Introduction: HDTV Multipath Shield

Picture of HDTV Multipath Shield

In the "Good Old Days" of Television, if you lived in an urban environment where there were a lot of tall buildings or things like water towers, you had to deal with a problem known as "Ghosting". The technical term for this is "Multipath". The signal from the TV station bounced off of something and got to your antenna just a fraction of a second later than the direct signal. What you ended up with on your TV was one signal overlapped over another. The TV didn't know the difference between the two signals, so It just displayed both.

Step 1: ATSC TV

Picture of ATSC TV

Ever since American TV went to digital you got one of two results. Either the error correction in the digital signal eliminated the Multipath signals, or YOU GOT NOTHING AT ALL! The above picture shows an off-air digital TV signal. See the little spike to the left? That's the ATSC pilot carrier. It's kind of a "Hi there!" greeting to all of the televisions in the area. Without this signal the TV just ignores the channel completely.

Step 2: Downtown TV

Picture of Downtown TV

That narrow gap between the two tall buildings just so happened to be where the TV station I wanted to pick up was located. The Multipath was so bad I couldn't receive anything on that channel at all.

Step 3: Roller Coaster

Picture of Roller Coaster

Remember that nice flat topped ATSC signal earlier? Well, this is what I was receiving thanks to the Multipath of the signal bouncing back and forth between the two buildings.

Step 4: The Cage

Picture of The Cage

I've been a "Cable Guy" for over 35 years, so I know how TV signals behave (or at least how they're supposed to behave). I used an antenna that would receive the frequency of the TV station I wanted and built a wooden frame around it. I covered the frame with steel screen (like screen door screen) and left one end open that I could point toward the TV station. I suspended the antenna inside the cage with plastic zip ties so that it wouldn't touch the screen. I attached a grounding block to the screen and some #6 copper wire to the ground block. This is the key. If you are familiar with electronics you might recognize this as a modified Faraday Cage. The signals that strike the screen (aka Multipath) are shorted out to ground.

Step 5: Before and After

Picture of Before and After

As you can see from the two pictures, the results are like night and day. The only problem was that I knew the wood frame and zip ties wouldn't stand the test of time. I needed a more permanent solution.

Step 6: The Trash on TV These Days...

Picture of The Trash on TV These Days...

Yes, it's a garbage can. You can see my old Faraday cage in the picture too. I placed a UHF Bowtie antenna inside a steel garbage can and mounted it to a pole. A coax barrel connector goes between the inside and the outside of the garbage can and supplies the grounding effect for the unwanted Multipath. Usually the first thing people say when they see this is "Oh, it's like the Pringles can for Wi-Fi." No, it's not. The Pringles can is a signal AMPLIFIER. The garbage can is a signal SHIELD. It repels the unwanted Multipath.

Step 7: Even Better

Picture of Even Better

The signal strength from the garbage can antenna was actually stronger than from the modified Faraday cage. One note though: This will only work with TV stations that are transmitting on a UHF frequency. Most are since the digital changeover, but there are still a few on VHF. Also, this will only work for one TV station at a time.

Comments

BrettHacks (author)2015-06-28

My neighbors thought I was crazy before, when I mount the garbage can to the antenna mast, they will know it for sure!

RF is so weird. I would think the can would work well for reflections coming from the side, but would have very little affect on multi path distortion located in the front facing beam path. Thanks for a cool idea.

bnaivar (author)BrettHacks2015-06-29

Actually it does. The multipath that enters the can is attenuated enough that the primary signal is easily distinguished by the receiver.

BTW: If you're going to put it on an antenna mast, remember that the garbage can handles can be used to attach guide wires to. ;)

BrettHacks (author)bnaivar2015-06-29

Oh no, I would have to but it on a rotator and have it cycle back and forth like a redneck radar dish when it was not in use.

bnaivar (author)BrettHacks2015-06-30

:)

BeachsideHank (author)2015-06-28

This is a novel idea, so the trash can kinda acts like a... ahem..., a condom?

bnaivar (author)BeachsideHank2015-06-28

Get your mind out of the trash. :)

DIY Hacks and How Tos (author)2015-06-28

Cool project. Thanks for sharing.

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