We’re going to deploy a dual frequency sensorstation in Yosemite in a month or so, and I was curious about vertical or horizontal mounting for the yagis in this location. Our site is on Glacier Point, which is 2,200 meters above the valley floor and a really amazing vantage point with a good wide view of a huge portion of Yosemite Valley. We are primarily interested in bat activity patterns in the valley floor and surrounding cliffs, but also of course want it to function as a general Motus receiver. We were going to aim 2 or 3 yagis for each frequency down into the valley at different angles, but I was curious what peoples thoughts were about horizontal or vertical mounting orientations in this case, to maximize coverage of the valley. Would it make too much of a difference either way? Any discussion on this would be much appreciated, as I don’t understand exactly how the orientation changes the reception, I just know that it has an effect. Thanks!
Theoretically you want to match the polarization of tag and receiver antenna but since the tag orientation is unknown the antenna orientation is noncritical. However, if using multiple Yagis you want to avoid interaction that would affect the antenna beam pattern. The easiest mechanically would be to stack vertically oriented yagis a wavelength or so apart on a single pole. You can point each yagi in the best direction to have full coverage of the valley based on the 3dB beam width of the yagi.
Hope this helps but maybe there are some antenna engineers out there that can model different arrays to optimise the desired coverage.
According to motus docs: https://docs.motus.org/en/stations/station-installation
“The orientation of an antenna changes the horizontal range relative to the vertical range. …Yagi antennas can be rotated along the boom to select a wider horizontal or vertical beamwidth.”
and “Most stations across Motus will orient their Yagi antennas horizontally (antenna elements pointing horizontal).
We recommend all collaborators do this to maintain consistency across the network.”
As I understand it - The main lobe beam width of a horizontal polarized motus yagi is sort of like a slightly squashed eggplant shape. ie wider in the horizontal and somewhat shallower in the vertical when pointed straight out. 3, 6 and 9 etc element yagis get progressively narrower horizontal coverage but gain significantly in distance reception. It seems reasonable that flipping it 90 degrees one can create a slightly more curtian-effect which might give a better view to seeing what is passing through an area as opposed to what is occupying a certain horizontal slice (and missing what is above or below) In practice I have no idea the real significance is, however given the receiver elevation above your target area it certainly would seem a concern if you were pointing straight out.
You mentioned pointing ‘down into the valley’ and a desire ‘to maximize coverage’ which would imply to me you want horizontal orientation to match the along the valley axis. Also - if its still an option - “down into valley” would also imply to me you would want fewer element yagis for the broader beam-width as the gain in range of a 9 element may be lost as the distance to the valley floor and walls is only a couple of kilometers. Fewer element antennas also would have less wind ‘sail-area’ and snow/ice loading to factor for.
That same motus docs page also gives recommendations for antenna spacing based on orientation and frequency as well as a lot of other station installation advice.
For the installation I did we have three 166Mhz and three 434Mhz yagis. We starting with the smallest 433 at the very top as it has the smaller ‘wind loading’.
Then we alternate 166 and 434’s down the mast. The goal was maximize the mast real estate - thinking the large ‘blank space’ between two 166Mhz might as well be used to hold a 434 Mhz antenna.
A minor point re spacing - we used a telescoping mast and I now wish we had not raised each section to full height so we would have more ‘overlapping steel’ at the junctions for strength.
Another thought to consider would be to plan extra space now for adding a 433MHz antenna in the future. That would enable research using multiple low cost CTT Nodes on the valley floor. (note it must be a 433 not a 434MHz) It might open up some opportunities for detailed studies of distribution and movement. (assuming you are using a CTT SensorStation with available unused channels). Klamath Bird Observatory has done some amazing detailed work in their Vesper Meadow study area using CTT. Nodes.
Good luck with you installation - it sounds like an amazing and challenging location!
-Rich
Antenna efficiency is best when the transmitting signal polarization matches the receiver polarization. When a bird is flying straight and level it would be horizontal polarization. Now that just to get the birds to do that. I remember an old Song were the lines read straighten up and fly right.
Glenn Pollock
6736 Laurel Ave
Omaha NE 68104
402 203 5789
Receiver antenna orientation depends on transmit antenna orientation. A vertically polarized transmit antenna requires a vertically polarized receiver antenna for maximum range. Likewise with a horizontally polarized antenna. Any angular difference between the two antenna reduces the signal strength realized at the receive antenna with the minimum (theoretically zero) signal reception being at 90 degrees of one another (one vertical and the other horizontal). So if the antenna on a tagged species (bat) lays flat on it’s back, you’ll get the best chance of detection with a horizontally polarized antenna.
Of course animals don’t play nicely and change their orientation relative to the receiving antenna often. But the majority of is time in flight would be relatively horizontally polarized.
I hope that helps.
Corey Wotring
For the valley, I think Richard’s point about using yagis with fewer elements is key. A 9 element yagi might be predicted to give you 10-14km range. If you only need 4, you could go with a shorter antenna with a wider detection bubble.
In discussing angling antennas up or down from horizontal with Bob Morton (recently of Maple Leaf antennas), he felt the ground plane effects might offset any gains in detection range you might get by angling the antenna. Ground plane effects can do things like introduce nulls or dead spots in the detection bubble. So a wider beam, mounted up high horizontally might give the best range with less ground plane induced dead spots.
Patrick Lorch
plorchgm@gmail.com
Thanks everyone! I feel like the advice about the orientation of our yagi antennas and the orientation of the bats is tough to follow given that bats do lots of things (like roosting vs foraging), but I did have an idea… What if we mount two pairs, so one horizontal and one vertical axis, angled down into the valley the same, and then we can test their reception range by driving the loop road with a gps track and tags on the roof so we can compare reception. If they are similar then we can just keep the best one, and if they compliment each other we can go overboard and have 4 antenna mounted to the railing we are using. I think we enough space that we could mount them 1 or 2 meters apart from each other.
I’ll report back once we deploy this if I have any insights! It’s a pretty unique situation, so not sure how helpful it will be, but it might be helpful to someone.
Thanks!
-Gabe