. Data, voice and SSTV were "loud and clear". Other than Gregson, the weakest signal was the equivalent of "S8" and that was at the dump site in Anaconda. The smelter site was a full "S" unit higher. Not bad for less than 1 Watt of r-f! Greater distance is possible but there is a range of mountains behind Anaconda and I did not attempt to go farther. |
| Summary: What has all this proved? A low powered SS, digital radio can get good range even in a noisy environment. A simple, inexpensive CODEC accessory can be built to provide voice and low bandwidth video. For the basic half duplex radio, a conventional TNC, packet interface is "plug and play". Repeaters are a "cinch". The mobil unit is no more complicated to install than a typical 2 Mtr rig. The cost of a station can be about the same as other UHF ham gear---there are several Part 15 tranceivers that perform well and, when purchased in reasonable quantities, their cost is near that of high end HT's. SS works! So, why not get started using SS on our under used UHF bands and prevent loosing our spectrum? |
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| Chuck Hartley, W6OOC
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