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

Bill;

I haven't looked at the back of a radio in a long time, but those that I
have looked at usually had a trimmer capacitor that helped tune the
antenna to the radio input. This is the impedance match you are looking at.

Dave WD9BDZ

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 3ÔÂ15ÈÕ, ÏÂÎç7ʱ11·Ö, 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



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"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. :-) )

"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.)

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.

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

AI4QJ wrote:

[snip]

Over the air HDTV is superior to cable and satellite HDTV. They may sell
more outdoor antennas as soon as it becomes clear that, from the 1996
telecommunications, not only did the FCC ruling on small satellite dishes
overrule the homeowner association covents restricting dsmall satellite
antennas, they ALSO overruled covenants restricting outdoor TV (i.e.yagi
type) antennas...this was done to promote fair competition for cable,
satellite and broadcast TV. I did put up a regular radio shack yagi style
outdoor TV antenna back then and the HOA did contact their lawyer and found
they could do nothing about it. If I ever splurge and buy my 1080p, I will
do it again because the OTA picture is absolutely the best.

AI4QJ

Put up one of these: http://www.digitalhome.ca/ota/superantenna

A home brewed double Gray-Hoverman. Apparently, they work pretty well
and if you build a really impressive one, the HOA will really get their
panties in a bunch.

--
Paul Hovnanian
------------------------------------------------------------------
The only tools one needs in life:
WD-40 to make things go and duct tape to make them stop.

On Mon, 17 Mar 2008 22:13:35 -0800, "Paul Hovnanian P.E."
wrote:

AI4QJ wrote:

[snip]

Over the air HDTV is superior to cable and satellite HDTV. They may sell
more outdoor antennas as soon as it becomes clear that, from the 1996
telecommunications, not only did the FCC ruling on small satellite dishes
overrule the homeowner association covents restricting dsmall satellite
antennas, they ALSO overruled covenants restricting outdoor TV (i.e.yagi
type) antennas...this was done to promote fair competition for cable,
satellite and broadcast TV. I did put up a regular radio shack yagi style
outdoor TV antenna back then and the HOA did contact their lawyer and found
they could do nothing about it. If I ever splurge and buy my 1080p, I will
do it again because the OTA picture is absolutely the best.

AI4QJ

Put up one of these: http://www.digitalhome.ca/ota/superantenna

Nice. Too bad it only goes from about 400-800MHz, which covers the
UHF channels. It doesn't work for the VHF channels. Looking at the
4NEC2 plots, it's not very good on VSWR, has reasonable gain, but only
on the lower UHF channels (no problem because the higher channels are
being auctioned off by the FCC).
http://www.qsl.net/va3rr/hdtv/hoverman.htm

A home brewed double Gray-Hoverman. Apparently, they work pretty well
and if you build a really impressive one, the HOA will really get their
panties in a bunch.

The local OTA digital TV is on Channel 8 (KSBW) at roughly 87MHz. The
reflector of a suitable Gray-Hoverman antenna should be about 1.5
meters wide. I think the aesthetics committee will be banging on the
front door rather quickly. What's that giant fly swatter doing on the
roof? I wonder if a TV antenna can legitimize a solar cell array if
the array is mounted on the TV antenna?

"Comparing some commercially available antennas"
http://www.hdtvprimer.com/ANTENNAS/comparing.html
I really like the negative gain (i.e. loss) in some of the commercial
VHF antennas. Too bad they didn't run antenna patterns, as many such
TV yagis have more gain in the reverse direction, than forward.

Incidentally, I once built a vertically polarized omnidirectional OTA
TV antenna intended as a disguise antenna (if you consider a 2.5 meter
white fiberglass pipe to be suitable disguise). It worked well enough
from about Channel 8 to about Channel 47. Not much gain (about 4dbi)
but the big problem was that it was prone to receiving ghosts
(reflections). I should probably resurrect the design, rename it
"Ultimate Digital Disguise Magic Super-Antenna", and join the hype
instead of fighting it.



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

Jeff Liebermann wrote:

On Mon, 17 Mar 2008 22:13:35 -0800, "Paul Hovnanian P.E."
wrote:

AI4QJ wrote:

[snip]

Over the air HDTV is superior to cable and satellite HDTV. They may sell
more outdoor antennas as soon as it becomes clear that, from the 1996
telecommunications, not only did the FCC ruling on small satellite dishes
overrule the homeowner association covents restricting dsmall satellite
antennas, they ALSO overruled covenants restricting outdoor TV (i.e.yagi
type) antennas...this was done to promote fair competition for cable,
satellite and broadcast TV. I did put up a regular radio shack yagi style
outdoor TV antenna back then and the HOA did contact their lawyer and found
they could do nothing about it. If I ever splurge and buy my 1080p, I will
do it again because the OTA picture is absolutely the best.

AI4QJ

Put up one of these: http://www.digitalhome.ca/ota/superantenna

Nice. Too bad it only goes from about 400-800MHz, which covers the
UHF channels. It doesn't work for the VHF channels. Looking at the
4NEC2 plots, it's not very good on VSWR, has reasonable gain, but only
on the lower UHF channels (no problem because the higher channels are
being auctioned off by the FCC).
http://www.qsl.net/va3rr/hdtv/hoverman.htm

A home brewed double Gray-Hoverman. Apparently, they work pretty well
and if you build a really impressive one, the HOA will really get their
panties in a bunch.

The local OTA digital TV is on Channel 8 (KSBW) at roughly 87MHz. The
reflector of a suitable Gray-Hoverman antenna should be about 1.5
meters wide. I think the aesthetics committee will be banging on the
front door rather quickly. What's that giant fly swatter doing on the
roof? I wonder if a TV antenna can legitimize a solar cell array if
the array is mounted on the TV antenna?

Does your HOA forbid solar arrays? Some of them can be pretty anal
retentive. Mine won't let me put up my thermonuclear clothes dryer. :-(

"Comparing some commercially available antennas"
http://www.hdtvprimer.com/ANTENNAS/comparing.html
I really like the negative gain (i.e. loss) in some of the commercial
VHF antennas. Too bad they didn't run antenna patterns, as many such
TV yagis have more gain in the reverse direction, than forward.

Incidentally, I once built a vertically polarized omnidirectional OTA
TV antenna intended as a disguise antenna (if you consider a 2.5 meter
white fiberglass pipe to be suitable disguise). It worked well enough
from about Channel 8 to about Channel 47. Not much gain (about 4dbi)
but the big problem was that it was prone to receiving ghosts
(reflections). I should probably resurrect the design, rename it
"Ultimate Digital Disguise Magic Super-Antenna", and join the hype
instead of fighting it.

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.

All of our DTV stations broadcast on UHF as the VHF band in the Seattle
area is pretty much filled up with the legacy analog stuff. I'm not
certain what the stations plan on doing once the digital cut over is
complete. They might move their digital signal down onto their analog
slot or abandon the analog slot altogether. That will be a factor in
what kind of antenna I put up.


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

--
Paul Hovnanian
-----------------------------------------------------------------------
Procrastinators: The leaders for tomorrow.