| [GMCnet] CB, SWR meter [message #296750] |
Sat, 05 March 2016 16:29  |
BobDunahugh
Messages: 2465
Registered: October 2010
Location: Cedar Rapids, IA
Karma: 11
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Senior Member |
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Below is something that was sent to me as to a CB radio. Any input as to if I need it. Bob Dunahugh
CB radios operate on channels 1-40, and one antenna cannot provide ideally tuned reception for all of those. Most people opt for the middle ground and tune the antenna for best reception at the mid-point. One easy way to tune an antenna is to cut it to length according to the frequency that you want it tuned for. The measurement instrument that's used for tuning is an external Standing Wave Ratio (SWR) meter. That meter will also help with trouble-shooting your equipment to find a bad coax, a poorly grounded mount or a weak or non-functioning ground plane.
I personally think this is a good idea for the trouble shooting aspect of any cb
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| Re: [GMCnet] CB, SWR meter [message #296752 is a reply to message #296750] |
Sat, 05 March 2016 17:01  |
habbyguy
Messages: 896
Registered: May 2012
Location: Mesa, AZ
Karma: 3
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A SWR meter is a good idea when installing / setting up any radio transmitter. To overly simplify it, an SWR measures how many watts are going forward (out) and how many are reflected (back). Ideally, nearly all of your power (a massive five whole watts in the case of a CB radio) is going into the antenna and out into the ether, with almost none reflected. An SWR meter will tell you if you have a gross defect, such as a pinched or open transmission line, or defective antenna or ground. And of course, it will give you an indication of how much power your CB radio is transmitting as well (pretty important).
And yes, it's difficult to get perfect tuning across all 40 CB channels, but if you just trim the antenna for minimal reflected power in the middle of the band (say, channel 20), it's going to be plenty close enough at either end (channel 1 or 40) that you'd be hard pressed to measure any difference in efficiency at either end... the small losses at the extremes would be kind of like hanging a single-ply tissue in front of your mouth in a crowded NYC bistro.
Mark Hickey
Mesa, AZ
1978 Royale Center Kitchen
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| Re: [GMCnet] CB, SWR meter [message #296756 is a reply to message #296750] |
Sat, 05 March 2016 18:00 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
Karma: -74
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Most of the current crop of DCB sets have a reflected power position on the meter. Have a look at it and tune the antenna for lowest without dropping forward power.
--johnny
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
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| Re: [GMCnet] CB, SWR meter [message #296780 is a reply to message #296758] |
Sun, 06 March 2016 04:24 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Senior Member |
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The theoretical 1/4 antenna length on Channel 1 is 109.5 inches. The theoretical length at channel 40 is 107.75 inches. So you are looking at a length difference of 1.75 inches end to end or .75 inches from center to either end. So what you are discussing is a nit. Notice I said "theoretical" as there are other things that affect antenna length. As others said here throw a meter on it and adjust for minimum SWR in the middle of the area you wish to cover and be done with it.
Keep in mind that you are discussing a radio that generates 1/2 of the power of what a single tail light on your GMC consumes or about 1/8 of the power of a single brake light uses.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: [GMCnet] CB, SWR meter [message #296789 is a reply to message #296750] |
Sun, 06 March 2016 07:47 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
Karma: -74
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Senior Member |
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Given the minuscule power we're talking about, it behooves you to pay some attention towards making the system radiate what's there. No sense in wasting any you don't have to.
--johnny
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
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| Re: [GMCnet] CB, SWR meter [message #296806 is a reply to message #296789] |
Sun, 06 March 2016 10:12 |
tphipps
Messages: 3005
Registered: August 2004
Location: Spanish Fort, AL
Karma: 9
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Trimming to frequency that you desire to use (Channel 19?) makes sense for transmitting. If you are only going to listen to the trucker's chatter, no real improvement on the receive side of the question. Channel 19 is 27.185, mode used is AM. SSB is normally on Channel 36 - 40 (27.365 to 27.405).
I totally agree with Ken, not worth troubling yourself over. Plus, I don't think that you are going to install a 1/4 wave antenna. Most, if not all of the CB antennas are compromises in any case.
Tom, MS II
See you in Dothan.
2012 Phoenix Cruiser model 2552
KA4CSG
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