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Just wanting to learn more about this stuff but can't find any resources based on what i know to ask.

This tower has several of these strange looking sets of 2 bent tubes all facing the downtown area. What are they? On an FM tower. Just wondering the function of such strange configuration since all FM transmitter picture i find look nothing like them.

enter image description here

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  • \$\begingroup\$ Are you sure it's an antenna and not just somebodies idea of modern art. \$\endgroup\$
    – Trevor_G
    Mar 10, 2017 at 19:41
  • \$\begingroup\$ haha. i doubt modern art would be up 1,100 feet. \$\endgroup\$
    – Emery King
    Mar 10, 2017 at 19:42
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    \$\begingroup\$ I found it: eriinc.com/Catalog/Antennas/FM-Antennas/… \$\endgroup\$
    – Emery King
    Mar 10, 2017 at 19:52
  • \$\begingroup\$ YOu might be surprised... check the pole on the top of the building in this image... google.ca/…: That was supposed to light up and change with the wind when they built it. It never worked since day 1 10 years ago.. Costs too much to take it down though. \$\endgroup\$
    – Trevor_G
    Mar 10, 2017 at 19:52
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    \$\begingroup\$ Good to know. In case anyone is wondering about the Tesla flower... it's called a "lightning dissipation spur" \$\endgroup\$
    – Emery King
    Mar 10, 2017 at 20:03

1 Answer 1

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If you look in a text book for FM antenna, you will often find a straight thin-wire centre-fed dipole illustrated. This is not, there are at least 4 features that are different, and for good reasons. Marshall House is correct with his identification of the antenna.

a) The antenna has fat tubes rather than thin wires. Thin wires result in a fairly well-defined resonance frequency. Fat tubes increase the bandwidth to cover the whole commercial FM spectrum

b) Each dipole is curved. A straight dipole has a well defined fore-and-aft beam radiation pattern, a curved dipole spreads it out into a more omni pattern

c) It uses two crossed dipoles, rather than a single. The physical spacing of the dipoles introduces a phase shift, which results in the signal being circularly polarised. Initially, FM transmission used horizontal polarisation, to horizontal receive antenna on rooftops, which could be pointed at the transmitter. Unfortunately, cars can point in any direction, so the antennae have to be vertical so the reception is independent of the facing direction. Transmitting circular polarisation caters for both groups of receiver.

d) It uses a stack of dipoles, rather than a single one. A single dipole would launch most of its energy skywards and into the ground. A vertical stack allows a pancake beam to be formed to improve the efficiency of broadcasting to ground-based receivers.

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  • \$\begingroup\$ Fascinating stuff. I was not aware that a circularly polarised signal could also be (somewhat) omnidirectional. Can't quite get my head around that. \$\endgroup\$
    – user98663
    Mar 13, 2017 at 11:35

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