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I am at a location about 8 miles from all of my local towers (in Cincinnati), but on the backside of a hill. So, while signal strength doesn't appear to be a problem, multipath most definitely is. I was hoping to install an antenna in my attic, rather than my roof, and it started me wondering whether it is possible to shield an antenna from multipath (reflected) signals. All of my local broadcasts eminate from roughly the same direction, so I could probably leave a directional antenna at one azimuth and still get all of the signals.


My question is: Is it possible to install some sort of shielding around an antenna to reduce its reception of reflected signals, without affecting reception of the primary signals? And if so, what type of shielding material would work best? Since this would be an attic install, I will probably have alot of leeway in terms of locating the shields around the antenna (aesthetics, wind, etc. would not be a concern)
 

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The best directionality I have been able to find is two 4248 Channel Masters side by side.


I started with a Parascope and added mesh. This made the F/B ratio great. Side lobes were still bad, and unfortunately my multipath was mostly from the sides.


Next I added mesh to extend the sides to the Parascope. This made my pick up angle at the front go from say 5 degrees to like 1 degrees. Side lobes at 30-60 degrees off axis, got narrower but were still disasterous in my situation.


That is why I had to switch to the dual yagi arrangement.


If your reflections come from one direction to the side of the main signal, your best bet, instead of shielding may be to set up two antennas, and use a phase canceller box. (check this out at Microwave Filter Co.)


If you try to use shielding, I think the main problem is that it can not only block signals, it can pick them up only to be re-radiated back at the antenna, just like antenna elements do. I think shielding has the best chance of working if the multipath is from the rear.


Also note what is strongest. I get one channel by receiving the reflection on rainy days when the ground is wet. In these condidions the reflection is stronger than the main signal. So this is another shielding problem, unless it can move with the antenna!


Good luck, and don't give up. I experimented a long time and went through lots of antennas, before I got something that worked.

 

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jhe has the right idea. Because your source is relatively in the same plane as is you antenna it is not easy to shield as are parabolics used for satellite communications (where the reception plane is generally acute to that of reflected sources). There was a thread here about 3-4 weeks ago that had a link to an excellent page covering dual and quad antennae clusters. I highly recommend locating that link (someone reading this may have it bookmarked).


Good luck!



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Most of us don't know which direction our multi-path problem is coming from http://www.avsforum.com/ubb/smile.gif What do you use to know all that?

Microwave Filter Co


The third diagram describes ghosting and the solution proposed.


Would a static setup (no adjustments needed for each channel) be likely to work? Does multipath tend to come from the same reflection source regardless of frequency? How can one figure out where the source of multipath reflection is, such that the phase canceling approach could be employed?



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Quote:
Originally posted by cbgenrich:
Most of us don't know which direction our multi-path problem is coming from http://www.avsforum.com/ubb/smile.gif What do you use to know all that?

....................

Would a static setup (no adjustments needed for each channel) be likely to work?

Regarding where the multipath is I use as directional an antenna as possible. (I have played with turning my big outdoor rig as well as just waving my Silver Sensor indoor unit around.) I then look for peaks on a signal meter. (I got the Sencore digital readout meter) Finally I compare the peak directions to a topo map of my area and look for the hills that could reflect. The maps and the signal meter correlate pretty well. The maps are necessary since even very good antennas have some side lobes.


As to a static set up, it could work in some situations, but there are multipath variations with frequency, so it would take luck. I need a phase canceller for only one very tricky channel that is super weak, behind a hill, and reflects off other hills. Even here I expect I may need some tweaks with weather changes. I will put the box beside my rotor for easy adjusting, once I run a new line and get the cable lengths matched.
 

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Quote:
Originally posted by jhe:
Regarding where the multipath is I use as directional an antenna as possible. (I have played with turning my big outdoor rig as well as just waving my Silver Sensor indoor unit around.) I then look for peaks on a signal meter. (I got the Sencore digital readout meter).
I have no such signal meter http://www.avsforum.com/ubb/frown.gif and have observed that the signal strength indication on my Samsung SIR-T150 STB actually displays the "headroom" of my signal. If it can't get a picture out of the signal it goes to zero. If it can it shows how much more signal it has than what it needs. So it does help some with determining the direction of the main signal, but is no help in analyzing the source of the multipath interference.



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cbgenrich,


If you can't get a hold of a signal meter you're limited to what you can see from maps, your eyes (buildings that could reflect), and your tv.


(I went for years thinking meters were too costly, and trying to make do by experimenting with aim and attenuators. I consider a meter well worth it for people in difficult areas, and am glad I finally got one.)


If you have analog stations near your desired digital ones in frequency, it may help you to just watch the ghosting as you rotate your antenna, to help locate your multipath.


In my location I have big problems with ch 42 digital, but the ghosting problems with ch 56 analog are very similar in scope and in direction.


 
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