Can't get SWR down below 3?
- Thread starter KSLB
- Start date
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Trilux
New Member
- Posts
- 10
- Location
- Southwest Wales
I've never been involved with CB, but am qualified in radio communication, so I hope that by explaining what happens with SWR, it might help you find the fault. I'll try to keep it simple 
.
The output of the transmitter has a thing called impedance which is a bit like the resistance to ac current flow. The cable and antenna together also have impedance. When the impedance of the transmitter and cable and antenna together are different, you get an effect like a wave travelling along a swimming pool and then bouncing off the side. The wave that travels back is basically what SWR is. The more that travels back, the higher the SWR because it fights against the outgoing signal. Now, if the impedances are exactly the same, in theory there is no standing wave and you get maximum power transmission.
So, the first things you must be absolutely sure about is that you have the right cable and that it is not kinked. The length is not too much of a factor. Then make sure you have the right antenna for the purpose and that the connections are 100%.
Until you've done that, don't even think about anything else. Trust me, I used to be a radio doctor
. The output of the transmitter has a thing called impedance which is a bit like the resistance to ac current flow. The cable and antenna together also have impedance. When the impedance of the transmitter and cable and antenna together are different, you get an effect like a wave travelling along a swimming pool and then bouncing off the side. The wave that travels back is basically what SWR is. The more that travels back, the higher the SWR because it fights against the outgoing signal. Now, if the impedances are exactly the same, in theory there is no standing wave and you get maximum power transmission.
So, the first things you must be absolutely sure about is that you have the right cable and that it is not kinked. The length is not too much of a factor. Then make sure you have the right antenna for the purpose and that the connections are 100%.
Until you've done that, don't even think about anything else. Trust me, I used to be a radio doctor
if all else fails theres always one of these
Zetagi MM-27 ATU Mobile Antenna Matcher - Ideal for CB Radio or Ham Radio Use on eBay!
i used them back in the day and they actually work quite well
Zetagi MM-27 ATU Mobile Antenna Matcher - Ideal for CB Radio or Ham Radio Use on eBay!
i used them back in the day and they actually work quite well
Makes sense, I will have a look.
Last edited:
brian47
Well-Known Member
- Posts
- 8,867
Got to agree with Trilux, you must get the aerial connections right first.
Without getting into the technicalities of impedance, matching or loading coils … yet, check the cable with an ohm meter (a multimeter on resistance).
Disconnect the aerial plug from the rig and check the following:
Outer shell to vehicle ground or ground plane – low resistance
Centre pin to the aerial – low resistance.
Outer shell to centre pin – high resistance
The power wiring should have both positive and negative wires, and no part of the aerial in the power chain, preferably taken back to the battery but seeing as it’s just a 5 watt CB rig and not a high power police mobile radio, it’s not really important where you take it from.
The aerial cable must be 50 ohm coax (uniradio UR76 or similar) and not TV coax.
Any grub screws used for adjusting the length of the aerial must bear down tightly onto clean metal; check for varnish or paint and remove any in the area. The same applies to the connection of the cable at the base of the aerial mount.
You say that the aerial is mounted on the wing, so that should provide a reasonable basic ground plane, although the size and shape might have some effect on the VSWR as well as on the directivity, but we won’t worry about that yet.
You could also check the cable between the rig and the SWR bridge.
I too used to be a “radio doctor” and a licenced radio amateur (GW3YSP).
Without getting into the technicalities of impedance, matching or loading coils … yet, check the cable with an ohm meter (a multimeter on resistance).
Disconnect the aerial plug from the rig and check the following:
Outer shell to vehicle ground or ground plane – low resistance
Centre pin to the aerial – low resistance.
Outer shell to centre pin – high resistance
The power wiring should have both positive and negative wires, and no part of the aerial in the power chain, preferably taken back to the battery but seeing as it’s just a 5 watt CB rig and not a high power police mobile radio, it’s not really important where you take it from.
The aerial cable must be 50 ohm coax (uniradio UR76 or similar) and not TV coax.
Any grub screws used for adjusting the length of the aerial must bear down tightly onto clean metal; check for varnish or paint and remove any in the area. The same applies to the connection of the cable at the base of the aerial mount.
You say that the aerial is mounted on the wing, so that should provide a reasonable basic ground plane, although the size and shape might have some effect on the VSWR as well as on the directivity, but we won’t worry about that yet.
You could also check the cable between the rig and the SWR bridge.
I too used to be a “radio doctor” and a licenced radio amateur (GW3YSP).
Freddy 007
Gone but not forgotten
- Posts
- 845
- Location
- Somerset, UK
You can't say 'Just for clarity' and then talk sh*t.
Can and did.
Freddy 007
Gone but not forgotten
- Posts
- 845
- Location
- Somerset, UK
:d
New top loaded antenna got it down to 1.4 also tried the Orbitor antenna on the roof and that to gave me 1.4, I know it's not perfect but usable. I am going to leave the Orbitor on the roof rack as it looks tons better than the 2m fire stick. I have left a Wilson mount on the front wing and left the patch lead attached so can switch from one to the other if I need to.
Thanks for all of the advice.
Thanks for all of the advice.
Last edited:
Pikey
Dummy Ejection Facilitator..
- Posts
- 47,185
- Location
- in Baron Wet Bum's Slave pit..
ooh glad ya sorted it. told ya to stick it on the roof Oh and keep the tall aerial they look sooo much better. and if ya stick bit of ribbon on the top ya can find it easier in a carpark. .
Oh and keep the tall aerial they look sooo much better. and if ya stick bit of ribbon on the top ya can find it easier in a carpark. .ooh glad ya sorted it. told ya to stick it on the roof
Yep, you did, didn't want to put a hole in the roof for it but mounting it where I have I managed to use the hole for the spot light wires.
I am going to give the 2m top loaded antenna a go on the roof as well and see how that works.
Still stuck with choosing a handle torn between 'Twisted Budgie' and 'Brass Monkey'
timtheenchanteruk
Well-Known Member
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- 4,633
- Location
- Leeds
got yer cb in same place as mine, i put me extension speaker on top of the dash though, still found it too quiet my me knees
