Horizontal vs. Vertical Polarization
Ramon Gandia
James wrote:
>
> Why is it that Commercial broadcast stations, both radio and TV
> (AM&FM, audio, video) use vertically polarized antenna's, yet the
> high-gain beam/Yagi's available to consumers (Radio shack, ect) for
> their TV's and FM radios are horizontally polarized elements?
Well, A.M. broadcast stations, 540-1700 kHz use vertical, because
with horizontal there would be very little ground wave: the soil
would absorb it, and the range would be very short. On the other
hand, the horizontal antenna would put out a good sky wave, and
interfere with far away stations. This is why since the late 1930's,
no AM station is allowed to transmit horizontally polarized signals.
However, T antennas, or top loaded towers have equal and opposite
current in the horizontal section, and no horizontal radiation
takes place, and is allowed. An inverted L has strong horiz polari
zeed radiation.
At the receiver, the antenna for AM sets is a loop, and is basically
polarization independent. It picks up whatever comes by.
A horizontal receive antenna will probably still pick stations up
because as the radio wave travels it "tilts" and the field is
not opendicular to the earth, resulting in a wave that will
induce a response in the antenna. But not much signal, for sure.
Remember, a single wire from the radio to a tree is NOT a horizonta
antenna, it is an iverted L.
On FM and TV stations use horizontal polarization. ON VHF there is
no ground wave to speak of, and the signal is picked up on what is
called "space wave". The ground does absorb signals near the
surface, which is why horizontal receive antennas are best up in
the clear air, like rooftop yagis or dipoles. Lower antennas work,
but not as well. Laid on the ground, an FM or TV dipole will pick
up almost nothing because there is no signal there to speak of.
Horizontal is better at VHF because there is less noise and
static there, which is important on TV channels which are A.M.
modulated (sound is FM).
FM stations are horizontal. In later years, automobile makers
complained that dipoles on cars were awkward, and the FCC allowed
vertical signal to be added. Most cars with windshield antennas
(little wires inside the glass) are dipoles. Look at it closely.
The outside mounted whips are vertical receive antennas. FM
transmission is very resistant to noise with adequate signal.
No FM station is required to send out any vertical signal, therefore
the performance of a car whip on FM may vary from station to
station.
Interestingly, the FCC allows equal transmit power vertical as
hoirzontal. The vertical power may not exceed the horizontal
power. FM stations are licensed for certain power horizontally,
and the vertical power is "extra" or free. For instance,
WXYZ FM may have a llicense for 100 kilowatts at the antenna
feed point. This is horizontal. WXYZ FM would also be allowed
to add up to 100 kilowatts to the vertical antenna as well, but
of course, needs FCC approval, but it is basically paperwork and
filing details, no problems at all.
For vertical component you could use a 2d transmitter, or one
transmitter with a power splitter, and feedline to each antenna.
Or, you could use an antenna that already proportions the
power between horizontal and vertical. Typically FM antennas
have elements that are dipoles or halos (horizontal loops open
at the point oppsite the feedline. To have both polarization,
the dipole has the ends bent. One arm is pointed up, the other
down. Say the dipole is 5 ft long, each arm being 2.5 ft long.
A dual polarization antenna would have the left arm going, say
1.25 ft horizontal, and the outer 1.25 ft bent up vertically.
The other arm is 1.25 ft horizontal and 1.25 ft downwards vertical.
The overall length is still 5 ft. Power division in this type
of dipole is about 70% or 80% horizontal, and 20% - 30% vertical.
The antenna is omnidirectional in the vertical wave, but
is bidirectional in the horizontal sense. Often, the dipole is
bent into a horizontal vee, with horizontal arms, or the ends
bent up or down. This makes for a more directional antenna,
useful in valleys or sea shore locations, etc.
Halo antennas are dipoles bent into a horizontal loop, the ends
almost touching. Often, a disc is at each end of the dipole
arms and the antenna can be somewhat smaller, which is more
rugged and has better directional properties; ie, it is NOT
directional, only slightly so. Some antennas of this type
overlap the ends (without touching) and get even better
omnidirectional coverage with horizontal polarization. This
halo antenna was very popular in the 1960's for 6 and 2 meter
ham use in homes and cars. It is passe now.
The circular antenna can have the ends of the arms bent up and
down for dual polarity. Such an antenna is surprisingly small,
and if made tubing or pipe is very stout. With a gamma match
to one of the arms, there are no weak insulators, etc. The
left arm, before bending into a loop is bent with half pointing
up, and the other arm with half pointing down, like on the
dual polarization dipole. The horizontal part of the dipole is
then bent in a curve to form a circle, with the point where
the arms bend up and down opposite the feed point and almost
touching. This antenna, again has about 25% radiation vertically.
The manufacturer of the antenna makes very careful tests on the
prototype, and certifies whte percentage of horizontal and
vertical power. Assume 20/80 ratio. The station is licesedd
for 10,000 watts. This is horizontal power. So, if
80% of the transmitter power is 10kw, the transmitter power output
must be 10,000/0.8 = 12,500 watts (actually at the antenna feed
point, not at the trasnmitter, but I digress). At the antenna,
10 kilowatts will go horizontal, and the remaining 2.5 kilowatts
will go vertical.
Hams on FM VHF use mobile whips and everyone is vertically polarized.
Even beams are vertical.
Hams on VHF CW and SSB generally use horizontal polarization since
it has a slight edge in weak-signal work. This type of antenna is
awkward (halos being perfect examples) in automobiles, so most
CW and SSB work is done from fixed stations only, not mobiles.
EME (moonbounce) and satellite use CIRCULAR polarization. This is
not dual vert/horiz polarization, but one in which the polarization
rotates round and round. It can be made with two dipoles, one
horiz and one vertical, and each fed 90 degrees out of phase to
induce the rotation. Other antennas that work are helical antennas,
and a few others. The reason for circular is that as the satellite
tumbles, or changes orientation with respect to ground station,
the polarity changes. Cross polarized transmit and receive have
a 20 db or more loss in signal between them, so circular overcomes
this and the signal then is steady. The moon also moves as well as
the reflections being unpredictable due to terrain. Circular
polarization means that polarization need not be worried about.
The disadvantage is that circular polarization has a 3 db loss
in signal strength, all else being equal. You need twice the
antenna.... but that is better than being 20 db down due to
crossed polarization!
73
would have the left ar
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James
Why is it that Commercial broadcast stations, both radio and TV
(AM&FM, audio, video) use vertically polarized antenna's, yet the
high-gain beam/Yagi's available to consumers (Radio shack, ect) for
their TV's and FM radios are horizontally polarized elements?
-pylon
Hank Blackstock
What makes you think that TV and FM signals are vertically
polarized?
Hank WA5JRH
Jason Williams-Lock
py...@nwlink.com (James) says:
>
>Why is it that Commercial broadcast stations, both radio and TV
>(AM&FM, audio, video) use vertically polarized antenna's, yet the
>high-gain beam/Yagi's available to consumers (Radio shack, ect) for
>their TV's and FM radios are horizontally polarized elements?
>-Just a thought that popped into my head while in the 'loo....
>-pylon
>
>
I think vertical polarisation is easy to pick up on an omni-direction aerial
such as a vertically mounted dipole, you don't need to point it broadside
onto the transmitter. Here in the u.k, vertical polarisation appears to be
used by low power relay stations, but we also have something called "mixed
polarisation", that is *both* vertical and horizontal components, this is to
improve reception on car and portable radios.
Regards,
Jason
Paul J. Dobosz
James - Your information is in error ....
Comercial FM stations (almost without exception) utilize
circular polarazation to accomodate both vertical masts
on cars and horizontal antennas for residential use. The circular
polarized signal accomodates both polarizations with only 3dB of
polarization mismatch loss as opposed to 20dB+ if linear polarization
was transmitted
AM stations utilize vertical polarization exclusively, but that is
seldom an issue since receiving antennas in most cases are so small
with respect to the wavelength that they exhibit no real polarization
and are almost totally reactive (capacitive) . They behave like a
capacitive probe than an antenna.
Television stations (VHF & UHF) use horizontal polarization because
their major target audience is residential. This lends itself well to
high gain horizontal yagis of the type you commonly see on rooftops
all over the US.
I'm not sure where you got your information on broadcast engineering
practice, but most of the polarizations you describe haven't ever been
in common use in the US for the past 40 years.
Paul Dobosz - K8PD
Nick Wymant
Hank Blackstock <wa5...@ionet.net> wrote:
>py...@nwlink.com (James) wrote:
>>Why is it that Commercial broadcast stations, both radio and TV
>>(AM&FM, audio, video) use vertically polarized antenna's, yet the
>>high-gain beam/Yagi's available to consumers (Radio shack, ect) for
>>their TV's and FM radios are horizontally polarized elements?
>>-Just a thought that popped into my head while in the 'loo....
>>-pylon
>>
>>
>
>
>polarized?
>
>Hank WA5JRH
>
>
>
>
least) is that TV transmitting antennas are mainly horizontally polarised
in main cities, and either horizontal or vertical in outer areas. The
polarisation is used to provide some protection between adjacent zones
where the same channel is re-used. ie: as added insurance against a far
away transmitter on the same channel interfering with the close
transmitter.
In Australia, nearly all FM stations use Circular polarisation (see
preceeding messages for description). Circular polarisation has the
benefit that the recieving antenna can be oriented at any angle, and
still pick up most of the signal. In fact, a halo antenna with the ends
bent up/down can be made to produce CP, and is the basis for many CP
antennas made in the USA.
Another variation used in FM antennas is Slant polarisation, where the
dipole is oriented to 45 degrees from vertical. Like CP, recieve antennas
which are either VP of HP can pick up the signal transmitted from a slant
polarised antenna. Again like CP, there is a 3 dB loss unless the RX
antenna is also slant polarised.
A good example of a slant polarised FM antenna can be seen at the top of
the Empire State Building. At the base of the spire at the top, is a ring
of dipoles (about 20 around the base of the spire), slanted at 45
degrees, which I believe are FM antennas. You can just make them out from
the ground, but you can see them very clearly from the upper observation
deck.
Regards
Nick Wymant.
Ramon Gandia
Nick Wymant wrote:
> In fact, a halo antenna with the ends
> bent up/down can be made to produce CP, and is the basis for many CP
> antennas made in the USA.
See my post related to this question. Such a halo antenna does *not*
produce circular polarization. It merely sends out a certain
percentage of the signal horizontally polarized, and another portion
vertically polarized.
To create C.P., you need two antennas, one vertical and one
horizontal, fed in quadrature, or 90 degrees phase. Example
would be two dipoles assemble like a cross. The feedline
goes to one dipole, and a quarter wave transmission line
connects the other dipole to the first. The quadrature
feed imparts the proper twist to the wave, so that depending
on where in the r.f. cycle the received signal is at, the
polarization changes from horizontal, slowly around to vertical
then continues around to horizontal again, and smoothly again
to vertical on once r.f. cycle.
In the U.S., the main reason to use dual polarization (the halo
or dipole with ends bent up and down) is to provide both
signals so radio sets with one type or the other antenna can
receive it. Our government allows broadcasters to transmit
at full licensed power horizontally, and to add as much vertical
power as wanted, as long as it does not exceed the horizontal
power. Theoretically, this could be equal power, for a
transmitter then putting out double the licensed power. Such
a scheme must be approved, of course, but is automatic and does
not gain the station any extra coverage (on paper) but merely
fills in the areas that a automobile radio would have fades, etc.
Most stations do not devote that much power to vertical. Vertical
is typically automobile radios, which operate under adverse
conditions and do not have as much range as a home with an
external F.M. antenna.
As far as I know, TV stations only go horizontal here in the U.S.,
but your description of the Australian system to bring co-channel
stations closer together but using different polarization is
good thinking. I bet you in those cases, the stations involved
usually carry the same programme and share one channel with various
transmitters in a given region.... just a guess!
COPY OF THIS POST BY E-MAIL too.
Bill Turner
py...@nwlink.com (James) wrote:
>Why is it that Commercial broadcast stations, both radio and TV
>(AM&FM, audio, video) use vertically polarized antenna's, yet the
>high-gain beam/Yagi's available to consumers (Radio shack, ect) for
>their TV's and FM radios are horizontally polarized elements?
>-Just a thought that popped into my head while in the 'loo....
>-pylon
>
>
------------------------------------------------
Commercial TV and FM don't use vertical polarization. They're horizontal.
Commercial AM uses vertical because groundwave propagates better when it's
vertically polarized. To anticipate the next question, TV and FM depend on
space wave, not groundwave. See any ARRL handbook or antenna book for a
discussion of the difference.
73, Bill W7LZP
w...@eskimo.com
Bob Bruhns
Paul J. Dobosz (pjdo...@msmail.rey.delcoelect.com) wrote:
: AM stations utilize vertical polarization exclusively, but that is
: seldom an issue since receiving antennas in most cases are so small
: with respect to the wavelength that they exhibit no real polarization
: a capacitive probe than an antenna.
Also, it is essentially impossible to broadcast any usable
horizontally polarized signal along the earth's surface at AM
broadcast frequencies. The ground reflection cancels it almost
completely at this zero-degree angle.
Bob Bruhns, WA3WDR, bbr...@li.net
Nick Wymant
Ramon Gandia <no....@see.signature.part> wrote:
> Such a halo antenna does *not*
>produce circular polarization. It merely sends out a certain
>percentage of the signal horizontally polarized, and another portion
>vertically polarized.
>
>To create C.P., you need two antennas, one vertical and one
>horizontal, fed in quadrature, or 90 degrees phase.
You are not quite correct !
If you can get the phase correct, and the length of the bent up ends
right, then you get CP, or very close to it.
If you dont believe it, then visit the following site which has a picture
just as you described, complete with gamma match, of a sidemount CP
antenna.
http://www.broadcast.net/jampro/jampjlst.html
You are correct about the two dipoles fed in quadrature. This is the
basis for the FM panel antenna which places two dipoles at right angles
(sometimes four dipoles) in front of a reflecting screen. This produces a
directional pattern. placing three or four of these panels around a mast
produces an omnidirectional pattern.
http://www.broadcast.net/jampro/jampjlst.html
Nick Wymant
Ramon Gandia <no....@see.signature.part> wrote:
http://www.broadcast.net/jampro/jampjlst.html
Nick Wymant
Christopher W. Boone
David Copperhead wrote:
>
> On Fri, 18 Oct 1996 04:56:32 GMT, py...@nwlink.com (James) wrote:
>
> >Why is it that Commercial broadcast stations, both radio and TV
> >(AM&FM, audio, video) use vertically polarized antenna's, yet the
> >high-gain beam/Yagi's available to consumers (Radio shack, ect) for
> >their TV's and FM radios are horizontally polarized elements?
> >-Just a thought that popped into my head while in the 'loo....
> >-pylon
PYLON,
got news for ya...
TV stations are mainly HORIZONTAL...just because the antenna LOOKS
vertical doesnt mean it is.....the TV antennas are typically batwings or
a design based on "SLOT" antennas....they radiate in the horizontal mode
though they are vertical in height....Alfred Slot antennas are
similar...they radiate horizontally because their diameter is small
compared to their wavelength and this causes the RF to radiate in a
circle around the antenna mast (not the true technical description but
its the easiest I can think up at this time you should be able to
understand)...DO NOT trun a TV antenna to vertical if you want the best
signal!!! Leave it horizontal..
FM has been mostly horizontal but in the past 20-30 years been using a
mix of hor and vertical polarization (crossed dipoles will do this...also
a halo with vertical elements on the tips will also do this in a simple
fashion though BC FM antennas are not that simple)..
nowadays, both FMs and TV are now using Circular Polarization(both horiz
and vertical in a corkscrew signal) or CP as it is abbreviated...to
reduce multipath and allow both vertical (car antennas) and horizontal
(home antennas ) to rcv the signal equally....also rabbit ears can get
best signal this way since they are NEVER true horiz or vertical most of
the time....BUT there is a giveaway for this.....you lose 3db trying to
rcv a CP signal with a LINEAR (H or V) antenna....only if you used a CP
antenna yourself woulf you get full signal....AND then the direction is
critical....there is left hand and right CP (just like Vertical and
Horizontal) and yes there is a 20db difference between the two!....BUT
with a linear antenna (H or V), it doesnt matter when trying to pick up a
CP signal..all you lose is 3db.
So the story here is LEAVE THE ANTENNAS alone...they ARE correct for the
stations you are trying to pick up
73
Chris
WB5ITT
ABC Radio Network Engineer - Dallas
> >
> >
> Simply change the polarization by mounting your yagi 90 degrees...but
> I'm sure you knew that...didn't quite get your point.