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]

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)?

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

"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

billcalley wrote:
Thanks; I'm very confused on this subject!

The AGC circuit in receivers has enough dynamic
range to compensate for low amplitude signals
from a mismatched HF antenna. The S/N ratio
on HF is mostly external to the receiver.
--
73, Cecil http://www.w5dxp.com

Dear Bill (no call sign):
It depends. The goal of a receiver is to have a satisfactory
signal-to-noise & distortion ratio. Many factors enter into that ratio.

Let us consider just the antenna: Below something like 30 MHz, it is
easy to have receivers that are quiet relative to noise picked up by most
antennas. In other words, below something like 30 MHz, the SNR is
determined outside of the receiver and tweaking the tuning between the
antenna and receiver just increases or decreases the volume. At VHF and
higher frequencies, the noisiness of the receiver tends to be a significant
player. It has long been known (see MIT Radiation Lab volume 18) that the
maximum SNR occurs close to a conjugate match between the antenna and
receiver's first stage.

However, many consumer VHF-UHF receivers (FM broadcast, TV, scanners)
are presented with such large signals that there is little point in tweaking
the interface between antenna and receiver for an even larger SNR.

In the early days of radio astronomy, when we were still using vacuum
tubes, a lot of attention was paid to having the right amount of mis-match
so as to maximize SNR.

So: HF and below: be concerned about the directivity of the antenna
(is it reasonably sensitive in the direction of interest).
VHF and above: have the antenna's impedance, transmission line's
impedance, and receiver's input impedance be within a factor of 2 of each
other AND point the antenna in a desired direction.

Regards, Mac N8TT
--
J. McLaughlin; Michigan, USA
Home:
"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

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

On Sat, 15 Mar 2008 04:11:21 -0700, 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

In areas where it matters receive antennas are matched, and are something
other than random wire. This is why you can go to Radio Shack and buy TV
antennas* -- they're designed** to be both directional and a good match
over the broad frequency ranges of TV signals.

Get into amateur radio or military communications and you'll find many
different permutations of directional, matched antennas on receive.

Note: For many LNA designs, the best signal/noise ratio occurs at an
impedance that is close to, but not really, a perfect conjugate match.
The signal is coupled to the amplifier best at the conjugate match
impedance, but sometimes the noise is enhanced even more.

* or could -- does Rat Shack still carry antennas?
** kinda -- they're really designed to _look_ like they'd make a good
antenna, but they're better than rabbit ears.

--
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

Note: *For many LNA designs, the best signal/noise ratio occurs at an
impedance that is close to, but not really, a perfect conjugate match. *
The signal is coupled to the amplifier best at the conjugate match
impedance, but sometimes the noise is enhanced even more.


That brings up an intersting question I never did get a good answer
to...

It is my assertion that an LNA that is physically at room temperature
(290K) can have a noise figure no better than 3 dB (i.e. its effective
noise temperature is 290K) IF it is also conjugatly matched i.e.
looks like 50 Ohms.

Yes you can make the noise figure better than 3 dB, but then you must
either cool the device or MISMATCH it to the line.

In other words if it looks like 50 Ohms and it is physically at 290 K,
it's effective noise temperature must also be 290K.

How could it be otherwise?

Comments plese.

Mark