Mods in general

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Jonathan_lemoine
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Mods in general

Post by Jonathan_lemoine » Thursday 7th May 2009, 5:12

On the website http://www.shadowstorm.com/cb/CB_Myths_Exploded.html the guy says that "An AM signal is fully modulated when the "swing" doubles the carrier. That is to reach 100% modulation a 4 watt key should swing no more than 8 watts. Anything more than that is distortion to your voice. The AM carrier is necessary for the receiver to properly demodulate the audio from the signal. If you add excessive swing it will distort your audio. It will not help you get out better." Now this sounds reasonable i guess. Does this mean you should increase your dead key if you are going to turn up the radio? I lowered my dead key to run my amp (I'm not sure if I had to though I know most amps want a low dead key)
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lbcomms
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Re: Mods in general

Post by lbcomms » Friday 8th May 2009, 1:20

If you need to "lower your dead key" then you are overdriving your amp, causing distortion (at RF, this means unwanted nasties such as harmonics and spurious mixing products) that will interfere with other services such as TV reception. A "linear" amplifier (most backyard amps are anything but linear) will amplify different power levels by the same multiple when operated within its limits. For example, a 1000 watt rated linear amp with a gain of 10:
0.1 watts in gives 1 watt output
1 watt in gives 10 watts output
5 watts in gives 50 watts output
33 watts in gives 330 watts output
and so on. If you give it 150 watts in, it will be pushed beyond its limit and distort. A good amp (a proper commercial design, such as a Harris, Barrett, etc, not a cr*p backyarder "pillbox") will have an ALC circuit to limit gain and warn the user. They are specified to the "1db compression point" - in the previous example, that means the maximum input would be about 125 watts, which would limit the output to 1000 watts with RF distortion limited to about -30dB, which shouldn't worry the local TV reception too much.

The quote from shadowstorm is basically right. Some simple maths will confirm this.

Speech or music audio is symmetrical - it goes equally up and down, in other words equally positive or negative.
In an AM transmitter, when it goes positive, it boosts the carrier, and when it goes negative, it reduces it.
You can't see this on a power meter - it's happening too fast to see with a meter - you need at least an oscilloscope, but preferably a spectrum analyser to see it (especially if you overdo it).
On the negative side of the cycle, the limiting factor is cutting off the carrier, this happens when the negative cycle audio power equals the carrier power. Any more than this, it gets ugly real quick. This is our "100%" limit. Since the positive cycle is opposite and equal to the negative one, 100% positive will double the carrier peak VOLTAGE at the 100% point.
Since doubling the voltage doubles the current, and power equals voltage times current, then it follows that the peak power (the PEP wattage) will quadruple. Any "swing" above this will be over 100%, and because the negative is the equal opposite of the positive, the carrier will be cut off, resulting in distortion and splatter.

This limitation results in wasted power, not a concern for a 4 watt or even a 100 watt transceiver, but it is an issue for a 5000 watt+ AM broadcast transmitter. To overcome this, they operate on reduced carrier and go over 100% on the positive cycles only. This can't be done with a simple "swing kit", it requires complex circuitry. The Harris broadcast transmitters use DSP (digital signal processing) modulators to achieve this, and cost five or six figure dollars. Have a look at:

http://www.broadcast.harris.com/product ... =WWWDXDEST

CB Swing kits are snake oil material. They might sound "loud" on air - most people associate loud with distorted, if you turn up a $5 portable radio or a bedside alarm clock radio they go "loud" distorted.

Add your "pill box" backyard amp and you've added spurious to the list - at best this might get your coax pinned by another local operator or irate TV viewer, at worst you might knock another service off the air in an emergency or get a visit from the ACMA (or your local equivalent if you are not in Oz).

If you are going to run over the legal limit, at least keep your signal clean.

Cheers

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Jonathan_lemoine
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Re: Mods in general

Post by Jonathan_lemoine » Friday 8th May 2009, 4:23

A very informative post. I am actually saving up to buy a scope. I may have one next month. I just fix and tweak cb radios as a hobby. I don't have any courses on electronics but I have been sort of teaching myself stuff and asking questions as I go along. I have a Palomar 225 (http://www.firecommunications.com/p225.shtml) I'm not sure of the gain. It's hard to find much info on them.

From what I can gather from your post if I wanted to have a deadkey of 10 watts with a swing of 20 then as long as it didn't overdrive the amp (over 200 rated watts PEP) it would be ok. Is this correct?
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lbcomms
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Re: Mods in general

Post by lbcomms » Sunday 10th May 2009, 17:11

It's actually fairly easy to find the info, you just need to know what to look for...

From the web link of the amp:
- 200 Watt PEP*
- 1.5db. Rf Compression Limit
That means that it's 1.5db past its limit at 200 watts. OK, math time again.

200W - 1.5dB = 141.59 watts, but that's under ideal conditions. In use, you will have factors like a non-perfect antenna SWR and voltage drop on the DC feed line to contend with. Assuming that you have a good SWR and nice thick power cable, allow 10% or so. 141 - 14 = 127 watts. We'll round it to nice 125 watts.

The next clue is the transistors in the amp. Nice of them to show an internal picture, from that we can determine they use SD1446's. Google the datasheet, find out what they can do:

Gain: 10dB
Power output: 70 watts

It uses a pair in push-pull, this will ADD the powers but keep the gain the same.
Our earlier calculation ties up OK: 70 + 70 = very close to our earlier 141 watts. So far so good.

125 watts / 10 db gain gives us our maximum input before it overdrives. This works out to 12.5 watts. PEP.
Divide this by the magic number (the square root of 2, doubled, i.e. 2.83) to get the RMS watts your dead key is measured in. 12.5 divided by 2.83 equals 4.4 watts deadkey MAXIMUM. Anything over that, it will distort when you approach 100% modulation.

So, if you are using this amp under close to ideal conditions, set your radio to 12 watts SSB and 4 watts AM.
Given that the amp does not have an ALC circuit , I'd set it a bit lower to be on the safe side - say 10W SSB and 3W on AM.

Now you can work out how much it will increase your transmit signal and range.

An "S" point (that your signal meter measures) is equal to 6dB. Your signal boost is 10dB. Therefore, your signal will increase by roughly 1 and a half "S" points. The "S" points are usually 1, 3, 5, 7 ,9, 19 (S9+10), and 49 (S9+30).

A signal of "3" will increase to 4 and a half.
A signal of "5" will increase to 6 and a half.
A signal of "7" will increase to 9.
A signal of "9" will show no increase.

In other words:the person at the other en d will barely notice any difference!!!

You might think "But I have increased my power by 10 times"... sorry, thats the way our ears (and radios) work.
You will, however, be 10 times more likeky to interfere with TV / radio reception, audio systems, alarms, etc.

How much will it increase range?
If you are talking to the locals on AM, not by much. Range is determined by obstacles such as hills and buildings, and the curvature of the earth, as well as the frequency band in use. The extra power is useful, however, if you are using "skip" - bouncing your signals off the outer atmosphere to talk to people hundreds or thousands of miles away. You'll need SSB for this in most cases.

Hope this helps.

Cheers

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Jonathan_lemoine
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Re: Mods in general

Post by Jonathan_lemoine » Friday 15th May 2009, 10:32

Thank you. Another very informative post.
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