SWR Saga (part 2)

Motorhome/Caravan CB Radio advice and troubleshooting

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Matt C
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Location: Nairnshire, Scotland.

Sun Jun 18, 2006 2:13 pm

Hello again folks!!

Right, I've spent the day trying to sort out the SWR on the motorhome...

I've swapped the 'Firestik' for a Thunderpole 'Super Flexi 48"', and through gradually lowering the length of the aeriel, and lengthening the 'no groundplane' aeriel on the mount by about 1cm, I've managed to lower the SWR down to about 1: 2.3 which is still a little high, but not in the red, thankfully!!

The SWR reads 1: 2.3 uniformly throughout all 40 channels (I can get the SWR lower on channel 1, but this makes it go higher on 40, so I guess having it set a tiny bit higher, but uniform through the channels is correct?)

I just can't get the aeriel any higher on the van. About 1/3 of the aeriel sits within 50cm of the vechicle body, I can't do much about it.

1: 2.3 won't damage my rig, will it? How dramatically will my signal be reduced? I've calculated that I'm losing about 1 watt on transmit, but I'm not sure if thats right?

Any ideas folks?
Call sign: 2H2SQ
Handle: 'Turtle'
Home is where you park it!...

Paul (2H3CK)
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Sun Jun 18, 2006 8:29 pm

Matt,
The signal could be reflecting off the body and causing the high SWR. I have seen aerials mounted on metal motorhome bumpers, but as a wild guess i bet yours are plastic!! Lol. I don't suppose you want to drill holes in your vehicle but is the cab flat fronted or does it have anywhere like a front wing that you could body mount the aerial? You could use a dv mount and adjust it to any required angle. So if you have the option either use the bumper or front wing. Failing that you may need a bracket that increases the distance away from the body!
Hope this is useful....Please let me know so perhaps i can offer some further help.

Cheers....P
All the best.....73....P

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Matt C
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Mon Jun 19, 2006 8:59 am

Hi Paul!!

As far as the bodywork on the motorhome, the whole van is a an 'A' class coachbuilt, which means only the engine, chassis and running gear is commercial, the rest is done at a coachbuilders using aluminium and GRP. The whole front end is pretty much GRP, nearly an inch thick!!

On Sunday, I reverted back to the Firestik 4', and I'm currently using the ERF NGP kit on a wing mirror. I've found that if I 'loop' the excess coax around the rear-view mirror in the cab, the SWR drops down to about 1: 1.7, which is fantastic!! Ive checked the whole run of coax for kinks or damage (there are none), I've also removed and re-soldered the plug at the end (just in case), and I've checked for shorting between the shielding and the core using a multimeter, and it all checks out!

*** HERE'S THE IMPORTANT QUESTION ***

When using an ERF NGP Kit, Is the length of the coax supplied critical to it's operation, or can it be shortened?

Is it possible to shorten the coax to a 'multiple' (e.g. exactly 'half length')?


(I know when using a mag-mount, it is recommended NOT to shorten it because of 'grounding' issues)

This is the hardest CB installation I've ever had to do!! Normally they go in like clockwork! Ive tried different rigs, aeriels, SWR meters and coax plugs. That just leaves the mount and coax itself, which Ive inspected thoroughly, and even attached an earth-strap to the chassis, via it's mounting bolt. I'm running out of ideas folks, although the fact that the SWR drops when the coax is run parallel to the aeriel is interesting!!

Any ideas? This is starting to get beyond me.... :oops:
Call sign: 2H2SQ
Handle: 'Turtle'
Home is where you park it!...

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Matt C
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Wed Jun 21, 2006 12:23 pm

**** UPDATE ****

Problem solved folks!!

Ripped out the ERF NGP kit, and replaced it with the slightly more expensive Thunderpole 'Electronic' ground-plane kit. Re-installed the aeriel in the same position, and the SWR dropped right down to 1.5:1, before I even tuned the Firestik!! Matched it up, and the SWR is now at a very respectable (for a motorhome) 1.4:1 :lol: :lol: :lol:

Stripped the ERF NGP mount down, and discovered that excessive corrosion had caused a short between the centre core and the mount itself. Although not enough to 'peg the needle', it was enough to send my readings way out!! :oops: (I had previously checked the coax for continuity of the centre core, and that there was an open circuit between the shielding and the core, but I never thought to check the mount itself!!!) :cry: :cry:

I've made sure this new mount, coax and connections are all taped, and liberally coated in 'Sikaflex' sealant, so I should have no more problems!!!

One cracking install, and it only taken me 2 YEARS to get it right!!! :roll:
Call sign: 2H2SQ
Handle: 'Turtle'
Home is where you park it!...

Round Baler
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Wed Jun 21, 2006 10:24 pm

Good to hear!

- Gordon
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"We Cut The Crops That Feed The World"

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mrbass
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Mon Jan 22, 2007 6:32 pm

nice one m8. ive got a autosleeper just put a maycom in not been on for years as ive been on the ham side of things..

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