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Discussion Starter #1
Hi,

I'm hoping someone out there can help me with this. I lost the instructions to my Radio Shack model 21-533 SWR meter. This is the cheaper one that has a large meter on the left side, a two position switch (SWR/Calibration/FS) near the middle and then a calibration knob on the far right side.

My question is when I put it in Calibration mode, where to I adjust the needle to? Once it is calibrated, what SWR reading am I trying to tune the antenna to? I seem to recall that 1.5 to 2 is a desired strength. Is that correct?

Thank you,

Chris
Santa Rosa, California
 

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Commnly you set the switch to calibrate and turn the calibrate knob till the needle is on the "set" position or far right edge of the meter range. The perfect tune is 0 swr but that of course is unattainable. Anything higher than zero means more power reflected back to the CB and thus more heat until its fried. Under 2 is ok but won't give you a long life out of the CB. Try and get to a reading of 1-1.5 on your most often used channels. Most people will tune the radio at channel 19 ( midrange) or 1 or 40 depending on their common usage.
 

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I don't have one of these, I use a regular power meter and measure forward and reflected power and calculate VSWR from this.

However, when in Cal mode, I would guess you adjust for a Full Scale reading based on your description. Ideally, a VSWR of 1:1 means no reflected power. Anything higher than 2:1 can cause your CB output transistors to fail due to excessive standing voltage waves.

Keep your RG-58 coaxial cable at 18' in length and start your antenna tuning on channel 19 since this is the geometric center frequency of the band. Also use a braided ground strap, or very small gage (large size) multi-strand wire, for your antenna connection.

Good luck and if you have trouble tuning, check for shorted connections by removing the cable and checking center conductor to ground. If that is good, make sure the ground is solid at both ends, then make sure your power is clean by connecting directly to the battery for your positive lead and engine block tie point for your negative.

Ken
 
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Discussion Starter #4
SWR meter

Do not worry about the SWR damaging your radio if it is under 2:1, (although you should tune it as described for lowest SWR). The CB output transistors will cut back power as the SWR increases. You are dealing with a small amout of power in CB, you probably won't damage the radio even if the SWR is over 2:1 due to the power cut back. The biggest reason to attempt to achieve best SWR is this protection circuitry. The reflected power is not why the CB reduces output, it is the mismatched load a high SWR represents.

You should tune for lowest SWR, and if 2:1 is the best you can do, don't worry, you will never be able to hear the difference. If you can't get it to at least 2:1, something is wrong, probably a problem with adequate ground.
 
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Chris I have the 21-524, but I believe the instructions are the same for cal.

1. Set Function to CAL (Duh!)
2. Select a channel and press the transmit key on the transmitter. Do not speak into the microphone while you make this measurement.
3. While the singal transmits, rotate CALIBRATION until the meter's needle aligns with CAL.
4. Let go of the transmit key (The instructions don't tell you this.)

Now go on to check your settings.

Ron
 
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Discussion Starter #7
Rubimon and everyone else,

Thanks, I knew this forum wouldn't fail me. So much easier than writing to Radio Shack.

Thanks,
Chris
 
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