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Re: [TowerTalk] "bias Tee" from MFJ and others

To: "'Jim Lux'" <>, <>
Subject: Re: [TowerTalk] "bias Tee" from MFJ and others
From: "JC Smith" <>
Date: Thu, 7 Sep 2006 11:06:31 -0700
List-post: <>
I'm going to be running some DC powered equipment on my tower.  Preamps at
least, maybe a downconverter and perhaps a 1.2 GHz brick.  I've been
planning to run power to the tower and install a DC bus in a raintight box.
I have a number of surplus power supplies that have terminals labeled +s
and -s (sense).  They are currently jumpered to the + and - output terminals
but if the jumpers are cut and two separate pairs are run to the load, the
power supply is supposed to adjust the voltage at the output terminals to
maintain the correct voltage at the load (compensate for the loss in the
connecting wires).  Does anyone have experience with this?  I don't really
want to run super-heavy gauge wire to the bus box and this looks like a good
alternative.  Will the power supply adjust over its fill range to
compensate?  Some of them are variable from 0 - 32 Volts; that could allow
the use of fairly small wire, but there must be some limits on how this
system works.  Unfortunately, I have no manuals for the supplies, but I am
going to look for them.

Anyone with experience with using the +s and -s feature of these supplies?

Thanks & 73 - JC, K0HPS

-----Original Message-----
[]On Behalf Of Jim Lux
Sent: Thursday, September 07, 2006 7:26 AM
Subject: [TowerTalk] "bias Tee" from MFJ and others

Does anyone have practical experience with HF bias tees used to send
DC power up the coax?  For instance, MFJ has one that is about
$50/pair. (MFJ-4116)

I'm sure they are just a series capacitor in the RF path and a RF
choke in the DC path, but what sort of values do they have? and, more
interestingly, what are the component ratings?  The MFJ site says 1
Amp and 50VDC, which is the DC path (what's the series resistance of
that choke, though?), but I'm curious about the RF path.

We have some MFJ remote switches at W6VIO which use the same
technique to send the relay current up the coax, and as I recall,
there's just a couple of disk caps for the DC block.  At 1.5 kW,
those caps have to handle 6 Amps or so of RF without significant heating.

Other sources than MFJ? (e.g. Tessco?)

What about designs to handle more DC power?

What about interaction with transient suppression devices?   What
order do you put the bias tee and transient suppressor?

Feedline to antenna: transient suppressor: bias tee: rig?

What about at the antenna end?

How could you do this and still have a DC grounded antenna? (the load
at the antenna end is presumably low Z for DC, and you could bypass
around it for RF that gets through the choke

I'm looking at schemes to run equipment that's a bit more than just a
bunch of relays at a remote antenna location and sending the the
power and signalling up the same 1000 ft run of coax.


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