On 1/27/2017 11:52 AM, Jeff Liebermann wrote:
On Fri, 27 Jan 2017 08:22:43 -0500, rickman wrote:
I had a WISP unit upgraded for better reception and they used a Yagi
antenna, at least I guess it's a Yagi. Here's a photo.
http://www.netwifiworks.com/images/a.../Yagi/yagi.png
Ok. That's an Ubiquiti AirMax AMY-9M16 900MHz antenna.
16dBi gain and dual simultaneous polarization (i.e. not switched):
https://www.ubnt.com/airmax/airmax-yagi-antenna/
https://dl.ubnt.com/datasheets/airmaxyagi/airMAX_900MHz_YAGI_Antenna.pdf
You need the dual polarization to get double the normal speed by using
2x2 MIMO streams, one per polarization.
I got a whole new setup. The old unit was one piece with what must have
been a panel antenna as the case was flat and broad. You can see what a
monster this one is. The beam is nearly three feet... no, I just
checked the data sheet and it around 4 feet long! It's not hugely
heavy, but to move it around I have to unbolt the mounting bracket and
it's a PITA while on a ladder.
This doesn't seem to fit the mold in a couple of ways. The elements are
not spaced at all regularly. The spacing seems to vary around a bit.
I agree. It does look weird. However, having extra aluminum near the
antenna in the form of the other polarization, and getting sufficient
isolation between the two polarizations, is going to do strange things
to the design. Send me dimensions and I'll analyze it (time
permitting), like I did with the 2.4GHz MFJ1800 yagi:
http://802.11junk.com/jeffl/antennas/mfj1800/index.html
If we get a warm day when I feel like messing with it I will. I spent
over an hour yesterday trying to optimize the orientation. I had
mounted a bracket under the deck thinking they would just move my
existing unit (NanoStation M900 Loco). I spent some time positioning
the unit and found a sweet spot that was two or three dB higher than the
surrounding area, next to a pole. Ok for this unit, but in the way for
the Yagi so it couldn't be aimed optimally. So I spent the time to
unbolt the bracket (everything was in the way of everything else so it
nearly all had to be dismounted) and move it to the other side of the
post. It can be pointed perfectly, but now it may be out of the sweet
spot so the numbers didn't change.
When the weather is better, I'll try moving it back to the sweet spot in
a way it can be aimed better. The old unit was easy to hold with your
hand and test. The Yagi is not so light and unwieldy, especially on a
ladder. I also want to move it to a less conspicuous spot. That will
be hard. I store kayaks under the deck mounted from pulleys.
But more importantly, I've read that the director elements are
*insulated* from the support beam while these are all welded.
That's done so that the mounting boom does not become part of the
element length. Were it not insulated, half the circumference of the
boom would need to be added to the element lengths. Insulating the
elements also provides slightly fewer sidelobes and possibly (not
sure) better isolation between polarization. Looking at the patterns
on the data sheet, it looks much better than I would normally expect
from a single polarization yagi.
Not sure if you understand me. This Yagi is *not* insulated. But
remember, it has 17 passive elements! It's not like they were just
fooling around. lol
Obviously it works.
Assumption, the mother of all screwups. However, the data sheet does
seem to show that they actually made some measurements.
I mean I'm getting some 6-7 dB better signal. Regardless of the SNR,
the receiver provides quality indexes that show a lower bit error rate
and higher overall throughput. That's why they put this up. It hasn't
improved my throughput so much, but it lowers the retransmits and
improves the utilization of their network.
I'm getting about 6 dB stronger signal than before although
it's a bit hard to compare as the location changed and I measured about
a 3 dB gain with the old unit in that location.
But more importantly, with the signal rising by 6 dB, the reported
background noise also rose 6 dB. Isn't SNR what is important that the
antenna should have improved?
Yep. You want to maximize the SNR. Increasing both equally is about
what I would expect if your antenna were also pointed at other sources
of interference. The 900MHz smartmeters are a likely culprit. Plenty
of other possibilities. Can you move your new yagi around a little to
see if you can minimize the background noise?
I'm not certain whether SNR or signal strength is most important. That
would depend on the noise factor of the receiver, no? I know in lower
frequencies the environmental noise is high enough the receiver noise
nearly doesn't matter. At higher frequencies I thought the limitation
was in the receiver front end. So until the noise gets to be high
enough that it approaches the receiver noise, it won't matter.
This Yagi claims something like 20 or 25 degree beam while the old
antenna had a 60 degree beam.
A narrow beamwidth is good for reducing interference coming from off
axis interference sources. However, if the source of interference is
along the antenna axis, or in your case, within less than +/-10
degrees off axis, you can easily make things worse.
Got a 900MHz spectrum analyzer handy? Any of the USB RTL-SDR
(RTL2832U) dongles should do the trick, although a 900MHz RF amp will
probably be needed. Use it to see what you're dealing with.
http://www.rtl-sdr.com/?s=spectrum+analyzer
What would a spectrum analyzer show me that would be useful. No, I
don't have one, but I could get one...
--
Rick C