Do receiver antennas need matching or not?
 

Jeff Liebermann wrote:

Ghosts are a big problem where I live. Or multipath and the resulting
dropouts the case of DTV broadcasts. That's why I like highly
directional designs (both horizontally as well as vertically). Other
than that, I can get all of our local stations with rabbit ears and a
UHF loop.


I live too far away from the local digital TV xmitters to get reliable
reception. My rule of thumb is that if OTA analog TV reception is
marginal, digital TV will be worse.

Highly directional antennas are the right way to eliminate ghosts
(reflections). However, I keep running into problems with f/b (front
to back) ratio problems, where the ghosts are reflected from behind
the antenna. That's where the lower gain, but higher f/b ratio
antennas, such as a barbeque grill backed bowtie array, makes more
sense. My preference is to use single channel narrowband yagi's for
maximum gain, but that gets really ugly as one per channel is
required.



Actually, with modern high quality receivers (which may not include the
$40 DTV to NTSC converters), they use a rake receiver to coherently
combine the multiple paths, so it's very likely that you'd get a better
signal with digital (i.e. the receiver would lock and you'd get ANY
output) than with analog in a multipathy, but not faint signal environment.

One of the design goals for DTV was to be multipath immune.


Note that in Europe, they actually transmit the identical signal from
multiple transmitter sites, guaranteeing multiple arriving signals with
different timing. Different modulation scheme, but the same multipath
issues. The same has been proposed for use in the US


However, with DTV, signal strength is more of an issue,mostly because
there isn't a "degraded mode" like there is with analog. Most viewers
are willing to tolerate remarkably degraded signals on an analog
channel, so folks are used to being able to receive a TV signal well
beyond the nominal service boundary. With digital, either you got it or
you don't. Some gain in the antenna helps..

Do receiver antennas need matching or not?
 

On Mar 19, 11:44 am, Jim Lux wrote:
Jeff Liebermann wrote:

Ghosts are a big problem where I live. Or multipath and the resulting
dropouts the case of DTV broadcasts. That's why I like highly
directional designs (both horizontally as well as vertically). Other
than that, I can get all of our local stations with rabbit ears and a
UHF loop.

I live too far away from the local digital TV xmitters to get reliable
reception. My rule of thumb is that if OTA analog TV reception is
marginal, digital TV will be worse.

Highly directional antennas are the right way to eliminate ghosts
(reflections). However, I keep running into problems with f/b (front
to back) ratio problems, where the ghosts are reflected from behind
the antenna. That's where the lower gain, but higher f/b ratio
antennas, such as a barbeque grill backed bowtie array, makes more
sense. My preference is to use single channel narrowband yagi's for
maximum gain, but that gets really ugly as one per channel is
required.

Actually, with modern high quality receivers (which may not include the
$40 DTV to NTSC converters), they use a rake receiver to coherently
combine the multiple paths, so it's very likely that you'd get a better
signal with digital (i.e. the receiver would lock and you'd get ANY
output) than with analog in a multipathy, but not faint signal environment.

One of the design goals for DTV was to be multipath immune.

Note that in Europe, they actually transmit the identical signal from
multiple transmitter sites, guaranteeing multiple arriving signals with
different timing. Different modulation scheme, but the same multipath
issues. The same has been proposed for use in the US

However, with DTV, signal strength is more of an issue,mostly because
there isn't a "degraded mode" like there is with analog. Most viewers
are willing to tolerate remarkably degraded signals on an analog
channel, so folks are used to being able to receive a TV signal well
beyond the nominal service boundary. With digital, either you got it or
you don't. Some gain in the antenna helps..

I am hoping that because of the narrow filter requirements of ham
radio
a fast acring multipath antenna switch would allow the dominant
polarisation to over rule all otherssince the ear can only accept a
few samples at a time.
Art

Do receiver antennas need matching or not?
 

Not sure about the higher frequencies but in the HF band we absolutely need
an antenna coupler that matches the impedance to the selected frequency. If
a coupler fails we can barely throw a signal a few miles whereas when the
coupler does it's job we can bounce a signal off of the ionosphere at night
for a few thousand miles.

Claude
Montreal

"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

Do receiver antennas need matching or not?
 

Not sure about the higher frequencies but in the HF band we absolutely need
an antenna coupler that matches the impedance to the selected frequency. If
a coupler fails we can barely throw a signal a few miles whereas when the
coupler does it's job we can bounce a signal off of the ionosphere at night
for a few thousand miles.

That's certainly true at the transmitting end. A good impedance match
is needed in order to enable the transmitter to deliver power
effectively into the antenna, from whence it can be radiated.

It's rather less true at the receiving end, at least in the lower-
frequency HF bands. In these bands, the ability to receive a usable
signal is often dominated by the amount of natural and man-made noise
in the band, and not by the receiver's own self-generated noise. Even
with a serious impedance mismatch between the antenna and the
receiver, enough signal reaches the receiver front-end to overcome the
receiver's own internal noise.

If you happen to live in an area which is blessed by a very low
background-noise level (e.g. out in the country, away from power
lines) and you're DXing in the HF bands, then a good impedance-matched
antenna will let your receiver take best advantage of the low noise
level.

If you're SWLing in a city, surrounded by power lines and electric
motors and neon lights and computers, the background noise level is
going to be much higher, and the weak distant stations will be drowned
out by the noise anyhow... and an inefficiently-matched antenna such
as a whip or longwire will give you enough signal to hear the stations
which are _not_ drowned out by the noise.

As an example - if your receiver has decent sensitivity, and a low
internal noise level, you may find that you can hear a signal with
decent audio quality all the way down to S0 or below (if there's no
noise obscuring it). You then find that with an impedance-matched
antenna the band's background noise is S6 or so. Assuming that you
can make use of a signal which is somewhat below the broadband noise
level, let's say that you decide you can copy stations whose own
individual signals are S5 or better, and that lower-level signals are
blanketed by the noise.

At this point, you realize that you can use an antenna which is 5
S-points (nominally, 30 dB) less efficient, and still receive the same
set of stations. With a less efficient antenna, the stations' signals
will be weaker... but so will the external band noise, by the same
ratio, and thus the signal-to-noise ratio of each station will remain
unchanged.

As an example, my ARES/RACES group has a multiband HF setup in our
city's police and fire admin building, which is downtown near the main
commercial-and-restaurant street and the light-rail system. We have a
trap-dipole antenna strung up above the building's roof. On the
80-meter band, the broadband noise level across the whole band is
rarely less than S9! We could probably receive the same set of
stations using an antenna consisting of two coat-hangers and some damp
string!

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Do receiver antennas need matching or not?
 

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

Do receiver antennas need matching or not?
 

Joel Koltner wrote:
This implies only that the antenna/receiver *matching* is good enough... yes?

*** No, it says nothing about the matching. It only says that the
signals
coming down the antenna from the cosmos are greater than the receiver
noise. If the antenna is matched to the receiver, whatever is picked
up
will be more efficiently fed into the receiver, resulting in a still
higher level.
If the antenna is not matched, well, there may be a heck of a lot of
both noise and signal, and even unmatched the results are strong
enough to override the rx noise..... One caveat , tho, ... in some
conditionsm,
a matched receiver input results in a higher receiver noise level....
not much,
but enough for purists to argue the point incessantly :))))) .


(I'm thinking that you would still sometimes prefer a highly directional
antenna over just a dipole even though both increase the background noise.
I.e., in both cases the antenna matching is good enough, but without the
directionality the antenna itself might not be good enough to eliminate
interference, overloading, etc. from sources other than the one you're
interested in.)

**** Of course, and a good point..... I was only talking about desired
signal
and atmospheric noise.... If there is a coherent interferer, then
that's a
whole 'nuther thang...... :)))

Andy W4OAH

Do receiver antennas need matching or not?
 

On Sat, 15 Mar 2008 04:11:21 -0700 (PDT), 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 a short answer to your question, NO!

The finals won't burn out as there are none and you won't be feeding
radiation into your room.

However, as you probably noticed from the FM radio with the extendable
antenna, you sometimes get a better signal when you extend the antenna
nd move it to the right location.

A good (and matched) antenna may allow you to receive signals you
could not have received with a random length antenna.

Most radios that I have seen usually have a matching network built in
to match the antenna that comes with the radio. In most cases they
aren't that elaborate, just a small coil and a trimmer capacitor.

My father learned that as great as his 50 foot long copper wire worked
for listening to his favorite short wave broadcasts, the signals
improved immensely when I installed an fan-dipole for his three
favorite bands. Did he 'need' that antenna? no, he could hear his
stations without it, but the signal strength was improved and he was
able to pick up more stations.

Dad also ordered an antenna tuner kit, assembled it and attached it
to his 50 foot wire. He found a great improvement in signal strength
using the tuner, almost equal to the multi-band dipole I installed.

Signal to noise ratio are not synonymous with gain. One can have a
lower-gain antenna with high s/n that outperforms a higher-gain
antenna with low s/n. When it gets critical, the s/n can be the
determining factor as to whether you receive intelligent communication
or not.

You are probably digging for technical, theoretical information more
than practical, but just in case...

If you look in stereo magazines, you seldom see radio ads bragging
about how much better they receive than the competition. If you look
in Ham magazines, that's a very important feature. Most consumers are
looking for stereos that play music well and they listen to local
stations. Most ham operators want to pull that weak signal out of the
noise to make the contact.

Just some thoughts.

Hope this is helpful to someone.
Buck
N4PGW

--
73 for now
Buck, N4PGW

www.lumpuckeroo.com

"Small - broadband - efficient: pick any two."

Do receiver antennas need matching or not?
 

I did not see all the replies nor the original post but in the world of
radio frequency (RF) match yes this would be an ideal situation since
maximum energy transfer occur when Imepdance of the antenna (Zant) =
Imedance of the Receiver Antenna Port (Zrx_port).

All this talk about noise while important to minimum detectable signal more
greatly influenced by the internal Noise Figure (NF) of the receiver (RX).
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.

The short end answer on HF you will probably not notice a big difference as
long as the antenna is close between a 1/4 to 1/2 wavelength long [e.g.
wavelength = Velocity of Light (C) / Frequency of Operation (Fo)]. 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 (think
of catching a baseball with and without a glove, the probability of catching
the ball is higher with a glove due to it larger capture area).

Well that is enough to put someone to ZZZzzzzzzzzzz. Hi Hi

73, Homer J
on all lower and upper channels Thane-Fer

Do receiver antennas need matching or not?
 

"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

Do receiver antennas need matching or not?
 

On Mar 30, 6:54 am, "Homer J" wrote:
I did not see all the replies nor the original post but in the world of
radio frequency (RF) match yes this would be an ideal situation since
maximum energy transfer occur when Imepdance of the antenna (Zant) =
Imedance of the Receiver Antenna Port (Zrx_port).

All this talk about noise while important to minimum detectable signal more
greatly influenced by the internal Noise Figure (NF) of the receiver (RX).
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.

The short end answer on HF you will probably not notice a big difference as
long as the antenna is close between a 1/4 to 1/2 wavelength long [e.g.
wavelength = Velocity of Light (C) / Frequency of Operation (Fo)]. 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 (think
of catching a baseball with and without a glove, the probability of catching
the ball is higher with a glove due to it larger capture area).

Well that is enough to put someone to ZZZzzzzzzzzzz. Hi Hi

73, Homer J
on all lower and upper channels Thane-Fer

I never knew that "aperature" could be measured in square feet or
metres!
Is there a book that describes it in such a way ?
Art