[amsat-bb] RX-only Antenna "tee" connector?
daniel at destevez.net
Wed Jan 11 14:46:53 UTC 2017
El 11/01/17 a las 02:11, Scott escribió:
> Hi everyone.
> I suppose that any deviation from the ideal setup of a near-perfectly
> matched antenna & receiver will have SOME negative effect, but for a
> receive-only antenna, how badly am I hurting myself to split the
> antenna between two receivers?
> Since frequency is probably important to answer this, I’m referring
> to 70cm.
> Also, if it matters, I have a very good preamp (SP-70 from SSB) at
> the antenna feeding 25 meters of LMR-400. Both receivers are SDR
> devices (receive-only).
A perfect splitter introduces a loss of 3dB, since half of the signal
goes to the other receiver. A real world splitter will have a slightly
greater loss due to imperfections, but something like 3.1dB or 3.2dB is
usual, so I'll call that 3dB for the sake of the argument.
If you have no preamp, this raises your noise figure by 3dB. However,
since you have a preamp before the splitter, you have to divide by the
gain of the preamp. Say your preamp has a gain of 20dB (which is
typical). Then your noise figure is only raised by 0.03dB, which is tiny.
The important question is how much signal-to-noise you lose because of
this increase of 0.03dB in noise figure. This depends on your antenna
temperature (which is the amount of noise that your antenna grabs from
the environment). Noise figure is normalized at 290K, so if your antenna
temperature is 290K you lose precisely 0.03dB of SNR. Your antenna
temperature is almost never 290K. At 2m it is likely that the antenna
temperature will be much greater than 290K, on the order of thousands of
K. Therefore, your losses in SNR are much smaller than 0.03dB. On 70cm
and higher, the antenna temperature can be lower than 290K (much lower
on the high microwave bands), especially in quiet rural zones.
Therefore, your SNR losses will be much greater than 0.03dB but still a
fraction of a dB, so you won't even notice the losses.
These are the exact calculations (by the way, the same calculations can
be used for coax losses and any other sort of losses after the preamp).
In layman's term it's much simpler: you lose (almost) nothing, because
your preamp provides enough signal gain to feed both of your receivers
with adequate signal level, despite the fact that the signal power is
split in half.
There might be a problem if the gain of your preamp is especially low
(say 10dB) and you're in a very quiet area.
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