How important is it for a Freeview UHF antenna to be exactly aligned with the tower?

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geek4me

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Topic # 115342 22-Mar-2013 15:18

There are a few topics on antenna alignment on Geekzone but none say whether one should exactly align the UHF antenna with a line of site tower (which is 2 to 4 km away) to get the best picture signal strength. Will the picture always have the optimal signal if its bar points directly towards the tower or will the signal sometimes improve if it is offset maybe 10 or 20 degrees or more?

The reason I ask is a relatives aerial points exactly at the tower but the signal strength is low. The cabling is old and needs replacing but I was wondering if moving the aerial from exact line of sight could perhaps boost the signal strength before I get on the roof and move it and before I get the cabling replaced.

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Reply # 785574 22-Mar-2013 17:04

...none say whether one should exactly align the UHF antenna with a line of site tower (which is 2 to 4 km away) to get the best picture...

Your use of the term "best picture signal strength" is totally confusing. With digital, if you have adequate signal, picture quality entirely depends on your TV's video processing and h.264 decoder chips/firmware in the STB and/or TV.

Will the picture always have the optimal signal if its bar points directly towards the tower or will the signal sometimes improve if it is offset maybe 10 or 20 degrees or more?

I'm going to assume you're referring to signal strength here.
It is difficult to predict signal strength without a good quality signal meter. In urban locations there are tons of objects for the signal to bounce off and obstacles that block it. In general it is desirable to have line of sight to the transmitter and point the aerial directly at that, but in many circumstances (obstacles, long distances etc.) infill transmitters or reflections can give better results than going for the main transmitter.

The reason I ask is a relatives aerial points exactly at the tower but the signal strength is low. The cabling is old and needs replacing but I was wondering if moving the aerial from exact line of sight could perhaps boost the signal strength before I get on the roof and move it and before I get the cabling replaced.

Are you saying the signal strength is too low to receive channels properly?
If no, no amount of aerial movement will improve the picture quality. It is what it is once the channel is being received.
If yes, are you sure the aerial is a UHF aerial?
I'd suggest don't bother mucking round for too long with moving the aerial. Sure, give it a twist and check the aerial and cable condition. However if the aerial and/or cable are compromised then you may get signal on a sunny day but experience rain fade when the weather isn't as good. Also, there is no guarantee that the aerial is mounted in the optimal position on the roof.

turb

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Reply # 785575 22-Mar-2013 17:05

I can't think why that would work - where did the idea come from?

Make sure the aerial is the right way up - ie with the bars sticking out sideways (eg for Waiatura in Auckland) or up-and-down (eg for the sky tower).

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Reply # 785576 22-Mar-2013 17:14

At 2-4 km you're more likely to run into "too much signal" rather than "not enough". I can point 90 degrees away from the local transmitter (27 km) and it still works fine - admittedly that's analogue but I can't imagine that digital would be too different (the digital transmitter is 120 km away so alignment is much more important there!)

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Reply # 785588 22-Mar-2013 17:39

mm1352000:
...none say whether one should exactly align the UHF antenna with a line of site tower (which is 2 to 4 km away) to get the best picture...

Your use of the term "best picture signal strength" is totally confusing. With digital, if you have adequate signal, picture quality entirely depends on your TV's video processing and h.264 decoder chips/firmware in the STB and/or TV.

Adding to that, digital either works or it doesn't unlike analogue which was more forgiving.

My wee drama I had was the UHF aerial at home got affected by the heat pump being installed. It created enough interference with the substandard coax cable running past the heat pump for the signal to be degraded to the point at times where it would not come on at all.
I replaced the cabling with a good quality RG-6 coax and a direct cable run direct from the UHF aerial to the TV, thus bypassing the diplexor the VHF aerial was using about 5m away on another part of the roof.
So a 20m run of joined cabling (mixture of RG-56 cabling) became a 5m run RG-6 direct into the TV, it's ran perfectly ever since!
I have line of sight to transmitter here in Dunedin receiving via a small standard UHF aerial. I have it pointing directly at the transmitter which is roughly 15km away.

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Reply # 785618 22-Mar-2013 18:38

There's also the possibility that you have two signals arriving at the antenna out of phase with each other causing cancellation of the signal. This can occur if there is a direct reflection of the signal off a roof in front of the aerial. The best fix for this is to raise or lower the aerial by half a metre or so. This will usually sort it out. As above though, be sure your polarisation is correct and matches the transmitter site.

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geek4me

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Reply # 785637 22-Mar-2013 19:38

Thanks for these replies. Yes, the signal is too weak receive the channels properly. The TV has a scale of 1 to 10 for signal strength - it varies between 4, 5 or occasionally 6. None are strong enough to give a steady picture. I get a program guide for some Freeview channels only. Sorry if my descriptions were confusing.

By bar I mean the central bar with the elements positioned along the bar. It is pointing the same way as other nearby houses.

There is a clear line of site to the transmitter with no roof in the way.

I know the cabling needs replacing and from what mm1352000 has said that "no amount of aerial movement will improve the picture quality" it sounds like trying to get a temporary improvement by moving the aerial is not going to help. It will need a new cable.

mm1352000

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Reply # 785742 23-Mar-2013 00:21

geek4me: I know the cabling needs replacing and from what mm1352000 has said that "no amount of aerial movement will improve the picture quality" it sounds like trying to get a temporary improvement by moving the aerial is not going to help. It will need a new cable.

No amount of aerial moving will improve the picture quality. It might improve the signal quality.

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Reply # 785764 23-Mar-2013 07:36

Behodar: At 2-4 km you're more likely to run into "too much signal" rather than "not enough". I can point 90 degrees away from the local transmitter (27 km) and it still works fine - admittedly that's analogue but I can't imagine that digital would be too different (the digital transmitter is 120 km away so alignment is much more important there!)

I would have thought the same, but I can tell you from personal experience digital is different (at least they way it is transmitted in Christchurch). I am only 5km from the tower. Analogue UHF is very high at 60dBuV but digital is way down at 40-44dBuV when measured with a signal analyser.

geek4me

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Reply # 785775 23-Mar-2013 08:42

mm1352000:
No amount of aerial moving will improve the picture quality. It might improve the signal quality.

Thanks for this explanation on picture versus signal quality. The picture is either there or not there - there is no such thing as a poor quality digital picture. It is indeed the signal quality that is poor. No amount of tweaking the aerial position is going to improve that enough to bother.

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Reply # 785783 23-Mar-2013 09:00

If you are that close to the transmiiter, as someone said you will have too much signal. Too much signal can show exactly the same as not enough signal, the front end of the tuner overloads and the automatic Gain Control is swamped.
To be honest, the best way to fix this is to get an aerial technician who has a decent field strength meter who can put your aerial in the optimum location, put the correct ammount of attunuation in the line, run and terminate the cable properly.

I am a huge fan of DIY but sometimes you will save yourself a lot of grief by getting it done properly by a professional.

The key is that the have to have a field strength meter that can do DVB-T digital, if they dont have that you are spitting into the wind.

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Reply # 785799 23-Mar-2013 09:48

Out of interest have you tried checking the signal quality in the middle of the night?
I havent seen anyone get a perfect 100% signal strenght ever.... yad have to be plugged directly into transmitter IMHO !

How old is the UHF aerial? Have you checked the connection where the cable meets the aerial? (could it be rusted out)?

geek4me

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Reply # 785808 23-Mar-2013 10:26

I should get someone else to sort it out as it could be too much signal as said above. I think the UHF aerial was put up in 1989 when the house was built to get Prime TV. The connections look old and tired. I have not tried at night. The house will soon be sold so perhaps the new owner can sort it out or use the much newer Sky dish.

My original question was can the signal strength be improved by moving the aerial from line of site? Looks like it's not worth the bother trying.

Thanks for all your helpful replies!

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Reply # 785818 23-Mar-2013 11:15

Wouldn't have been 1989, maybe 1998??

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Reply # 785843 23-Mar-2013 12:13

Digital is weaker than analog because it can be and still work. 60dBuV isnt really that strong for analog, its prettymuch as low as you can go and still get a reasonably noise free picture.

Out of the antenna here I am getting in the mid 70's on analog, that is ages from the transmitter, no amplification and a relativly small antenna compared to some available. No idea on the digital levels since the vintage meter we were using was popping all over the place on the digital channels, but the TV's etc show full strength, and 99% or 100% quality.

It does still work when it has gone unamplified thru an 8 way splitter in the house, then another 8 way in the garage, and then a 2 way behind the tv. Strength is down to about 60% by that stage, quality still 100/99 almost all the time.

Richard rich.ms

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Reply # 785895 23-Mar-2013 14:42

geek4me:
My original question was can the signal strength be improved by moving the aerial from line of site? Looks like it's not worth the bother trying.

And the answer is - possibly. Depends on too many factors and needs to be trial and error. Usually beneficial only when not in line of sight and signal is low. You may be being overpowered with VHF analogue signal. If the antenna and cable is old they should be replaced. You may have an open circuit feeder? Have you tried an indoor antenna or even just plugging a short length of wire into the TV's antenna socket? I'm about 7km line of sight here in Auckland and can get a useable signal on a short piece of wire on the end of a short co-ax lead. Moving my antenna thru 360 degrees doesn't seem to make any difference to the signal.

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