Contents: Basics; Antenna Considerations; Power Considerations; ALC; Keying Interfaces; Second Battery Info; Mounting; Odds and Ends;
There is much more to installing a mobile amplifier than most folks realize. What follows are the major considerations, but not all of them, as every installation is different in some way or another. The most important one is the antenna, and that's why that section is first. Wiring techniques, and alternator capacity are important too.
One often overlooked aspect is the total cost. Amplifiers are not inexpensive, but the other needed accessories and upgrades can often double the cost. These facts will become evident as you read further.
Before you spend your hard earn cash on an amplifier, you should take a long, hard look at your antenna installation. Mating an amplifier to a cheap antenna and mount is counterproductive.
What ever antenna you use, it must be capable of handling a true 500 watts RMS. Since far too many manufacturers stretch the truth, here are the antennas to avoid: Any vinyl covered one especially those with large metal end caps; Any screwdriver antenna with more than 10 turns per inch, or smaller than 2 inches in diameter, or wound with less than size 14 awg wire; Any spirally wound antenna like a Hamstick®, or Outbacker®.
Here's a rule of thumb applicable to any brand antenna. If the unmatched input SWR is less than 1.7:1 on any band from 160 through 17 meters, then it isn't mounted correctly, or you haven't done enough bonding, or you need a better antenna.
Trunk lip mounts, license plate mounts, cheap ballmounts, mag mounts (no matter how many magnets they have), are all suspect mounting systems. If you use one of these mounts, and it will adequately hold up your antenna at highway speeds, then instead of buying an amplifier to increase your ERP, you should invest in a more efficient antenna, and mounting scheme!
Remotely tuned antennas (HiQ's RT models, and High Sierra's HS-1800 are examples) are very popular. A large portion of the users of these antennas purchase devices to automatically change bands, either by pushing a button or using the radio's built in controls. Some count the turns the requisite adjustment screw makes, and some rely on the SWR in one form or another.
Both types suffer from RFI ingress which is exacerbated by high output power levels (I don't care what their brochures say). The liberal use of shielded cabling, bypass caps and split beads are basic requirements of any controller installation. Read my Antenna Controllers article for more information.
Readers should peruse my Wiring, Wiring Protection, and Alternator articles. They explain the basic wiring installation requirements, fuse selection, and more.
There are two DC power considerations with mobile amplifier installations. First is the radio itself. Most 
miniaturized radios are designed to operate on 13.8 VDC. When an amplifier is being used, it is not uncommon for the voltage to dip below the operational cut off voltage (11.6 VDC). This can and does cause distortion problems for a variety of reasons. One way around this is to use a battery booster on the radio. There are several designs for them on the net, and at least three commercial units aimed at amateurs. The W4RRY unit, as featured in the pages of QST (October 2005, page 73), is reasonably priced at $120. It doesn't need a power switch as it's standby current is in the microamp range. If you run a mobile amplifier without a second battery, the booster is almost a necessity.
Running a mobile amplifier with 500 watts output, without an excessive amount of IMD (inter-modulation distortion) products, can be a difficult nut to crack. There are several reasons for this. Properly designed tube amplifiers will take a moderate amount of overdrive, however, this is not true of solid state amplifiers. Adding insult in most mobile installations, is the lack of a good stiff 13.8 VDC supply capable of 40+ amps average with peaks as high as 1oo amps. Even with good installation practices, it isn't uncommon for the input voltage to drop below 11 VDC on peaks. As the voltage goes down, the IMD products go up!
Another item you need to be aware of is output power versus input voltage. All currently available mobile amplifiers are rated at a nominal 13.8 VDC for full output. For every one volt drop, the power output will drop 50 to 90 watts depending on the unit. If you're sitting in your driveway with the engine idling (or off), do not expect the amplifier to deliver its full rated output. Increasing input power to make up for this drop is asking for a ton of problems. Aside from the lousy sounding signal due to excessive IMD, overdriving solid state finals will cause premature failure regardless of any built in protection scheme. This leads us to another problem; no ALC output.
ALC, which stands for Automatic Level Control, is essentially a feedback circuit to the transceiver (internally and/or externally) which gain-limits one or more stages before the finals. Its purpose is to prevent over driving the finals as well as any attached amplifier thus maintaining some moderate level of IMD which will meet or exceed the limits set by the FCC. Due to design considerations of most legacy tube-type transmitters, the first ALC circuits (which started to show up on amateur rigs in the late 60s) were negative going. That is to say, their resting output (no ALC action) was zero. As overdrive was approached, the voltage started going negative, typically up to -100 VDC. This was fed to the grid circuitry of the driver stage lowering its drive. A backplane port for an external amplifier was usually included.
Most of the first generation amplifiers designed for SSB required much less than 100 watts for full output. Running without the ALC connected was a prescription for amplifier melt-down. As better tubes and GG designs became available, drive power requirements increased to around 65 to 90 watts. Most amateurs erroneously believed (then and now) that all amplifiers require 100 watts of drive. Since this is all their transceivers delivered, many didn't bother with the ALC line. Unfortunately the manufacturers haven't helped any as most advertisements still specify drive power at 100 watts regardless of the final type, solid state or tube.
The truth is, most tube amplifier only need about 65 watts for full rated output, and some as little as 40 watts. Depending on their input circuitry, some solid state designs require just 25 watts! In either case, over driving results in excessive IMD, manifesting itself as splatter and distortion so commonly heard on today's ham bands. Not connecting the ALC just exacerbates an already grievous situation.
While ALC is all and good, most manufacturers have not stepped up to the plate to provide ALC output circuitry in their mobile amplifiers. Doing so would require a negative DC power supply, so I suspect it is a cost-saving step. This makes it doubly important for mobile operators to be aware of the limitations of mobile solid state amplifiers.
Some designs have swamping resistor networks to literally turn the excessive drive into heat (MFJ and Metron are examples). Others have input attenuators which automatically switch in and out as required (SGC). The Tokyo High Power unit has a manual switch for selecting either 10 or 100 watts in. Even on the 100 watt setting, it only requires 60 to 70 watts of drive for full output. It should be obvious that over driving these units can and does result in damage to the input attenuators and/or swamping resistors as the case may be. Over drive can cause the finals to fail regardless of any built in protection scheme, especially if the input circuitry is also damaged. This is an especially important point because some of the finals for the aforementioned amplifiers are no longer available at any price! The only solution is to watch the amount of drive power we use, and how me measure it.
A mobile station and amplifier can be setup without a peak-reading watt meter, it definitely helps so we'll assume there is one in use. We'll also assume these tests will be done using a dummy load, not an antenna, and that the motor is running. At this point battery voltage measured at the back of the amplifier should be 13.8 to 14 volts. If it isn't, you've got a problem. In SSB, the peak power should be approximately the same as it is in CW mode, but there may be a small difference either way.
Average power out is a whole new ball game as variations in voices patterns, meter damping, mic settings, and other factors effect what you read. Don't be surprised if it reads just 25 watts, as the peak will still be 100 watts. If it reads more than say 40 or 50 watts, either your mic gain is too high, or you have the speech compressor turned on. Incidentally, running compression while in motion will garner you a lot of bad reports about background noise. It also adds greatly to the electrical requirements which are most-likely already stressed.
Before switching on the amplifier, it is important to know just how much drive the amplifier needs to produce its practical SSB output (never its full-factory rating). The best output level is not solely dependent on the brand or power rating of the amplifier. Electrical system dynamics, wiring size, whether or not you use a second battery, your transceiver's power requirements, alternator size, and your voice pattern all effect it. As an example, a Tokyo High Power HL700B is rated at 600 PEP out, and will do it using a 15 VDC bench supply. In a mobile installation, 450 PEP output is about all you can hope for.
Using CW to set up the drive level is counterproductive as most electrical systems cannot keep up with 100+ amp loads for more than a few seconds. This is why we use SSB peak power, not CW, to adjust the drive level. So, you'll need to turn down the drive power. Icom makes this easy as there are separate power levels for HF, 6 meters, and for VHF/UHF. The adjustment goes from Lo 2, 3, 4, 5, 6, 7, 8, 9, and Hi. On my personal 7000, the numbers are very close to the actual power out times ten, with 3 equalling 30 watts. However yours is set up, you should start with 20 watts or so and work up.
What we're trying to do here is to increase the drive power incrementally to a point where more drive does not produce a corresponding change in output power. Then we'll back down the drive one notch (the correct level of PEAK power is shown by the red line in the chart at left). We're not after the absolute maximum output, instead we're after a reliable output as free from IMD as we can get. After all, no one can tell the difference on the air between 400 watts out and 450 watts out except for the extra splatter you cause by over driving your amplifier. Remember this, we don't have ALC to watch our back, so the key word is moderation.
Your speech patterns are important here and rather than say "test, test", use your call. "This is KX9XXX testing" is a good start. Key up and slowly start increasing the input power until the output power no long increases incrementally. Let me explain this another way. Assuming 30 watts in is 300 watts out, and 40 watts in is 400 watts out, and then increasing the input to 50 watts only increases the output to 450 watts, you're into the non-linear portion of the amplifier's power curve. This is what we're trying to avoid as non-linearity equals excessive IMD, or splatter.
Depending on the amplifier, your electrical system, and the transceiver you use, actual output from the transceiver may be as low as 10 watts, or perhaps as much as 70 watts, but will seldom be full power. Once this point is found, reduce the power to the next lower setting, or by at least 20%. The peak power out will be from 350 to 450 watts, perhaps a little higher if you electrical system is stiff. And please, don't drive it harder just because it is rated higher. And watch your mic gain too. We're all guilty of getting excited about that rare DX station and yelling into the mic.
Once you've finished setting the drive level, switch from the dummy load to your antenna. Don't be surprised if the peak reading is lower or higher. Any HF mobile antenna will always have some reactance, even at resonance. Even a little bit can effect a wattmeter no matter how good it is. Increasing the drive from where it was previously set is a prescription for splatter.
Operating responsibly is every amateur's duty regardless of where his/her station is located. Besides, good clean signals always garner more contacts than over modulated, non-linear ones do.
All of these solid state amplifiers use 5 to 12 VDC keying with currents varies between 28 and 850 mils (the SG500 is the lone exception). This means most of the popular mobile rigs like the FT100 and IC-7000 will require an external keying circuit. If you don't care to build one, there are two alternatives. The first is the AmpKeyer. Mobile or base operation, this unit has three separate outputs to match any amplifier. At $50 (plus shipping) it's a bargain considering its capabilities. Details are on their web site.
Ameritron (MFJ) makes the ARB-704 amplifier interface which retails for $60. Both of these units work on nominal 12 VDC. If you're using an SG235, you won't need this interface as the coupler has one built in.
If you want to build your own, here is Bob Wolbert's (K6XX) home brew interface with full instructions. He has a pdf version too. It's simple, it's inexpensive, and can be built-dead-bug style into a plastic thimble. If you're careful, you can even build it inside an Icom accessary plug.
Incidentally, the Icom accessary plug only comes in a prewired, pig-tailed version. However, the accessory plug for the Kenwood TS-2000 is identical (but no pig-tails), and is widely sold by AES, HRO, and other distributors for about $10.
There is another recent mobile accessory entry, and that is the Ameritron ARI-500M interface. This device, which sells for $120, uses the band reference voltage and/or data lines (of Icom, Yaesu, and others) to automatically set the correct bandpass filter of their ALS-500M amplifier. It also contains a keying interface, which Ameritron uses to address a potential drawback that plagues similar devices.
Unlike a regulated power supply, mobile input voltages vary from 11 VDC to 14.5 VDC. This is true whether or not you run an amplifier; a second battery notwithstanding. This fact causes the band select output signal (0 to 8 volts depending on the band in use) and/or the data lines to vary. This causes similar devices to select the incorrect bandpass filter (not something you want to happen). The Ameritron ARI-500M avoids this pitfall by ignoring the input data when the key line is low (radio in transmit mode). It further appears this device could be adapted to work with the THP HB700L with just the addition of properly wired cables. There is one small caveat if you use an Icom IC-7000. To wit, you'll have to make the internal mod as described on page 140 of the owners manual. Otherwise, there will be no band select output signal on pin 5 of the accessory socket.
Although I cover alternators and batteries in another article, it's important enough to mention them here too. Batteries are the most often overlooked aspect of high-power mobile operation. It is not uncommon to use a second, trunk-mounted battery as I do, but the type to use with a mobile amplifier is in debate. Far too often I read where folks have selected deep-cycle marine batteries for this application. While they are the battery of choice for powering QRP rigs where current draws are low, their use with mobile amplifiers is ill advised. SLI (Starting, Lights, Ignition) batteries are a better choice as they are designed to provide large amounts of energy for short periods of time. Remember, it is the alternator which is powering the amplifier. The battery is just acting as a buffer.
You should use a battery of the exact type (plate alloy). Based on my personal experience the size should be at least as large as the vehicles existing SLI battery and perhaps larger if you have the space. Age isn't too important, but it should be in good working condition. If it is going to be mounted in an inclosed trunk area, you should select an AGM type which is sealed and will not out gas under normal operating conditions. Remember, the battery is acting as a buffer to handle the peaks while the alternator is doing the hard work. Deep cycle batteries typically cannot supply the necessary peak current for this type of operation. If you need more information on which type to use, peruse these two sites: Exideworld.com and Optima.com
Here are a few more caveats you should consider. Battery isolators have their place. As an example, if you are charging deep-cycle batteries from an vehicle alternator (a common occurrence in RV's) they make sense as they protect the deep cycle from over charging, and protect the starter (SLI) battery from discharging when using the deep cycles for power. But in a mobile amplifier scenario, forget about them! This goes for heavy-duty isolation relays as well. Their complexity outweighs any benefits they may offer. If you want to protect your vehicle battery from discharge, don't run your amplifier unless the engine is running. Remember, this isn't long-term portable operating, it's high power while under way.
Any trunk-mounted battery should be installed in a battery box and properly restrained. The rule of thumb for battery restraints is 6Gs lateral and 4Gs vertical. The last thing you want to happen is to have a 60 pound battery flinging acid all over your trunk! If you use a flooded type battery, the box should be vented to the outside. If this isn't possible, you should use a sealed AGM battery such as the Optima red top. While they cost about twice as much as a standard car battery, they quickly solve the vent problem.
Mounting a mobile amplifier inside a passenger compartment isn't easy, and for no other reason but safety, I discourage it. If you have to mount it inside (i.e.: early model Ameritron ALS-500s without remote mod), I suggest under the seat or other out of the way place, but where the band switch can be reached. This is a moot point if you run a mono band antenna. You have to stop to change the antenna, so band switching the amplifier is a minor hassle.
Obviously, remote controlled amplifiers can be mounted in the trunk or other out of the way location. This complicates wiring, but if a second battery is used, the I2R losses will be reduced. Wherever you mount your amplifier, mount is solidly! It pays to remember that some amplifiers weigh as much as 25 pounds, so simple straps and bungie cords won't do the trick.