[OZAPRS] Newb HF questions ;)

Terry Neumann tfneumann at internode.on.net
Sun May 24 14:27:02 EST 2009 

Damien Gardner Jnr wrote:

> Hrrm, I thought an advanced (I used to be AOLCP before they changed  
> everything..) license let you transmit on any ham band??  That's what  
> the ACMA site seems to say?

As I see things Damien, you "are go!" for all bands, all modes - no 
restrictions.


> But yeah, anyone have suggestions on a 'good' but cheap antenna for  
> 30m that'll mount on the roof of my wagon or on the boot of the 
> wife's  car?  (Idea is to have the same loom in the back of my wagon, 
> and the  boot of her car, and just drop the radio and gps into 
> whichever we're  going away in..)
>
As far as mobile HF whips -  the way to go is a bit more complicated.    
Mobile HF antennas (especially those for the lower bands) are always a 
compromise.  At VHF - and possibly down to 10 metres, it is possible to 
use something approaching a quarter wave whip  - or even better.   Once 
you get down to frequencies below that, things get messy, and you need 
to look at some sort of inductance in the overall picture to "fool" the 
radio  into thinking it's "seeing" a quarter wave.   You can make 
antennas shorter but then efficiency is reduced and bandwidth  also suffers.

The prime consideration is mechanical integrity.  Any antenna on a 
vehicle must be 100% secure and safe with no risk of it breaking off and 
hitting other vehicles or pedestrians.   This means rugged engineering 
for any whip of worthwhile length.    Bull bars and towbars can be 
utilised here with good results.   Not too many family cars have a bull 
bar, but here is what I used on our  last car with excellent results: 
http://www.users.on.net/~tfneumann/Amateur/Magna/Magna.html 
<http://www.users.on.net/%7Etfneumann/Amateur/Magna/Magna.html>  It was 
a bit directional  with best signals over the front of the vehicle, but 
gave good results on 20 metres where it was always in use for phone 
contacts, and also for HF APRS on 30 metres.    I was always comfortable 
with the mechanical integrity of the system.   It was probably 
over-engineered but that's how farmers do things. 

I'm still to work out a HF antenna system for our current car - a 
Misterbishi Lancer VRX.   I don't want to put a towbar on it, so I'm in 
the same boat as you are here.   A second consideration comes into play 
here too - namely the antenna installation must be acceptable 
aesthetically for the co-pilot.  For the moment I can't think of 
anything which is like to satisfy both criteria adequately, so it's in 
the too hard bin for the present.

As far as the antenna goes - assuming you have got a safe and acceptable 
means for mounting it,  there is not a lot of flexibility here either. 

I have used the Mobile One 
<http://www.mobileone.com.au/antennas/defaultamateur.htm> M20-1 mobile 
whip  - once sold by DSE - with reasonable results on 20 metres.  It's 
not all that long, but does the job.  You may be able to put a longer 
tip on it and possibly drag it down to 30 metres - I did some 
experiments in that area but I can't recall whether I did that worked 
successfully or not.  Their M30-1 would appear to be a good choice for 
30 metres, but it is six feet long.     Mechanically, it might be 
plausible to mount one of these using a roof rack - depending on your 
travel speeds.    If you drive under a low carport or roof with it in 
position, it will not not work as well afterwards  :-(   .

Mounting one on the boot lip starts to stretch things a bit - it's hard 
to get the required strength in a boot lip mount to resist distortion or 
damage to the surface you want to attach it to.   You need to watch with 
boot amount antennas that you can still open the boot itself.     
Magnetic bases are another option but those with sufficient grab to hold 
a HF size whip safely get rather large and, well, ugly, and that may not 
get past the second criteria.

I eventually concentrated my work in the top loaded whip series similar 
to the old Scalar resonators.  One of these is in use in the pictures 
used in the Magna pages referred to above.    These work well but put 
the greatest wind loading at the top of the whip so they lean back a 
bit.   Don't forget that if you drive at 100 kph into a 40 kph wind, the 
effective loading on the antenna is the same as if you were driving at 
140 kph in calm conditions.       I've always been going to  try 
rewinding a standard 5 foot 27 Mhz CB helical whip for 20 or 30 metres 
just to see whether it would work. These offer a much lower wind profile 
and therefore less load on the base and its mount, but I imagine that 
the performance will be lower as well.   I have re-wound one of these 
for 15 metres many years ago and it worked very well indeed.

Incidentally if you want to use the same whip on two vehicles, it's 
desirable to make sure that the mounting and locations on the bodywork 
are similar if not identical.  An antenna tuned for the best results on 
a roof rack will almost certainly need re-tuning if it's then lowered to 
boot or towbar level on the same or another vehicle.

So that's a long answer with no easy or simple solutions at the end of 
it.  Once again, Google around for "mobile hf whips/antennas" and see 
what others are using.

73
Terry
VK5ATN









> Cheers,
>
> Damien
>
> Damien Gardner Jnr
> VK2TDG. Dip EE. GradIEAust
> rendrag at rendrag.net -  http://www.rendrag.net/
> On 23/05/2009, at 4:24 PM, Shaun Pitcher wrote:
>
>>
>> Balun may be appropriate for 80 and 160, although not sure about you
>> licence for those bands, however you can always listen.
>>
>> Regards,
>> Shaun Pitcher
>> ozaprs-bounces at aprs.net.au wrote on 20/05/2009 12:17:31 PM:
>>
>>> Hi All,
>>>
>>> All I can say is WHOA!@#!  Very temporary replacement for the balun
>>> (buggered if I can find my heatshrink box, OR my self-amagamating  tape
>>> :( ) :
>>> http://photos-f-0.ak.fbcdn.net/photos-ak-
>>> snc1/v4704/250/124/506862762/n506862762_1874629_2686781.jpg
>>>
>>> Turned on the transceiver, hit tune on the AT-897, which took it  about
>>> two clicks, then TX on AM, and my current 50mv of 50hz ripple turns  
>>> into
>>
>>
>>> a fun 120mv of 10mhz with 50mv ripple, but I'm pretty happy with  
>>> that..
>>> Pressed send beacon on UI-View, she TX'd happily without messing up  my
>>> monitors, killing my mouse, or dumping crap to the serial ports,  
>>> and by
>>> the time firefox had loaded up, there was my packet sitting on  
>>> aprs.fi!
>>>
>>> VK2TDG-15>APU25N,GATE,WIDE2-1,qAR,VK4UN-4:=3312.35S\15130.12E- 
>>> Testing on
>>
>>
>>> HF {UIV32N}
>>>
>>> Now I'm even *hearing* other stations! (well its either that or  
>>> pagers..
>>
>>
>>> there's plenty of somethingorother that sounds like a machine gun.. )
>>> Though the DCD light on the MFJ isn't coming on, so I'll have to  
>>> grab a
>>> couple of recordings and shove them through the spectrum scope in
>>> audacity and see what shows up :)
>>>
>>> So one more question ;) What's a 'good' but not overly expensive  
>>> antenna
>>
>>
>>> for 30m, which will happily be mounted on one of the roof racks on a
>>> commodore wagon, or on a boot side-mount on a commodore sedan? :)
>>>
>>> Cheers,
>>>
>>> Damien
>>>
>>
>>
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