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by Pete Putman KT2B
902 MHz Linear Transverter
Box 178 New Boston, NH 03070 (603) 547-2213 Price Class: $600
The 33 centimeter band (902-928 MHz) is to many amateurs today what 1296 MHz was 20 years ago: uncharted territory with lots of potential The increased availability of commercially-manufactured equipment for 23 cm has taken away much of its mystery. SSB Electronics of West Germany has been in the forefront of supplying such equipment with their now-famous LT-23S linear transverter. It stood to reason that when 902 MHz became available* they would follow up with a similar unit, the LT-33S. (Photo A.)
Both the LT-23S and LT^33S share many things in common, not the least of which is overall appearance. The same housing has been used with a slightly modified front panel From left to right, rocker switches control LO selection, TX, and Power On. A meter has been included to show relative output power unlike the LT-23S it is illuminated, which fs a nice touch.
Rear panel connections are also similar: BNC input for the 144 MHz IF, BNC input for the 902 MHz receive input, and a type N connector at the output of the PA board. Note that (as on the LT-23S) no T/R switching is included and an antenna relay must be added. SSB also brings + 13.8 VDC out to a separate binding post which activates on receive and drops out on transmit. This scheme, incidentally, protects mast-mounted preamps, though J stilt encourage using a sequencer instead.
Most of the circuitry in the LT-33S derives from the 1296 MHz unit. The major difference is in the final amplifier which incorporates a pair of Phillips ON4284 devices in parallel, as opposed to the LT-23S which uses 2 BLU99s in the same configuration. Amplifier operation is in Class AB1 mode, grounded emitter for a truly linear signal. What goes in comes out, whether it be SSBh CW, AMt or FM.
The final amp now uses ON4284 devices because the BLU99s kept failing at 902 during high VSWR stress tests, Conversely, the ON4284 does not have significantly more gain than the BLU99 at 1296; hence the two different types of finals in the two transverters. The good news is that the 902 final configuration
Photo A. SSB Electronics LT-33S 902 MHz transverter.
produces over 20 watts output saturated, which is a good amount of drive when using an outboard lube amplifier. It's also plenty of power for QRP work as well.
The front end device is an active RF amplifier using the time-honored Mitsubishi MGF1302, rated at about 1,3 dB noise figure. Early models of the LT-33S ran only 6 watts output and a lesser-quality GaAsFET was selected for the front end. The consequences of this were low gain and poor compression performance! The MGF1302 works much better in this regard, making the unit slightly more of a "bunny rabbit" than an "alligator1' (more ears than mouth).
As on the LT-23S, a 144 MHz !F is the standard configuration, although you can specialorder 28 MHz IF frequencies. The use of a 144 MHz IF allows for better filtering of the LO signal. With a 28 MHz IF, it would fall at 874 MHz and be considerably more difficult to filter out than if a 2 meter IF was used. In this case, the LO would be at 758 MHz and is easily trapped out. On-board resistors allow drive with up to 12 watts to interface with the popular multtmode radios, most of which run 10 watts or have adjustable power output.
Practice shows that a lower drive level results in more linear operation. Typically, 1 to 2 watts drives the transmit mixer and the output is clean and stable. An adjustment for drive is available near the power resistors and it should be set just below the point at which the output saturates—typically in excess of 20 watts.
I used the LT-33S extensively during the ARRL January VHF Sweepstakes with a Down East Microwave 33 element loop yagi at about 45 feet. The feedline was 9913 (what else?} and no external power amplifier was used. It is certainly a challenge to work DX on a band where activity levels are low and most contacts are made with schedules. It's even more of a challenge with 20 wattsT but the LT-33S came through with flying colors.
From my location in FN20, central Bucks County, 22 contacts were made in 6 different grid squares Several were long-haul to FN42 (W1RJL) and FN32 (WA1MBA). There were many CW schedules and the LT-33S heard them all after some jockeying of the rotor box. No external preamp was used, and I'm not sure one is needed with the stock setup. Should an external amplifier be used, however, it might be worth considering.
One problem (if it could be called that) was extensive warbling of the signal (also called FMing), due to LO instability. I attribute that instability, however, to poor voltage regulation caused by too much of a voltage drop in the DC power leads from an Astron RS-7 suppfy. This was confirmed in on-the-air tests with K2SMN and WB2WIK, so the power leads were cut to 2 feet. The problem completely disappeared! Another cure for this condition on the LT-23S has been to re-route the coax to the final amplifier with a pairof 90 degree BNC connectors around the LO crystal, (I'm not sure why that latter problem should have existed in the first place; but the fix works 100% )
The LT-33S was also used on the ARRL 903 Spring Sprint, Its small size and ease of switching are well suited to portable operation and grid-hopping. With a storage battery as the power source, it would be an excellent idea to disconnect the lamp from the power output meter to save on current drain. Such radios as the Yaesu FT-290R are Ideally suited for portable IF stages. By using a coax switch and a small 2 meter beam, schedules can be quickly coordinated and completed,
SSB Electronics also makes a 902 transverter kitT using the UEK-3 and USM-3 modules (RX Mixer and TX Mixer). This combination uses a great deal of the circuitry from the LT-33S and the output stage is a BFQ34 running about 5 watts. The advantage of this scheme is that it allows customization of a transverter housing and antenna/DC switching.
The SSB Electronics LT-33S is a well-designed and engineered linear transverter for all-mode operation in the range 902-906 MHz The front end exhibits excellent sensitivity and the power output is more than adequate for external amplifiers or straight-through operation. It is ideally suited for portable and/or contest operation, which should encourage more 902 MHz grid-hopping!EI
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