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.
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
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.
Obviously it works.
Assumption, the mother of all screwups. However, the data sheet does
seem to show that they actually made some measurements.
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?
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
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558