High SWR reading on CB

[Madcowz]

Hi,

I have just installed a Midland 48 Plus CB from 4x4 CB and am getting high (2.7-3.0) SWR readings despite what I think is a good install and would appreciate some advice.

I have the aerial mount connected at the back top left of the roof using a side mount, and the cable running behind the headlining and down to the radio.

On:

CH 1 I get 2.7

CH 20 I get 2.9

CH 40 I get 3.0

The tests I have run are:

There is no continuity between the ground and live on the cable (tested at the radio end) so that means no short (?)

If I test continuity from the aerial plug (radio end) ground side to anywhere on the body I get a good result. That includes roof, doors, chasis, etc.

If I test continuity from the live pin of the connector (radio end) to the base of the aerial spring (or the grub screw holding the aerial into the spring) I get a good result.

So that says to me I have no short and full earth and live connections from the radio to the aerial, but I am still getting high swr readings.

Where have I gone wrong?

thanks.

EDIT:

I have run the following tests as per the guide on http://www.firestik.com/Tech_Docs/swr-trblshoot.htm

The following list shows the most probable causes of high SWR in order from the most common causes to the least common causes. If experiencing SWR problems, check each of the following conditions in the order shown until problem has been uncovered.

Shorted Stud Mount

Disconnect coaxial cable at antenna mount. Test continuity from mount to antenna coupling nut. There should NOT be any continuity. If shorted, reposition or replace insulators and retest.

Checked.

Improper Type and Length of Coax

Single antenna installations require RG-58 type coax and dual antennas require the use of RG-59 type coax cables. We recommend 18 foot long coax leads from the radio to each antenna, especially on installations that are displaying high SWR problems.

I have heard that the 18 foot rule is a pile of b*ll*x

Shorted Coaxial Cable

Disconnect coax at radio end. Test for continuity between center pin and threaded sleeve. There should NOT be any continuity. If shorted, repair and retest.

Checked, no short

No Chassis Ground at Antenna Mount

Test for continuity between the antenna mount and the vehicle's common ground. There MUST be continuity. Repair and retest as required.

Checked, continuity throughout the vehicle

Open Coaxial Cable Center Lead

Disconnect coax at radio end. Test for continuity from center pin to antenna base. There MUST be continuity. If necessary, repair and retest.

checked.

Open Coaxial Cable Ground Shield

Disconnect coax at radio end. Test for continuity from the antenna mount to the coax connectors threaded sleeve at the radio end of the coax. There MUST be continuity. If necessary, repair and retest.

Checked.

Improper Installation Location

Transmit antennas need free space around them if expected to perform properly. If more than 30% of the antennas overall length is parallel to the side of the vehicle and within twelve inches of that surface, SWR problems are probable, i.e. between truck cab and shell, corner of truck bed near cab, low mount position on motorhome, etc. Relocate the antenna to a position of performance (versus convenience or appearance) and retest.

Spring top is higher than roof, all of aerial is above roof

Insufficient Ground Plane Available

Fiberglass, plastic and thin aluminum vehicle bodies lack the reflective characteristics needed for proper antenna performance. On some occasions, running a 12ga or heavier wire from the antenna mount to the vehicles chassis ground will be sufficient. Otherwise, a no-ground-plane antenna system may be required.

Might need to do this.

Low Quality Coax Cable

Delivering radio frequency to an antenna via poor quality coaxial cable is the equivalent of watering your lawn with a hose full of holes. The bitterness of low quality will linger long after the sweetness of low price is gone. Use high quality coax only.

As far as I know the cable is good

Antenna Tuned Without Tip

If the antenna is tuned without the tip then put on after tuning is completed, the SWR will change when the tip is installed. All readings must be made with the antenna tip in place.

Aerial and tip are all in one

Defective SWR Meter

Some are made bad and others have been known to go bad. Try to confirm readings with a second meter.

?

Damaged Antenna

Aside from apparent damage that is visible from the antenna striking immovable solid objects, you can test for any internal damage by checking continuity from the antenna base to the tunable tip extender or end of wire. There MUST be continuity. If there is no continuity, the antenna must be repaired or replaced.

Tip is not removeable

[Kim Horsevad]

Hi,

I have just installed a Midland 48 Plus CB from 4x4 CB and am getting high (2.7-3.0) SWR readings despite what I think is a good install and would appreciate some advice.

What antenna are you using?

On most antenna the swr is adjusted by changing the lenght of the antenna. Somewhere on the antenna there should be someway of adjusting the lenght.

Otherwise your installation sounds OK.

[Madcowz]

What antenna are you using?

On most antenna the swr is adjusted by changing the lenght of the antenna. Somewhere on the antenna there should be someway of adjusting the lenght.

Otherwise your installation sounds OK.

It's a large springer http://www.4x4cb.com/public/item.cfm?itemID=700

It has a large coil then a separate whip aerial locked in with a grub screw. I can move the aerial up or down in the spring but this has no effect.

I don't want to start chopping lengths off the aerial until I know that is what I need to do.

Otherwise I will end up with a 3 inch aerial and an even higher reading.

[pinny]

if the swr is higher on channel 40 than ch1 the whip needs shortening

[western]

Insufficient Ground Plane Available

Fiberglass, plastic and thin aluminum vehicle bodies lack the reflective characteristics needed for proper antenna performance. On some occasions, running a 12ga or heavier wire from the antenna mount to the vehicles chassis ground will be sufficient. Otherwise, a no-ground-plane antenna system may be required.

Might need to do this.

don't have ant problem with my much more powerful Ham radio antenna's SWR, 2 are on gutter mounts on opposite sides of the roof as is my CB antenna.

sounds like you need to cut some off the whip this might help explain SWR better

[cipx2]

Those type of antennas are not tuned and they are supplied longer than they're supposed to be.

If you're afraid to chop the antenna then replace the tip with a piece of bare solid steel or copper wire aprox the same diameter and exactly the same length and do the chopping tests on it. If you find a suitable length on the replacement wire then cut the original antenna tip a little bit longer and come to the final length in small steps (say 2 mm at a time).

The 18 feet rule is not entirely b****x. The fact that it's better to use a certain length of coax is true, only the "18" and the "feet" parts are wrong (i.e they're not 18 and using feet to measure it is much too coarse).

[Madcowz]

Those type of antennas are not tuned and they are supplied longer than they're supposed to be.

If you're afraid to chop the antenna then replace the tip with a piece of bare solid steel or copper wire aprox the same diameter and exactly the same length and do the chopping tests on it. If you find a suitable length on the replacement wire then cut the original antenna tip a little bit longer and come to the final length in small steps (say 2 mm at a time).

I followed your advice and replaced the antenna with some high tensile fencing wire and then proceeded to happily chop bits off until I got a good reading. Then did the same, but a lot more cautiously with the antenna and now I have:

CH1 1.7

CH20 1.8

CH40 1.9

I assume that I could shave a little off these figures by reducing the length a bit more but I managed to break my pliers and had to resort to cutting with a hacksaw.

Thanks for the advice.

mad

[newclear]

I take it a reading of 9 ish (in the red) is a bad thing?

Think I have some sort of serious issue with the Aerial maybe? Or is it more the fact I have it near my (mostly steel constructed) Snorkel?

Already created my heavy gauge earth strap to the Fusebox Earth point, didn't seem to help much if at all...

Mad: How long did your aerial end up? Currently mine is around 80cm plus the spring etc. just wondering if the length needed to be a LOT shorter?

Cheers

Matt

[Madcowz]

I take it a reading of 9 ish (in the red) is a bad thing?

Think I have some sort of serious issue with the Aerial maybe? Or is it more the fact I have it near my (mostly steel constructed) Snorkel?

Already created my heavy gauge earth strap to the Fusebox Earth point, didn't seem to help much if at all...

Mad: How long did your aerial end up? Currently mine is around 80cm plus the spring etc. just wondering if the length needed to be a LOT shorter?

Cheers

Matt

The instructions I have say that anything over 3 means your antenna or cabling has a problem and you shouldn't use it until the problem is resolved. My antenna was a 160cm and I chopped off about 15cm to get the current readings.

Double check with a meter that you don't have any shorts and that you have continuity where you should.

See the checklist I have in the post above and work your way through this.

mad

[Kim Horsevad]

I take it a reading of 9 ish (in the red) is a bad thing?

Think I have some sort of serious issue with the Aerial maybe? Or is it more the fact I have it near my (mostly steel constructed) Snorkel?

Already created my heavy gauge earth strap to the Fusebox Earth point, didn't seem to help much if at all...

Mad: How long did your aerial end up? Currently mine is around 80cm plus the spring etc. just wondering if the length needed to be a LOT shorter?

Cheers

Matt

Do you actually mean a SWR reading 9 ???

This means that nearly all of the electromagnetical energy - which should be converted to radio waves by the antenna - is actually beeing feed back into the transmitter, and probably doing some damage on the output transistor.

There is something totally wrong with your setup. Most likely a broken connection between output terminal on transmitter and antenna. You should not use the transmitter before you have rectified the problem.

[newclear]

Believe me I didn't (read the How to on SWR linked above, as soon as I got the reading I stopped and turned off the Unit).

Good job I have a week off work, hopefully it should be quite obvious when I start to look at it and investigate.

Just sort of needed a confirmation that 9 was certainly as bad as I thought

Cheers

Matt

[steve_d]

.....Just sort of needed a confirmation that 9 was certainly as bad as I thought

Bader than a bad thing and then some.

Steve

[ThreeSheds]

Mad: How long did your aerial end up? Currently mine is around 80cm plus the spring etc. just wondering if the length needed to be a LOT shorter?

Be aware that you can't rely on two different setups requiring the same length of whip - there are too many other variables at work here for that.

Rog

[newclear]

Thanks Rog,

I was more after a bit of a general "idea" - I know there is no way if I just go straight for the length Mad removed it will work fine.

I was looking for about how much is "usually" taken off, as I didn't want to get to like 10cm removed (for example, need to do a lot ore than that to fix my problem) and be thinking I was taking off too much.

The more I look into it the more I'm understanding its more of a "you take off as much as it needs, there is no too much"

Thanks Steve - I got that impression as soon as the meter indicated it, thus the reason I turned off the unit and stopped

Cheers for the help,

Matt

[jimfoo]

I had an antenna that had an ok SWR, but it was very stiff, I added a spring, and the SWR was past 3 on any channel no matter how I tuned it. I ended up finding a thin strip of copper foil taped near the tip brought the SWR down to a reasonable level, 1.5 on 1 & 40. I had to do a lot of trial and error on strip width and placement. It works by adding capacitance to the antenna, thereby lowering it's resonant frequency.

[steve200TDi]

Ok.....Holy thread res-erection blah blah...yeah yeah yeah!!!

Right, I have done all the continuity checking which is all good. Admittedly I could sand some paint off on the inside, but I have SWR of infinate/off the scale!

Please help!

Do I just start chopping the antenna down or could there be another issue??

Thanks

 

Steve

[Ex Member]

Infinite means there is no connection.  Broken wire or broken antenna or no ground.

[Mo Murphy]

Aerial earth Steve.

Mo

[Ex Member]

The aerial does not have an earth to speak of.  It is critical that the wire going to the aerial is not grounded and is very well isolated from the ground.  The radio waves bounce off of the metal of the vehicle, which is called the ground plane, but it is not really important as to any actual reference to electrical ground.  The coaxial cable sheath needs tp be grounded to prevent the cable acting as an antenna.

[steve200TDi]

Ok, but I did attach a wire from the antenna ground to my chassis with no change. Do I need to check continuity from antenna connection on the bracket to antenna tip?

More investigation needed!

Steve

[Ex Member]

Can you take a picture of the mount and the wiring connections?

[steve200TDi]

I'll do that next week when I'm off again!

Thanks

 

Steve

[Happyoldgit]

You should get continuity from the centre of the PL 259 plug up [centre conductor of coax] to the antenna whip. Likewise continuity from the outside of the plug, through the coax shielding up to the plug or mount connection at the antenna. The mount, or parts of it, should generally be in good metal to metal contact with the body of the vehicle. What mount and antenna are you using and where do you have it located. An off the scale SWR sometimes indicates a short between the coax centre and shield or connected parts.

Don't start chopping bits off the whip as you can't glue them back on! When I was installing this stuff many, many years ago I would carry lengths of cheap rod to use when working on those on tricky installations.

[Mo Murphy]

Very well Red90 for your clarity, bare metal connection for the aerial mount.

We just call it the earth here because we're not proper like you.

Mo

[geoffbeaumont]

Wandering mildly off topic (probably) - if a bare metal connection is important for the aerial mount, how does that work with magnetic mounts, which usually have a plastic pad under them?