Do receiver antennas need matching or not?
Joel Koltner wrote:
"Homer J" wrote in message
.. .
All this talk about noise while important to minimum detectable signal more
greatly influenced by the internal Noise Figure (NF) of the receiver (RX).
My understanding is that this is not the biggest influence at HF -- there's so
much atmospheric noise down there that even with a pretty poor receiver (noise
figure-wise) the MDS is usually just about the same as with a much better
receiver.
In my line of work, which is Radar engineering, we use a standard
temperature T = 270 Kelvin to model the noise originating by natural
extgernal sources of which the Sun is the biggest contributor.
Have you seen the graph in, e.g., Krauss's antenna or EM book? T=270 is a
poor model at many frequencies. (Granted, if you're doing narrowband designs,
it'll just be some offset error that's probably not too much worse than, say,
+/-3dB.)
However, if you use one of those collaspable whips found on the portable
shortwave receivers you will. This is because the anyenna impedance is a lot
less than the usual 50 Ohm impedance of the RX antenna port (e.g. Zant
Zrx_port ).
You can match very short antennas with antenna tuners to make them transfer
efficently to the RX antenna port but now the nasty parameter of effective
antenna aperature (square feet or meters) reduces it caoture ability
From watching this thread I get the impression that -- at least on HF again --
the (lack of) capture area is the much bigger problem than the mismatch is.
---Joel
yes
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