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Discussion Starter · #1 ·
For all you RF engineers out there:


What is the proper spacing for a UHF two antenna system? Not an array, but two antennas pointing two different directions.


I'm northeast of Boston, and I've got one antenna (a "weatherized" Silver Sensor) pointed towards the Needham towers 20 miles away, the other (a R/S U-75R) roughly 90 degrees from that to pick up the PBS-DT station in New Hampshire, 50 miles away. They're coupled together with a ChannelMaster 0538 VHF/UHF/FM joiner.


With the Silver Sensor directly connected to my downlead, I get all the Boston DT stations at 98-100 on my Mits SR-HD5. With just the U-75R, I get WENH-DT (ch 57) at about 30-40. If I add a UHF pre-amp to the U-75R, signal gets up to about 60-65.


But, when I couple them together (with or without the pre-amp), ch.42 in Boston and ch.57 in NH drop out completely, and the other stations drop to about 70-80. Right now, I've got them on a short mast attached to my DBS antenna on the roof, so the two antennas are only about 8-10" apart vertically. Other than using a ch.57 Jointenna, what should the vertical spacing be between the two? I've got an extra 5' mast section I can use.



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Jon Gauthier

"If I'm not sailing I'd rather be watching sailing in HD!" - Me
 

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Can you simply losen one and move it up/down until you get good results. It should only be some fraction of a wavelength.
 

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I don't see how you can get this to work without a jointenna. Each antenna will be adding both multipath and sidelobe noise to the signal from the other.


The source for additional multipath interference I think is obvious. For example, the u-75r picks up reflections from ch 42 coming from the direction of New Hampshire, and adds this signal to the signal from the Silver Sensor.


You get sidelobe interference because no antenna is purely directional. It may be only 10db more sensitive in one direction than another. So for example, the u-75R will pick up the direct signal coming from ch42, attenuate it by 10 db, and mix it in with the signal from the Silver Sensor. This signal will most likely be out of phase with the one from the Silver Sensor and degrades the picture.


Why not use a Jointenna?


I'm not an RF engineer, and my last courses in antenna design were in college, so I may be mistaken.


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Alex
 

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Discussion Starter · #4 ·
Quote:
Originally posted by KenLand:
Can you simply losen one and move it up/down until you get good results. It should only be some fraction of a wavelength.
I'm using a mast from Winegard that attaches to the support pipe of a standard DBS dish. It's S-shaped, and the vertical section is only about 8-10" long. I thought, given UHF's short wavelengths, moving one of them that distance would effect some change, but I couldn't detect any. That's why I'm willing to try the 5' extension.


Also, I can't explain the ch.42 vs. ch.57 interference. 729.25MHz divided by 639.25MHz gives a 1.14079 ratio - is that any special harmonic? I mean, it seems they were cancelling each other out completely!




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Jon Gauthier

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Jon,


Coupling two good antennas together that are pointed 90 degrees apart, makes one very bad antenna.


First, as for the vertical spacing, keeping them 2 wavelengths apart should do the trick. At least 3.5 ft. should help.


Also, you need a frequency-selective coupler, so that at any frequenyc you're interesting in receiving, you're only getting signals from one of the antennas. A Channel Master Jointenna should work as long as the signal you're adding from the second antenna isn't too close to a desired channel on the first antenna (if it is, you'll need a very selective and very expensive filter from a supplier such as Microwave Filter Co.).


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Hi Jon

Quote:
Also, I can't explain the ch.42 vs. ch.57 interference
Quote:
Each antenna will be adding both multipath and sidelobe noise to the signal from the other.
Or more succinctly put
Quote:
Coupling two good antennas together that are pointed 90 degrees apart, makes one very bad antenna.
Do you feel differently? I'm curious what I'm missing.



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Alex
 
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