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Adjusting CB aerials

 
 
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Difflock Royalty


Joined: 19 Jun 2002
Odometer: 40007
Location: Northern Ireland's Gold Coast


2009 Land Rover 110 CSW

PostPosted: Thu Jul 16, 2009 5:27 pm    Post subject: Adjusting CB aerials Reply with quote

As this question gets asked over and over here's couple of pretty good guides

Quote:
CB Antenna Tuning Guide

So you’ve wrestled your CB radio into the dashboard and you’ve got your antenna mounted on a space with decent ground plane. Everything is connected and ready to go, right? Wrong. It’s vital that you tune your antenna before using your new CB radio. If you’re not familiar with the importance of the concept behind SWR or the necessity of adjusting it, let us give you fair warning: improper tuning of your antenna has the potential to cause much worse than a weak broadcast signal – it can end the life of your radio before you get a chance to enjoy it.

The good news is that this article will walk you through the process of properly tuning your antenna (aka adjusting the SWR). It’s really not a terribly difficult process, as long as you can follow directions and are patient enough for a little trial-and-error. Assuming that everything else in your system is properly installed, the only additional equipment necessary is a short length of coaxial cable (known as a jumper lead), an SWR meter, and something on which to record your readings.



1. The first thing you need to do is find a suitable location to park your vehicle. There should be no obstructions, such as trees or buildings, within 10-15 meters of your antenna. Neither you nor your buddies should be hanging out around the car either – make sure that you’re inside with the doors and windows closed to ensure an accurate reading.

2. The next step is to hook up the SWR meter. First, disconnect the coaxial cable from the back of the radio. Reconnect this end of the cable, which is going to the antenna, to the SWR meter in the connector marked “antenna” or “ANT.” Next, use the jumper lead and connect your radio and the SWR meter through the connection marked “transmitter” or “XMIT.”

3. Now you are ready to measure the SWR on a few different channels. Remember throughout this process that it is important to keep the microphone the same distance from the meter for each test.

4. Set the switch on the SWR meter to “FWD.”

5. Turn the radio the Channel 1.

6. Key the microphone (depress the button and hold it).

7. Turn the knob on the SWR meter labeled “SET” or “ADJUST” until the needle reaches the setting position at the end of its range.

8. While still keying the microphone, flip the switch on the SWR meter to the “REF” or “SWR” position.

9. Quickly record the reading given by your SWR meter and release the transmit key on your microphone.

10. You are now going to repeat this process for channel 40. Follow steps 4-9.

The objective behind tuning your antenna is to make these two readings as close as possible. Getting down to a 1.5:1 ratio or below makes for a passable broadcast signal. There are two basic points to understand before adjusting the length of your antenna:

* If the SWR on channel 40 is higher than that on channel 1, your antenna is too long.

* If the SWR on channel 1 is higher than that on channel 40, your antenna is too short.



If your antenna is too long, it is necessary to reduce its physical length. There are several methods for shortening an antenna which vary by manufacturer. Consult your owner’s manual for detailed instructions on how to shorten your antenna. While many antennas feature a “tunable tip” that makes use of a small screw, some antennas may need to be cut to be shortened. Do so in 1/4 “ increments and then get new readings to determine your progress.

If your antenna is too short, it is necessary to increase its physical length. Most instances where the length of an antenna is too short are caused by a lack of ground plane. In modern antennas, there is usually a method for adding length built into the antenna. Other options, such as adding a spring or a second antenna, are also legitimate options.

Readings on both channels that are less than 2.0 mean that your radio is safe to operate, but transmission may not be optimal. If readings on these channels are in the red zone on your SWR meter or above 3.0, do not attempt to use your radio. This problem must be remedied before attempting to use your radio.

Let’s review the most common problems that cause your SWR meter to scream danger on all channels: poor grounds, a short in the coaxial connectors, or an improperly installed mounting stud.



1. A large percentage of high SWR readings are caused by ground plane problems. It is a good idea to run ground straps from the body of your vehicle to the frame, doors, trunk, etc. Everything except your dog. Running the shortest possible ground strap from the antenna to the chassis or your vehicle is generally a good solution for ground plane problems. Simply put – grounding everything that can be ground together will improve ground plane.

2. It is essential that your mount is properly grounded. Most improperly grounded mounts are connected to places on your vehicle that themselves are not thoroughly grounded. Any part of your vehicle that has a plastic or nylon bushing separating it from the chassis is probably not grounded. Additionally, chassis paint can often prevent a mount from being properly grounded. You can check the grounding of suspect parts with a voltage meter.

3. A short in the coaxial connectors may also be the culprit behind abnormally high SWR readings. Issues with the coaxial cables are often identifiable by eye, such as severe bends or pinches. You should know that it is essential to use 50-ohm coax for single antennas and 75-ohm for dual. When all else fails, sometimes it is just necessary to replace the coaxial cable because there is a failure inside the line.

By following the steps outlined in this article, you should be able to successfully tune your antenna for optimal performance and transmission.


Quote:
* The antenna should be outside and at least 15 feet from any buildings or large objects such as trees.
* Do not touch the antenna while transmitting.
* Do not let anyone stand close to the antenna. it will throw your SWR reading way out.
* Objective is to obtain an SWR of less than 2 to 1 (2.0:1) on all channels from 1 to 40.
* If you experience a high SWR on all channels you are probably experiencing a ground plane deficiency. recheck all your cables and grounds.
* If the SWR on channel 40 is greater than that on channel 01, the antenna is considered to be 'long' and reduction of physical length is necessary.
* On m/c antennas with a center loading coil, adjustment is made after loosening the top Allen screws.
* If it is required to cut the antenna, the amount that you cut off should be in increments of no longer than 1/8th of an inch.
* Remember, cutting is your last resort after all possible trouble areas have been eliminated.
* If the SWR on channel 01 is greater than that on channel 40, your antenna is considered to be 'short'.
* You should recalibrate the meter each time you make a channel change or adjustment in antenna length.
* Coax for 50 ohm systems employing a single CB antenna should be RG-58A/U.
* Cables supplied with CB antennas should not be cut or shortened as they are a predetermined length.
* Do not tightly coil excess cable as this will effect your SWR.

__________________________________
Bert the Defender 110 XS - because it's Cool (work it out yourself!)
Lolita the Lightweight
???? the V8 90 CSW
Suzuki DL1000 V-Strom - yes that's right, I have a Zook!
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chrisvitarasport94
Off-Road Guru


Joined: 17 Jun 2005
Odometer: 1255
Location: South Devon



PostPosted: Mon Feb 23, 2015 8:59 pm    Post subject: Reply with quote

what do we think about the CB aerial mounts that are "arms" connected to the spare wheel mount, and locating the base of the aerial high up but just below the roof line?

any good? can I get a good SWR with such? ( soft top Vitara )

__________________________________
'94 3dr s/top Sport, roof bars, roof lights, Calmini bumper, SuperwinchEP9, CB & handheld, GPS, fwh's, 8x15 alloys, 31x10.5x15 BFG MT K/M's, 2" body lift, skid plates, 2.5" susp lift, & hi-lift. Safari Snorkel. R Lockright
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Difflock Royalty


Joined: 19 Jun 2002
Odometer: 40007
Location: Northern Ireland's Gold Coast


2009 Land Rover 110 CSW

PostPosted: Tue Feb 24, 2015 12:21 pm    Post subject: Reply with quote

So long as it makes a good connection to enough metal to create an effective groundplane then I can see no reason why they wouldn't be OK, especially if the aerial clears the roof so that there won't be reflection from the body.
__________________________________
Bert the Defender 110 XS - because it's Cool (work it out yourself!)
Lolita the Lightweight
???? the V8 90 CSW
Suzuki DL1000 V-Strom - yes that's right, I have a Zook!
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