Tuning in a CB aerial goes wrong

A story and some notes to help.
(Warning a lot of Geek speak)

He did everything right.
The aerial mounting was fitted correctly on the metal roof of the car that is plenty big enough and high enough that the aerial wasn’t near to anything that could “throw it off”. He’d used the correct coax and connectors. The CB radio was secure and well mounted and then he’d started tuning the aerial in using MY trusty SWR meter and patch lead.

oneswrOnly it won’t, tune in that is.
He could get both the low and high channels to balance but the overall SWR was sat at around 3:1.
It means that 25% of your possible transmitted power is being reflected BACK into your radio. Or, in simple terms, your radio output stages start to heat up and that’s bad. Keep transmitting on full power and the probable result will eventually be “POP”. End of transmitter. Not good in a survival scenario.

This was something that happened to a friend who luckily called me in before “POP”. From the radio to the aerial , as said,  everything was perfect except:-
The coax was coiled by three or four turns and cable tightly tie wrapped tied into a bundle. That’s a bad thing to do to coax. It stresses it physically and it a sign that there is just too much cable between the radio and the aerial. (Hold that thought).

BUT some aerials come with a preset length which is common in No Ground Plane aerials you find on marine craft and no metal vehicles. Then you shouldn’t cut cable under any circumstances BUT it wasn’t one of them!

So what to do?
The usual reasons for a poor tuning (or SWR) of an aerial are:-

  1. Radio badly mounted (Usually a bad earth)
  2. Bad earthing of the antenna mount.
  3. Location of antenna is not in the best position.
  4. Little or no ground plane.
  5. Bad soldering of connectors
  6. Cheap or the wrong sort of coax.
    For CB you need 50 ohm coax with a good 100% inner braid.
  7. Coax damaged.
  8. Coax is too long. (Hang onto that thought)
  9. Excess coax is coiled.
    Apart from the strain, you’d need a lot of turns to make a difference.
  10. Antenna not tuned.
  11. Bad SWR meter and or patch lead
  12. You’ve cocked up somehow!

I checked everything and gave him a gold star except for the coax length.
It was WAY too long and a weird length. 1.4 meters actually.

It’s a funny thing but some installations are really sensitive about coax lengths and some you could stick a metal coat hanger on the end to act as an aerial  and it would work. Still back to the coax and a little bit of math.

Cutting COAX to get the best performance.
It is best to use coax in multiples of  ¼, ½ or a full wave length of the center frequency of the antenna you are connecting to.

For CB in the UK and Europe the two frequency bands in use are:-
UK 27.60125 to 27.99125 MHz (FM only)
CEPT/EU 26.96500 to 27.40500 MHz (AM/FM/SSB)

And center frequency (CF) of that lot  works out as 27.478125 MHz
You’ll need that figure in a minute.
The coax used was RG58 which has a velocity factor (VF) of 0.66
You’ll also need that figure too.
Bearing in mind all he needed was 1.4 m in length. This length was REALLY bugging me but I couldn’t quite work out why.

Now the maths.
To calculate the cable length you use the formula:-
length (in Meters) = (300/CF) * VF
VF = Velocity Factor
CF = Center Frequency

So, using the above numbers we worked out,
Length = (300/27.478125) x 0.66
OR         = 10.92 x 0.66 = 7.2 meters.
¼ of that is 1.8m and that is the minimum length you should use and he had 1.4 m.

Ah-Ha! So, I was wrong!
It looked too much BUT was actually a bit short !
wrongSo, cutting a new bit of coax we re-soldered the connectors on to the new cable and plugged in. As for the excess, that was coiled loosely is this way:

As a result the aerial SWR DROPPED to a really safe level of 1.2:1 (Just over 1% loss of transmission power) across all 80 channels with only a tiny tweak to the aerial length. Not bad eh!

Now some notes for you that might help.

  • Popular Coax VF figures.
    RG-8     VF  0.66
    RG-11    VF 0.75
    RG-58    VF 0.66
    RG-213  VF 0.66
    RG-400 VF 0.695
  • SWR readings
    1:1        No losses what so ever
    1.5:1    4% loss Normal power
    2:1      11% loss Keep to low power
    2.5:1  18% loss Keep to low power
    3:1      25% loss Low power but be very brief
    4:1      36% loss Listen only
    5:1      44% loss Listen only
    10:1   67% loss Probable short or break in aerial wire.
  • Wound fiberglass pole aerials supposed range at 4 watts
    2′ Length: 2 to 3 miles
    3′ Length: 3 to 4 miles
    4′ Length: 4 to 6 miles
    5′ Length: 5 to 7 miles
  • Magnetic based aerials supposed range at 4 watts
    3‘ Magnet Antennas: 3 to 4 Miles
    5′ Magnet Antennas: 5 to 7 Miles
  • 102″ whip
    You don’t need a ground plane as 102″ is almost a perfect 1/4 wave pole (EU/CEPT channels) but might need a little cut off its length for UK Channels.
  • SWR
    Higher on top channels compared to low channels, SHORTEN THE AERIAL
    Higher on lower channels compared to high channels, LENGTHEN THE AERIAL

Radio notes in general:- (Very old stuff!)
CB Set Basics
Wire Aerials

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