In my 24+ years of working with amateur repeater stations, it has become obvious to me that tube-type transmitters were easier to make duplex. The first 2 meter repeater I helped build was for the local amateur radio club and the radio equipment used was GE Mastr Pro. We had a new Sinclair Hybrid-Ring duplexer. This duplexer worked fine on the tube Mastr Pro, however when I built a brand-new Hamtronics repeater some years later, I had trouble. The transmitter output power was the same, the only difference was the all-solid-state exciter and Amplifier.
The 'problem' was not the Hamtronics repeater; one of the cans of the duplexer was not tuned properly (wasn't even close) and this prevented the solid-state repeater from working, even though the tube-type repeater worked fine. BUT WHY?
One reason was receiver sensitivity. The Hamtronics receiver was quite a bit more sensitive (8 or 9 db) so the duplexer had to make up for this. Also, solid-state amplifier stages that are not tuned are therefore broadband, amplifying everything within a reasonable bandwidth the same amount. The stages in the Hamtronics exciter are tuned but the 2 to 15 watt amp is somewhat broadband.
Tube-type amplifiers use tuned input and output stages that are "High-Q" meaning their operating bandwidth is VERY limited without re-tuning. This is a definite plus in repeater service because they amplify the repeater's desired signal but not the exciter side-band noise. Since most repeaters don't QSY often, tuning is not a problem.
What am I getting at?
It has been my experience, on 2 meters with a 600 kHz. transmit to receive split, that you can run up to 100 watts of transistor power using a good duplexer and a single duplex antenna with good results. Over 100 watts of transistor power, with even the best duplexer and a single antenna, won't give enough isolation for NO desense. On the contrary, tube power can exceed several hundred watts with good results.
On my 145.270 KQ3M repeater in Hays Mill PA we run a GE Mastr Pro 4CX250R tube-type amplifier model 4EF5A1 capable of 330 watts. This amplifier has been driven with solid-state Hamtronics and Motorola exciters to this power level with a 4-can duplexer (93 dB isolation) into a single duplex antenna with NO desense. The receive sensitivity on this repeater is under 0.125uV for 12 db SINAD (yes, receiver sensitivity plays a part in desense). I have tried to run transistor PAs at one-half of the tube level and, all else being the same, with unacceptable results. Unacceptable to me is more than a dB or two of receiver desense. This repeater normally runs 250 watts into the duplexer; about 175 out (-1.5db) with several dB of reserve isolation.
Always strive for NO desense if at all possible. The use of a simple toggle switch in the PTT line, and listening to the receiver on a local speaker, is the only good way to test for desense. Use a weak signal leaking into the Antenna System, not by inserting it into the receiver or duplexer line with a "tee" connector or some other unacceptable manner, (everyone else has to go through the antenna). Don't be afraid of tweaking the last drop from the equipment, especially the duplexer. Duplexers that are "factory tuned" are done so with laboratory test equipment with near perfect 50 ohm terminal impedances. Is your antenna, transmitter and receiver a perfect 50 ohms? I doubt it. Terminal impedance affects duplexer tuning. This is no big deal in tube type equipment because you can tune out reactances with transmitter output and receiver input tuning controls, but with no-tune transistor equipment you are stuck with whatever the terminal impedance is, and therefore the reactances must be tuned out by duplexer tuning, or interconnecting cable lengths.
Most quality repeater antenna systems have no tuning, so here again is good reason to tweak the duplexer. After all, the duplexer has to satisfy your transmitter and receiver, not the manufacturer's test equipment.
Dec 19 1996 Kevin K. Custer W3KKC
Updated June 2003 due to aging text (first line).
Last updated: 16-February-2007.