[OZAPRS] Need a hand

Sun Dec 3 21:43:01 AEDT 2017

cool. I realise there are practical realities.

A way to deal with this on cars is to provide a capacitive coupling to 
the panels by means of some adhesive copper tape or aluminum foil . Use 
some thin plastic between the metallic tape/sheet and the paint, hold it 
does with some insulation tape or whatever.

You want less than 5 ohms of capacitive reactance for a short antenna 
like this. at 10 MHz this is 3nF of capacitance.

with a 0.5mm insulation between the base metal and the conductive sheet, 
(thats really quite thick) , you need an area of ( C = epsilon0 * (area/ 
distance)) = 0.177 m2

about a foot square..like a square foot sheet of say blank PCB, or al 
foil, or copper sheet, or aluminium sheet, or aluminium sarking (really 
thin- perfectand cheap from bunnings- like foot wide, 10m long 
$25)anyway does not have to be super conductive, anything (ideally) non 
magnetic will do...

half the distance (thickness), half the area required....

just on cars be mindful how big the panel is . you want the whole 
vehicle at 10 MHz. you coudl run multiple ground wires (say 1mm2)  going 
to multiple capacitance pads each of say 0.1m2

-------

and I hear people say, what has grounding the end of the coax at radio 
got to do with it, well if the VSWR changes when the end of the coax at 
the radio is grounded /ungrounded tells you if the coax outer is acting 
as part of the antenna.

And if it is, it tells you the ground on the antenna base is not doing 
the job. IE it is choosing the small coax ground compared to a big slab 
of metal. Ground losses will be very high. Remember all the antenna 
currents generated by the 'inner ' - the whip must return back down the 
inside of the shield . If this return current returns via lossy ground, 
coupling to the chassis etc, this is all very undesirable.

the variation of ground currents / presented ground impedance of the 
coax (being connected/disconnected from an alternative ground) , and 
resultant change in impedance or VSWR tells you that you are changing 
the current distribution of the antenna system.

-g

On 3/12/2017 9:19 PM, vk4tec at tech-software.net wrote:
> Thanks Glen
>
> Will revisit the car tomorrow
>
> A
>
> -----Original Message-----
> From: OZAPRS [mailto:ozaprs-bounces at aprs.net.au] On Behalf Of Glen English VK1XX
> Sent: Sunday, 3 December 2017 8:00 PM
> To: ozaprs at aprs.net.au
> Subject: Re: [OZAPRS] Need a hand
>
> OK good stuff thanks for the answers.
>
> 1) Magbase has insufficient capacitance at 10 MHz to give you any sort of ground. I wouldnt recommend a base base below 60 MHz or so, depending on the construction and the antenna.
>
> 2) Bonnet mount good but are the screws deep into your paintwork right through to the base metal  to get a good ground? probably not.
>
> suggest take a earth wire off the baseof the magbase body or the bonnet mount to a chassis  or panel bolt, this probably means relocating the antenna, maybe. if onto the bonnet you might want to connect to the bonnet and then connect the bonnet to a bolt in the engine bay to ground that panel (rather than the hinges) . some vehicles have ground straps for that.  the wire to the immediate ground should be a good choice for the RF compared to the coax , so maybe something at least 2.5mm2 or flat strap and less than a meter long.
>
> you'll need to re evaluate the antenna location/ mountaing to get a suitable immediate ground connection for the antenna (as the ground is half of the antenna, without it, the antenna wont feed)
>
>
>
> On 3/12/2017 8:22 PM, Andrew Rich wrote:
>> Both a bonnet mount and maggy mount in center of roof
>>
>> Diamond 30 meter vertical
>>
>> Radio is only connected via cigarette lighter
>>
>> Getting 6:1 and 9:1
>>
>> Vna dips but vswr is 6:1
>>
>> Might try my vhf whip on vna
>>
>> Not sure car and antenna base are grounded
>>
>> Like something is floating
>>
>> I need to measure ohms to chassis
>>
>> Rg58 only 4 Meters
>>
>> Radio ground only by power lead
>>
>> Vna is floating
>>
>> Not tried any ground straps yet
>>
>> A
>>
>>
>> Sent from my iPhon
>>
>>> On 3 Dec 2017, at 5:56 pm, Glen English VK1XX <glenlist at pacificmedia.com.au> wrote:
>>>
>>> hi Andrew
>>>
>>> Please do ya best to answer the following barrage of questions and I may be able to assist :
>>>
>>> but all questions need to be answered to solve the crossword...
>>>
>>> 1) what is the whip.
>>>
>>> 2) where is it located, precisely
>>>
>>> 3) does the ground on the base of the whi[ measure 0 ohms to the chassis ?
>>>
>>> 4) how long RG58 lead in ?
>>>
>>> 5) Is the radio grounded via a local ground short strap/thick wire to earth bolt or only its negative power lead ?
>>>
>>> 6) does it dip below 2:1 VSWR anywhere when you sweep it with a VNA ?
>>>
>>> 7) With the VNA, is the VNA grounded, or floating on the end of the  coax?
>>>
>>> 8) With the VNA, if floating, does the VNA indicated sweep/match
>>> change if you ground the outer of the VNA coax connector to a local
>>> ground like ground bolt ona  door hinge etc
>>>
>>>
>>>
>>>
>>>> On 3/12/2017 6:42 PM, Andrew Rich wrote:
>>>> I bought a 30 m whip for the car
>>>>
>>>> Can’t get better than vswr 6:1
>>>>
>>>> I was told I need to ground the base
>>>>
>>>> Also cigarette lighter maybe not enough current ?
>>>>
>>>> 10.147 MHz
>>>>
>>>> Andrew
>>>>
>>>> Sent from my iPhone
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