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AM CB Myths???

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  #1  
Old 08-08-2010, 05:40 AM
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Default AM CB Myths???


Quote:
Lets Explode a Few CB Myths
4) To get out you need lots of swing in your AM signal.

Wrong. 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.

If you really want to see the needle swing when you talk purchase a sideband capable radio. Sideband is a much more efficient means of transmitting. You will be able to talk much further with alot less power. The signal to noise ratio on sideband is much better than AM which means you should be able to hear weak signals better on sideband than on AM.

5) The myth of CB AM swing.

What most CB operators do not know is that AM does not just swing forward. It also swings backwards. The reason a watt meter only shows the forward swing is because the watt mater contains something called a diode. The RF signal from the transmitter is passed through the diode before it gets to the actual needle on the watt meter. The diode only allows electricity to flow in one direction, so the meter only sees the postive half of the "swing". If the diode was not in place the meter would not work correctly, because the needle could not possibly move fast enough in the positive and negative directions to show the RF signal in it's true state.

Let's say you have a 4 watt AM carrier and 2 watts of forward audio swing. 4 watts plus 2 watts equals 6 watts. Ok that works. Now the audio swings 2 watts in the negative direction, so the 4 watt carrier minus the negative 2 watts of audio swing equals 2 watts of carrier at the most negative portion of the "swing". Ok that works too.

Now lets suppose you have a 4 watt carrier and 6 watts of audio "swing". On the forward portion of the "swing" 4 watts plus 6 watts equals 10 watts so that works. Now the audio attempts to swing negative by 6 watts. Well you only have 4 watts of carrier, so it is impossible for the audio to swing negative 6 watts because 4 watts of carrier minus 6 watts of audio swing equals negative 2 watts. You can't put out less that zero watts much less two watts less than zero watts, so the negative portion of the swing gets chopped off after the carrier reachs zero. The chopping off of the negative swing of the audio translates in to distortion of your voice over the air.

So what do you add to your AM audio when you add a "swing kit" to your radio? Well the answer is distortion of your audio. The greater the forward swing ratio to carrier output the greater the distortion of the audio. Swing kits are snake oil non-sense, and so are the "Elkin" type CB amps that are designed to generate large amounts of "swing".
Is any of this true? And If so why do we put swing kits in our radio's in the first place? I have been searching on what the right deadkey with and without and amp and this just tops the icing on the cake
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Old 08-08-2010, 06:07 AM
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Originally Posted by Simbalage22 View Post
Is any of this true? And If so why do we put swing kits in our radio's in the first place? I have been searching on what the right deadkey with and without and amp and this just tops the icing on the cake

Wrong. 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.

That part is partly true,the swing should double the carrier peak voltage,but it quadruples the pep power not doubles it p=v squared/r

ie if peak carrier voltage was 50v then 50v x 50v /50ohms = 50w

if peak voltage is doubled with 100% modulation added,then it becomes 100v x 100v /50ohms = 200w pep

if you want avg of peak power (avg peak power = rms of 100 peak volts squared /50 ohms) then it would be 70.7v (the rms voltage of the 100v peak) x 70.7v /50 ohms = 99.9698w avg peak.which is probably where he got his doubling of power from by using a non powered peak reading meter.

a 4w key should swing approx 15.6w pep for 100% modulation.

whoever wrote that has confused voltage and power.anything over 100% modulation is distorted whether you can hear it or not.

have a look at the bottom pic (a pic is worth a thousand words) in this link you'll see what happens when you overmodulate,the green graph is a spectrum analyser representation,watch as the harmonics either side of the fundamental frequency grow the more you overmodulate:

Amplitude Modulation

Keep it clean,sound mean.its not rocket science!!!
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Last edited by jazzsinger; 08-08-2010 at 06:54 AM.

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Old 08-08-2010, 06:16 AM
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I too have read this article. Seems pretty interesting and informative. The ratio of modulation to carrier should be about 2:1 on a watt meter. So for every watt of carrier you need 2 watts of swing. This should equal about the right amount modulation. Anything more will just add to distortion of audio. So unless you want to watch the needle swing to the right as far as possible and don't care about someone understanding you, than I would say try to keep swing relatively low. I'm guessing if you add an amp, you would still need to keep the same 2:1 ratio to sound the best.

Truckers and competition CBers like to put a swing kit in their radios. Yes, it DOES make the radio put out more power, No it probably doesn't make it "get out" any better, or sound any better. And if it does get out better, you probably won't understand it anyway. I've moded my buddies radio's to swing on their request and I warn them about what it will do, but most people don't care, they just want to see the needle swing and tell everyone their radio is badass. Honestly, a CB "peak and tune" usually just consists of the guy cutting the modulator limiting resistor, and then tuning the coils out. So the radio will splatter like hell.

The newer radios' I've bought all seem to mostly have tuneable modulator pots that can be used to increase or decrease the modulation level to get that sweet spot, but it does it all within the limits of the radio, so you don't have to worry as much about splattering it unless you cut something out and tamper with it.

Backward swing is also true, though I don't know the math or the theory behind it. Watt meters will only read forward power.

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Old 08-08-2010, 06:38 AM
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Originally Posted by Simbalage22 View Post
Is any of this true? And If so why do we put swing kits in our radio's in the first place?
people put swing kits on radios because they listen to folk who don't know half of what they make out they know,its that simple.louder isn't necessarily better and thats the concept they have difficulty getting their heads around,go turn your hifi up full blast till it starts clipping and you will see sure its louder,but it sounds rough as f@ck,exactly what you are doing when you modify radios.a small amount of clipping can help cut through in difficult conditions when signals are low,but it comes at the expense of audio fidelity and is tiresome on the person receiving its' ears.
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Old 08-08-2010, 07:19 AM
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People put swing kits on radios because the average person wants to "SEE" something happening. The more they "SEE" happening, the happier they are. Since you cant "SEE" radio waves, per se, the swings the thing.
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Old 08-08-2010, 07:23 AM
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Originally Posted by packrat View Post
People put swing kits on radios because the average person wants to "SEE" something happening. The more they "SEE" happening, the happier they are. Since you cant "SEE" radio waves, per se, the swings the thing.
Nailed it on the head. Great answer, wish I had thought of it.
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Old 08-08-2010, 07:51 AM
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Originally Posted by pro151 View Post
Nailed it on the head. Great answer, wish I had thought of it.
pity you can't see distortion either.if the average person could see and understand a spectrum analysis of their signal after adding a swing kit,they'd never do it.

not to mention the increased strain they put on their final transistor/s which can only result in shortening their lifespan.
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Old 08-08-2010, 08:15 AM
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Tune the radio for "optimum" not "maximum" performance, install a good antenna and if that doesn't do it for you, add an "antenna stretcher".
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