[Lowfer] W1VD modified Amrad whip tested (matching jfets not needed?)

[Steve Ratzlaff]

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