[OZAPRS] Need a hand

vk4tec at tech-software.net vk4tec at tech-software.net
Mon Dec 4 08:02:48 AEDT 2017 

Good Outcome 

Thank you 

-----Original Message-----
From: OZAPRS [mailto:ozaprs-bounces at aprs.net.au] On Behalf Of Glen English VK1XX
Sent: Monday, 4 December 2017 7:00 AM
To: ozaprs at aprs.net.au
Subject: Re: [OZAPRS] Need a hand

yep and what you did  with the engine mount strap confirms with you the need for a ground connection.

You should try dufferent options. Some will work better and not make any sense.

I think the days of people going into their paint with the grub screws is gone . Guys how many rusted out gutters did we see with gutter grippers screwing into the base metal in the gutter !

Once the match is 2:1 or better, that is probably quite reasonable.  2:1 is OK. dial up 50% power on the radio  it wont hurt it.  IE do the radio wont try and do 100%.

I think your bonnect mount is OK in conjunction with a ground strap. 
should be plenty of ground bolt opportunities

but watch out for ground straps in the engine bay, they can pick up quite a bit of noise.  but if you are onyl receiving  that doesnt matter !


On 4/12/2017 7:50 AM, vk4tec at tech-software.net wrote:
> The bonnet mount has hex grub screws and a little aluminium plate onto the protective coat
>
> But no where does it touch the chassis except coming back from the radio through the negative lead
>
> A
>
> -----Original Message-----
> From: OZAPRS [mailto:ozaprs-bounces at aprs.net.au] On Behalf Of Glen English VK1XX
> Sent: Monday, 4 December 2017 6:47 AM
> To: ozaprs at aprs.net.au
> Subject: Re: [OZAPRS] Need a hand
>
> Good stuff
>
> yeah there is no such thing as ground independent. There is ground less dependent.
>
> The only time you can get ground less dependent from an end fed antenna is when it is electrically loaded to a half wave AND at least for simple 2D structures, the physical length is at least 1/4 wave.
>
> It does this being an electrical half wave which is a voltage mode feed, rather than a current mode feed. For a voltage mode feed there is almost no current , and if there is nearly no current, an nearly open circuit doesnt matter.
>
> And on 30m, the end fed whip would need to be up around 7m long to be able to properly  support ground independent operation .
>
> For short compared to a wavelength end fed whips, with low impedances - IE current mode feeds, being high current, the ground connection is thus very important. The ground serves two purposes, a) for the return currents (the other half of the antenna) , and b) preventing the whip seeing the lossy ground- ie  preventing the return currents from travelling through the lossy ground around the vehicle.  In fact the taller whips see more of the lossy ground than the shorter whips. A infintesimally short whip over a big ground cannot actually see the lossy ground around it,whereas a tall whip can see over the edge.
> However, losses for shorter whips increase due to the reduction in radiation resistance, higher currents and higher losses. The ground loss appears in series with the antenna feed.
>
> Centre loaded antennas may be slightly less needy of ground than a base loaded type of the same length, but the difference for a low HFwhip on a vehicle is NEGLIGIBLE.
>
> I expect you will need some sort of antenna tuner. if a HF whip is a 1:1 match with good bandwidth this is suspect and usually indicating very poor efficiency . As I said, the ground loss appears in series with the antenna feed. If the ground loss was 45 ohms and the antenna was 5 ohms
> (typical) , then the 50 ohms  you get, with say, 1 amp of current in it, the 1Amp is 45W dissipated as heat in the ground and 5W in the antenna !
>
>
>
>
>
> On 4/12/2017 6:57 AM, vk4tec at tech-software.net wrote:
>> Ok an earth strap to the engine mount has resulted in a VSWR 2.01 : 1
>> at 10.100 MHz
>>
>> I did not realize that this antenna was not ground independent
>>
>> Andrew
>>
>> -----Original Message-----
>> From: OZAPRS [mailto:ozaprs-bounces at aprs.net.au] On Behalf Of
>> vk4tec at tech-software.net
>> Sent: Monday, 4 December 2017 5:43 AM
>> To: 'Australian APRS Users' <ozaprs at aprs.net.au>
>> Subject: Re: [OZAPRS] Need a hand
>>
>> Good Morning
>>
>> I did a test with my Diamond VHF and I am getting 1.13 : 1 at 143 MHz
>>
>> Must be ground independant
>>
>> I thought my super gainer was a dual band - the VSWR up at UHF is
>> horrible
>>
>> I need to look at that
>>
>> //
>>
>> I just tried a jumper lead from the base of the bonnet mount to the
>> engine mount and the VSWR dropped from 9:1 to 3:1
>>
>> Getting there
>>
>> Andrew
>>
>> -----Original Message-----
>> From: vk4tec at tech-software.net [mailto:vk4tec at tech-software.net]
>> Sent: Monday, 4 December 2017 4:56 AM
>> To: 'Australian APRS Users' <ozaprs at aprs.net.au>
>> Subject: RE: [OZAPRS] Need a hand
>>
>> Hi Glen
>>
>> Bonnet mount to car chassis is open circuit
>>
>> Trying a strap
>>
>> Andrew
>>
>> -----Original Message-----
>> From: OZAPRS [mailto:ozaprs-bounces at aprs.net.au] On Behalf Of Glen
>> English VK1XX
>> Sent: Sunday, 3 December 2017 8:43 PM
>> To: ozaprs at aprs.net.au
>> Subject: Re: [OZAPRS] Need a hand
>>
>> 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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