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INSIDE THIS ISSUE

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Comparison of Cobra 2010 to 2000
Power & Modulation Adjustments
New DF 10,000 Low Pass Filter
Coax Types & Lengths
Linear Amp Selection &Setup
Mobile Radio Interference

SEPTEMBER 1996
VOLUME 1
ISSUE 2

Last Update
 01/05/13 10:05 AM

 

 

 

 

 

Comparison of Cobra 2010 GTLW & 2000 GTL

Back in the late 1980s, rumors were floating around that Uniden told Cobra that they would have to find a new manufacturer for their CB radio line. First came the Plus line of CBs. They were all Korean made low cost AM radios and sold through mass merchants as well as dealers. These radios performed OK but meter & channel display burnouts were common. I was waiting for a replacement sideband unit, but it never happened. I assume the Korean manufacturer wouldn't or couldn't design a SSB radio.

After a few years I gave up looking for new Cobras to emerge. I figured that Cobra and Uniden came to a new agreement. If they did, it didn't last. Suddenly without warning dealers across the country were receiving Cobra 148 GTL radios that were imprinted Made In The Malaysia. They even copied the label Made In The Philippines replacing Philippines with Malaysia. It shows they were in clone mode by not omitting the word "The" before Malaysia. Uniden no longer makes Cobra CBs. Cobra has Ranger Communications Inc. (RCI) cloning the GTL mobiles and have dropped the 142 GTL base. The radios look exactly as the Uniden made originals on the outside but as with any clone there is something lost in the translation. Inside the component layout is identical but the Uniden made parts such as the filters and ICs are replaced with RCI or 3rd party replacements. The transmitters put out about the same as the originals but the receivers seem a little noisier and less selective.

For many years the Cobra 2000 GTL was their flagship and the dream radio of many. It was a quality radio that worked as well as it looked. Features like the fine/coarse clarifier and 6 digit frequency counter made it a desirable out of band radio. Clarifier swing of 20KHz was easily controlled and monitored.

The following is a list of the features that have changed between the Cobra 2000 GTL & 2010 GTLW:

(+) Improved (=) Same (-) Worsened

Cobra 2000 GTL

Cobra 2010 GTLW

Stock Mic + Power Desk Mic
External Speaker -  Internal Speaker
Speaker 1 &2 Switch -  1 External Speaker Output
Fine/Coarse Voice Lock (Clarifier) = Fine/Coarse Voice Lock (Clarifier)
Sloped Front Panel  + Straight Vertical Front Panel
Brushed Aluminum/Chrome Front Panel -  Dark Gray Front Panel
Wood Grain Cover   - Black Plastic/Vinyl
Brushed Aluminum Knobs + Rubberized Black
Clock/Timer -  Clock
Meters: 2 Back Lit -  Meters: 2 White Background Front Lit
Frequency Counter: 6 Digit -  Frequency Counter: 5 Digit
Channel Selector Switch -  Channel Encoder Switch
  + Weather Alert & Channels

 

My first experience with the 2010 was when they were first available. It wasn't a pleasant one. The radio had two severe problems. First the channel encoder knob didn't function properly. The channels would go up and down in the same rotation of the knob. The speed of rotation didn't have any effect. It would go up when you wanted to go down and vise versa. The second problem was on sideband. The audio was very distorted out of the box and after a tune up it was just as bad. The radio went back the same day.

The second radio I tried months later showed improvement. The channel encoder worked almost all the time, I did notice it malfunctioned at slow speed rotation. The sideband audio was clean out of the box and better after tune up. The transmitter was off frequency about 240 Hz. This was common with the 2000 out of the box. Most operators have the radios peaked and clarifiers unlocked anyway.

Every time the 2010 is plugged into the electrical outlet it lets out a screech for about a second. This is quite annoying if not startling. It happened in both evaluation units. It's the weather alert alarm. It comes out a separate speaker in the rear of the radio. It should get your attention if you ever use it.

The power mic is weighted and is the best power mic Cobra has offered. I haven't found a Cobra power mic that I've liked. This one is acceptable although I'd swap it for a better feature.

They have eliminated the external speaker. The internal speaker is OK but it doesn't have the sound or took of the 2000 speaker. There is only one jack for external speaker. This will be disturbing to may operators. If you want better sound I recommend the Uniden ESP-25 extension speaker the cost will be around $20.00 and it will give you excellent vocal response.

The Fine & Coarse clarifier is unchanged. When unlocked it swings approximately from + 5Khz to - 15 KHz. It seems to work as well or better than the 2000.

The traditional sloped panel is gone. Many won't miss the dust collecting ability of that feature. What I miss is the brushed aluminum and chrome front I find the, dark gray quite plain. And the two meters with the white background may be easy to read but stick out like a soar thumb. A light gray background would have been better.

The knobs have a rubberized finish that makes them easy to turn without your fingers slipping. The controls have a smooth quality feel.

The on/off timer and the wood grain cover have been eliminated.

The conventional channel switch has been replaced with a channel encoder switch. This is nothing more than up/down buttons in a rotary switch form. Once the radio is unplugged the channel will come up on I no matter what channel was used last. The 2000 switch selected a particular code for the channel and it didn't mater if the power was turned off or disconnected. The real problem isn't in incorporating the encoder although if it's a poor quality encoder it may cause problems. It seems the CBs and 10 Meter radios that incorporate them aren't buffered properly to prevent the channels from going in the opposite direction of the turn of the knob.

The weather alert feature can be useful. If there is a weather emergency announced on the weather channels, an alarm will sound and you can turn on the weather and listen to latest report. This feature works while your listening to your favorite CB channel.

The last feature is the frequency counter. I think this was the key to the 2000's success, and could be the downfall of the 2010. A five digit counter will only give you 1 KHz resolution. That is you could be off almost 1KHz before the last digit of the counter would change. On sideband you could sound like a buzzing mosquito to someone that's on frequency and you wouldn't know it. The one benefit to this counter is that it will display extra channels. I would have gladly traded the desk mic for an extra digit. Surely Cobra would have made more profit that way. The mic was to make up for the external speaker. It does make the box bigger. It seams they lost sight of what will happen after operators get on the air with the 2010. It's the single most important thing that would make me want the older 2000. 

The following is a representation of a 6 digit frequency counter:

10MHz 1MHz 100KHz 10KHz 1KHz 100Hz
2 7. 8 0 5. 0

The main circuit board in the 2010 is the same that goes into the 148 except the PLL is a MB8734. The same chip that was in the 2000. To channel it replace the MB 8734 chip with a MB8719. The conversion should be about the same and yield the same channel compliment.

The radio functions reasonably well and should being a cost reduced clone of the 2000. They reduced the cost in many areas mentioned earlier in this article. In addition they replaced the LED frequency and channel display with a single liquid crystal display (LCD). This is less expensive in itself, but also reduces the current requirement of the power supply. With the decreasing cost of technology I expected a lot more than Cobra is offering. In my opinion this is a radio that should have a list price of no more than $299.95.

Cobra better hope that Uniden doesn't decide to come out with a Madison XL incorporating a 6 digit counter. Hmmm ... I bet that would be a hot item! CBWI

Proper Power & Modulation Adjustments

Everyone seems to be looking for more power and modulation. The 40 channel radios have their modulation limited on AM by the AMC (automatic modulation control). Most have a potentiometer (POT) that controls the amount of maximum modulation the limiter allows the radio to put out. The reason it has been limited is to reduce the adjacent channel splash on the band. The primary cause of splash is by over modulation distortion. But many radios are turned down well below the point of distortion. Also the AMC may be slow and allow distortion during some syllables. The radios without an adjustment pot are quickly modified by clipping out a resistor, diode, or transistor, but this eliminates the AMC. This is OK if the output is constantly monitored and good sense used. The AMC constantly monitors the output and automatically reduces the input sensitivity to maintain a constant maximum modulation.

Below are illustrations of an AM transmitter under dead key and audio test. These illustrations are representations of what you would see on an oscilloscope if you were monitoring the RF output of the transmitter under test. Notice the over modulated signal is clipped off at the top and bottom. There is much audio information missing. The ideal situation is to get the maximum modulation out without loosing any part of what is going into the audio circuit.

Above is an un-modulated AM transmission.

Above is an under modulated AM transmission.

Above is an over modulated AM transmission.

Above is a 100% modulated transmission.

How many times have we heard or said, "You're breaking my speaker?" Over modulation distortion does sound like a breaking or broken speaker. But everyone thinks that they're so powerful that they are overloading the other parties receiver. This isn't true. Yes, you're loud, but the more you over modulate the more voice information is being lost.

The dead key power should be one third of the peak power. Say the radio is capable of 18 watts peak without distortion, then the dead key should be 6 watts. Less if you're driving a linear amp. We'll cover amps in more detail later in this issue. This ratio will make your signal loud and clear. Not only will that impress other operators, it will increase your range of transmission.

If you don't have access to a scope, you can get an idea of what the peak level of modulation is available without distortion. With the radio tuned for maximum whistle loudly into the n@dc and take a reading on your peak reading watt meter. Usually reducing the peak by 15% with the AMC will give you clear audio. Then reduce the carrier to 1/3 of the peak power. For example we have a radio the produces 20 watt peak modulation. Reduce it by 15% by adjusting the AMC to read 17 watts peak with a whistle. Then adjust e carrier to 5.5 watts. Check the peak for original 17 watt setting. Now you have approximately 100% clean modulation.

An ALC (automatic level control) is incorporated on sideband radios to limit the maximum PEP output. A radio with a miss adjusted or defeated ALC can clip in a manner similar to the illustration of over modulation on an AM transmitter. When the signal clips, information is being lost. It may look good on the meter, but it doesn't sound right at the receiver's end. Sideband is a different animal from AM. It doesn't take as much audio from a mic to carry a great distance. Therefore sideband is more susceptible to background noise and an overpowering mic gain adjustment is counter productive.

In understanding how to adjust your audio on sideband you must first know what the receiver AGC (automatic gain circuit ) does as it receives a sideband signal. The AGC has the job of varying the receivers gain to prevent overloading from strong signals. Without it, signals from around S-5 and above would start or completely overload the receiver rendering the signal unintelligible. In controlling the gain the AGC keeps the audio compressed to a somewhat constant maximum level. The AGC has a recovery delay time that varies from radio to radio. This delay keeps the gain according to last signal received. If a weak signal comes in after a strong one, the first word or two may be very low or completely missed.

Now, understanding that to increase a signal by 3db the power must double and a calibrated "S" meter reads I- S unit for every 6db. We can use the reverse for an example. If we cut the output power in half we loose 3db of signal. Receiving an 18 watt signal at S-9 we would receive a S-8 at 4.5 watts, a S-7 at 1.125 watts, a S-6 at .14 watts, a S-5 at .07 watts and a S-4 at .018 watts. That's 18 milliwatts to give a S-4 signal and 1000 times that (18 watts) to give a S-9 on the meter. Background noise that wouldn't be close to registering on your RF meter would be received with a decent signal strength. Imagine if the background was flicking the meter at around I watt, approximately 20% of full output. Your background noise would be giving a S-7 and your peak signal only 2 "S" units more. The background is desensitizing the receive with the AGC kicking in, making your peaks have less impact or punch. The background would get mixed in with the voice and make it difficult to understand what is intended to be heard. On sideband the name of the game is signal to noise ratio, the less noise the less distraction from the intended transmission. There's enough noise out there to get through without creating your own to overcome.

In setting the ALC it is good practice, with a loud whistle, to open the ALC up all the way and then back it down until you see a reduction of about 2 watts. This will give you peak performance without clipping in most cases. Some radios won't distort with the ALC wide open while others will distort terribly once the mic is cranked up. If you control the mic gain carefully you can leave the ALC open.

For true reproduction of your voice a good quality mic is essential. Using only the amount of gain needed to amplify your voice to a respectable level, holding the hand mic or desk mic close to your mouth and talking into the side or talking across the screen will give your transmission the most natural sound . CBWI

The New DF 10000 Low Pass Filter by RF Limited

Yup a 10,000 watt low pass filter. Most of you don't need anything quite this heavy duty, but it's a pleasure to see some new serious equipment coming down the pike. There are some out there that could use this gem. There are some that could use a 50,000 watt version. But believe it or not that's not the most impressive specification this unit offers.

RF Limited has three models to choose from, the DF-4000 a 4KW PEP unit, the DF-7000 a 7KW PEP unit, and the DF-10000 a 10KW PEP unit. All are rated half of their PEP rating for DC. That's only were the specifications differ. All other specs. are identical.

DF Series Low Pass Filter Specifications

Insertion Loss DC - 3OMhz, Less Than 0.3db
Input/Output 50 W, Non-Polar
VSWR 1 To 1.2
Cutoff 32 MHz
Attenuation 70 - 100 db Above 38Mhz
Design 13th Degree Cauer-Parameter

 Above is the from the specification sheet supplied with the unit. To the right is the graph of frequency verses attenuation.

After removing the 19 screws I peeked under the cover and found some of the best workmanship I've seen in a long time. All the connections are secure. This is were most failures occur in filters. A connection problem may not be obvious to the operator making it a more undesirable problem. One bad connection could cause the filter to allow the harmonic emissions along with the fundamental frequency to pass right through. Continued interference after installing a filter leads you to believe it could be one of a hundred other problems and not the filter because your match and signal strength are OK. There isn't much to go wrong with them beyond connections. The coils are heavy gauge copper and all insulators are Teflon including the SO-239 connectors. Everything is screwed together or soldered. No pop rivets in these units!

The two specifications that impress me are the cutoff frequency of 32Mhz and the attenuation of 70 - 100db above 38Mhz. The low pass filters I've encountered, Nye Viking, Barker & Williamson, Para Dynamics, and the old CPI, have cutoff frequencies from 45Mhz to 50Mhz. This is very close to the lst harmonic of CB and the bottom of the TV frequencies. And the above mentioned filters had attenuation figures from 60db to 80db at 54Mhz. As you can see the DF series filters are at approximately l00db at 5OMhz.

The DF filters have a ground connection with a wing nut to connect the filter to earth ground. None of the other filters have this connection even though it makes sense to ground the filter to bleed off the unwanted emissions. Under an actual use test I replace a RF Limited DF-2000, which has decent specs., there was no interference on channels, 2, 4, 5, or 7. Without the filter Channel 2 was a washout, 4 had a few lines, 5 was terrible and 7 had intermittent interference. The DF-2000 improved everything but 5 had a few lines and 2 became blurred with interference. Adding the earth ground did show a slight improvement. I'm impressed, the DF-10000 is the best I've seen.

The manufacturers suggested list price is a little inflated but I'll include that for reference. The DF-4000 MSRP is $219.95, the DF-7000 is $249.95, and the DF-10000 is $299.95. In real life you can expect to pay about 2/3rds of suggested retail. BCB will make them available for a limited time by special order at a reduced amount.  CBWI

To order call: 1-800-473-9708
DF-4000 $119.95 DF-7000 $139.95 DF-10000 $159.95

Coax Types & Lengths

Did you know there are many types of RG-8/U coax? This is what most people refer to as the heavy stuff. Wen, RG-8/U coax is heavy but there are a number of different grades and if you get one of the cheaper grades you might as well get the thin stuff. Belden has about 4 grades of 8/U and the RG-213/U which is a military spec. cable equivalent to RG-8-A/U. Belden coax shields range from 95% to 97% and 100% on the 9913 their low loss 8/U.

Below is a table of Belden specifications on the coax I normally use or recommend. These are not all the specs. available but are the most important for our use in this article.

Belden Coax Specifications

Description Insulation Nom. Imp.
W
Nom. Vel. of Prop.  Nom. Atten. @ 3OMhz. Per 100'

RG-58-A/U
Polyethylene 50 66% 2.35 db

RG-8X type
Cellular Polyethylene 50 78% 1.75 db

RG-8-A/U or RG-213/U
Polyethylene  52 66% .93 db

Belden 9913 or JSC 3500
Semi-solid Polyethylene 50 84% .65 db

Other manufacturers make 8/U type cable with shields that range from 66% to 80% and you can't tell without stripping them back to look at the shielding. The worse cables I've found are the pre-made type. You're better off making your own or having cables made from known high quality cable. I've had experience with JSC Wire cable and found their RG-58-A/U, RG-213/U, and their 3500, a 9913 equivalent to be an excellent low cost alternative to the Belden counterparts.

The insulation column indicates the center conductor insulation material. Mil. spec. cable such as the RG-8- A/U, RG-213AJ, and RG-58-CIU use the solid polyethylene insulation. These cables have a nominal velocity of propagation rating of 66% and a shield consisting of 95-97% copper braid. This is an important number in calculating coax lengths, as you will see. The RG-8/U type, RG-8/X type, and RG-58/U type have a cellular polyethylene insulator and are normally referred to as foam. These less rigged foam cables, have a slightly improved attenuation figure and a velocity factor of 78% and a shield consisting of 95-97% copper braid. The 9913 and 3500 have a semi-solid polyethylene insulator. A spiral air space is created between the center conductor and the insulator making it semi-solid. These cables have a velocity factor of 84% and a shield consisting of 100% aluminum foil plus 90% tinned copper braid.

As you can see the nominal impedance is 50-52W. Don't worry about the 52W impedance, you'll never see a difference on your SWR. As mentioned earlier the nominal velocity of propagation for polyethylene is 66%, cellular polyethylene is 78% and semi-solid polyethylene is 84%. It is used to determine the length of cable at a particular frequency to keep from making your coax a factor in your SWR. You determine the length and use that multiple to get the total length you need. If your antenna is resonant and your SWR doesn't show 1:1 it most likely is the coax or the coax length.

Mobile and base systems use different formulas. Base antennas work best using multiples of 1/2 wave lengths of cable. Mobile antennas work best using multiples of 3/4 wave lengths of cable. You may recall that one antenna that seemed impossible to match, this could have been the problem. I've had truckers come in and say they've tried everything, even new coax, and nothing works. The first question I asked; How long was the coax cable? The response usually was 6-12 feet and once it was replaced with an 18 foot piece of RG-58-A/U the match came down, usually below 1. 5: 1.

Here are the formulas used to calculate the coax length:

  1. Base Coax = 468 + Freq. in Mhz x Velocity of Propagation
  2. Mobile Coax = 702 + Freq. in Mhz x Velocity of Propagation

Now say you have 50 feet of coax cable in your base station and want to replace it with 9913. First you must decide where on the band you spend most of your time or if you bounce around a lot take the lowest frequency and subtract it from the highest frequency then divide that number by 2 and add it to the low frequency. This will give you the center frequency you use.

  High Frequency 27.905
  Low Frequency 26.185
Formula with your information plugged in is: 468 27.045 X.84 Bandwidth In MHz 1.720
First 468 27.045 = 17.305 Then .84 X 17.305 = 14.535 Feet    
Approximately 14.5' is your multiple 1.720 2 = .860
To determine how many multiples you need 50 14.5 = 3.448    
So you need either 3 X 14.5' = 43.5' or 4 X 14.5' = 58' Low Frequency 26.185
  Mid. Bandwidth     .860
  Center Frequency  27.045

 Now you can calculate for any other type of coax by plugging in the velocity factor from the coax table and use the other two figures as before. For mobile using RG-8/X coax, just divide 702 by 27.045 then multiply that figure by .78. 702 + 27.045 X .78 = 20.25' Using RG-8/X cable your mobile multiple is 20 feet 3 inches. For most mobile installations I multiple is enough.

Use these formulas for all types of coax cables and frequencies. The correct length of cable is much more critical on mobile installations. I've seen a 4:1 match corrected with the proper length of coax. Base installations are less critical, but keeping the length close will give you an acceptable SWR across a broader bandwidth.

Coax cable types can be mixed as long as the correct multiples are used for each length. Maybe you want to use 9913 outside your house and RG-58-A/U inside because it's easier to run thinner cable in the house. For 27.000Mhz you would use 14.5' multiples of 9913 and you would use 11.5' multiples of RG-58-A/U. If you're replacing a single 50' piece and 11.5' is enough to reach your equipment, then you'll need a 43.5' piece of 9913 and a 11.5' piece of RG-58-A/U coax. Use a double female or barrel connector to splice the two together. This will work fine and won't create any problems as long as you wrap it with coax seal. Coax seal is a plastic putty that can be molded to form a weather tight seal that will never leak. It stays soft and pliable and connections can be unscrewed at any time.

The placement of and the length of coax to the SWR meter has an effect on the accuracy of your readings. If a linear amplifier is used, the meter should be in line with the antenna side of the amp, otherwise it should be after the radio. A very short piece (6-12") of cable should be used between the radio and the meter. If you require more than that, use a 1/2 wavelength of cable. Use the formula in this article or an easy to find 12' pre-r made RG-58-A/U cable. This will give you the most accurate reading. There's nothing worse than troubleshooting a problem that may not be there. If you start with known test equipment your project will take less time and fewer patients.  CBWI

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 broad band 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 broad band 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 their 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 air flow 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 a 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 wave length 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 power use it. 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 reduce 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. CBWI

Mobile Radio Interference

Here's a problem that has been escalating as the automobile industry brings out the newer models. Computers that aren't shielded, electric fans that replace the old belt driven ones, electric fuel pumps, and higher output alternators all of which have less than adequate RFI filtering. The RF emissions from the devices can cut your receive range substantially. And why listen to any more noise than you have to.

Before any action can be taken the cause needs to be narrowed down. Disconnect the antenna with the vehicle running. If all the noise goes away it's RF and coming in through the antenna. If you hear a whine that increases in pitch when the engine is revved up, it's coming through the power leads. Run the power connection to the battery and if that doesn't work use a power line filter (LM-P-10) in the positive lead at the radio.

Radiated interference that is received through the antenna can be reduced or eliminated in many ways. I've found that the use of a Wilson 1000 antenna reduced interference on half and cured the problem on the other half of the tests made at my store. A popping sound that increased with the speed of the engine is ignition noise. Insure that your vehicle has suppresser plug and wires. Wires may need replacement, one bad wire can create an intermittent problem. Install a LM-20RF on the battery side of the ignition coil.

This is the most common new problem, Electric fuel pump whistle. It's a steady hashy whistle as soon as the ignition is turned on. Fords are the worst offenders. The fuel pump is in the tank. Locate the positive and negative feed lines and install a LM-20RF on each line. Mount the filters to a good chassis ground cleaned to bare metal. Test and make sure you have the feed lines and not the fuel level indicator leads.

A less common problem is the radiator fan interference. It causes an audible static when the fan is running. Disconnect the fan to verify the problem, if it's a culprit install a LM-20RF in line with the power lead.

These LM-20RF filters will work with other motors such as the heater blower motor and the windshield wiper motor. CBWI

More information and these products are available from: 

MTI Land Mobile Division PART# MSRP
Marine Technology, Inc.  LM-P10  $25.95
2667 E. 28th Street, #505 LM-20F $10.95
Signal Hill, CA 90806 LM-25ORF $27.95
(800) 772-0796, (310) 595-6521 LM-TVX2 $37.95

Also Available From Bob's CB & Wireless
1-800-473-9708

 

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