HF autotune mounting ?

Submitted: Wednesday, Mar 15, 2006 at 09:23
ThreadID: 31751 Views:2031 Replies:7 FollowUps:0
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HI All
I have a codan 9323 with autotune mounted on kaymar mount on swing-arm behind spare wheel. This means that the autotune is about 300mm above the roof height when in position.... so I have for the last 12 months been taking it off when back in town and putting it on to go away.

Question: IS there likely to be any significant impact on trasmit/receive if I cut the Kaymar by about 300mm so that when mounted it is almost (just above) at roof level so I can leave it on all the time and just screw the whip on for trips?

thanks
JohnN
VKS 737 Romeo 4263
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Reply By: Peter - Wednesday, Mar 15, 2006 at 09:35

Wednesday, Mar 15, 2006 at 09:35
We have a Barrett that is on kaymar wheel carrier and top of autotune is level with roof line. Have seen no differance with it being up above roof line. when you break it down the transmission comes from the wip not the auto tune.

AnswerID: 160596

Reply By: Member - Barry W (VIC) - Wednesday, Mar 15, 2006 at 10:05

Wednesday, Mar 15, 2006 at 10:05
Hi JohnN
I have the 9323 as well works great for me
I have it mounted on a rear carrier on 100s LC
I was advised to have the screw secction just above the cabin top
Here is an extract from Codan mounting instructions
" Automatic tuning antennas will often not tune if mounted in a position low on the vehicle body As well there can be tuning problems caused by the proximity of vertical body components such as rear body panels, truck cabs exhaust stacks and so forth"
You can also try these www.codan.com.au or e-mail HF@codan.com.au or Phone Codan Head Office 0883050311.
Something to consider, I had the same Codan Radio stolen when my 80s LC was stolen now I remove Auto tune and handset from vehicle when not in use - cheap insurance
Hope this helps
Barry

AnswerID: 160603

Reply By: JohnN - Wednesday, Mar 15, 2006 at 10:45

Wednesday, Mar 15, 2006 at 10:45
Thanks Peter and Barry
I just rang CODAN and spoke to a tech guy (why didnt I think of that?) and he said that the top 300mm of the autotune is part of the antenna and that was why Kaymar set it up that way. He said that it was preferable to leave it above the roofline but that many people had chopped the kaymar bracket and not noticed significant signal loss.
He said there would have to be some loss but that it may not be enough to concern me.... but I wouldnt know till I did it.

SO... who's done it.... any 1st hand experience out there before I decide to chop?

JohnN
AnswerID: 160611

Reply By: Member - Mike DID - Wednesday, Mar 15, 2006 at 11:04

Wednesday, Mar 15, 2006 at 11:04
Barrett, Codan and Moonraker Autotune antennas are fundamentally different.

The Barretts have electrical components inside that do not radiate, but will still have some electric field. The main radiation comes from the whip. The 160mm clearance will be more of a problem on a straight-sided vehicle like a bus or a van. For rear-mount on a 4WD where the top of the housing is at roof height, there isn't a problem due to the slope of the rear window.

Codan Autotunes have a vertical coil inside the housing which does contribute to the radiated signal. So it really helps to keep the top part of the housing as clear as possible.

Moonraker Autotunes have the coil higher up from the base so they are less sensistive to nearby metal around the base of the antenna.

As Codan said, there will be some loss of signal, but won't notice anything unless your signal marginal.

Mike
AnswerID: 160616

Reply By: Member - Loco Locums - Wednesday, Mar 15, 2006 at 15:28

Wednesday, Mar 15, 2006 at 15:28
Yes, the top 300 mm is the tuning coil that the impedance matching pick up runs along and is part of the radiating element of the antenna.

The suck it and see priniple will always be the winner on the day...if you change it's position and it still tunes and gets out as before....good result.

If I may relate a little experience of the last century when we were Codan dealers, we had a display set up at a local show. An 8528 with an 8558 auto tune antenna was set up on the display table. We ran it all from a power supply, but the thing was that the display was indoors..a good thing too because it rained a fair bit that weekend.

So here we are in a pavillion that could only be described as a BIG tin shed with a tin roof. Not the best for radio comms with the outside world.

Anyway, we had the autotune antenna mounted on a piece of quarter inch plate steel that was around 500mm square and we ran an earth from the antenna plate to the steel upright of the shed.
Well, it shouldn't have got a lot of signal radiated out of that shed...except maybe the window and door. However, we frequently hit the Darwin Radphone beacon along with Sydney beacon and we made Radphone Direct Dial phone calls as part of the demo as well.

So, if it works in practise...who cares if the theory sounds a little less than optimum.

Just my thoughts..hope they help you out.
Cheers
Rick & Julie
AnswerID: 160651

Reply By: VK3CAT - Wednesday, Mar 15, 2006 at 17:31

Wednesday, Mar 15, 2006 at 17:31
John, up to recently I had the 9350 mounted to the kaymar bar usung some 8mm aluminuim angle using the big main wheel carrier bolts. Top of the tuning unit was just above the roof on the 80 series. Just enough clearance between the back of the carrier & the tailgate. I think I have a picture of it on my website in the mobiles area at http://www.qsl.net/vk3cat.
You may be able to do something similar & sell the kaymar mount!
Cheers, Tony
AnswerID: 160683

Reply By: JohnN - Thursday, Mar 16, 2006 at 09:38

Thursday, Mar 16, 2006 at 09:38
Thanks all for the ideas/comments.

I think I will look at chopping the bracket I have to give minimal clearence above the roofline and see how it goes.

cheers
JohnN
AnswerID: 160864

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