swr reading

[mfm]

hi, i installed a cb today.mounted aerial through the roof(def 90)and earthed it to the battery.i put the swr meter on and by cutting an inch off the aerial i got the reading to about 2.5.it's an orbiter aerial from thunderpole but a fairly short one,they also had one about 1.5 m long.would i be better off with the longer aerial as i've been told 2.5 is a bit high.also when i removed the earth to battery it didn't affect anything,do i need it?
any tips would be appreciated.thanks

[PinkSeriesIII]

hi, i installed a cb today.mounted aerial through the roof(def 90)and earthed it to the battery.i put the swr meter on and by cutting an inch off the aerial i got the reading to about 2.5.it's an orbiter aerial from thunderpole but a fairly short one,they also had one about 1.5 m long.would i be better off with the longer aerial as i've been told 2.5 is a bit high.also when i removed the earth to battery it didn't affect anything,do i need it?
any tips would be appreciated.thanks

For a start you don't need to earth them to a battery. The antenna grounding has NOTHING to do with the electric earth of the vehicle. It is the ground plane of the antenna, the counter part to the upright whip. So in the case of the mobile antenna it the bigger flatter face of metal the better. Hence saying middle of a roof it the ideal so there is an even propagation, moving the antenna front or back, left, right will bias the propagation of the transmission in the other direction. But aerial in middle of roof are unpractical and look silly so we make compromises.

Usually no need to cut those orbiter as they are in the right ball park for the UK 40, just need fine tuning via the grub screw. It probably too short now to ever be right and best you get a new one!. If you had SWR issues it was due to poor grounding or objects close to the vertical whip. The short ones do have a slightly higher but SWR 2:1 is acceptable.

Make sure there are no vertical panels or objects near (within 2 foot) of the vertical whip or spring/coil of that antenna as they will be acting as part of the ground plane and therefore would be as though you are laying the aerial flat down on the roof. Closer an object or the more of a vertical panel that covers the length of the antenna then the worse the SW ratio will be. Having the mount on the rear vertical face of the roof would be acceptable.

[mfm]

hi pink series 111,thanks for your help.i've changed the aerial to a longer one(1.5m).i get a swr reading of just under 2 now after adjusting it with grub screw.the aerial went into an existing hole in the roof but its next to the radio aerial(as in pic).will the radio aerial affect the cb aerial?
i can hear people on the rig but only one person has replied when i ran a rig check on 19.is this because i can recieve them but they're too far away to transmit to.the person that did reply was about 7 miles away.

[PinkSeriesIII]

hi pink series 111,thanks for your help.i've changed the aerial to a longer one(1.5m).i get a swr reading of just under 2 now after adjusting it with grub screw.the aerial went into an existing hole in the roof but its next to the radio aerial(as in pic).will the radio aerial affect the cb aerial?
i can hear people on the rig but only one person has replied when i ran a rig check on 19.is this because i can recieve them but they're too far away to transmit to.the person that did reply was about 7 miles away.

That actually should be fine. Have you had the mount off recently? mounts on alloy roof tend to need cleaning up now and again as the alloy oxidizes, just clean it up an get a good electrical contact there. But 2:1 is 'ok', it none damaging at least.

[Rex Hunt]

Radio aerial may well be de-tuning the CB one,
This may cause reflected energy going back into the long whip giving a raised VSWR, It would be worth checking the reading again but with the broadcast antenna folded down, and preferably away from the new one.

Another little tip.

When I first fit an antenna/aerial, I check the swr.
IF it is high, then I usually replace the upper section with (wait for it!)
a length of coat hanger wire of the same length.
I then trim this, checking with the srw meter every 1/4 inch, until the reading comes down. (very often get own to a 1.2 or 1.5 to 1 reading (nigh on perfect this way).
Coat hanger wire is MUCH easier to cut than the stainless steel whip!

Having got a good result, I then remove the coathanger wire, and replace it with the original whip section, having cut it to match the modified length of the coathanger.

A final check......and away you go chasing the DX!

Coverage to the horizon is all you can expect on CB frequencies at the moment (horizon a nominal 7 miles away). :depressed:

However when conditions improve
i) during the summer :dj: and
ii) over the next few years as the sunspot cycle improves,

You can expect to start hearing continental and stateside signals. :Cowboy:

:chatter:

Hope this helps

Rex (G0CLR)

[mfm]

thanks rex.i'll give it a go