A portable computer sounds like a great idea. You could toss it in your briefcase and work on a budget while waiting for an airplane or use it to take notes in meetings. A portable would work smoothly with other computers so you could work on whichever was handy and easily move information between them. A portable could even make a desktop computer unnecessary, since you can take it everywhere.

But no single portable computer can do all this. Any computer small enough to be easily portable does not have a keyboard and display screen big enough to be comfortable for your hands and eyes. And if you want a hard disk drive, the portable gets so heavy it's no longer easy to carry.

If you want a portable computer, be prepared to compromise. And you probably shouldn't buy one as your first computer. Portables are generally a little more complicated than their desktop cousins, so if you are a beginner, you should start with a desktop model. Portables fall roughly into three classes:

* Miniature computers for taking notes and storing addresses, in effect replacing a notepad.

* Laptop computers with full-size keyboards and screens large enough to read without much discomfort.

* Powerful computers with fast processors and hard disk drives. These models weigh 14 pounds or more and can often substitute for desktop computers.

Miniature Computers
There's a whole crop of tiny computers that fit in one hand, such as the Sharp Wizard, Casio B.O.S.S., and Psion Organizer II. They all have undersize keyboards and screens limited to a few short lines of text. If you love gadgets and have lots of patience, you might try one. Most people will find a notebook and a calculator much easier to use.

Two larger -- but still miniature -- models are more likely business tools. Atari's Portfolio and the Poqet computer both weigh about a pound and can fit in a large coat pocket. Both can run some software designed for the IBM PC. Neither comes with a disk drive; you store programs and data on memory cards instead. With a cable you can connect them to larger desktop computers to exchange programs and data. The much cheaper Portfolio (the basic unit lists at $400) has a 40-column by eight-line display and comes with just 128 kilobytes of RAM. These limitations mean Portfolio cannot run most MS-DOS software adequately, so it will probably suit you only if you like the built-in software: a simple editor, spreadsheet, scheduler, and address book.

The Poqet, at a $1,995 list price, is quite a different machine. It boasts a tiny but complete screen display -- 80 columns by 25 lines -- and 512 kilobytes of RAM memory, so it can run many MS-DOS programs, provided they can fit on Poqet's memory cards. The Poqet is an impressive package, but it may be too small for many jobs.

Laptops
The lightest computers with full-size keyboards cannot run IBM-PC software; instead, they come with built-in software. The best of these, the new Tandy WP-2 word processor, is good for taking notes on the run. It weighs three pounds and is just one inch thick. Its text-editing software works adequately, but exchanging text files with other computers is an awkward process. At present there is no software for spreadsheets or other applications.

You need a portable with a full-size keyboard and a standard screen to run IBM-PC software without difficulty. The three smallest models capable of doing this -- Toshiba T1000, NEC Ultralite, and Zenith Mini-Sport -- represent the best truly portable computers available today, so I will compare them in some detail.

Weight: The NEC Ultralite has no disk drive and weighs four and a half pounds. The Toshiba and the Zenith each weigh in at about six pounds.

Screen: All three have a liquid-crystal display. The Zenith screen and the larger NEC screen are backlit and the easiest to read. Toshiba's screen, which lacks backlighting, is the dimmest.

Battery: All have rechargeable batteries. Zenith's easily replaceable battery is most convenient; NEC and Toshiba's batteries are permanently installed. Fully charged batteries run the Toshiba for more than four hours, the Zenith for more than two hours, and the NEC for less than two hours.

Data and program storage: All three have MS-DOS permanently installed in their circuitry. Toshiba and Zenith both incorporate a single floppy disk drive with 720 kilobytes of storage. Toshiba uses a standard 3.5-inch floppy disk, which you may already have in a desktop computer; Zenith uses a tiny new 2-inch floppy disk. Either disk format works fine, although if you already use 3.5-inch disks, Toshiba is more convenient. Because it doesn't have a built-in disk drive, the NEC is much less flexible. NEC has a few programs available on memory cards; otherwise, all programs and data must be stored in short-term memory circuits until you can transfer them to an external disk drive or another computer. NEC's accessory 3.5-inch disk drive requires AC power, so it isn't really portable, and if you carry the disk drive with you, the NEC loses its weight advantage.

Price: The Toshiba sells for $600 to $700 in stores; the Zenith is likely to sell for about $1,400, and the NEC goes for about $2,700 with essential accessories.

Overall: I like the Zenith the best of these three computers, but you have to decide whether it is worth twice as much as the Toshiba. NEC gives up too much flexibility to achieve its sleeker package.

Heavy Portables
The next step up in size and weight are IBM PC-compatible portables with two floppy disk drives. I don't think these portables are very appealing; they weigh 10 to 14 pounds, can't run software designed for hard disk drives, and their processing is slow. You might be better off putting up with a pound or two more and getting a more powerful portable with a hard disk drive and a much faster processor -- an 80286 or an 80386 CPU. While the more powerful portables are expensive, they may be all the computer you need for your office as well as for travel.

If you are in the market for an 80286 portable, you can choose from many effective models. For a good survey of 80286 portables, see July's PC Magazine. One key feature to look for is a VGA screen, which can display both graphics and text well. The best-packaged model is the Zenith SuperSport 286, which has a VGA screen in its latest version.

The portables with 80386 processors are the high end in weight and price; some models weigh over 20 pounds, and prices can exceed $10,000. I wouldn't buy one now, but if you want to read more, August's Byte magazine surveyed 80386 portables.

A few companies have repackaged a standard Mac into a portable case, but the results have not been very satisfactory. Apple's new portable Macintosh performs well but was designed for long battery life rather than portability; it weighs 16 pounds.

For the most part, portable computer users run the same software on a portable as on a desktop model. But neither MS-DOS nor Macintosh software is particularly well suited for portable computers. MS-DOS software is hard to learn and use, especially when you are simply trying to take a few notes. All Macintosh software depends on a mouse to point at items on the screen; when you are working in an airplane or waiting for one, you probably won't have anyplace to run a mouse. Apple substitutes a track ball in its portable, but track balls are awkward to use since they give you little sense of how far you have moved the pointer on the screen.

One solution would be a family of software with different versions tailored for different computers. All versions would share files. The portables' software would work well from floppy disks or small memory cards and leave out features such as fancy printing. But no one has developed a family of software yet, so an effective portable computer is still some years away, awaiting improvements not only in software but in screens, storage, and battery life. n