[Lowfer] W1VD modified Amrad whip tested (matching jfets notneeded?)

[Dave Brown]

Steve/Jay- thanks for the replies on this- your test results Steve are 
just the sort of detail I was interested in.
Think I have about 20 in  a bag here so will go through them and see 
what the spread is like.
73
 Dave
ZL3FJ

----- Original Message ----- 
From: "Steve Ratzlaff" <steveratz at wildblue.net>
To: "Discussion of the Lowfer (US, European, &amp;UK) and MedFer 
bands" <lowfer at mailman.qth.net>
Sent: Thursday, December 09, 2010 8:17 AM
Subject: Re: [Lowfer] W1VD modified Amrad whip tested (matching jfets 
notneeded?)


> Just to see what happens when the two jfets aren't matched at all, I 
> went
> through my bag of J310's and tried to find two with very different 
> Idss. I
> picked one with 51.8 mA and one with 30.6 mA and put them in the 
> same
> circuit I tested yesterday. Here are the IMD results, same setup as
> yesterday. (My results from yesterday, with two matched 36 mA jfets 
> are
> shown too.)
>
> 24.0 volts/59 mA
> 1 MHz +80.5 OIP2
> 2 MHz +34.3 OIP3
> 5 MHz +34.1 OIP3
> 7 MHz +79.8 OIP3
>
> 13.5 volts/41 mA
> 1 MHz +80.7 OIP2
> 2 MHz +33.6 OIP3
> 5 MHz +32.7 OIP3
> 7 MHz +73.1 OIP2
>
> It doesn't appear that matching jfets does much if anything, but 
> being able
> to adjust the bias with an IMD test setup is the main thing. Since 
> few folks
> have that capability, perhaps just setting the bias for similar 
> currents to
> what I obtained would be good enough.
> 73,
> Steve AA7U
> NE Oregon
>
> ----- Original Message ----- 
> From: <jrusgrove at comcast.net>
> To: "Discussion of the Lowfer (US, European, &amp;UK) and MedFer 
> bands"
> <lowfer at mailman.qth.net>
> Sent: Wednesday, December 08, 2010 3:09 AM
> Subject: Re: [Lowfer] W1VD modified Amrad whip tested
>
>
>> Dave, LF
>>
>> Matched them for Idss. Only had to go a half dozen deep into a bag 
>> of 50
>> to get a match. The spread
>> didn't appear to be too great - at least in the batch I had. Quick 
>> and
>> easy to do the test ...
>>
>> Jay
>>
>>
>> ----- Original Message ----- 
>> From: "Dave Brown" <tractorb at ihug.co.nz>
>> To: "Discussion of the Lowfer (US, European, &amp;UK) and MedFer 
>> bands"
>> <lowfer at mailman.qth.net>
>> Sent: Wednesday, December 08, 2010 1:58 AM
>> Subject: Re: [Lowfer] W1VD modified Amrad whip tested
>>
>>
>>> Jay/Steve
>>> Did either of you use any special selection/matching of the J310s?
>>>
>>> ----- Original Message ----- 
>>> From: "Steve Ratzlaff" <steveratz at wildblue.net>
>>> To: "Discussion of the Lowfer (US, European, &amp;UK) and MedFer
>>> bands" <lowfer at mailman.qth.net>
>>> Sent: Wednesday, December 08, 2010 11:54 AM
>>> Subject: [Lowfer] W1VD modified Amrad whip tested
>>>
>>>
>>>>I built a version of Jay's modified Amrad active whip and tested 
>>>>it
>>>>on the
>>>> bench today, checking IMD at supply voltages of 24 and 13.5 
>>>> volts,
>>>> adjusting
>>>> the bias for optimum 2IMD for each voltage. I used 10 pF as the
>>>> series input
>>>> cap to simulate a 36-39 inch whip, and my standard 3 and 4 MHz 
>>>> IMD
>>>> test
>>>> tones. This circuit won't output my normally-used 0 dBm to the
>>>> spectrum
>>>> analyzer so I used -10 dBm output. (I used 17 bifilar turns on
>>>> FT50-75
>>>> toroid for the transformer.)
>>>> supply voltage 24.0 volts (60 mA optimum bias)
>>>> 1 MHz +76 OIP2
>>>> 2 MHz +36.5 OIP3
>>>> 5 MHz +35.8 OIP3
>>>> 7 MHz +76 OIP2
>>>>
>>>> supply voltage 13.5 volts (36 mA optimum bias)
>>>> 1 MHz +79.8 OIP2
>>>> 2 MHz +34.2 OIP3
>>>> 5 MHz +33.7 OIP3
>>>> 7 MHz +72 OIP2
>>>>
>>>> Not much IMD improvement is gained running at 24 vs. 13.5 volts. 
>>>> As
>>>> Jay has
>>>> found, this modified circuit should work OK for many uses and is
>>>> inexpensive
>>>> to build compared to the expensive transistor needed for the
>>>> original Amrad
>>>> circuit.
>>>>
>>>> 73,
>>>> Steve AA7U
>>>> NE Oregon
>
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