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 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

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

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.


I make amplifiers with 50 Ohms input impedance and 300pV/rtHz input-
referred noise. By your assertion, that should have been impossible.
The trick is that the input impedance obtained by feedback: The
cooled resistor trick. It works.

Jeroen Belleman

On Mar 17, 4:06*am, Jeroen Belleman wrote:
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.

I make amplifiers with 50 Ohms input impedance and 300pV/rtHz input-
referred noise. By your assertion, that should have been impossible.
The trick is that the input impedance obtained by feedback: The
cooled resistor trick. It works.

Jeroen Belleman- Hide quoted text -

- Show quoted text -

good point thanks
Mark

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

"Tim Wescott" wrote in message
...
* or could -- does Rat Shack still carry antennas?

Yes. If anything there's been a bit of a rebirth in antenna sales as people
start hearing about free over-the-air (ATSC) HDTV.

(Speaking of HDTV... Fry's is advertisiting ATSC-NTSC converters for $59,
coming very close to the $49 I was predicting a while or so ago... Joerg will
be pleased. :-) )

On Mon, 17 Mar 2008 09:56:01 -0700 in rec.radio.amateur.antenna,
"Joel Koltner" wrote,
(Speaking of HDTV... Fry's is advertisiting ATSC-NTSC converters for $59,
coming very close to the $49 I was predicting a while or so ago.

I saw two models at Wal-Mart for $49 each. Magnavox and some other
name I don't know. That comes even closer.