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Re: Bias-T's and preamps.

> 1) How critical is the construction?  A cap, encapsulated inductor, and
> connectors doesn't appear to be that complicated.

No, not terribly.  Just make sure you have the right sizes for cap and
inductor.  It's a simple circuit though.

> 2) Do preamps and converters pass the DC up the feedline to "upstream"
> components?  e.g.:
> "T"
> Antenna--- 2.4 GHz ---- 2.4 -> 2M ---- 2M - 28MHz ------- rig
> preamp       converter      converter     |
> (if needed)  (Drake or      (maybe)      12V DC
> similar)

Probably not as you have it.  Generally, such devices have blocking caps
somewhere in their lineup.  Even if it didn't specifically have a blocking
cap, there are plenty of RF circuits that will by nature not pass DC, either
because of a series cap or an inductor to ground.  Matching networks for
amplifiers, filters, etc. will all cause DC problems if you try to pass it.
A filter for example, can end up looking like nothing to DC, like an open,
or like a dead short depending on the configuration and type of filter.

Now you could do a design where you take the DC off the input, block it from
the RF circuit of the module, route it to the output where you have another
bias T to inject it back into the feedline.  I am not sure any commercial
equipment is made that way, but you could modify it.

> Or should I assume that I need to put the bias T before the component(s)
> needed power on the feedline?  Or simply make sure everything uses
> external power and run a 12V DC line?

Personally, with multiple devices, I'd run a completely separate DC line.
This is particularly true if you have a transmit converter and amplifier
mounted at the mast.  If you have 12 Volts inside the shack and you run it
up 60 feet of feedline and are drawing several hundred milliamps or even
amps of current, you will not have 12 volts at the mast.  The best bet is to
run a separate supply that is higher than 12 Volts and then regulate the
voltage at the mast.  That way, you'll always give your components the
voltage they need. 

I would avoid sending DC up the coax for anything but a single RX converter.
Some may disagree but it's my opinion.  Running DC lines is easy.



Jon Ogden
NA9D (ex: KE9NA)



"A life lived in fear is a life half lived."

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