Preamplifiers

As you are reading this, no doubt Sania Claus has come and gone and hopefully left you with some nice VHF/UHF goodies! Maybe it was a new beam a new amplifier , a multimode. perhaps or a transverter . ? One thing is for certain; There's no shortage of equipment today for the VHFOJHF enthusiast, To my count, right now there are more than 20 companies lhat come to mind that make a broad range of equipment—from exciters, amplifiers, and receivers to antennas, converters, and trans-verters. And this doesn't include the basement operations that show up at flea markets selling home-brew preamps, antennas, and the like.

What's the most popular piece of VHF equipment? My guess is that it would be a preamplifier. ! have yet to meet a ham of any type who feels his or her radio "hears" well enough, especially at 6 meters and above, Which brings me to thts month's topic: preamplifiers and how to determine if ihey are working as they should.

The number most of us are concerned with when discussing preamplifiers is gain, expressed in decibels (dB). Many preamplifiers are sold strictly on gain figures with no consideration for any other parameter—yet those other parameters are just as important (if not more important). Such things as MDS, IMD, and 1-dB COMP need to be considered as well. "Hey, what were those abbreviations? L I hear you asking. Let s go through them one at a time.

MDS stands tor minimum discernible signal. This is the absolute lowest level signal that can be detected by the preamplifier and is usually measured with a signal generator driving the preamplifier into a spectrum analyzer. usuaMy in a very narrow bandwidth (say 1 kHz). The limitations here are the atmospheric noise {more of a limitation at 220 MHz and down)t the device noise (more of a limitation at 432 and up)T and the gatn of the device used in the preamplifier Typicaf MDS readings might be as tow as -120 dBm for a well-de signed MOSFET preamplifier and -130 dBm for a GaAsFET device.

Another parameter that goes hand in hand with MDS is dynamic range, the range over which the preamplifier is linear. How is this determined? First, we need to determine the 1 -dB compression point (1-dB COMP) as mentioned earlier. This is the point at which the preamplifier's gain figure drops by 1 dB for a given signal input level. Usually, ihis level will be very strong—on the order of -20 dBm or better. When a given input signal can no longer be amplified by the specified gain figure, we say the preamplifier is compressing at that point—hence, the term 1-dB compression point.

For a welj-designed preamplifier, the 1-dB compression point should be in excess of 0 dBm. Some preamplifiers Ive measured have been as high as +7 dBm (outstanding!) and as low as -6 dBm (mer -ere). What does it mean to you as a preamplifier user? Well, consider that most preamplifiers available today have enormous gain bandwidths. At 432 MHz, a gain bandwidth of better than 10 dB over more than 20 MHz is not unusual A preamplifier rated at 20 dB at 432 MHz could easily have 18 or better dB of gain at 440 MHz.

If you live next to or near a strong repeater at lhat frequency, your preamp wilt amplify that un-desired signal to the tune of i8dB, But if that signal is already better than -20 dBm to begin with (a very strong signal, indeed) and your 1-dB COMP point is only -4.5 dB, your preamplifier will Start to compress and become nonlinear. And we all know what that means—mtermoduialon distortion (IMD) products are created on the signal you wish to hearr creating ail kinds of signaEs and garbage on that weak signal. Horrors!

If you live in a high rf density area (such as a major metropolitan area) or near a hilltop with multiple radio services (such as repeaters, TV or FM stations, and public-service links), you could be asking for trouble by using that super-duper gain preamp to the point that you1d be better off without it. No question about it, you need a preamplifier with a wide dynamic range figure!

Let's get back to that preamp with 20 dB of gain at 430 00 MHz. The MDS is 125 dBm. not bad. The 1-dB compression point is only -2 dBm, not so good. That means the dynamic range is only 103 dBm, which, although adequate, can be improved.

Now, let s look at a typical GaAsFET preamplifier running only 12 dB of gain, The MDS tests outto -130 d8m in a 1-kHz bandwidth. The 1-dB compression point is +5 dBm, which is very good- Now the dynamic range is 123 dBm, a full 20 dB better than the first preamplifier and probably a better choice in your installation, since strong adjacent signals arenTt going lo blow your front end away while you're trying to work that new grid square just one-half S-unit out of the noise.

Past experience has led me to several conclusions: First of all, gain isn't everything. Preamps I've tested with only 10 to l2dBof gain often far outperform preamps with 18 to 20 dB of gain when it comes down to those magic letters IMD, MDS, and 1-dB COMP. Next, it makes no sense in any event to run 20 dB of gain into your multimode's MOSFET or GaAsFET front end if it will exceed the 1 -dB compression point of that same front end. Then you're really asking for it!

"How much are you giving up if you forsake that 20-dB preamp for 10-12 dB of 'clean' gain?"

The best designs for widest dynamic range and gain employ a balanced mixer with a well-designed low-noise preamp running about 12-15 dB gain ahead of it. This results in the best receiver performance possible, and indeed designs like this are now showing up in amateur equipment for 50 and 144 MHz. One thai comes to mud is the new Microwave Modules MMT-144/28 R transverter, which employs an NEC GaAsFET running about 12 dB gain to a balanced diode ring mixer. Let me tell youT tt is an outstanding contest performer

So how much are you giving up if you forsake that "wild." unrestrained 20+ dB preamp for 1012 dB of "clean1' gain? On a sig nal that is S1, a 12-dB preamp wilt raise it two S-units to S3, Fifteen dB will raise it to S3-1/2, whiie 18 dB wilt raise it to S4P but might also create some other interesting junk, such as a television program from an adjacent television transmitter reading S7, driving you crazy trying to figure out what it is on SSB or CW, Of course, if things get slow, you could actual ty switch to FM mode (if you have it) and try to listen io thai 200-kHz-wnde signal for kicks.

At my station, I use the MMT-144/28R with the companion MML-200-S power amplifier, which also has a 12-dB preamp with excellent dynamic range. At limes I switch in ihe preamp, putting i2dBahead of 12 dB for a total of 24 dB of gain. Now, when there's no sirong local activity, i can get away with this. Should a local come on with some power, "good-bye, DX contact'' as I'll hear noihing but hash Therefore, the preamp stays out of the line most times as the GaAsFET front end in the transverter is more than adequate.

Just remember those magic let* lers—MDS, IMD, 1-dB COMP— and you won't go wrong with a preamp. Reputable manufacture ers will readily make this data available to you upon request.

KLM Price Increases

What's up, KLM? Has anyone looked at the prices for KLM aniennas recently? Holy cow! An average of 80% increase in many cases for such items as the JV-2 J-Pole 4-element, 2-meter beam and more than 50% on such items as the 144-16 LBX and 22C anten* nas! I'm not sure what's going; on at KLM, but I can ten you that many amateurs are very upset at these price increases, tt seems that the VHF/UHF line took the hardest shots The aforementioned J-Pole went up nearly 70 dollars in price unbelievable.

This certainly puts KLM at a competitive disadvantage with such manufacturers asCushcrafi, TonnaH Hy-Gain. and others! Couple this with KLM's previous prob-iems of miscut coaxial baluns for their long-boom 144 and 220 antennas (resulting in feedpoinl impedances of 2:1 or higher) and what you have is a line of antennas that may become extinct. Already, many dealers are grousing about dropping the product line due to excessive pricing. That would be too bad, for KLM has long made some of the best antennas around, including those work

27th ANNUAL

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  • markus friedmann
    What means 1 dB comp.?
    9 years ago

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