David Harmon wrote:
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.

Does Walmart honor the $40 coupon at:
https://www.dtv2009.gov/ApplyCoupon.aspx
--
73, Cecil http://www.w5dxp.com

On Mon, 17 Mar 2008 18:51:33 GMT in rec.radio.amateur.antenna, Cecil
Moore wrote,

Does Walmart honor the $40 coupon

Duh.

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

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

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

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

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

(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.)

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

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

"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

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