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

Bill,
That's one of those "sort of" kind of questions. It depends a lot
on the particular receiver and what frequency range you plan to listen
to, and then on the amount of space you have for an antenna.
In general, receivers that listen to the lower frequency bands, HF
for example, are more sensitive than those that are used for the
higher bands (VHF, UHF, SHF). That means that to hear a typical
signal a specifically designed antenna having the exact output
impedance as the receiver's input impedance isn't required. Almost
anything will work to some extent. The receiver just isn't that
'picky', since it typically has more 'hearing' ability than required.
There's a 'catch' though. Reducing the amount of signal losses in
that 'whatever' antenna is going to make the resulting signal getting
to the receiver that much stronger. Something always nice to have,
but there are practical limits. If that 'whatever' antenna meets your
requirements, then it's as 'good' as anything, sort of. If it
doesn't, then making that antenna less 'lossy' is also nice. That
"less 'lossy'" thingy also includes making it directional, longer,
shorter, higher, whatever, to increase the amount of signal getting to
the receiver. So, looking at it from that point of view, the antenna
ought'a be reasonably 'close' to what will typically 'work' well on
the received frequency. Huge range in that 'close' quality and the
definition of what 'works' means.
There is no 'perfect', 'do everything', antenna. Just too many
factors involved. 'Higher' and 'longer' tends to 'work' better than
'lower' and 'shorter', in general. Which, like any generalization, is
never always true.
That's the sort of 'long', half-assed, technical answer. The
'quick-n-dirty' answer is, no, they don't have to be 'matched' to the
receiver. Which says nothing about transmitters.
- 'Doc

[all puns intended]

On Sat, 15 Mar 2008 05:14:53 -0700, ltdoc wrote:

Bill,
That's one of those "sort of" kind of questions. It depends a lot
on the particular receiver and what frequency range you plan to listen
to, and then on the amount of space you have for an antenna.
In general, receivers that listen to the lower frequency bands, HF
for example, are more sensitive than those that are used for the higher
bands (VHF, UHF, SHF).

More correctly, it's _easier_ to make an HF receiver with a good noise
figure, and harder to do so as the frequency goes up.

However, atmospheric noise goes _down_ as the frequency goes up. So for
weak-signal work a receiver designer has a lot of motivation to make
really quiet front ends on VHF and higher equipment. Basically if the
radio is cheap (i.e. if it's for consumer use) then the front end may as
well be made of wood. If the radio is used for long-distance
communication (i.e. microwave links, space communication, some military
or amateur radio) then designers will go to great lengths to get the
noise figure down.

-- snip --


--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

The difference between antennas used for receive only and antennas
used for send/receive is that in the former case you are not going to
fry the radio's insides because of the mismatch.

But I'll bet that as you change the settings on your antenna tuner you
hear the received signal go up and down-- in particular, reception
goes up as you get a better match.

"billcalley" wrote in message
...
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

The best transfer of energy is achieved when the antenna impedance matches
the input circuit impedance of the receiver.
This should be the case for the entire band of the desired received signal.
It is not always feasible to do this and it is often not necessary. Today's
receivers have very high gain and excellent selectivity.
They need very small signal strengths to operate and excess signal is
attenuated.
So a piece of wire used as an antenna in an area where the signal strength
is large will not do any worse than a perfectly matched antenna.
In an area where the signal strength is weak (like Mars) matching of the
antenna to the front end of the receiver is desired.
There are other considerations for transmitters.

Tom

"Tom Biasi" wrote in message
...

"billcalley" wrote in message
...
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

The best transfer of energy is achieved when the antenna impedance matches
the input circuit impedance of the receiver.

OH NO! now you have done it! i hope cecil doesn't see this or you have just
openend another endless energy sloshing around thread! what does happen if
the antenna isn't matched to the radio? where does the mismatch energy
go???

On Sat, 15 Mar 2008 16:35:00 GMT, "Dave" wrote:
The best transfer of energy is achieved when the antenna impedance matches
the input circuit impedance of the receiver.

OH NO! now you have done it! i hope cecil doesn't see this or you have just
openend another endless energy sloshing around thread! what does happen if
the antenna isn't matched to the radio? where does the mismatch energy
go???

Your neighbours ;-)
Less of the energy is "taken" from the received EM field.

Joop

Dave wrote:
"Where does the mismatch energy go?'

It is mostly reradiated from the receiving antenna.

Best regards, Richard Harrison, KB5WZI

Dave wrote:
"Tom Biasi" wrote in message
...
"billcalley" wrote in message
...
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
The best transfer of energy is achieved when the antenna impedance matches
the input circuit impedance of the receiver.

OH NO! now you have done it! i hope cecil doesn't see this or you have just
openend another endless energy sloshing around thread! what does happen if
the antenna isn't matched to the radio? where does the mismatch energy
go???



A good deal of it is re-radiated by the antenna.

Cheers,

Phil Hobbs

"billcalley" wrote in message
...
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)?

Correct. Those are downsides. The upside is convenience and simplicity.
It's sub-optimal; but it works!