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

On Mar 15, 9:11*am, billcalley wrote:
Hi All,

* *I always hear that antennas have to be matched to their radio, but
in receivers (such as FM and shortwave radios) I see mostly long
random length antennas used, and these antennas -- be they a
telescoping whip or a long wire out a window -- are used over some
really wide bandwidths. *How is this possible if an impedance match
must always be maintained for radios? *And since there cannot be a
good match over such wide bandwidths with any (typical) wire antenna,
what is the downside to using these completely unmatched long antennas
for receivers? *(Poor gain patterns with lots of nulls? *Lower
sensitivity due to bad noise figure or gain match for any LNA or
frontend amp? Degraded overall antenna gain)?

Thanks; I'm very confused on this subject!

-Bill

What has not been mentioned much is that AM band broadcast receivers
and FM band receivers are designed to to tune over a fairly wide band
of frequencies; so very difficult to build antenna that will mtach at
all those different 'wavelengths'.

For example; the broadcast band (North America) is roughly 550
kilohertz (that's 545 metres wavelength) to about 1.7 megahertz (about
176 metres). That's 3:1 ratio!

On FM, 88 to 108 megahertz (3.4 to 2.8 metres) the ratio is less but
still cnsiderable at 1.2:1 So again very difficult to design and build
an 'all frequencies' antenna.

For stations designed to receive only one frequncy the antennae can be
constructed for that only; hence the matching can be as optimum as
possible.

In article
,
terryS wrote:

For example; the broadcast band (North America) is roughly 550
kilohertz (that's 545 metres wavelength) to about 1.7 megahertz (about
176 metres). That's 3:1 ratio!

HOWEVER, if you will look at the Antenna Design for a AM Broadcast
Receiver, using a Ferrite Loop Antenna, you WILL notice that the
Loop IS Tuned to the specific part of the band that the receiver is
being tuned to, by Linked Ganged Variable Capacitors, in the Receivers
Frontend. These designs have been around for MANY years, (1940's anyway)
and are a very mature technology.