[Lowfer] JFET (J310) Biasing

[Clive S Carver]

Hello John JD, Tom, John W1TAG and Mike

Thank you for your replies.

The reason I asked the question was that I have recently purchased off eBay
a couple of rather nicely made, double-sided pcb's with
through-hole-plating, solder resist and screen printing for PA0RDT based
e-probe.

I use a PA0RDT e-probe to good effect, however from the circuit could see
that it was not optimum for VLF. Value of coupling capacitors and feed
choke.  I did modify my bias-T to help improve VLF response. I had purchased
e-probe and bias-T from Roelof Bakker PA0RDT in 2007 and his construction is
superb. I felt guilty taking a soldering iron to it, but reminded myself
that English Electric Lightnings and Nimrods had survived my soldering, so
went ahead. (I worked for the UK MoD as a civilian in the late 60s/70s) 

So my plan was to build a couple of PA0RDT e-probes myself and see if I
could improve VLF reception, by using higher value coupling capacitors and
feed choke.

That said, SAQ's 17.2 kHz transmission on Sunday 29th from 1070 km away from
Hawarden, north Wales, was at a good strength, during key-down for tuning
virtually the same as JXN on 16.4 kHz 1783 km away. The Alpha transmissions
were also clearly visible on the SpectraVue waterfall, the receiver being an
SDR-IQ. 

However, re the eBay pcb's, noticing less resistor location markings than
expected, I traced out the tracks and found the 'third' J310 biasing option
that I referred to originally.  Whilst Roelof uses the three resistor
method, I have come across the simple potential divider elsewhere. I will
stick with Roelof's method, which I can achieve with resistors mounted
vertically.


73's

Clive
GW4EYO


From: Lowfer [mailto:lowfer-bounces at mailman.qth.net] On Behalf Of Clive S
Carver
Sent: 28 June 2014 18:20
To: Lowfer at mailman.qth.net
Subject: [Lowfer] JFET (J310) Biasing


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