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General SWR

conkers

Big Landy Fan
Getting a decent SWR reading is nigh on impossible anyone got any decent tips. the antenna is supposedly pre set at the factory.
I'm a complete novice at all this so any tips welcome.:)
 
Hello.

First check all the contacts are clean and corrosion free. use a multimeter on the continuity check across all the joins. Also check the Co-ax cable that it has a low resitance. I have heard that water can get into it and corrode the inner so causing a short.

then As copied from another forum. Credit to Budgie for this usefull advice.

SWR meter:
Plug the coax cable from the aerial into the "Ant" side of the meter, you'll need a "Patch Lead" which goes into the other socket on the meter with the other end into the aerial socket on the CB.

If you've got a 40 channel rig then put it on Channel 01 and set the SWR meter switch to the "FWD" "DIR" or "CAL" (which ever your meter has on it) side and transmit.
Using the rotory button on the SWR meter while you transmit, set the reading the infinity (as high as it will go on the scale & there should be a sign like a number 8 laid on it's side) and stop transmitting.

Flick the switch to "REF" and transmit again, reading off where the needle gets to on the scale. Remember this figure and do the same on channel 40 (remembering to reset the meter to infinity on channel 40).

If the figure is higher on channel 01 then on 40 then the aerial is too short and needs lengthening, likewise if it's higher on 40 than on 01 then it needs to come down.
High on the High it needs to come down, High on the Low it needs to go up!

You are looking for a reading that is the same on both 01 & 40 and below 2 on the scale. The lower you can get it the better!!

You can normally lengthen or shorten the aerial with the grub screw at the base of the whip, if it needs to be shortened more then you can cut small amounts off the whip (about 5mm at a time) until you get the right reading. Remember: You can't add material onto the whip so don't cut too much off!!

Have fun!!
 
Hello.

First check all the contacts are clean and corrosion free. use a multimeter on the continuity check across all the joins. Also check the Co-ax cable that it has a low resitance. I have heard that water can get into it and corrode the inner so causing a short.

Your multimeter will only tell the difference between a connection and a non-connection. It can't tell you if you have a poor connection because it is using DC to measure resistance. At radio frequencies (RF) things behave a bit differently, and a perfectly adequate DC connection can look like an open circuit to RF. The best way to check connections is to make sure that any solder has flowed properly, and is shiny silver, not powdery or matt coloured.

Similarly, your multimeter won't tell you the condition of your coaxial cable. It's important to strip a bit of the cable back and lok at the braided screen. If there's water you'll see a powdery substance or discolouring of the braid. BTW, if you can easily see the centre conductor insulation through the braid, then you have lousy coax, and you really should get some better quality coax.

An easy way to set up your antenna is to borrow an antenna analyser such as the MFJ unit I use. It's incredibly easy and quick, and it tells you exactly what your antenna "looks like" to your radio. An ordinary SWR meter simply tells you the SWR.

A common mistake that people make is to become maniacal about getting a "perfect" SWR. Any SWR below about 1.8:1 to 2:1 isn't going to make any difference in practical terms.

Three things have an affect on your antenna performance:
1) The condition and quality of the connectors and cable that you use. Joins in antenna cables are problematic, and must be dealt with properly.
2) Nearby objects (trees, carport, people, bull bar, roofrack etc.)
3) MOST IMPORTANT - the condition of the connection between the antenna ground and vehicle ground. Simply using a pop rivet into aluminium is NOT adequate, and relying on the antenna base "claws" to cut into aluminium is not adequate. You must prepare the surface of the aluminiun with fine sand paper until it shines, then mount the antenna, and cover it all up with a generous blob of silicone sealant. DON'T USE THE SEALANT THAT SMELLS OF VINEGAR. It causes oxidation of the copper and aluminium.

I've attached two photos.

The first shows a poor solder connection. Apart from being the wrong way to install the connector in the first place, insufficient heat was used, and the solder didn't flow. This would causea "dry" joint, and will cause problems.

The second shows how NEVER to join coaxial cable. The relationship between the dimensions of the inner and outer core and the material that the insulator is made from are critical for cable to work properly at RF. A mulitimeter would show no fault in this cable, but at RF all sorts of problems will be caused.

Regards,
Marc
 

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Another thing you need to check is that the solder on the tip of the coax connector isn't too big as this will prevent the connector going all the way into the CB. I couldn't get an swr below 3 no matter what I did. Untill I filed the solder off the tip of the connector and used pliers to tighten it to the CB. As soon as I did that the swr went to 1.5.
 
Another thing you need to check is that the solder on the tip of the coax connector isn't too big as this will prevent the connector going all the way into the CB. I couldn't get an swr below 3 no matter what I did. Untill I filed the solder off the tip of the connector and used pliers to tighten it to the CB. As soon as I did that the swr went to 1.5.

The most comon cause of a big blob of solder at the end of the pin on the connector is insufficient heat used while soldering. If heat is applied correctly, the solder will flow into the pin due to capillary action. There should be no solder that is proud of the ridge of the pin. (I'll post photos soon).

Marc
 
The most comon cause of a big blob of solder at the end of the pin on the connector is insufficient heat used while soldering. If heat is applied correctly, the solder will flow into the pin due to capillary action. There should be no solder that is proud of the ridge of the pin. (I'll post photos soon).

Marc

This one came with the aerial and was already soldered. when I got it.
 
They should shoot the manufacturer! :D

attachment.php

It's a bit difficult to see, but the connector on the left shows that the solder has flowed into the centre pin. The connector on the right is poorly soldered. More heat should be applied, and the solder will flow.

Marc
 

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They should shoot the manufacturer! :D

attachment.php

It's a bit difficult to see, but the connector on the left shows that the solder has flowed into the centre pin. The connector on the right is poorly soldered. More heat should be applied, and the solder will flow.

Marc

mine had flowed ok just too much of it. Got it from thunderpole about 2 years ago. fitted it 2 weeks ago.:rolleyes:
 
Just a simple rule of thumb to remember when SWRing CB aerials "LOW, LOW, LOW".

If the SWR is LOWer on the LOW channels than the high channels then the aerial needs to be LOWered (ie shortened) in order to centre the SWR across the chanels.

A bit of a simplisic approach to tuning an aerial I know but at least it gives you some idea as to whether you should be moving the whip up of down to achieve a good SWR (assuming the rest of the system is fault free of course).:)

cheers, Tim
 
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