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plk
Premium
join:2002-04-20
united state

1 edit

TV Antenna- metal roof- multipath

I need to throw a TV antenna up on the shop. It has a metal roof 12/12 pitch. Pitch faces due west 270 degrees. My antenna needs to point 280 degrees.
I am in the country and once on the roof, I should have a clear LOS for many miles. I can see 5 miles from the ground.

I am at 990 ft above sea level and tv station is about 1000 ft above plus 1500ft tower.

Will I have a multipath problem from the roof? Can I adjust the distance off roof to cancel it?
I'm 50 miles from towers



Tzale
Proud Libertarian Conservative
Premium
join:2004-01-06
NYC Metro

said by plk:

I need to throw a TV antenna up on the shop. It has a metal roof 12/12 pitch. Pitch faces due west 270 degrees. My antenna needs to point 280 degrees.
I am in the country and once on the roof, I should have a clear LOS for many miles. I can see 5 miles from the ground.

I am at 990 ft above sea level and tv station is about 1000 ft above plus 1500ft tower.

Will I have a multipath problem from the roof? Can I adjust the distance off roof to cancel it?
I'm 50 miles from towers
You should be fine... Just keep the antenna about 4 feet off the metal roof. The higher, the better.

-Tzale
--
They who can give up essential liberty to obtain a little temporary safety, deserve neither liberty nor safety.


mr_slick

join:2003-05-22
Lynnwood, WA
reply to plk

and dont forget to ground it!! check in the home improvement forum if unsure how to do it



tschmidt
Premium,MVM
join:2000-11-12
Milford, NH
kudos:9
Reviews:
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4 edits

2 recommendations

reply to plk

As Tzale See Profile posted as long as antenna is not too close to roof it will not affect reception.

How far away are stations and what channels are they using? That determines type and size of antenna. If you have not already done so may want to check out TVfool to model expected signal strength. I've found it to be more useful then CEA's Antennaweb site. Current channel assignment may be different then final Post June 12 assignment. Some stations are moving from temporary DTV channel to permanent once analog goes away. In your planning make sure you take this into consideration.

Be sure to ground antenna properly and bond it to metal roof.

I've documented my experience setting up a outdoor antenna in fringe area of rural NH if you are interested in the gory details.
»www.tschmidt.com/writings/Design···enna.htm

/tom

fixed typos



runnoft
Premium
join:2003-10-14
Nags Head, NC
kudos:1

3 edits
reply to plk

In searching the web on this question, it surprised me to learn that multipath interference, which as most of us know, can occur when signal bounces off structures, mountains, etc., causing the same signal to reach a receiver at slightly different intervals and cause ghosting issues with analog OTA TV, can indeed also occur with digital OTA TV, where it can in effect be worse because it can confuse the receiver to the point that it cannot tune the channel at all. (At least with multipathed analog, you see a ghosted picture, which is better than no picture.) Obviously, weak signal can cause No display on digital OTA TV, but so can multipath interference.

I'm sure Tzale and tschmidt are right that you'll avoid multipath from your metal roof if your antenna's high enough, but I was surprised that digital TV is bothered by it at all. Highly directional, properly aimed UHF antennas and high-quality tuners are considered the fix for digital multipath victims.



tschmidt
Premium,MVM
join:2000-11-12
Milford, NH
kudos:9
Reviews:
·G4 Communications
·Fairpoint Commun..
·Hollis Hosting

said by runnoft:

I was surprised that digital TV is bothered by it at all.
It was a huge problem with DTV because it prevent proper recovery of encoded data. Modem DTV RF tuners employ a number of methods to reduce the effects of multipath.

/tom

mocycler
Premium
join:2001-01-22
kudos:1

1 edit

3 recommendations

reply to plk

Click for full size
TV antennas are usually a log periodic design and have have a radiation width of 20-40 degrees, depending on how many elements. The more elements, the more gain, but less tolerance for being off beam.

In other words, think of your antenna as a directional "funnel" for signals. The radiation width would be how wide the mouth of the funnel is. You do not need to aim the antenna straight on. If you are only 10 degrees off (270 vs. the desired 280), you should be fine. As you can see in the diagram, the "angle" is not really an angle, but rather, a lobe. The diagram is from a foreign website so they spelled "lobe" as "lob".

Multipath is caused by large aircraft, mountains, and tall buildings. Your metal roof is too small and too close to be a multipath issue, but it can still screw up the performance of your antenna so make sure you have it at least a few feet above the peak of the roof. If you do get multipath, sometimes moving the beam just an inch or two off center will fix it.

At 50 miles out you will need at least 15-20 dB gain. By the way, if you live within 15-20 miles of the transmitter you could pretty much point the antenna any way you want and still get a useable signal.

mocycler


Radio Active
My pappy's a pistol
Premium
join:2003-01-31
Fullerton, CA

Bravo, Mocycler!

Great explanation.
--
I don't sleep... I wait.