IP-002 How does a UHF loop antenna work?
Reader Gene in Madison, WI tossed me a 300 pound pumpkin:
A.
Since TV transmissions are normally horizontally
polarized (and, I believe, a loop antenna is
polarized in the plane of its circle), how come the
standard vertical UHF loop that comes as part of
the usual “rabbit ears” indoor antenna gets anything
at all? I realize that I am so close I might even be
able to “tune in” with a paperclip shoved into the
input, but how about the people who have been (and
are still) on an indoor antenna in more marginal
places?B.
Wouldn’t the horizontally positioned loop antennas
I see offered be so omnidirectional as to pick up
not only the station but also a bunch of bounced
signals and, thus, be a poor choice for DTV?
That’s a pretty good question, and while looking for supporting documentation on the web I noticed the “experts” seem to contradict themselves. Wiki had some good info on it and http://www.electronics-tutorials.com/antennas/antenna-basics.htm has about the best explanation on all types of antennas.
We know VHF and UHF are different animals, and the way the antennas work is totally different. In VHF, the elements on the antenna, or even rabbit ears, resonate at the frequency or wavelength it is adjusted to. Remember my post on “A Lesson on Wavelength” where I give you the length of the wave for channel 4 at just under 15 feet? That would be a huge pair of rabbit ears, but you can divide the length by half, or a quarter, and still have an efficient antenna at that frequency. Pulling the ears out fully to about 3 feet would resonate perfectly for that channel. It is exactly the same principal as tuning a guitar to a piano. When the two notes are the same, it has a sweet sound, but if one is out of tune with the other, it’s very noticeable.
Remember in school when you took a wire and wrapped it around a nail several times, and connected it to a battery? You created an electromagnet, and the more loops you made, the stronger the magnet’s power. In theory, you are shorting across the battery with a wire, and naturally you’d think nothing would happen, but as current flows through the wire, an electromagnetic field is generated around the conductor. If you alternate the polarity of that field, say 717.25 million times per second, you’d get one heck of a field generated.
The loop antenna reacts to the magnetic field hitting it in the air, as opposed to vibrating with the frequency like a VHF antenna does. And yes, you are correct that it picks up in a figure 8 pattern equally well from the front or the back. That is its’ Achilles Heel for DTV; if there’s a reflecting surface behind it creating multipath it will naturally disrupt the integrity of the signal. Loops however, do an amazingly good job with UHF, especially in analog.
Now your question about polarization is another interesting point. A VHF signal will actually rotate its’ orientation due the the Earth’s magnetic field. It may start out horizontally, but it skews several degrees the farther from the transmitter you look at it. UHF does not react to the Earth’s field since it’s a much higher frequency. That’s one of the reasons UHF is very well suited for DTV.
A good way to look at polarization is to tie a rope to a tree, Swing it up and down and you’ll be generating a wave pattern in the vertical plane. If you move it from side to side parallel to the ground, you’re oscillating in the horizontal plane. If you, your wife, and two kids had a rope standing back to back in 4 different directions, you’d notice gaps between the ropes in vertical, and a smaller gap when oscillating left to right. If you saw my video on the tour of my transmitter, you saw the slots inside the antenna at the very end. These are mathematically calculated to give full 360 degree “ropes” with no gaps. If they overlap, there will be directions that get more signal if in phase, or no signal if out of phase. Those little knob thingies inside the antenna are adjustable to tune the antenna for the proper pattern.
I hope that explains it a little bit. In a nutshell, the VHF antenna reacts to the wavelength, and the UHF reacts to the magnetic field. That’s why the two types of antennas look so differently, and why the typical “V” rabbit ears are not suited for UHF.
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Hi Russ (& All),
I’ve been working my way through the “pumpkin
pie” you produced from my 300# pumpkin, but
it’s slow going because I have had some other
matters that took priority.
I was especially tuned in to your comment that
the “experts” seem to spend a lot of time
contradicting each other. That’s definitely
been my experience getting a bit of help on
the internet, LOL. I have made some progress
and ran across a reference to what you said
(in the matter of polarization),
“A VHF signal will actually rotate its’
orientation due the the Earth’s magnetic
field. It may start out horizontally, but
it skews several degrees the farther from
the transmitter you look at it,”
while searching for something else (of
course) What I found was a reference to:
http://en.wikipedia.org/wiki/Faraday_rotation
It definitely sounds like the magnetic effect
you mentioned even though the article seems
rather focused on visible light. What has me
puzzled, though, is the further comment that
“UHF does not react to the Earth’s field
since it’s a much higher frequency.”
Hmmm! That means the Faraday rotation can’t
be responsible for a loop antenna, set with
its flat plane vertical, picking up UHF that
was transmitted horizontally polarized.
So, are there other things that might cause
the polarization to differ from the design
intent?
Regards