Multipath Issues and possible solutions

[Cokeswigga]

I have an interesting problem.

I currently have a 9095 with an AP-4700 preamp

I think I am experiencing multi-path, but it's on one channel only.

I'm picking up the Los Angeles Stations (which are all on the same mountain about 70 miles away)

All the stations come in at 80+ on my Dish 811.

I have one exception. KABC-DT 7.1, 7.2, 7.3
It transmits on UHF 53.

This channel has about a 10% chance of tuning in. Once it does lock I get a 80+ signal on it with no pixelization or drops)

but most of the time it doesn't lock.

Now.... if I turn my antenna 60 degrees off axis, and point it towards a mountain, I can tune in this channel and get a low signal of 60-65 (which has pixelization and drops)

This ONLY happens with this channel.

Can multipath effect only one station

Next:.

What is the best way to fight this?

1.) Get the AP-4800 Preamp and hope the change in gain is enough for the receiver to ignore the multipath signal (or will it further confuse it)

2.) Try changing to an 8-bay antenna (and the ap-4800 preamp to compensate for loss in gain

3.) Change to the 91x antenna and hope for the best

4.) or should I get another 9095 and gang the antennas together (in which case which method do I use veritcal or horizontal stacking)

 

[Rick0725]

Try emailing this site for assistance...they did alot of testing in your area as you can see.

atechfabrication.com/reception_solutions.htm

and this one

bobmerritt.com/dtv/dtv.htm...shows his stacking project.

put the www, etc. in front of course...I was blocked from entering properly.

There is a link on the atech page for suggestions on reducing multipath also. From my experience, the 8 bay will be worse with multipath because it gets some signal from the back and is less directional than the yagi's. The 9x is similar ("copy") to the blake antennas that are mentioned at that site...as you will see. If you are interested in ganging antennas there is plenty of reading on the subject there.
Rick

 

[Tower Guy]

I have an interesting problem.

I currently have a 9095 with an AP-4700 preamp

I think I am experiencing multi-path, but it's on one channel only.

I'm picking up the Los Angeles Stations (which are all on the same mountain about 70 miles away)

All the stations come in at 80+ on my Dish 811.

I have one exception. KABC-DT 7.1, 7.2, 7.3
It transmits on UHF 53.

This channel has about a 10% chance of tuning in. Once it does lock I get a 80+ signal on it with no pixelization or drops)

but most of the time it doesn't lock.

Now.... if I turn my antenna 60 degrees off axis, and point it towards a mountain, I can tune in this channel and get a low signal of 60-65 (which has pixelization and drops)

This ONLY happens with this channel.

Can multipath effect only one station
)

There's another possibility, that your system has interference on channel 53, and aiming at the mountain reduces the interference. Note that there's an analog channel 53 in Fresno.

What is your exact location? That may help pinpoint a possible cause.

 

[Cokeswigga]

I think what happening is that I am getting two signals for channel 53.

One is directly from the transmission antenna.

the 2nd is a reflection of the transmission antenna off the mountain.

For example... direct to transmission is 283 degrees.
direct to the mountain is 223 degrees.

***My theory***
the problem is that the reflected signal is very weak and doesn't hold a lock.
the major problem is that the direct signal is being inteferred too much by the reflected signal, and it's confusing the receiver.

BUT this seams weird.. why would the stronger signal be more susceptible to the interferrence that the weak signal?


AND WHY DOES THIS ONLY EFFECT ONE CHANNEL?

Is the location of antenna in the highest point of positive interference of the signals?

 

[Cokeswigga]

Try emailing this site for assistance...they did alot of testing in your area as you can see.

atechfabrication.com/reception_solutions.htm

and this one

bobmerritt.com/dtv/dtv.htm...shows his stacking project.

put the www, etc. in front of course...I was blocked from entering properly.

There is a link on the atech page for suggestions on reducing multipath also. From my experience, the 8 bay will be worse with multipath because it gets some signal from the back and is less directional than the yagi's. The 9x is similar ("copy") to the blake antennas that are mentioned at that site...as you will see. If you are interested in ganging antennas there is plenty of reading on the subject there.
Rick

Thanks for the info, but I'm not yet ready to spend another $250 and another weekend working on my roof.

For $299 I can upgrade to the HD locals through Dish and get a DVR.

it's just annoying that only ONE Channel is effected by the issue when all the transmissions antennas are in the same location.

 

[Cokeswigga]

There's another possibility, that your system has interference on channel 53, and aiming at the mountain reduces the interference. Note that there's an analog channel 53 in Fresno.

What is your exact location? That may help pinpoint a possible cause.

Fresno is over 500 miles away!

and there is a huge mountain range in the way of fresno.

I live in Temecula 70 miles southeast of Los Angeles

 

[Tower Guy]

I think what happening is that I am getting two signals for channel 53. One is directly from the transmission antenna. the 2nd is a reflection of the transmission antenna off the mountain.

AND WHY DOES THIS ONLY EFFECT ONE CHANNEL?

I'd guess that a sidelobe of the antenna is aimed at the mountain on channel 53, but not on the other channels.

Changing preamps won't do anything.

If you decide to stack antennas, they should be stacked horizontally with a distance apart that nulls the multipath. That distance would be an odd multiple of .802 feet. If you select X 5, the horizontal distance would be 4.01'. This calculation is based on the 60 degree direction toward the mountain on channel 53 (707 mhz).

 

[Cokeswigga]

I'd guess that a sidelobe of the antenna is aimed at the mountain on channel 53, but not on the other channels.

Whats a side lobe?

I have a Winegard 9095

Changing preamps won't do anything.

If you decide to stack antennas, they should be stacked horizontally with a distance apart that nulls the multipath. That distance would be an odd multiple of .802 feet. If you select X 5, the horizontal distance would be 4.01'. This calculation is based on the 60 degree direction toward the mountain on channel 53 (707 mhz).

How did you come up with this calculation.


Thanks for you for your help

 

[Tower Guy]

Whats a side lobe?

How did you come up with this calculation.

Thanks for you for your help

A side lobe is a secondary peak in the radiation pattern of the antenna. Take a look at the following site to see the pattern of the Channel Master 4228. There is a side lobe on channel 60 at about 48 degrees.

http://www.hdtvprimer.com/ANTENNAS/16bay.html

The calculation of 4.01 feet is based on a horizontal distance where the signals from the mountain to the two antennas will result in an out of phase situation, nulling the multipath. This is a reference:

http://www.lindsayelec.com/antenna/commercial.catv/stacking_ant.html

You'll need a bit of trig to solve the distance calculation with the interference arriving from 60 degrees off axis.

 

[Cokeswigga]

A side lobe is a secondary peak in the radiation pattern of the antenna. Take a look at the following site to see the pattern of the Channel Master 4228. There is a side lobe on channel 60 at about 48 degrees.

http://www.hdtvprimer.com/ANTENNAS/16bay.html

That would make sense for that antenna, but I'm using a Winegard HD9095

Its pattern looks like this:
http://www.winegard.com/offair/pdf/hd9095p.pdf

it doesn't have the secondary peak.

Another thing thats weird to me, is that I have NO probelm tuning in the channel at 60 degrees off axis.. it's just really weak, why wouldn't the direct signal cause problems with the reception at 60 degress off axis

The calculation of 4.01 feet is based on a horizontal distance where the signals from the mountain to the two antennas will result in an out of phase situation, nulling the multipath. This is a reference:

http://www.lindsayelec.com/antenna/commercial.catv/stacking_ant.html

You'll need a bit of trig to solve the distance calculation with the interference arriving from 60 degrees off axis.

Awesome thank you. Trig I can do... I just wish Lindsays website was working
I've been trying to get on it all day.

I also found that they make parabolic UHF antennas. From what I have found, parabolics have very narrow beamwidths and have excellent gain. But the only ones I have found are several hundred pounds are are LARGE

Can you point me to anywhere in this direction, or are parabolics impracticle for home use?

I looks like most are mounted to towers, which I know my girlfriend will not let me install in our backyard

 

[Tower Guy]

That would make sense for that antenna, but I'm using a Winegard HD9095

Its pattern looks like this:
http://www.winegard.com/offair/pdf/hd9095p.pdf

it doesn't have the secondary peak.

Awesome thank you. Trig I can do... I just wish Lindsays website was working
I've been trying to get on it all day.

I also found that they make parabolic UHF antennas. From what I have found, parabolics have very narrow beamwidths and have excellent gain. But the only ones I have found are several hundred pounds are are LARGE

Can you point me to anywhere in this direction, or are parabolics impracticle for home use?

I looks like most are mounted to towers, which I know my girlfriend will not let me install in our backyard

Actually the 9095 does have secondary peaks. The graph scales are not the same on the Winegard site and the HDTVPrimer reference, so the lobes are harder to see. Note how the front to back ratio gets very poor on channel 69.

I have a nice tower but have decided to avoid a dish due to the size and windload. My tower can handle a dish, but my rotator can't. If you don't want to rotate it, it is possible to mount a dish on a roof tripod that is fastened securely to the roof joists.

I don't think that you need a dish, just a way to eliminate the reflections.

Can you mount your antenna in a location that is blocked to the ghosting mountain but not blocked to Mt. Wilson?

 

[Cokeswigga]

no unfortuntely everywhere at house has a great view.

 

[Cokeswigga]

Towerguy, thanks for all your help and info:

I was finally able to get onto lindsays site...

man what a great reference!

I wish I have several thousands of dollars to buy a few of their antennas..

 

[Cokeswigga]

**************
Update
**************

I just got a vip622, and all my OTA problems are gone.
The turner quickly tunes in ALL channels, and I don't have any issues!

Thank you DishNetwork for making a better OTA receiver.

 

[Jim5506]

I also had issues with one local DTV channel until I got my 622, now all lock in solidly.

I am, so far, impressed with this unit, and with VOOM HD, too.