Spring 2008 Issue
Building a 10-GHz FM
Our 24-inch offset dish and Bob Platt’s Ku-band LNBF at the receiving end of a 17-mile path.
In the Winter 2007 issue of CQ VHF in the article “Microwave TV, A New Approach!” I described the work we are doing here in central Pennsylvania using the 1280- and 3480-MHz bands. We have obtained very high-quality television pictures using FM for both the video carrier and the audio sub-carrier. Not very long after we went on the air with this system in 2002, the desire began to emerge to design and create a very high-quality linking system to cover a 17-mile path between two sites. The basic requirements for this system developed as follows:
• Super reliable 24/7—that is to say, never off the air (OK, almost never, hi!).
• The ability to withstand rain, snow, sleet, hail, heat, cold, and wind, all common in central Pennsylvania.
• The ability to send very high-quality FM video and
FM audio sub-carrier signals (commercial quality) over this path with no
apparent degradation of video or audio.
This is the simplest part and uses a Bob Platts, G8OZP, 1 Ku-band LNBF with an LO of 9.0 GHz mounted on a 24-inch/60-cm offset dish made for satellite TV reception. This dish has a gain in the range of 31.5–33 dBd. The IF is 1400 MHz for an input of 10.400 GHz to a surplus analog “headend” cable-TV-system satellite receiver. In our case, we have used the Scientific-Atlanta 9660, Blonder-Tongue, Holland, and PICO receivers. We know that the IF is right to use U.S. analog satellite receivers as well. There are no doubt other equally fine receivers, and these are just the sub-set of our experience.
Of these receivers, the Scientific- Atlanta Model
9660 has given us the best audio and video signals due to the ability to
adjust the audio de-emphasis to zero. This is an option not always
present in satellite receivers. The others will work, but their audio
quality will be reduced. The audio and video signals are taken from the
satellite receiver for viewing and/or retransmission, recording, etc.
This set of requirements started a search for what was available in the marketplace to meet our needs. We initially considered Gunnplexers, as Joe, WA3PTV, and I had exchanged full-color ATV pictures at 51 miles using about 250 mw at one end and 5–10 mw on the other end using 2-foot dishes. While the pictures were P5 (the best), we had no extra signal to combat fading or bad weather. We also had no 10-GHz audio, as we used 2 meters for the audio channel. We soon realized that both power and frequency stability above the Gunnplexer level were needed, plus an audio sub-carrier. That also suggested that using a Gunnplexer with a linear amplifier of 500 mw to 1 watt output was not viable due to cost and the drift problems inherent in Gunnplexers.
Next we looked at the Kuhne Electronics ATV
transmitters of 250 mw and 1 watt output, which use DRO oscillators and
require a baseband video/audio unit for use on the air. Although I
ordered the transmitter on a frequency of 10.400 GHz, it is adjustable
±50 MHz. The modulation is FM and the deviation can go to 10.75 MHz. The
1-watt model features a diode for monitoring the power output where 1
watt out equals 1.4 VDC.
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