[OZAPRS] Grounding of HF Antenna for APRS

[vk2tv]

Don't forget that a 1.4m whip on 30m will have a Zo <<50 ohms and the 
closer the VSWR is to 1:1, without utilising a matching network, the 
higher the system losses which make it "look" like a good match. A 25 
ohm antenna in a perfect world will have a VSWR of 2:1 for 50 ohm coax.

Old ARRL, et al, handbooks suggest resonating the antenna with a GDO 
(I'm old school although mine uses a FET) and then take care of the 
matching with a LC network. Having said that, I've always tuned the 
antenna for VSWR and ignored the losses. Scalar used to manufacture a HF 
antenna base that contained a 2:1 transformer to improve efficiency. Did 
they work? Haven't a clue.

FWIW a base loaded antenna exhibits lower Zo than one with the coil up 
higher. A centre-loaded whip with a capacity hat used to feature in ham 
handbooks as a means to improve efficiency. I've used both centre-loaded 
(no hat) and continuously wound HF whips, and if there was a difference 
in performance I didn't notice it.

Ray vk2tv


On 04/12/17 12:41, Glen English VK1XX wrote:
> BTW everyone, if this is too off topic I can carry this privately. I 
> have experience with MF/LF antennas.....
>
> HF30FX ahh yeah , right previous generation base loaded compared to 
> the centre loaded ones
>
> yes, very much ground dependent. anything under 2:1 is probably OK, 
> with various ground arrangements see if there is a pronounced dip 
> anywhere
>
> the sharper the dip, the more likely the efficiency is good .
>
> My approach would be to ginore the VSWR once below 2:1 and try and get 
> the sharpest looking VSWR versus frequency curve
>
> think of it this way :
>
> If it was a dummy load, it would be good at all frequencies
>
> if it was a high Q tuned circuit, then it would be good VSWR at only 
> one freq and infinite everywhere else
>
> Your antenna will be somewhere in between a dummy load and a pure 
> tuned circuit...probably about 2:1 SWR BW of 50kHz (assume centre 
> matched to 1:1)  indicates reasonable efficiency.
>
>
>
>
> On 4/12/2017 12:13 PM, vk4tec at tech-software.net wrote:
>> Diamond HF30FX
>>
>>
>>
>> -----Original Message-----
>> From: OZAPRS [mailto:ozaprs-bounces at aprs.net.au] On Behalf Of Glen 
>> English VK1XX
>> Sent: Monday, 4 December 2017 11:09 AM
>> To: ozaprs at aprs.net.au
>> Subject: Re: [OZAPRS] Grounding of HF Antenna for APRS
>>
>> well, there are alot of assumptions for pre cut antennas on vehicles 
>> for HF.
>>
>> So I wouldnt pay too much attention to SWR once it is below 2:1. What 
>> sort is it ? WHat model / URL ?
>>
>>
>> On 4/12/2017 11:58 AM, vk4tec at tech-software.net wrote:
>>> I went to see an engineer mate
>>>
>>> He gave me some brass plate
>>>
>>> I am cutting out a plate that will have a braid strap coming off it
>>>
>>> That should do the trick - he said the VSWR should come down even 
>>> further
>>>
>>> Nudge bars are $1000
>>>
>>> Andrew
>>>
>>> -----Original Message-----
>>> From: OZAPRS [mailto:ozaprs-bounces at aprs.net.au] On Behalf Of Glen 
>>> English VK1XX
>>> Sent: Monday, 4 December 2017 7:35 AM
>>> To: ozaprs at aprs.net.au
>>> Subject: Re: [OZAPRS] Need a hand
>>>
>>> yeah that will work. it would be out of the weather, and get covered 
>>> with oil/grease and probably weather seal it.
>>>
>>> being close to the feedpoint actually isnt that necessary, maybe no 
>>> more than 1/20 of a wavelength, though , that is probably a good 
>>> number. so max strap length 1.5m for that freq.
>>>
>>>
>>> On 4/12/2017 8:27 AM, vk4tec at tech-software.net wrote:
>>>> Glen
>>>>
>>>> I am thinking that maybe just a strap that fits in the gap between 
>>>> the bonnet and the side wall.
>>>>
>>>> I can sink a screw into the bonnet just inside
>>>>
>>>> That way it would be as close to the feed point as possible
>>>>
>>>> I would drill into the top side of the bonnet but that equals a
>>>> divorce LOL
>>>>
>>>> -----Original Message-----
>>>> From: OZAPRS [mailto:ozaprs-bounces at aprs.net.au] On Behalf Of Glen
>>>> English VK1XX
>>>> Sent: Monday, 4 December 2017 7:23 AM
>>>> To: ozaprs at aprs.net.au
>>>> Subject: Re: [OZAPRS] Need a hand
>>>>
>>>> In looking for a place to attach a ground strap, consider that 
>>>> ideally, you want the largest continous area of metal. That is 
>>>> probably the engine bay firewall (which would attach to the upper 
>>>> body, roof etc) , fender mounts,  or in the rear, hinge mounts of 
>>>> the rear hatch, etc.
>>>>
>>>> Anyone else have any input here  on where to connect on a modern 
>>>> vehicle to maximise surface area ?
>>>>
>>>> comeon. ???
>>>>
>>>> I am a bit out of tough on modern vehicles. (I have a 2005 XC90 which
>>>> if you pass a RG58 through a door seal will squash it flat and cause a
>>>> short... hence requiring coax entry through a bung in the spare tyre
>>>> well under the vehicle)
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
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