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Old 2011-01-10, 11:29 AM   #16
300ohm
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Technically, its elliptical polarization, weighted to the horizontal. Screwy huh ? Heh.

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In the early days of FM radio in the 88-108 MHz spectrum, the radio stations broadcasted horizontal polarization. However, in the 1960's, FM radios became popular in automobiles which used vertical polarized receiving whip antennas. As a result, the FCC modified Part 73 of the rules and regulations to allow FM stations to broadcast RHC or elliptical polarization to improve reception to vertical receiving antennas as long as the horizontal component was dominant.
http://www.astronwireless.com/topic-...larization.asp

A bi-quad gives a nice ball pattern, and a reflector less bi-quad for FM (just quickly rescaling my WiFi bi-quad) would basically be 2 squares about 29.5" X 29.5" each and would give about 5 to 6 dbi gain in two directions with a wide beamwidth. It could be constructed with PVC pipe and two crosses, notching the pipe for stringing the wire. Total size of about 84" wide by 43" high, fairly managable.

Odd that Ive never seen a crossed Yagi in use for FM, as it sounds like a good idea too.
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Old 2011-01-10, 12:50 PM   #17
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300ohm,

I have built a “cubical quad” for FM and funny enough I was considering an array of 4 bi-quads with reflector since the total height is almost in the ballpark for me. I'm guessing we probably don’t see the crossed yagi on our end due to the frequency specific delay network used on one of the two antennas in order to approximate circular polarization.
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Old 2011-01-10, 02:23 PM   #18
stampeder
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Cool FM TX Rototiller Antennae

Lineloss if there is ever a chance to drive by the antenna tower you would be able to eyeball what polarity your favourite station is using. There are some great photos of FM TX "rototiller" antennae in Posts #121 and 122 of the following thread, with older FM TX antennae in Post #124 and a different type of new rototiller in Post #128:

TV Broadcast Towers & Transmitters (photos, vids)

Question for all: any point in having a rototiller-style FM RX antenna?
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Old 2011-01-10, 08:48 PM   #19
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Cool pics!

On the net somewhere, I came across someone who wrote that the antenna pair compromising a rototiller better approximates a perfect circular polarized wave due to its symmetry while the helical design (no delay circuit needed) is not symmetrical and does not approximate a perfect circular polarized wave. Now I don't know, but I would assume that the rototiller used as a RX antenna would be band limited due to the fixed delay circuit (non issue for TX), but on the plus side we could change from left to right polarization just by swapping the delay circuit to the opposing antenna in the rototiller.. interesting thought for my dilemma. FM is a narrow band by proportion so a fixed delay may not be so bad.

Last edited by Lineloss; 2011-01-10 at 08:53 PM. Reason: 50% average in english.
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Old 2011-01-10, 09:23 PM   #20
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Maybe just tilting/slanting a yagi would help FM performance ?
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Old 2011-01-11, 11:55 PM   #21
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down the road from me, one of these elliptical polarization antennas is up in the air about 40 feet, cool now, after some reading i figured out how to model it thanks guys

http://www.ve3sqb.com/hamaerials/oz2oe/build_yagi.html
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Old 2011-01-12, 01:16 AM   #22
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As an example two antennas with SWVR of 1,2:1 will give an unbalance of 1,6 dB!
Hmm, he has his decimal point (decimal comma in Europe, heh) off by one place. A SWR of 1.2 will have a mismatch of .16 db.
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Old 2011-01-12, 12:55 PM   #23
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FM polarization may be either "clockwise" or "counterclockwise":
http://louise.hallikainen.org/FCC/FccRules/2010/73/316/
Apparently, in addition to Horizontal ONLY, it can also be Vertical ONLY:
http://www.fcc.gov/Daily_Releases/Da...C-93-306A1.txt

Unfortunately, the FCC's FMQ database does NOT require submission of
polarization direction, so you might have to look at the detailed application
requests and engineering data, or investigate the antenna part number or
call the station (hoping to talk to someone who REALLY knows):
http://www.fcc.gov/mb/audio/fmq.html

PS: If broadcasting a polarized signal, TV is always "clockwise (right hand)":
http://www.hallikainen.com/FccRules/2011/73/682/

BTW: A Helix would be preferred vs Crossed Yagi's due to the "quarter-wave"
mis-match across the FM Band (20% bandwidth). K6STI has analyzed several
Circularly Polarized antennas for FM, including a Cubical Quad. Someone might
want to perform further analysis using 4nec2 (or equivalent):
http://www.ham-radio.com/k6sti/

More information re Cir. Pol. Antennas and the quarter-wave transformer:
http://sv1bsx.50webs.com/antenna-pol/polarization.html

Some DIY Cir. Polarized Antennas:
http://jcgriffin.com/circularlypolar...%20antenna.htm
http://home.comcast.net/~ross_anderson/quadix.htm
http://www.af9y.com/helix.htm
http://www.tuc.nrao.edu/~demerson/helixgain/helix.htm
http://home.earthlink.net/~w6rmk/antenna/helixmatch.htm
http://www.wade-antenna.com/Wade/WH14-69.pdf

Exercise for OTA experimenter who has some FM Yagi's but isn't ready for a DIY project:
Mount one Yagi for horizontal reception and the other for vertical reception and
interconnect using the quarter-wave transformer described in the the first reference above.
It might not be an "optimal" antenna for circularly polarized signals, but it might
surprise you and for sure will receive signals that are predominantly Horiz. OR Vertical,
depending on what the station is transmitting. Not sure if they need to be matched.....

Unfortunately, like all Cir. Pol. antennas, it will also pick up noise & interference from
the "other" antenna when trying to receive a predominately Horiz. or Vertical signal....

Exercise for the 4nec2 student: simulate the above with various vertical stacking and
horizontal stacking heights....which will require figuring out how to REALLY simulate
AND analyze Cir. Pol. signals in 4nec2 (it isn't obvious....) Verify by looking for big drop
in Gain when trying to receive a left-hand signal with a right-hand antenna.
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Old 2011-01-12, 09:35 PM   #24
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Holl_ands always delivers... Some excellent info in your post!
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