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[TowerTalk] "bias Tee" from MFJ and others

Subject: [TowerTalk] "bias Tee" from MFJ and others
From: Jim Lux <>
Date: Thu, 07 Sep 2006 07:26:19 -0700
List-post: <>
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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