OK, three postings asking a similar question: How can you do it if they are
all the the same boom?
The designer of the dual pol yagi has the opportunity to accommodate the
frequency and impedance disturbance of the other polarization elements at
design time. So the effects of the other opposite polarization elements are
tuned out of each other.
You can't do that when just sliding two yagis up and down a common mast. I
can't say how much effect there would be if you got them within a couple of
inches of each other but there will be an effect and probably not a helpful
effect.
If I can find two identical yagis and find the ambition to mount them and
hook them up to a network analyzer, I can actually come up with a better
answer.
-----Original Message-----
From: Ken Hohhof
Sent: Wednesday, February 17, 2016 8:54 AM
To: [email protected]
Subject: Re: [AFMUG] 450i 900 using existing yagi
If you have tried to align a 900 MHz yagi, you know just a piece of metal or
your hand within a foot or so will affect its performance. I assume putting
the cross pol elements on the same boom avoids that issue.
-----Original Message-----
From: Wireless Administrator
Sent: Wednesday, February 17, 2016 9:34 AM
To: [email protected]
Subject: Re: [AFMUG] 450i 900 using existing yagi
+1
-----Original Message-----
From: Af [mailto:[email protected]] On Behalf Of Jay Weekley
Sent: Wednesday, February 17, 2016 10:26 AM
To: [email protected]
Subject: Re: [AFMUG] 450i 900 using existing yagi
Dr. McCown. If separation is important then how do they get away with
making both antennas essentially the same unit? I'm sure there is a simple
explanation.
Chuck McCown wrote:
Somewhere, in one of my 50 or so antenna books, there is a graph
showing mutual coupling of yagi antennas mounted on the same mast with
respect to separation. Right now, I am 50 miles away from those books.
As I recall, if they are cross polarized, there is very little
coupling at close distances.
I think there is theoretically zero coupling of the far field. But
you get strong reactive nearfield effects at 900 MHz around the 7 inch
range or less. That just means that the two antennas will start
throwing off the impedance and resonant frequency of the other one
the closer they get.
However there is the Radiating Nearfield Distance too. If you are
inside that limit the two will mess with each others patterns pretty
bad. That is on the order of 22 inches. However due to being cross
polarized the effects should not be strong.
So, yeah... 2 feet is good if cross pol. I don’t think I would not go
under 1 foot.
If you have some time, experiment a bit.
*From:* Brandon Yuchasz <mailto:[email protected]>
*Sent:* Tuesday, February 16, 2016 9:09 PM
*To:* [email protected] <mailto:[email protected]>
*Subject:* [AFMUG] 450i 900 using existing yagi
Since we are now deploying some 450 900 gear we are pulling down lots
of old FSK with M2 Yagi antennas. In general I really like the Cambium
yagi they are well thought out and the SM being mounted on it is
really clean. Swaps are a breeze. That said I am still thinking about
using the M2 in specific situations like tripods. I did some
retrofitting today to convert two to slant and we went and tested and
got good results. There were separated from each other vertically
about 8 inches at the mounting brackets and I didn’t see any issues .
We got 8x8x line of site.
So my question to the group is, in general is there a rule of thumb to
use for vertical separation on dual polarity antennas being mounted
like this? Could they be stacked tight near each other?
Best regards,
Brandon Yuchasz
GogebicRange.net
www.gogebicrange.net <http://www.gogebicrange.net/>
