Dan,
If you and the group will pardon a somewhat long response, let me
offer some thoughts. I think you said you were not particularly
"radio literate" and it appears you aren't really understanding the
difficulty with what you propose to do. I will attempt to explain.
> I am trying to find a DC injector that will power a preamp which can
> "bypass" for an output amplifier.
As others have said this is technically possible but be prepared to
pay for it. This type of system won't be cheap. Hint: I haven't
really looked into it but I suspect a building closer to the antenna
mast would cost far less. The DC injector isn't the problem, it's
the extra hardware you're going to need to split the transmit and
receive signals at the preamp (see below).
> From the GM400 I run to the duplexer at 25 watts. then, from the
> output of the duplexer to the antenna, I connect a dc injector, then I
> connect the amplifier for the output power of 150 watts.
No. The amplifier has to go between the GM400 (GM300?) and the
duplexer. The purpose of the duplexer is to isolate the receiver and
transmitter from each other while allowing you to use one run of
coax and one antenna. Everything associated with transmitting (such
as the amplifier) goes on the transmit port of the duplexer.
Everything associated with receiving (such as the preamp) goes on
the receive port of the duplexer. At the antenna port you have
transmit and receive signals on the coax at the same time - you
cannot put any transmit devices (such as amplifier) or receive
devices (such as preamp) here without a lot of extra hardware!
What you propose to do is put a receive preamp in the part of the
system that is carrying both receive and transmit signals. If we
were talking about a normal base station, which is either receiving
or transmitting (but not both) at any given moment, this would be
reasonably easy by providing a switching system to bypass the preamp
when the radio transmits. It's not that simple in a repeater which
receives and transmits *at the same time*.
In a single-antenna repeater system, the only way you can put a
preamp near the antenna is to essentially have the equivalent of two
more duplexers up there with it! It may be called "diplex filtering"
as someone else here mentioned, but make no mistake it is going to
require some expensive hardware. You would essentially have to split
the receive and transmit signals, and then recombine them after the
preamp. That's basically the function of *two* duplexers (one to
split, one to recombine). You're also going to have some losses
(both transmit and receive) in this hardware.
In one of your posts you mentioned 250' of coax and in the other
350'. Which is it? Your transmit power figures indicate you are
expecting 9 dB loss in the cable. That implies fairly poor coax in
either case. What type of cable are you using?
You say the budget allows for "reasonable expenses". I would
seriously look into getting a good low loss feedline. The cost of
new 1-5/8" or even 7/8" heliax may be prohibitive, but in some areas
you might be able to get it on the used or surplus market for less
than it would cost to erect a building closer to the antenna. At 350
feet, 7/8" would have about 3 dB loss, a lot better than the 9 dB
you seem to expect from your current cable. 1-5/8" would come in
somewhere around 2 dB for 350 feet. The catch is, reading between
the lines I get the feeling you are not "well connected" to radio
people in your area, so you may not be able to find the deals if
they are out there.
> My uneducated guess on the final output power on a 350' run of coax will
> be about 18.75 watts out.
That is correct for 9 dB loss between the amplifier and antenna.
However, as I said before the amplifier goes on the transmit port of
the duplexer, so you will have the feedline loss plus the duplexer
loss. Depending on the duplexer, it probably won't be much...
probably around 1 dB give or take a bit.
> A 20 db preamp connected to a 6db gain
> antenna would be 1.625db at the repeater. Is that about right?
I'm sorry, I don't follow what you mean here, but I'd have to say
no, that's not right.
Per your additional post about separate transmit and receive
antennas, yes that may be an option. I'll let someone with the
proper tools work out how far apart they would have to be. You're
still going to lose a lot of trasmit power if you use the lossy
coax, but the receive can be improved by running two antennas with
the preamp up at the receive antenna... provided you can get
adequate spacing between antennas. Which antenna goes on top depends
on your needs and circumstances. I generally prefer the receive
antenna on top, as receive performance is often the limiting factor
in repeater installations.
Without knowing exactly what coverage you need or whether you need
to fully exploit the potential of your repeater site to achieve it,
it's hard to say what measures are really appropriate and/or
necessary. Upgrading to a really good feedline would drastically
reduce losses and improve performance without needing to put a
preamp at the antenna with all the extra expense and complication
that comes with it. Not to mention it would improve both receive and
transmit. If the budget or circumstances won't allow that, then
separate antennas with preamp at the receive antenna may be a
solution, provided you can get the antennas far enough apart... and
provided your coax shielding is adequate to prevent transmit signal
from "leaking" into the receive line (I'm assuming the cables would
be parallel and quite close together for much of the run).
One other option might be to use one antenna and put the duplexer
and preamp in a weatherproof enclosure on the tower. You would have
to run separate receive and transmit coax lines down to the rest of
your repeater in the building. As with any other approach there are
some caveats, but it can be done.
If you didn't understand something or have additional questions, I
will try to help.
Paul
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