You would b better off googling a trc-493 owners manual .
SOURCE: Radio Shack TRC-502 cb radio
Here is the owners manual I'l see if I can get my hands on a internal schematic
http://support.radioshack.com/support_electronics/doc62/62220.pdf
SOURCE: radio shack trc-502 cb radio
YOU CAN GET SCHEMATICS FOR THAT MODEL
I JUST LOOKED IT UP FOR YOU........THE BOOK IS SAMS PHOTO FACT...WEBSITE IS
WWW.SAMSWEBSITE.COM
THEY HAVE EVERY CB RADIO EVERY MADE
GOOD LUCK
SOURCE: tunning the antenna on radio shack cb trc-447
After installing the antenna, follow these steps to adjust the
standing wave ratio(SWR).
1. Turn on the CB.
2. Set S/RF/SWR/CAL to CAL.
3. Key the Mic and rotate SWR CAL so the meter points
to CAL.
4. Unkey Mic.
5. Set S/RF/SWR/CAL to SWR.
6. Key Mic again and note the actual measurement on
the SWR scale. Refer to the following table to interpret
the reading.
1:1 - 1.5 Great
1.5 - 2:1 OK
2:1 - 3:1 Not So Good
Higher than 3.1:1 Terrible
The ideal standing wave ratio (SWR) is 1:1, or a meter
reading of 1 on the SWR meter's top scale. A SWR ratio
of 1.5:1 to 2:1 is excellent for most mobile CB antenna
applications.
Check the SWR on Channel 1 and Channel 40
If the SWR is higher on 1, make the antenna longer
If the SWR is higher on 40, make the antenna shorter
Most antenna's are adjustable. Make small adjustments at a time.
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