Topband: Array of MiniWhips?

[Lee STRAHAN]

Hello Ash,
   It depends on how complex an array you want to build  as to whether these amps would work OK. Two or three elements might work fine.
      There are several things that concern me about these amps. I do not have the time to test them here however.
    First there is no way to make sure the gain would be equal between several amps as the gain in these amps is pretty well
defendant on the semiconductor gain parameters and operating currents.
    Secondly there is no protection against static discharges on the elements. Or induced currents in the feedline for that matter. If you are near any winter storms or lightning events 
I don't think the amps would survive.
    The third item is that the output impedance of this amp is just a few ohms due to transistor operating current, transistor  Beta, and Rb parameters, plus more and each amp would likely be different causing 2 things. One is gain change and the other is driving a coaxial cable off the design impedance will cause extra phase shifts not intended. Yes, I know if one end of a coax is terminated there is supposed to be no unintended phase shifts however I have found with typical RG-6 coax cables, impedance tolerances, and typical matching transformers etc it is best to match coax impedance at both ends which has produced much better results and predictable results here. To fix this would require the amp to have a 75 ohm output impedance. 
    The last item is, it is my belief that this amp would not have a very high second order IMD capability and if you were too close to any broadcast band high power stations it could be a problem. 
   Lee  K7TJR  OR


-----Original Message-----
From: Topband <topband-bounces+k7tjr=msn.com at contesting.com> On Behalf Of Ashraf Chaabane
Sent: Sunday, February 20, 2022 11:43 AM
To: VE6WZ_Steve <ve6wz at shaw.ca>
Cc: topband <topband at contesting.com>; Boubaker Ahmed <Boubaker-AH at hotmail.com>
Subject: Re: Topband: Array of MiniWhips?

Hi Steve,

Thanks for your email and information.

We've been looking at the preamp design again but didn't manage to confirm if it would deliver accurate phase/amplitude.

Can you (or anyone here) check the design and advise. It can be found
here: http://www.kiwisdr.com/docs/pa0rdt_whip.pdf

If an array of Miniwhips can fly, it would be a perfect solution for us.

73 Ash 3V8SS 3V8SF


On 19/02/2022 16:49, VE6WZ_Steve wrote:
> Hello Ash,
>
> The small vertical (HI-z) arrays like the 8 circle and 9 circle depend on VERY accurate phase and amplitude between elements.
> Even small variation in amplitude, or phase can destroy the pattern and RDF which is what makes these arrays effective.
>
> The HI-z amplifiers used in these arrays have very accurate phase and amplitude which is normalized between all units.
> I do not know what the PA0RDT amps are like, but this is a crucial thing you need to look at.
>
> 73, de steve ve6wz
>
>> On Feb 19, 2022, at 1:25 AM, Ashraf Chaabane<ash.kf5eyy at gmail.com>  wrote:
>>
>> Hi All,
>>
>> We recently built a couple of PA0RDT MiniWhip antennas that worked fine on low bands (seehttp://www.kf5eyy.info/technical.htm) and we wonder if these were used by anyone to make a steerable array.
>>
>> Please advise.
>>
>> 73 Ash 3V8SF 3V8SS
>>
>> www.kf5eyy.info
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