Question - CB installation (setting SWR's)greenspun.com : LUSENET : TimeBomb 2000 (Y2000) Preparation Forum : One Thread
I'm setting up a mobile cb unit as a base station. It will be powered by two 6 volt batteries joined together to make 12 volts. I'm using a 5' fiberglass whip antenna (meant for mobile - mirror mount) which is attached to the side of a wood building. When I tested the SWR's, they read a 3. I thought that maybe the antenna might need to be grounded so I ran a 16 ga. wire from the antenna mount to the negative post on the battery bank. This dropped the SWR's down to about 2.3. I can't seem to get any lower than that. Preferably, I would like to get them below 1.5. The antenna has a brass bolt at the top to adjust the SWR's but it makes very little difference. Does anyone have any idea why I can't get the SWR's any lower? Does it have anything to do with not being grounded to something large like a car (ground plane?). Is there much difference in transmission when the SWR's are 2.4 as compared to below 1.5? I did get a radio check and the radio seemed to be transmitting to at least 4-5 miles. The reception seemed very good. Thanks in advance for any info.
-- NokternL (email@example.com), December 24, 1999
1. You might try running the battery negative to a ground rod in order to create a large ground plane (the earth).
2. The SWR is an indication of mismatch between the transceivewr and the antenna. It can also be expressed in terms of 'return loss,' meaured in dB (decibels). In other words, a high SWR indicates that some of the power going from the transmitter to the antenna is not going out the antenna, but is being reflected back into the transmitter (and that some of the received signal is not being absorbed by the receiver but is being reflected to the antenna).
At a 1.5 SWR not much power is being reflected (about 1-2% if memory serves). Above 2.5 you would be losing some power, but the loss would not be really noticeable unless you were fighting for eavery foot of range. Given the limited power output from a mobile unit, you probably won't notice much difference.
-- (firstname.lastname@example.org), December 24, 1999.
You would be better off to make a simple dipole antenna. Take 16 gauge strande wire and cut two pieces 102 inches long. Solder one to the positive coax lead and one to the negative. Stretch it parallel to the ground and there you go.
design your own dipoles and Inverted Vees
Another thing that can give you a high Standing Wave Ratio (SWR) is a bad feedline length. FOr 11 meters (cb band) try to use coax lengths that are proportional to the wavelength transmitted..... roughly this works out to 18, 36, 56 feet.....1, 1 1/4, 1 1/2 wavelentgh etc.
Your best bet for an antenna would be an Antron or similar base antenna, but I have had excellent results with homemade wire antennae.
-- Forrest Covington (email@example.com), December 24, 1999.
Mobile CB radios use an antenna which is attached to the metal in your car's frame/chassis/etc. This creates a GROUND PLANE for your antenna (in essence, the entire car). Without such a ground plane, high SWR ratios WILL burn out your "primary" transistors ("Primaries").
I took a bed-spring unit, and used vise-grips (with an antenna mount welded onto them, available at ALL truck stops) to attach the antennas to the bedsprings. Worked like a charm. SWR about 1. BUT, when transmitting, EVERY TV and RADIO in the house heard what I was saying!
Best of luck in your installation.
-- Dennis (firstname.lastname@example.org), December 26, 1999.
You can make the ground plane part for your antenna by using 3 or 4 radial wires. join them all together at the entenna end, and ground them to the metal mount. Then spread the radials out equal spaced. Angle them somewhere close to 45 degrees down from the vertical. They should be somewhere between 8.5 and 9 foot long, each, terminating with an insulator. (This can be a store-bought one, or just a plastic coat button or similar). Then attach the other end of the insulators to nylon cord and anchor them to somewhere convenient. Have fun. It is simple and it works. You just have to be prepared to test and tweak a bit.
-- David Harvey (email@example.com), January 02, 2000.