[R-390] Multicouplers - active vs. passive

[Lester Veenstra]

Don:
    This sounds like the right way to go; Can you post a URL that points to
the schematic? I do not come up with it on my search.
   Thanks
       Les


Lester B Veenstra  MØYCM K1YCM W8YCM
lester at veenstras.com

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-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net]
On Behalf Of 2002tii
Sent: Tuesday, January 15, 2013 8:37 PM
To: r-390 at mailman.qth.net
Subject: Re: [R-390] Multicouplers - active vs. passive

If you build an RF amplifier like the one in a TenTec RX340 (not the
switchable BFG16A RF preamp, the RF amp comprising six J310 FETs in
push-pull parallel) [schematics are available on the web], you will get 6 dB
of gain with input-referred noise approaching -130 dBm in a
3 kHz bandwidth, harmonics around -80 dBc at 100 mV input (-7 dBm), and
enough headroom to shrug off nearly anything that doesn't melt your antenna.
Run with a 12 V supply, the 1 dB compression point is greater than 4 Vrms
input (+25 dBm).  Follow this amplifier with a passive splitter, and you
will have a state-of-the-art multicoupler.  System gain will be ~ +3 dB with
a 2-way splitter, 0 dB with a 4-way, and -3 dB with an 8-way.

I know of no commercially-available multicoupler, past or present, that can
match this performance (of course, I have not seen every commercial
multicoupler ever produced).

For anyone who wants to build one:  I typically select FETs for Idss of
45-50 mA at 12 V (this needs to be pulse-tested with a duty cycle ~30%, to
mimic in-circuit operating conditions and prevent overheating the FETs) --
but 6 unselected FETs should work fine unless you are very unlucky in the
draw.  L59 and L60 are unnecessary and can/should be omitted.  You will need
to wind or find three suitable ferrite transformers, one of which (T3) must
operate as a
1:1 transmission-line transformer ("current-mode balun") at the lowest
frequency of interest.  I replace C115 with a 100 uF tantalum, and add
another 100 uF tantalum in parallel with C113.  Make sure all of the
multicoupler outputs, including any unused outputs, are terminated in 50
ohms to maintain the rated isolation.

Best regards,

Don