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Discussion Starter · #1 ·
You did not say how it worked without all the shielding? I don't think aluminum foil would hold up at all outside and the large wire cloth box may have a high wind load. Are you getting any new channels now with the shielding?


Glenn

Quote:
Originally posted by cbgenrich:


I can't really say how well it worked because I get plenty of signal and most of the time I get all the reception I could want.
 

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Quote:
Originally posted by Glenn_L:
You did not say how it worked without all the shielding? I don't think aluminum foil would hold up at all outside and the large wire cloth box may have a high wind load. Are you getting any new channels now with the shielding?


Glenn



When I said that I get plenty of signal and all the reception I need in good weather, I meant before and after.


When I said that I took the double bow-tie outside, I meant on my driveway (with my STB and a portable TV). I have no plan to put this outdoors, rather I'm looking to put something in the attic since I get plenty of signal and can afford to lose some.


I get all the channels in Atlanta routinely and have never tried to get out of town signals and doubt I would or could.


The problem I am trying to solve is this: I get plenty of signal all of the time, which I know because I can put 5 to 7 3db attenuators inline and still get locked on. But I feel I must be just above the threshold of a multipath problem, because weather (even just wind on a sunny day) causes some of my channels to become intermittent. I would like to improve the reliability of my reception. I can afford to give up 7/8ths or more of my signal in exchange for reduced multipath (I'll just take out 3 3db attenuators).


I started out with the one-piece foil shield trying to figure out which direction my multipath was coming from (never did).


I am hoping this will help and am curious what everyone thinks.


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-CB-


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Since I was the one who originally posted the question about a multipath shield, let me just say thanks for playing around with shielding. You've inspired me to do a little playing around myself.


Let me give a brief update on my situation. I'm about 8-10 miles from the towers in Cincinnati, but just on the backside of a hill, so I figured I would have multipath problems.


Rather than fiddle around myself, I got an antenna installer to come out over this past weekend (combination VHF/UHF, but don't have the make/model handy). Since it was raining, he tried the antenna up in the attic with a preamp, and I am now getting digital channels 29 and 31 in Cincinnati with no problem, and 35 with a few dropped frames here and there. Channel 10, however, has some pretty bad multipath problems, where I get a freeze frame picture at best around every 5 seconds. So, I figure I'm about 70% of the way there.


Before I have the installer come out and put the antenna up on the roof, I'll try to get up in the attic and play around with a shield, and see if that helps with VHF 10. I'll post my results when I get a chance.
 

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Discussion Starter · #4 ·
Some antennas use directors that are not electrically attached to the boom. So any metallic element could interfere with the signal if it is too close to the other elements. I know the bowtie does not use director elements, but it could still have the same effect. These problems can be minimized by placing the shielding at least one wavelength from the antenna. So your box would need to be larger to get the same blockage. On the other hand, the shielding is just a reflector and can simply catch multipath signals and reflect them back into the antenna. So, I guess the best type of shield might be one that is insulated (with some sort of RF aborbing material? Stealth Fighter?) on the inside, with reflecting material on the outside.


Glenn


[This message has been edited by Glenn_L (edited 06-04-2001).]
 

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A simple half window screen is the simplest to use as a shield and determine if multipathing is indeed present. Simply place it between the bowtie and the suspected direction of the multipath signal. (Large building/Hill etc, off to the side of the actual transmitter direction.) Keep the screen at least a couple feet away from the bowtie and take care that the placement is not actually enhancing multipath reception from another direction. Check ALL your digital OTA stations after placement of the screen to insure the fix on one channel hasn't caused problems with another.

FWIW I have actually ocassionally found a multipath signal to be better than the direct transmitter signal!

Good luck.


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Larry
 

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This weekend I took my Radio Shack double bow-tie outdoors to do some testing. I had read here talk about using a reflector or shield to affect multipath signals so I wrapped aluminum foil over a piece of cardboard and waved it around the antenna. I went to the next level by wrapping 5 sides of a cardboard box with aluminum foil and set the antenna way back in the back.


I ended up buying a roll of 1/4" hardware cloth and building a box 3' long and 16" on a side. Again I put the antenna way back in the back and pointed the tube towards the transmitters.


I can't really say how well it worked because I get plenty of signal and most of the time I get all the reception I could want. My problem is weather related, so I won't really know if this is more robust until I experience a variety of weather patterns.


I would like to hear some thoughts on the effect this should have.
  • 1/4" hardware cloth would totally block the signal?
  • would the aluminum foil work as well?
  • would a 3' depth be effective at blocking multipath?


I still get plenty of signal, determined by inserting a variable alternator.


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-CB-


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I don't understand how EM radiation travels, bends and reflects. The whole thing is difficult for me to believe, really, and I have a degree in EE (computers, not antennas).


If this were visible light, and the cage I built were made of mirrors, then I guess Glenn_L's note about the interior reflecting implies that it doesn't reduce the multipath effect at all. I think these waves bend around corners and pass through things in ways that visible light cannot, but I just don't have a feel for what that means.


I am very interested in the idea of lining the interior with a non-reflective material. But what would that be? Preferably something I can get at Home Depot and not very expensive. And it must be effective, else why bother?


I have calculated the wavelength of my stations 10, 19, 27 and 39. The longest UHF is 0.6 meters or about 2 ft. To make the box have that much room around the 16" square antenna would make it 5 ft on a side and it would need to be who knows how long, maybe 10 to 20 ft. Wouldn't fit in my attic, but maybe somebody's. But I don't understand the comments on "directors not electrically attached to the boom". I'm using the Radio Shack double bow-tie, and the bow-ties are held 1.5" away from what I believe to be a reflector by plastic mounting brackets.


What is the deal with the amount of spacing that these reflectors can have between the metal? I've used 1/4" hardware cloth, but the double bow-tie appears to have metal bars 2" apart acting as a reflector. And the V shaped portion of a yagi is acting as a reflector? For the full 14-69 UHF space, what is the maximum spacing that is effective? Is closer better? What is the corresponding visible light analogy?


Also, my channel 10 has a wave lenth of 1.54 meters. Doesn't this suggest that I should make the length of my rabbit ears telescoping VHF antenna be a certain size, either a full or a half wave length? Would that be the size of one or both telescopes?




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-CB-


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Discussion Starter · #8 ·
CB,


I don't know of a non-reflecting material, but the only reflection you need to worry about are those coming in from the front of the box, since the back and sides are shielded.


If this is the type of antenna scenario that works for you, then you are better off with a yagi. They will not pick up as many multipath signals as a bowtie. The acceptance angle for the signal is a lot smaller. A yagi would also be much smaller in size than a "bowtie in a box".


You mentioned ch.10, this is a VHF channel. I don't know about the RS bowtie, but the CM 4-bay is optimized for ch. 55-65, the farther you get from that, the worse the antenna will perform.


Glenn
 

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Quote:
Originally posted by cbgenrich:
When I said that I took the double bow-tie outside, I meant on my driveway (with my STB and a portable TV).
Hahaha! That must have been quite a site for the neighbors! More "evidence" that's we're a wacky bunch of geeks! "Why doesn't he just get cable like everyone else?"

http://www.avsforum.com/ubb/biggrin.gif http://www.avsforum.com/ubb/biggrin.gif http://www.avsforum.com/ubb/biggrin.gif



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Tom


Sony SAT-T60 DirecTc>ic>Vc>oc> w/2.0.1

Sony SAN-24MD 24" elliptical dish
 

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I think K1UO's idea of a window screen would work- to identify the presence of multipath. I also think screening makes sense if you decide on a permanent installation because the wind-loading is low, so it might actually stay up. As long as the holes are at least 1/4 inch or so or smaller, the material should be solid from an RF perspective (at UHF).

But really, the best way to eliminate multipath is to eliminate the side and back lobes. That can be done by using high gain antennas and stacking them in bays. I've seen hams use 4 X 4 bays at UHF to achieve really tight beamwidth and then bounce signals off the moon to communicate. Really long path!

John in VA
 

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Hey cbgenrich, If you are using the RS Double Bowtie, be wary of the 300 Ohm twinlead cable that comes attached to the antenna.


Based on a recommendation in another thread here, I trimmed all but a couple inches of of this cable off and attached one of the cylindrical 300->75 Ohm adapters. My reception is hugely improved after making this change. It seems that the twinlead is soaking up a LOT of extra signal & causing multipath-like problems.


Since making the update, I have removed my Silver Sensor antenna from Antenna B of my DTC-100, I can get all available channels with my Double Bowtie with no adjustments needed.
 
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