[opendtv] Re: MIMO
- From: Ron Economos <k6mpg@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Wed, 02 May 2007 16:18:24 -0700
Can't say that I've ever used one. When I was
active in amateur radio satellite communications,
I used crossed Yagi antennas. Either the elements
were co-located and fed out 90 degrees out
of phase, or the elements of one yagi were
positioned 1/4 wave ahead/behind the elements
of the other yagi (and both are fed in-phase).
Here's a picture of a circular polarized yagi
using the 1/4 wave offset technique.
http://www.m2inc.com/products/vhf/2m/2mcp14.html
Ron
Manfredi, Albert E wrote:
Ron Economos wrote:
Google for "Lindenblad antenna".
What a messy-looking antenna.
Seems like a way to receive a circularly polarized signal,
omnidirectional on the horizontal plane, using the mechanical alignment
of four folded dipoles. The dipoles are each tilted 30 degrees from the
horizontal, to follow the rotating waveform, along a nominally N/S and
E/W set of orthogonal axes.
The E field is rotating to the right or to the left as it propagates,
and it flips 180 degrees for every 1/2 lambda of forward motion.
Assume the two dipoles along either axis were located 1/2 lambda apart,
and the two dipoles were wired on opposite phase but each oriented
horizontally. They would each receive the same rotating E field as it
travels through the antenna.
Now untwist these two dipoles to be only 120 degrees apart, rather than
180 degrees apart, and the separation between them has to be reduced to
(120/180) * 1/2 = 0.333 lambda apart.
Does this sound reasonable? The key step is to think of each dipole pair
as being twisted 120 degrees apart, rather than 60 degrees apart. I
found this one site that shows the design in enough detail to find out
if this explanation works:
http://www.qsl.net/dl2lux/lindenblad/lindenblad.html
It looks lie the "upper" side of the dipoles are all four connected
together and sent to the center conductor of the coax downlead, and the
"lower" sides go to the shield. So this means that in fact, along either
axis, the two folded dipoles are connected out of phase with each other
(Figure 5). And it shows that the folded dipoles are 0.3 lambda apart
(Figure 1). So perhaps that explanation works.
Figure 3 shows that this makes for a better OTA antenna than satellite
antenna, unless the satellites are close to the horizon.
I would think that you could achieve the same results by connecting one
vertical dipole and two horizontal dipoles together, with a 90 degree
delay line between the vertical and two horizontals. And get something
less weird-looking. Thene again, you'd have to work out the impedance.
The nice thing about four half-lambda folded dipoles paralleled together
is that they work out to 300/4 = 75 ohms. How convenient for RG-6 or
RG-59.
Bert
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