900 MHz propagation over a river 1

[kc7cjo]

What kind of propagation issues are we looking at using a 1-Watt 900 MHz Spread Spectrum signaling system, shooting over a river approximately 1500' wide?

The "A" & "B" sides are approximately 30' above the water level (which does vary approximately 6'-10', depending on the time of year).

The control station is a River Ferry which uses the system to control RED & GREEN signal lights for traffic flow on and off the Ferry.

Would the distance and frequency, shooting over the water have an effect on the systems response?

 

[Comcokid]

I would think you are will not any propagation issues. Fast fading/multipath/Rayleigh issues (where there is constructive/destructive interference as the signal travels different paths from one end to the other) should not be a problem.

You are basically in a line-of-sight situation. You have more than enough power for 1500 feet even if your antennas are onmi-directional (the products I work on give 1 mile at 900 MHz DSSS with just 40 mW into a simple PCB antenna on a clear day). The river will probably never be glass-smooth, and even it was your other factors (distance above ground, ant-to-ant distance, and TX power) by far dominate.

I have not checked the absorption factors for rain or snow at this frequency, but with 1 Watt power I do not see how even this will be an issue even under the worst conditions. Assuming, your coax from the receiver or transmitter to the antenna are fine and your receiver is working properly. It would take a very poor receiver, bad coax, bad antennas, or antennas on the wrong side of the tower to degrade any link-budget needed for this setup.

With this power at this frequency, RF interference from other sources are unlikely. If your "spead spectrum" is FHSS, interference is slightly more likely than if you are using DSSS or some other pure digital modulation scheme.

Since you are a Civil/Environmental, I will place a disclaimer for this post that I am not a registered professional engineer (I work under an exempt status as an employee), and so anything I state is strictly opinion.

 

[MikeHalloran]

Do you get much barge/ freighter traffic on the river?



Mike Halloran
Pembroke Pines, FL, USA

 

[kc7cjo]

How do I change what shows on a posting as to what career field I'm in? I couldn't find anything close to what I am but I'm NOT a "Civil/Environmental"! I was thinking it was something else.

I'm an Electronics Technician!

 

[biff44]

"I would think you are will not any propagation issues. Fast fading/multipath/Rayleigh issues (where there is constructive/destructive interference as the signal travels different paths from one end to the other) should not be a problem. "

Why do you say that? That river will give a very big reflection that will interfer with the direct path.

If it were me, I would use high gain antennas on both ends, and point them somewhat up at the sky--angling them down just enough to get good transmission when there is heavy rain.

 

[Comcokid]

"I think you will not find any propagation issues. Fast fading/multipath/Rayleigh issues (where there is constructive/destructive interference as the signal travels different paths from one end to the other) should not be a problem. " (sentence corrected! oops!)

This is a matter of the incident angle. When the angle is very small, the river will look like a mirror. At a higher incident angle, more of the signal travels into the water and is absorbed. When you add in the possibility of waves, the reflected signal will be scattered from multiple points, and the probability is that the multiple reflections will actually cancel each other out and never get a chance to interfere with the direct-path signal.

The effect is similar to the mirror-image you see of the horizon when looking over a flat road on a hot day. If you get you head up even a few more feet, the mirage will disappear. If the road/terrain is very uneven, you won't even see the mirage effect.

This comes from my own experience of range testing 900 MHz signals over water and land many, many times. When receiver/transmitter are positioned 4' above the reflecting surface (ground, water) on a 1/2 to 1 mile range test, I find many nulling points were data reception goes to zero. When the height is 6', I find mostly points were signal strength drops off, but data still mostly gets through. At 10', I can't even find the effect when I plot the signal strength Vs distance. So, for the situation kc7cjo is asking about - 30' above the water and 1500' between points, I see no issue at all.

kc7cjo - no problem with being a technician. Welcome to Eng-Tips! You're obviously a ham, so am I! My disclaimer statement was because in some other eng-tips forums there are discussions of the nuances of what a PE means. Civil engineers work in a world where most need a PE. In the electronics world, most electronic engineers don't need a PE, and few go for it.

 

[kc7cjo]

I think one of our problems is that both ends aren't at the same height and the Ferry varies in height in relation to each end. Since 900 MHz is virtually Microwave and the antennas are high gain, the antennas SHOULD be pretty much on the same plane, or at least close to it.

Thoughts???

 

[Higgler]

What's your antenna gain? The water center is at best 2.3 degrees below your beam peak (1.5 degrees with 10 foot water rise) and you'd need a very large antenna to roll off the antenna pattern at that angle.

To really minimize the bounce off the water, you'd want both antennas to be circularly polarized so that the water bounce doesn't get into the other antenna, since one bounce makes the other polarization and ends up very weak into your receive antenna. Plus, when a boat goes past, it's probably helpful to use circularly polarized antennas to minimized the signal dips.

If by chance you've already purchased linearly polarized antennas, see if you can return them for circularly polarized antennas. If you're stuck with linear polarization, the following applies;
When the energy bounces off a flat ground plane (water), you get nulls in elevation spaced about every 1 degree angle in elevation, or every 27 feet your signal across the 1500' of water will change from a null to peak to null. Hence every 13.5 feet in height you go from a null to a peak in energy.

When you test on a water antenna range for single linear polarization, you move your transmit antenna to make sure the bounce off the water makes a peak signal at your receive antenna.

When your water rises, your signal will change alot it seems. If your height above water is 30 feet, for a vertically polarized antenna, you have signal nulls at 13.5&40.5 feet above that water and a peak at the water and a peak 27 feet above that water. When the water rises 10 feet, you have nulls 20 feet and 40 feet above the water and a peak at 30 feet above the water. Sounds like water rising 10 feet isn't good for you since a null 20 feet above the water makes you lose signal at the receive antenna. If your antennas are horizontally polarized, it has peaks where a vertically polarized antenna has nulls, so that's not so good when the antenna is 30 feet above the water. Go circular or tilt the antennas slant linear at a minimum, peaks and nulls alternate in elevation for V and H polarization and at least a slant linear antenna minimes the dropouts with water rise.

khiggins @ toyon dot com

kch

 

[kc7cjo]

I haven't had much schooling on antenna theory so I really didn't understand much of what Higgler said, but let me try:

Here's what I've done so far: I've replaced the high gain antennas with 1/4-wave antennas and that seemed to help some, at least the problem side. Now the problem seems to have changed to the other side. I'm going to be relocating the antennas on the shore ends 10-feet away from the transceivers.

My reason for this is because in VHF, at least, a 1/4-wave antenna will radiate in a circular pattern in all direction, kind of like a ball. Where as a high gain antenna tends to force most of the signal out towards the horizon.

Since the antennas aren't on the same plane and we're dealing with microwave frequencies, they seem to act like the signal is over or under shooting each other.

Am I wrong in this assumption?

Would what Higgler described be tilting the antenna down towards say the center of the river so as to use the water as a reflector to hit the Ferry? I don't necessarily want the two ends to talk to each other as they might have conflicts.

I have Absolutely NO Tech data on this system and it IS covered under warranty, but we do need to keep the costs down as much as possible. It is, after all, a Government agency!

Just so you guys know, I REALLY Do appreciate all your help in this!

 

[Higgler]

A quater wave antenna will radiate everywhere.

I'd suggest tilting them 45 degrees to each other, don't tilt towards the water, tilt along the bank. Tilt both antennas in the same direction. That'll help with the water bounce.

High gain antenna should have been ok too. If they are linearly polarized, tilting them 45 degrees will help the water bounce too.


kch