AM and FM antenna question
Kyle Cassidy
back of the radio, but an FM antenna must be straight?
kc
Richard Matthews
> back of the radio, but an FM antenna must be straight?
>
> kc
>
the center of this band is about 300 meters, so a 1/4 wavelength antenna would be about 75 meters long. FM broadcast band frequencies
have a center band wavelength of about 3 meters, so a 1/4 wavelength
antenna would be about .75 meters long. So to use an equivalent
wavelength antenna on the AM frequency band it is convinent to coil the 75 meters of antenna wire onto a small coil.You could have a straigth antenna on the AM band, but you would have to string it out the window.
In short the higher the frequency the shorter the antenna.
Richard
WA4NWW
Chuck Hawley
>>Why is it that a portable AM radio antenna can be merely a coil of wire in the
>>back of the radio, but an FM antenna must be straight?
>>>
>Basic RF Physics.
>This is because AM signals actually curve due to the amplitude variations
>that comprise this type of transmission.
>FM conversely has what are called in the industry "Straight Waves"
>that do not enjoy amplitude curvature.
>This means that a coiled antenna actually form fits the symmetry of
>an AM broadcast wave, thereby reducing SWR and enhancing signal to
>noise ratio. The same effect occurs for the FM Straight Waves
>with the Rod type or even a folded dipole antenna.
>To prove this to yourself, build a reasonable 160 meter band or AM
>broadcast band Helica antenna, and compare reception that of a
>5/8 wave colinear on AM signals on the se bands.
>Good luck, Happy Listening.
>Glad to help out
>73
>Ed
>KA9AHQ/7
I'll bet you thought someone was going to write a blistering attack on
your explanation and make a damn fool of themselves.
Ha!
Chuck Hawley, KE9UW in Urbana, Illinois
haw...@aries.scs.uiuc.edu
School of Chemical Sciences, Electronic Services
University of Illinois, Urbana-Champaign
Ed Wright (0x0E2 UNIX The saga Never Ends)
Richard Matthews <Richard_...@SHSVSMTP.Huntsville.Sparta.Com> wrote:
>Why is it that a portable AM radio antenna can be merely a coil of wire in the
>back of the radio, but an FM antenna must be straight?
>>
Basic RF Physics.
This is because AM signals actually curve due to the amplitude variations
that comprise this type of transmission.
FM conversely has what are called in the industry "Straight Waves"
that do not enjoy amplitude curvature.
This means that a coiled antenna actually form fits the symmetry of
an AM broadcast wave, thereby reducing SWR and enhancing signal to
noise ratio. The same effect occurs for the FM Straight Waves
with the Rod type or even a folded dipole antenna.
To prove this to yourself, build a reasonable 160 meter band or AM
broadcast band Helica antenna, and compare reception that of a
5/8 wave colinear on AM signals on the se bands.
Good luck, Happy Listening.
Glad to help out
73
Ed
KA9AHQ/7
--
I think I've got the hang of it now .... :w :q :wq :wq! ^d X exit ^X^C ~.
^[x X Q :quitbye CtrlAltDel ~~q :~q logout save/quit :!QUIT ^[zz ^[ZZ
ZZZZ ^H ^@ ^L ^[c $q ^# ^E ^X ^I ^T ? help helpquit ^D ^d ^C ^c help
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OMG
>In article <39r8go$1...@louie.udel.edu>,
>Richard Matthews <Richard_...@SHSVSMTP.Huntsville.Sparta.Com> wrote:
>>Why is it that a portable AM radio antenna can be merely a coil of wire in the
>>back of the radio, but an FM antenna must be straight?
>
>Basic RF Physics.
>
>This is because AM signals actually curve due to the amplitude variations
>that comprise this type of transmission.
>FM conversely has what are called in the industry "Straight Waves"
>that do not enjoy amplitude curvature.
...
>Ed
I am not a physicist, but I thought that FM transmissions were circularly
polarised these days to improve reception using car (vertical-ish) antenna.
I suppose the get-out-clause might be that the radius of the circle is large.
Does anybody know?
Regards,
Martin
MLe...@atlantic.cs.unb.ca
mor...@ipers1.e-technik.uni-stuttgart.de
Ed Wright (0x0E2 UNIX The saga Never Ends) <e...@sequent.com> wrote:
>
>Basic RF Physics.
>
Hi Ed,
I well and truely enjoyed your explanation and your sig too.
Since I will be giving a talk on the Internet shortly in our
local club, I shall present it there.....
73, Moritz DL5UH
Zack Lau (KH6CP)
: In article <39r8go$1...@louie.udel.edu>,
: Richard Matthews <Richard_...@SHSVSMTP.Huntsville.Sparta.Com> wrote:
: >Why is it that a portable AM radio antenna can be merely a coil of wire in the
: >back of the radio, but an FM antenna must be straight?
: >>
: Basic RF Physics.
: This is because AM signals actually curve due to the amplitude variations
: that comprise this type of transmission.
: FM conversely has what are called in the industry "Straight Waves"
: that do not enjoy amplitude curvature.
: This means that a coiled antenna actually form fits the symmetry of
: an AM broadcast wave, thereby reducing SWR and enhancing signal to
: noise ratio. The same effect occurs for the FM Straight Waves
: with the Rod type or even a folded dipole antenna.
So why doesn't the SWR change when you go from SSB (a form of
amplitude modulation) to a continuous carrier (no amplitude
variations at all)? This theory sounds awfully bogus to me.
: To prove this to yourself, build a reasonable 160 meter band or AM
: broadcast band Helica antenna, and compare reception that of a
: 5/8 wave colinear on AM signals on the se bands.
If you look at the shortwave group (rec.radio.shortwave?) you will
find that one of the preferred antennas for AM broadcast DXing is
an AM broadcast tower that isn't in use. Come to think of it, why
do AM broadcasters use straight towers instead of coiled antennas?
Don't they have the money to buy the best antennas?
The FM antenna does not have to be straight. Lots of people have
had excellent results with Quad antennas. Of course, bending
a wire antenna often detunes it--the length does have to be
readjusted for best performance.
However, it is true that AM broadcast antennas work just fine
even when they are little compared to the size of the radio
waves. This is due to all the noise at those frequencies.
Sort of like wearing sun glasses on a really bright day or
ear plugs next to a jack hammer. As you go higher in frequency
the noise is usually weaker--so more efficient antennas mean
improved performance. Not necessarily so at low frequencies.
--
Zack Lau KH6CP/1 2 way QRP WAS
8 States on 10 GHz
Internet: zl...@arrl.org 10 grids on 2304 MHz
Felix Yen
> The FM antenna does not have to be straight. Lots of people
> have had excellent results with Quad antennas. Of course,
> bending a wire antenna often detunes it--the length does have
> to be readjusted for best performance.
Having just built my second horizontal loop for FM reception, I can
attest to the fact that loop FM antennae can work very well. In fact,
if I could only use one FM antenna, it would be a loop. I still have
my old dipole connected, but I never find myself using it.
Felix
f...@cadre.com
Howard Lester
>
>Having just built my second horizontal loop for FM reception, I can
>attest to the fact that loop FM antennae can work very well. In fact,
>if I could only use one FM antenna, it would be a loop. I still have
>my old dipole connected, but I never find myself using it.
Perhaps you should consider a book on philosophy, if indeed you are
trying to "find yourself".
Howard Lester
Gary Coffman
>Ed Wright KA9AHQ/7 (0x0E2 UNIX The saga Never Ends) (e...@sequent.com) wrote:
>: In article <39r8go$1...@louie.udel.edu>,
>: Richard Matthews <Richard_...@SHSVSMTP.Huntsville.Sparta.Com> wrote:
>: >Why is it that a portable AM radio antenna can be merely a coil of wire in the
>: >back of the radio, but an FM antenna must be straight?
>
>: Basic RF Physics.
>
>: This is because AM signals actually curve due to the amplitude variations
>: that comprise this type of transmission.
>: FM conversely has what are called in the industry "Straight Waves"
>: that do not enjoy amplitude curvature.
>: This means that a coiled antenna actually form fits the symmetry of
>: an AM broadcast wave, thereby reducing SWR and enhancing signal to
>: noise ratio. The same effect occurs for the FM Straight Waves
>: with the Rod type or even a folded dipole antenna.
Heh, heh, I love it.
>So why doesn't the SWR change when you go from SSB (a form of
>amplitude modulation) to a continuous carrier (no amplitude
>variations at all)? This theory sounds awfully bogus to me.
He's hooked. :-)
>If you look at the shortwave group (rec.radio.shortwave?) you will
>find that one of the preferred antennas for AM broadcast DXing is
>an AM broadcast tower that isn't in use. Come to think of it, why
>do AM broadcasters use straight towers instead of coiled antennas?
>Don't they have the money to buy the best antennas?
They just don't want to screw the signals into the ground. :-)
>The FM antenna does not have to be straight. Lots of people have
>had excellent results with Quad antennas. Of course, bending
>a wire antenna often detunes it--the length does have to be
>readjusted for best performance.
OTOH, FM stations do use circularly polarized waves. They *like*
screwing the listening public. :-)
>However, it is true that AM broadcast antennas work just fine
>even when they are little compared to the size of the radio
>waves. This is due to all the noise at those frequencies.
>Sort of like wearing sun glasses on a really bright day or
>ear plugs next to a jack hammer. As you go higher in frequency
>the noise is usually weaker--so more efficient antennas mean
>improved performance. Not necessarily so at low frequencies.
To be serious for a moment, there's a bit more to it than this.
A ferrite loaded antenna *does* have a larger capture area than
a similarly sized bit of coiled wire. Theory says that the
greater permeability concentrates the field lines into the coil.
This increases the effective capture area of the antenna.
Gary
--
Gary Coffman KE4ZV | You make it, | gatech!wa4mei!ke4zv!gary
Destructive Testing Systems | we break it. | emory!kd4nc!ke4zv!gary
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Lawrenceville, GA 30244 | |
Jim Grubs, W8GRT
hle...@nemo.as.arizona.edu (Howard Lester) writes:
I know gurus recommend solitude and silence during meditation, so I
guess FM listening is why he never finds himself.
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