[color:"red"]Antenna Information -- including how to make CB antennas work better[/color] I commented above on a Kenwod antenna that didn't work well.
This next part applies to CB antennas as well: They're not all equal (and while size matters, so does how you use it). <img src="/forums/images/graemlins/evil.gif" alt="" /> Many antennas have the ability to be tuned by adjusting the length (or even just soldering on a lump of metal or wrap of wire).
Antenna primer: All antennas stink. It's tough to get a good match over more than 10% of their frequency band. So never expect greatness. That's why good antennas are either frequency specific, or have tuners that come with them so you can "on the fly" adjust them for a better match.
At work I have a network analyzer that I can use to inject RF power into an antenna, and measure how much power is reflected back (this tells me how good the match is).
Bigger return loss = better. If a device has 6 dB of return loss, the reflected energy back from that device (which goes back into your transmitter--not good!) is 6dB lower than the incoming energy. Remember that in decibels, 3db = 2x power (or 1/2 power, depending on the way you are measuring). So 6db lower means 1/2 * 1/2 = 25% power is reflected back.
Notice also that this is all exponential growth--A VSWR of 3 is 6x worse than a VSWR of 1.5.
VSWR = 1.0 --> Return Loss = infinity --> Reflected Power = 0%
VSWR = 1.01 --> Return Loss = 46. dB --> Reflected Power = 0.005%
VSWR = 1.5 --> Return Loss = 14. dB --> Reflected Power = 4%
VSWR = 2.0 --> Return Loss = 9.5 dB --> Reflected Power = 11%
VSWR = 3.0 --> Return Loss = 6.0 dB --> Reflected Power = 25% Ouch!
VSWR = 4.0 --> Return Loss = 4.4 dB --> Reflected Power = 36% Ouch!
VSWR = 6.0 --> Return Loss = 2.9 dB --> Reflected Power = 51% Super-Ouch! Over half the power you transmit out is reflected back.
* Note that the law of reciprocity applies--power coming into the antenna from other people's signal is likewise reflected away from entering the radio and allowing you to hear it.
I put my Kenwood HT antenna on our network analyzer--the Kenwood antenna is an 8 inch rubber antenna that came stock with the radio. The antenna has a VSWR/return loss that's all over the place as I wiggle it. That's bad--it should vary a little, but not much. Holding it steady, I read across the 420-450 MHz frequency band that the VSWR is 4 to 6. That is incredibly horrible for a fixed antenna.
I went with my better Comet antenna. (looks
like this but with only 2 sections). It's a fixed metal antenna with the extra ability to pull it up (pulling a spring in the base) then fold it over for garages. Neat trick, but generally the more mechanical connections, the worse overall match you get as the RF bounces off each new joint/connection. But I like the foldover and got it.
It also has a little impedance matching trap at the bottom. This means even without the metal wire poking out, it's a reasonably good match (but it wouldn't transmit much farther than out of the room without the vertical wire).
When I hooked it up, the VSWR ran from 1.7 to 2.5 at the 140-148 MHz band, and 2.5 to 4 at the 420.0-450.0 MHz band. Not so good.
To fix this, I loosened the hex set screws and pulled it apart. Next, I cleaned it to insure good connections. Then I put it back together. Trial and error while hooked up to the analyzer told me the bottom half was the 440 MHz antenna, and the entire antenna was for the 144 MHz.
I put it back together and adjusted the bottom length to get a better match on 440 MHz (now it's around 2 to 3) and tightened it down. I then adjusted the top section to get a better match at 2 meters (1.5 to 2). I tightened it down, verified my 440 MHz again, and called it good.
When I tested it on my portable ham radio (look
now I'm at work, the stock antenna could barely hear outside traffic, and could not transmit. The newly tuned (and much larger) antenna had full signal strength on transmit and receive.
So if you have a CB antenna (on your [color:"red"]Isuzu[/color]--nod to Jim)....ask someone to check and tune it with an SWR meter. Keep your connections clean, and your cables short. You will transmit farther and "hear" better, and even lengthen the life of your radio. If you have a loose or broken connection inside it, this will find it (replace antenna). If you have a crummy antenna, this will find it (replace antenna). If your match is off a little, tuning and adjusting can improve it.
If you want to understand reflected power (DON'T TRY THIS!), find someone with a ham radio in their Isuzu (not me), pull off the antenna (horrible match since there's nothing there), set it to 50 watts transmit (high power) and hit transmit. 50 watts leaves the radio, hits the end of the cable, and 100% of the power will reflect back into the radio and destroy it.
