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Author Topic: If man made noise is vertical.....  (Read 274 times)
KY202
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Posts: 45




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« on: November 03, 2006, 09:16:18 PM »

This has been laying heavy on my mind for some time now...

When driving up and down the roads, I always see these little vertical yagi antennas at the same type of locations.  Some are located above (just barely) electric sub-stations, and most of the others are beside the little bridges over creeks on the rural roads.  The ones by the bridges (the ones I've seen anyways) are only on like a 10' mast.  Sometimes I even see them along the road side on a short mast, with no creek or bridge in sight.  In fact there is one about 4 miles from here (just down from my moms church) on a short mast, which is secured to a wooden fence post along side the ditch.  

What are these transmitter stations for?  I guess the ones at the electric stations would be for information about the station or the grid.  I guess the ones by the bridges would be for monitoring the fog, possibly?  I say fog because a lot of the bridges are so high over the water, that flooding would be impossible.  But I have no clue as to the use of the transmitters located along these rural roads with no noticable reasoning.  What could they be?  And now for my main question... Why are they (whoever receives the signal) using these very low mounted vertical yagis?  I mean with some of these antennas being only around 10'-12' above the ground, and most man made noise being of the vertical polarity, would it not be better to run a horizonatal antenna?  

I am no where near being the smartest idiot, but I would think that vehicles driving by the ditch mounted verticals would cause noise in the transmission they emit.  That may be the dumbest thing you've ever heard, but thinking back to my CB days and the fact that mobile CB antennas were vertical and the fact that antenna position and type of vehicle and many others attributed to a noisy receive at the radio.  I have no Amateur license yet, so I must base this on my 11m experiences from years ago.  

I just had to ask this, because it is driving me crazy wondering why these vertical antennas are located so close to the ground and/or electrical transmission lines, when that puts them right there with all the man made noises.  Also, why would they be mounted so low?  

Sorry if this is just to "dumb" of a question for all of you Elmers out there, but I have been wondering for a good while and would really like to have an answer.  It just seems to me that higher and horizontal would make for a better quality of signal at the receiver(s).



Thanks for reading, and hopefully answering.

Jason
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K7KBN
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Posts: 2754




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« Reply #1 on: November 03, 2006, 10:42:16 PM »

Small antennas, such as you describe, are probably for VHF and higher frequencies.  This suggests that they use FM, rather than AM or SSB (if the transmission is voice), or they use some form of digital data transmission.

FM is inherently much less susceptible to most forms of noise than AM/SSB.  Digital communications generally uses some form of comparing what was received against what was transmitted, and automatically correcting any errors that might have cropped up.

I don't have a clue as to the exact purpose of the antennas, but I wanted to explain my theory regarding vertical/horizontal polarization.
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73
Pat K7KBN
CWO4 USNR Ret.
WB2WIK
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« Reply #2 on: November 03, 2006, 10:56:12 PM »

They are for telemetry applications (based on your description) and send digital signals back to a receiver using an omnidirectional antenna so it can hear hundreds of them, and they operate in the UHF band.

Vertical polarization is the norm for this application, so the base receiver can use an omnidirectional vertical antenna; these are mostly line-of-sight systems (or very close to line-of-sight), so signals are strong.  Horizontal polarization would work fine, but then the base receiver would need a horizontally polarized omnidrectional antenna, which are more difficult to arrange and install and aren't nearly as available.

Roadside emergency telephones usually also traditionally used vertically polarized UHF beams for the same reason: Lots of them per single base station; they all aim at the base station, which uses an omnidirectional vertical so it can communicate with a lot of the remotes in all different directions.  

Horizontal polarization actually does work better on VHF and UHF because it scatters over the horizon better, for weak signal work.  However in this case, the signals aren't weak and they come from many directions and vertical polarization has been the norm for these applications since the inception of such services, mostly because vehicular antennas are much easier deployed when they're vertical -- same reason VHF-UHF FM repeaters are all vertically polarized.

WB2WIK/6
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K7UNZ
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Posts: 691




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« Reply #3 on: November 03, 2006, 11:08:14 PM »

Those antennas you describe are most likely for telemetry from the site location to a central control point.  In other words, to enable remote monitoring/control or to simply gather information.

For example, road surface temps near a bridge to suggest frozen/icy surface and have the local "sanding" trucks dispatched to the area.  Sometimes it's nothing but a simple traffic counter to gather data on road usage.

Other uses include control of traffic signals at major intersections by responding emergency vehicles.

The list goes on and on....

73, Jim/k7unz
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AD5PE
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Posts: 67




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« Reply #4 on: November 04, 2006, 02:13:46 PM »

They're fairly common around here - relatively "flat" country means that a little rain over a wide area can all flow into only a handful of creeks, and only raise each one a little bit.  But "flood forecasting" uses the small rise in each (along with other parameters) to model and predict the flood peaks (flood stage) downstream where ultimately all of those creeks come together.

The use them for digital "rain gauge" telemetry even in areas away from obvious drainage to gather data on how much water should be making it to which creeks.
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KB4QAA
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« Reply #5 on: November 04, 2006, 09:06:42 AM »

Yeah, they are telemetry transmitters in the VHF or UHF bands, and are transmitting only a few miles to a central receiver.

Generally they are probably only transmitters, so noise sources nearby aren't going to have any effect on the transmitter.  It is the central reciever that needs good filtering and selectivity.

For the sites at bridges, remember that they are over rivers, and the Geologic Survey puts automated guage stations to measure water height and flow.  Much more efficient than years ago when my grandad used to travel to each bridge to make measurements.

For water and sewage pump stations there may be a control link that transmits back to the pump station orders to turn pumps on and off.  Some electric utilities send low frequency low speed command signals over the grid, but obviously dedicated radio nets are probably more reliable and flexible.

Keep thinking and asking questions!  Curiosity and the desire for knowledge are traits that bring so many people into electronics.  Now, hurry up and get your ham ticket!  See you on the air!

73, bill
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N6AJR
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« Reply #6 on: November 04, 2006, 05:43:39 PM »

we use them here on the call boxes on the road way to hit the central office, and some are used for traffic monitering and some are used for weather.
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N7NBB
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« Reply #7 on: November 05, 2006, 06:36:02 AM »

I sent you an E-Mail to the address you have listed, that will hopefully give you the informatiion you need.

I was a little "involved" to post here.

73
Cam
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KY202
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Posts: 45




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« Reply #8 on: November 05, 2006, 06:02:54 PM »

Thanks for the replies.  I have a better understanding of them, along with a lot more questions.  Looks like I will have to do a little research and few more "road trips"...  I would like to find the receiving location for some of these transmitters.  But I do appreciate the quick replies.  I know that I can always come to you guys with questions, no matter how directly related to Amateur radio they may or may not be.

N7NBB, I got the e-mail & appreciate it very much.  


Thanks Again,
Jason
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