Mobile Linear Amplifier Selection & Setup

Before selecting a linear amplifier, several things must be weighed. First, they are illegal to use on CB radio. You should research the decision carefully before taking the plunge into this very costly facet of the hobby.

This article includes some facts that I hope will help you make the right choice. Adding more power will give you increased range, but it may not be what you’re expecting. Sometimes using what you have efficiently will give you the results you’re looking for.

Here are some questions you need to answer first:

Is my radio peaked?	A peaked CB radio will do 6-8 AM and 18-20 watts SSB.
Is it a 12 or 25 watt radio?	An export type will do 10-12 AM and 35-45 watts SSB.
Where is my antenna mounted?	A centrally located antenna on the roof is best.
What is it’s power rating?	An antenna should be capable of twice the power used.
How efficient is it?	The most efficient antenna is the 102″ whip.
Do I have a quality amplified mic?	Astatic makes the best power mics.
Does the radio have speech processing?	A good speech processor raises you’re average audio.
Do I receive farther than I transmit?	There’s no sense transmitting farther than you receive
How noisy is my receive?	Filtering and a good antenna can help here.
What size alternator Is In my car?	A 250 watt amp will draw 20 Amps. A 500 – 40 Amps
Is there space for good air circulation for an amp?	Ventilation is very important for the life of an amplifier.

Remember the formula for power verses gain in the Proper Power & Modulation Adjustment article? Every time you double you transmission power the signal increase at the other parties receiver goes up 3db. That’s 1/2 of 1 S-unit, so the largest gain is from 4 watts to 250 watts.

This jump would give you a 18db gain or 3 S-units. To get another 3 S-unit gain you’d need to put out 16,000 watts. That’s right 16 Kilowatts to gain 3 more S- units! Now you can see where efficiency comes into play, because costs can add up. Right off I’d guess it would cost at least $60 for the trailer hitch to pull the $10,000.00 generator needed to supply the $8,000.00 linear amp. Then you have other incidentals like wiring, ground strapping, computer shielding, 20 KW antenna system. And then you wonder, will this interfere with my ABS system? After all it would be nice to have the ability to stop with this large investment on board.

This is a ridiculous scenario, but in reality going power happy does get crazy. Your gain for the dollar ratio changes exponentially.

Below are two graphs. The one on the left is a fictional graph reflecting what the average radio operator thinks happens when they run power. The one on the right is a true representation of power verses gain.

These are the sad facts. As you increase output power input supply current is required. This means a large alternator and heavy gauge wire from the battery to the amp. And that Radio Shack antenna must go. They would never manufacture an antenna that would allow much more than the legal limit on CB.

You’ve probably heard wattage isn’t always the answer. True most of us want to communicate. That’s a two way street. We need to hear as well as get out. One of the most important things that most of us don’t address is noise picked up by the receiver.

This could consist of ignition, alternator, computer, electric fan and or electric fuel pump noise. Running the radio power cord directly to the battery may help with some noise and will increase power to the radio. Antenna selection can help greatly. A shunt fed antenna such as the Wilson base load series antennas reduce or eliminate noise in almost all cases.

In case your wondering, the K-40 antenna isn’t shunt fed and won’t help in the elimination of noise. If you’re experiencing excessive noise, find a friend with a Wilson base load antenna and try it on your vehicle. If you still have a problem there are filters available that are designed for these specific interference problems and are covered in the next article.

If you’ve decided to buy an amplifier, you have your work cut out for you. As I mentioned earlier they are illegal for CB. In fact the broadband low drive type are illegal to rent sell or lease in this country ham license or not. If you’re a licensed ham operator you can build your own and use it on the allocated amateur bands.

Therefore it will be hard to find anyone that will talk to you on the subject of purchasing one. Without mentioning the company name, the best amplifiers have a built in low pass and other filtering with an adjustable sideband delay on the rear panel. Besides, you can’t go on name alone, there are many garage and basement operations putting the same name on their inferior products.

The low pass filter is very important in filtering the first harmonic. These broadband amps put out a great deal of spurious emissions. Without the filtering, you will be attracting attention from who knows where. These emissions will also make your antenna match look bad and could cause damage. The low pass filter can be added externally at the antenna output of the amp. MTI has one that will do fine. It’s the LM-TVX2 and the MSRP is $37.95. More on MTI in the next article.

The above mentioned no name amplifier is a true class AB amplifier not a class C unit. Class C is good for AM, but unless you want to sound as if you’re talking with your nose pinched on sideband you want class AB.

A quality amplifier cost more to manufacture, with price driving the market these days, almost all manufacturers have cut all the non-essential parts required to show power on the meter. If it reads power on the meter they’ve done their job. These guys don’t care how long it lasts. If it breaks, where do you return it? Besides they rely on the operator not to know what they’re doing.

Remember the cheap units don’t have filtering and will show a bad match on a tuned antenna. The paperwork will have warnings and disclaimers on running with a bad match, so when they break the poor consumers take the blame. They have it covered. As they say, buyer beware!

There are high drive and low drive amps. If you have a radio that does only 18-20 watts on sideband you want a low drive amp. If you have a radio capable of up to 45 watts you can use a high drive or a low drive if you have adjustable power or turn the output down to drive the amp properly. On a low drive amp of 175 to 250 watts in the high stage, the radio output should be set at no more than 18 PEP sideband.

On AM the carrier of the radio should be set to drive the amp at 1/3 the PEP rating or approximately 60 to 85 watts dead key. The peak swing is what’s going to make you heard! A lower dead key will increase the modulation and give the appearance of a more powerful signal. A properly tuned and installed radio/amp combination should sound good at close range and in the distance.

Only if you’re parked next to the other operator you may sound lousy by overloading their receive and distorting the receive of their radio. I’m sure you’ve heard, “kickers only sound good in the distance. It’s only true of poor quality or improperly installed units.

Amplifiers of up to 250 watts should be wired directly to the positive and negative terminals of the battery with #10 wire for runs up to 12 feet and #8 for longer runs. I recommend brass battery post extenders as always. A #10 quick disconnect is a good idea so you won’t have to cut wires to remove the unit if need be.

Don’t forget an inline fuse or circuit breaker on the positive lead at the battery. (BCB has wire, quick disconnects, inline fuse holders and both types of battery post extenders in stock.

Find a mounting location that will allow airflow to the heatsink. Don’t install it in the glove box or in a closed console. Under the front seat is a good possibility. Run a braided grounding strap from the case of the amp to close chassis ground. You can use the braided shield from a spare piece of coax cable.

In connecting the radio to the amplifier there are two lengths of cable that I recommend. If the amp is mounted to or close to the radio, use a I foot or less piece of coax. If one foot of cable won’t do it use 1/2 wavelength multiples of cable. Refer to the earlier article on coax for the formula.

SWR readings are done with the amplifier in line. Connect the meter input coax to the antenna output of the amp. If you connect it before the amp your reading will only be the match between the radio and amp when you turn on the power. The important match is the antenna.

First, check and set the match with the amp off. If there are any problems it’s best to find out with low power. The transistors in the amp are expensive and will quickly fail with an extreme mismatch. If all is well, switch the amp on and re-calibrate the meter. The match may go up a bit but a 2:1 is OK. If your match goes up more than that try a low pass filter.

Different power levels to me are bells and whistles that I never used if you need the power to use them. If you don’t, turn it off. Amplifiers are available in what they call 2, 3 and 4 stage. This means 2, 3 or 4 power levels.

Low/High, Low/Med/High, or Low/.N4ed/High/Max. They have a selectable attenuator on the input that reduces the drive from your radio into the amp. Most people think the more the better. Almost all amps are bi-amp. This is that they have a switchable receive preamp. Most of them pull in more noise than signal making them useless. But this also depends on the radio and installation.

The unit I mentioned earlier in this article has a sideband delay adjustment on the rear of the unit. This can be adjusted for different individuals. Some operators speak quickly and don’t take long pauses during transmissions. Others take frequent pauses. This adjustment allows you to tailor the delay to your individual radio habits.

Following these basic rules and taking the time to do a quality job will insure great long-lasting results. If you already have a linear, check and see if everything is set up as described earlier. You may improve the sound and range of an existing station.