Do receiver antennas need matching or not?
 

"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???

Do receiver antennas need matching or not?
 

On Sat, 15 Mar 2008 04:11:21 -0700 (PDT), billcalley
wrote:

How is this possible if an impedance match
must always be maintained for radios?

Hi Bill,

It is not always needed if the signal is strong enough. If the signal
is not strong enough, then you can obtain considerable gain through
tuning.

Tuning also brings other advantages by rejecting signals that could
depress your radio's sensitivity.

73's
Richard Clark, KB7QHC

Do receiver antennas need matching or not?
 

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

Do receiver antennas need matching or not?
 

On Sat, 15 Mar 2008 09:35:22 -0500, Tim Wescott
wrote:

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.

I once cranked out an NEC2 model of a Radio Shock TV antenna to see
what it really did over the 54-890MHz range. It was fairly horrible.
There were actually a few frequencies where the impedance was close to
300 ohms. There were also a few frequencies where it actually had
some gain. At some frequenies, it had more gain in the reverse
direction than forward. As an example of a "directional and a good
match" antenna, that typical Radio Shock TV antenna doth truly suck.

I'll see if I can find the model. Unfortunately, it may have been on
a hard disk that crashed a few years ago.

Not having a good match between the antenna and LNA has several
effects. The mismatch will affect the system noise figure thus
reducing sensitivity. Some LNA's are not unconditionally stable and
will oscillate when presented with a weird source impedance. At HF
frequencies, the atmospheric noise level is above the receiver input
noise level, so considerable mismatching can be tolerated. About
about 20MHz, this is no longer the case, and a match is required.

Antennas are also affected by their load impedance. A highly
directional yagi antenna pattern can easily be ruined by mismatched
coax or LNA input impedance. For the antenna to work as advertised,
it has to see the rated load.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Do receiver antennas need matching or not?
 

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

It is mostly reradiated from the receiving antenna.

Best regards, Richard Harrison, KB5WZI

Do receiver antennas need matching or not?
 

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

Do receiver antennas need matching or not?
 

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

Andy writes:

As a practical matter, if the background noise heard in the receiver
increases when the antenna is attached, the antenna is good enough.

This means that the atmospheric noise, in the frequency range that
the receiver is tuned to, is greater than the internal receiver
noise....

It also means that any signal that is to be received that exceeds
the
atmospheric noise, will be heard......

Unless you are using some signal processing that can detect signals
below the atmospheric noise level, this is a very good rule of
thumb....

Consider an airborne LORAN antenna, used on aircraft, to receive
100 khz signals. It works out that around 22 inches is the length
where
the atmospheric noise exceeds the general receiver noise for most
receivers. Making the antenna longer will pick up more desired
signal,
but also more atmospheric noise, in the same ratio.....so the SNR
doesn't
get much better. Note that a matched antenna for 100khz will be
many many hundreds of feet long.... but is never used either in boats
or airplanes, since a "matched" antenna serves no advantage to
sensitivity ( SNR)....

These are rules of thumb, and useful approximations, but, in
effect,
you don't need a great antenna unless you are trying to receive a weak
signal....or have a method to increase the SNR by signal processing.

Andy in Eureka, Texas W4OAH

Do receiver antennas need matching or not?
 

Mark wrote:
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


That beer in your hand was also cooled in a 300K ambient. How is that
possible? (Hint: the fluctuation-dissipation theorem only applies to
systems in thermodynamic equilibrium. The moment you turn on the power,
that assumption is violated, just as it is in your domestic refrigerator.)

An ordinary room-temperature diode has a noise temperature of 150K
(Tambient/2) as you can show in about 3 lines of algebra, starting from
the diode equation and the shot noise and Johnson noise formulas.

Cheers,

Phil Hobbs

Cheers,

Phil Hobbs

Do receiver antennas need matching or not?
 

On Mar 15, 3:11*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

Well, I 'm not an expert, but it seems that with a transmitting
antenna, the idea is to transfer as much power as possible to increase
efficiency, and so the antenna needs to be closely matched to the
output of the transmitter for best results. But the receiving antenna
is a different problem, since no power from the antenna is needed to
drive the receiver, and so who cares about the match? The idea with
the receiving antenna is to get the most voltage and highest S/N ratio
with no load. The input to the receiver should be buffered with a high
impedance FET amplifier, or some such, so the receiver draws almost no
power from the antenna. This leaves you free to design the antenna and
input tuning circuit for the highest Q and lowest noise figure without
worrying about impedance match.

Just my opinion.

-Bill

Do receiver antennas need matching or not?
 

On Sat, 15 Mar 2008 17:51:47 -0700 (PDT), AndyS
wrote:

Consider an airborne LORAN antenna, used on aircraft, to receive
100 khz signals. It works out that around 22 inches is the length
where
the atmospheric noise exceeds the general receiver noise for most
receivers. Making the antenna longer will pick up more desired
signal,
but also more atmospheric noise, in the same ratio.....so the SNR
doesn't
get much better.

Agreed. However, the short 18" antenna is commonly used for handheld
and aircraft Loran receivers. However marine Loran antennas are
typically 8ft long.
http://shakespeare-marine.com/antennas.asp?antenna=5220

That's not the only reason that Loran antennas are rather short. If
the antenna were longer, the impedance would increase, causing it to
pickup more percipitation static, atmospheric noise, and 60Hz
harmonics. A longer antenna would also not be as narrow band and low
impedance as a short (loaded) antenna. The relatively narrow
bandwidth is helpful for eliminating broadcast, beacon band, and other
forms of interference.

Incidentally, that's also one reason why some remote Loran systems
have a pre-amp that really burns some watts. It needs to handle the
out of band overload and stay linear. If the antenna were made
longer, the amplifier would need to handle proportionately more power
(and probably melt). Some details in the patent at:
http://www.google.com/patents?id=ONUrAAAAEBAJ&dq=4875019
The "background" section is worth reading. The other reason for the
amplifier is to give the antenna system a 50 ohm output impedance so
that cheap coax can be used.



--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558