[Lowfer] JFET (J310) Biasing

[Michael Sapp]

Tom, Clive: I prefer the W1VD modified amrad e-probe for it's inherent 
common mode suppression design feature.  The 470uH choke on the PA0RDT B+ 
line wont

suppress much RF at 100 kHz with 295 ohms of reactance, 10 times less at 10 
kHz.  The 10 turn BN73-202 isolation transformer & DC injector has 1mH 
windings

and the RF transformer isolation is highly desireable for noise suppresion 
as long as one uses a battery to keep the AC mains noise off the active 
antenna.....

Mike wa3tts


----- Original Message ----- 
From: "n8tl" <n8tl at woh.rr.com>
To: <clive at ancient-mariner.co.uk>; "Discussion of the Lowfer (US, European, 
& UK) and MedFer bands" <lowfer at mailman.qth.net>
Sent: Saturday, June 28, 2014 2:16 PM
Subject: Re: [Lowfer] JFET (J310) Biasing


> Hi Clive:
>
> I am currently working on w1vd's version of the e-probe.  You should 
> adjust for 20-25 ma or 2.0-2.5
> volts across the bias resistor which is 22 megohm in this case.
>
> Good Luck and Cheers,
>
> Tom N8TL
>
> On 14-06-28 01:20 PM, Clive S Carver wrote:
>> I am curious of the pro's and con's of three different ways of biasing a
>> J310 for use as an e-Probe front end stage.
>>
>> 1. For example, the PA0RDT mini-whip uses two 1Mohm resistors in a 
>> potential
>> divider across the +12ve to ground supply, with a third 1Mohm resistor
>> between the potential divider mid-point and the J310 gate.
>>
>> 2. Some designs omit the third 1Mohm resistor referred to above, and 
>> connect
>> the potential divider mid-point directly to the J310 gate.
>>
>> 3. I have also come across a design where there is a single 1Mohm 
>> resistor
>> between the +12ve rail and the J310 gate.
>>
>> So, what are the advantages/disadvantages in the three methods (besides
>> using less 1Mohm resistors!).
>>
>> 73's
>>
>> Clive
>> GW4EYO
>>
>>
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