Codan Autotune

Submitted: Wednesday, Nov 08, 2006 at 15:19

ThreadID: 39248 Views:2457 Replies:3 FollowUps:9

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Member - Craig D (SA)

Fellow forumites,

does anyone using the 9350 AT (am using it with an NGT) ever have trouble tuning VKS channel 5 (3.995 MHz)? SWR lowest achievable is 1:8.8 - not good.

The ATU is bullbar mounted and tunes all other freqs perfectly, and is brand new.

I have done the following already trying to get rid of the null point:

1. Moved the earth braid to many different locations.
2. Fixed the earth to chassis, bodywork (where it should be).
3. Ran extra earths direct to B-ve.
4. Earthed the TXer to body and chassis.
5. Said many expletives I won't repeat!
6. Moved vehicle to several different locations.
7. Used both Codan and Barett S/S whips.
8. Removed conductive grease from earthing points.
9. Added extra earths directly to ATU base.
10. Power cables are well seperated from coax and ATU control cables.

I have not yet shortened the coax as the installation of the Transciever is not yet fixed; it is sitting under the passegers seat.
And before the obvious is asked I did the install myself.

Any ideas fellow travellers?

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Reply By: traveller2 - Wednesday, Nov 08, 2006 at 15:26

Wednesday, Nov 08, 2006 at 15:26

Check the bullbar to chassis electrical connection, if it is High Resistance then you will have problems as you have described despite the presence of direct braids.
NEVER connect the negative connection direct to the battery ALWAYS connect it to the chassis or body end of the negative cable. If you get a crook battery connection then the engine starting current will go through the HF and it WILL NOT work ever again.
How close to the bonnet is the bullbar? if within 150mm then try earthing the actual bonnet to the body, large expanses of electrically isolated metal will affect tuning as well.

AnswerID: 203712

Follow Up By: Member - Craig D (SA) - Wednesday, Nov 08, 2006 at 15:31

Wednesday, Nov 08, 2006 at 15:31

Thanks traveller2. Yep fully aware of the implications of the direct B-ve connection - I was trying to negate the null point.

Have also checked the bullbar to chassis but will check again.

The ATU is within the 150mm distance of the bonnet - I will give your suggestion a go tonight. Strange that it's a lower freq giving me grief as it's normally the higher buggas that play up.

Thanks again for the hints.

FollowupID: 463337

Reply By: Member -Signman - Wednesday, Nov 08, 2006 at 16:04

Wednesday, Nov 08, 2006 at 16:04

Does the radio show 'tune pass' when tuning up 3.995??
What length whip are you using???
What are you using to measure the VSWR- do you have the meter between the ATU and the radio (stupid question I know)...
Try adding 3 or 4 feet of wire off the top of the whip- and give that a go..bearing in mind 3-4 mhz is a low frequency..

AnswerID: 203718

Follow Up By: Member - Craig D (SA) - Wednesday, Nov 08, 2006 at 16:14

Wednesday, Nov 08, 2006 at 16:14

Hiya Signman,

no it doesn't show tune pass - shows TUNE FAIL :(

Length whips are standard Codan and Barrett.

The NGT has a SWR meter built in as an indication; I am using this as I don't have access to a VSWR meter.

What gets me is that channels close to the 3.995 freq tune great (+/- about 100kHz).

FollowupID: 463348

Follow Up By: Peter 2 - Wednesday, Nov 08, 2006 at 17:42

Wednesday, Nov 08, 2006 at 17:42

I've found that when an installation won't tune one particular freq it is usually antenna related, grounds, crook coax or connectors etc.
You have got the control cable to the antenna away from power and preferably coax haven't you.
The control cable between the remote head and the actual transmitter should also be run away from power and coax as well.
The transmitter is earthed to a good ground point?

The installs (I used to install HF's in vehicles) that gave me the willies were the ones that had an antenna mount in the centre of the rear mounted spare tyre, nothing like a big steel coil (the steel belts in the tyre casing) mounted right next to the antenna and insulated from the rest of the vehicle to stuff up the autotune ;-))

FollowupID: 463388

Follow Up By: Member - Craig D (SA) - Thursday, Nov 09, 2006 at 09:24

Thursday, Nov 09, 2006 at 09:24

Peter, I did a lot of changes last night including seperating the coax and control cables, and the TXer is earthed to a good ground point,too. As I have an NGT the control cable and the coax have to be close together as they enter the TXer at the same spot, however I did try seperating them from the bullbar through the engine bay to where they enter through the firewall and then to the TXer.

I also tried various and different earth points, added extra earths, cleaned earth points again, even removed the autotune and reinstalled it (cleaning the mount points again as well).

I'll get the install checked over by an installer - may even have a duff AT yet.

FollowupID: 463540

Follow Up By: Member - Craig D (SA) - Thursday, Nov 09, 2006 at 12:26

Thursday, Nov 09, 2006 at 12:26

One thing I haven't done is to lengthen or shorten the coax...DUH...basic stuff I've forgotten:(
Will give it a go tonight and report tomorrow.

FollowupID: 463578

Reply By: anglepole - Thursday, Nov 09, 2006 at 14:25

Thursday, Nov 09, 2006 at 14:25

Hi Craig,

Sounds like you have done all the right things. The whip on the top should be around two metres.

As much as I hate to say it is the motor in the AT running the tuning "slug" to its right position to tune the 4 MHz freq?

I would have it checked out by a competent service tech.

Good luck

AnswerID: 203903

Follow Up By: Member - Craig D (SA) - Thursday, Nov 09, 2006 at 14:29

Thursday, Nov 09, 2006 at 14:29

Can tune to within 150kHz of 3.995 MHz so I reckon the slug is hitting the right spot. I'll be shortening the coax tonight as I posted above and if that doesn't solve this blessed problem then I will be looking at swapping over ATU's asap as there's not much else to check (except there is a small PA adjustment the Codan techs know about that could be the culprit, too).

Thanks dude.

FollowupID: 463592

Follow Up By: anglepole - Thursday, Nov 09, 2006 at 15:19

Thursday, Nov 09, 2006 at 15:19

Craig,

If you end up having to visit some about your Antenna and I notice you live in SA,
my neighbour had trouble with his Codan AT he was very happy with the service and price he got from Gawler CB and Auto at Paradise. He apparently installs a lot of Codan's in vehicles.

Cheers

FollowupID: 463596