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2 posts

Wannabe Geek

#42573 7-Oct-2009 11:44
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Hi, I've been trying for some time now to get a clean DVB-T signal but am still having issues with periodic breakup. There doesn't seem to be any pattern or regularity to the frequency of the breakup. It mainly affects the 538 Mhz transponder although is apparent occassionally on the 570 and 666.

My set-up is as follows:

Single medium gain UHF aerial with F type connector approx 12 months old running through three metres of RG6 duo-shield to a Kingray MHW34FS fully screened F type masthead amp running off a DC power supply. (I have no VHF aerial) The masthead amp is also about 12 months old.  From the masthead amp there is a 25 meter run of brand new Belden quad-shield RG6 to a four way fully screened F-type splitter. One of the splitter outputs is not used and is terminated with a 75ohm F type resistor. One output runs to my video recorder and loops out to the TV. All of this cable is new RG6 quad-shield too. The other two outputs from the splitter run to my Home Theatre PC which has dual DVB-T tuners in it (brand new). I use this for Freeview viewing and recording.  All connectors are crimp on F type other than the ones into the video and TV which are soldered on  Belling-Lee type. There are no tight bends or kinks in the coax.

I need the masthead amp to get a good clear picture on Prime analogue. Without it, the loss caused by the long cable run and splitter results in a noisy picture. The amp has variable gain and I have it on minimum (approx 19dB) On this setting I get excellent Prime analogue reception and no rain fade or signal loss on DVB-T. The amp also has VHF gain, this is on minimum with no connection to the VHF input. It also has a built in FM and pager -25dB band stop filter for 158Mhz which is enabled.
Some of the things I've tried:

·         Raising the aerial a further two meters above the roof (our house is reasonably low lying) It's now about 3 meters above the roof line

·         Varying masthead amp gain between min and max

·         Taking the video recorder out of circuit

·         Taking the splitter out of circuit

·         Taking the masthead amp out of circuit (signal then too weak for DVB-T lock)

·         Varying aerial direction (main direction set using compass and bearing to Waiatarua using Google maps)

·         Changed masthead amp power pack from AC unit to DC unit

·         Borrowed a DSE Freeview certified set-top box and got the same break up issues with this while my HTPC was turned off. Signal strength showed at around 50% on all 3 transponders. (I don’t have a signal strength indication on my HTPC)

·         Turned off everything electrical in the house (including hot water, WiFi, cordless phone, cell phone and fridge) other than TV and receiver

·         Tried switching on and off some appliances and lights to reproduce the interference. I couldn’t find anything that would reproduce it.

·         Independent earth rod and earth cable to earth tag of splitter

So far nothing has fixed my issue (or made it any worse) I contacted Radio Spectrum Management on the suspicion that I have some localised interference and they came out and took field strength measurements (but didn't really seem up to speed on Digital at all) Accoring to them my issue is that my signal is too weak to overcome backgroung interference. On digital I've got around 49 dBuv and Prime analogue is around 70. (I'm not sure why the analogue is so much better)

So I'm thinking my next step is a better aerial. I'm not sure though whether I should be looking at a 90+ element Yagi or a Hunter Phased array. I have no clear line of sight to Waiatarua from my roof.

Any help appreciated.


P.S. I've tried the Skytower too on V pol but not enough signal strength for a digital lock.

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7095 posts

Uber Geek


  #262023 7-Oct-2009 12:56
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Yep, if you've only got 50% strength roughly, you're going to need a better aerial to get a better base signal. The amplifier can't make up entirely for a weak signal, it will make your rough signal louder, but that amplifies any errors as well.

25 posts


  #262112 7-Oct-2009 16:04
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During the TVNZ digital trials , 98.9% of all problems were . 1: too small UHF Aerial with a thumping great mast head amp on it causing swamping of the receiver front end. 2: Rusty diplexers , and saddle and clamp splitters . Analogue days have gone one has to now understand that digital signals are easly corrupted . Home wired systems and add on connections with El Cheepo distribution amplifers were a killer of digital signals during the trials.

Normal analogue signals UHF require 65 to 75Dbuv + to provide fully snow free video/ audio pictures It has been proved that digital sets will lock up and work very happily thank you on 35Dbuv. whats 35Dbuv equate too an analogue picture which is flashing from snow to blue screen.

The problems are now simplified since Prime joined Free view all the networks are now on the same platform. So digital only enviroments can exist as a stand alone . If you have like most of us CRT tv's in other rooms that are working on a VHF / UHF distribution Network . Then the answer is simple leave that network run until the CRT TV's are gone and install a NEW UHF aerial directly to your New Free View enabled flat panel set . you've probably paid several grand for it whats another $500.00 or so to make sure it can provide you with the best quality reproduction it can .

Solves all the problems , other wise you are looking at in most cases rewiring the entire house with F connector type Splitters , perhaps even a UHF distrbution amplifer and perhaps even New Aerials and given the fact VHF is on its way out why bother.

Every problem referal from other installers , we've insisted that a stand alone aerial is connected directly to the freeview box or TV set and never had a problem yet.

There are few comprises with DTT reception and these TV's dont take kindly to being abused with sub standard signal and trying to balance analogue and digital reception normally leads to frustration and dissapointment.


481 posts

Ultimate Geek

  #262137 7-Oct-2009 16:36
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SATECH: Every problem referal from other installers , we've insisted that a stand alone aerial is connected directly to the freeview box or TV set

Wow - that's a backward step for them isn't it?

I realise that approach is cheaper and easier than actually solving their distribution issues, but over the longer term they will have to solve those problems if they want to connect TVs in more than one location in the house.

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  #262163 7-Oct-2009 17:06
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I agree , think of the alternative of telling some one that , their entire house and aerial system has to be upgraded when they might have just paid up to $10,000 on a new Tv, imagine having to break the news that now they also need to think about a new video recording device also .

It can take days and days to fully rewire an average home , Even if you could pull new cables down walls it takes time . Most people would rather wait and the stand alone diretly to the new TV can always be used by a UHF network later if and when new Tv's or set top boxes are added.

During the digital trials we found that saddle and clamp splitters caused problems , some homes we went to even had old resistive splitters in them.

When sky came along if the picture was'nt good enough , bang on a mast head with VHF by pass and the problem was solved . UHF DTT is not as forgiving.

Just stand in any store that sells DTT equipment and listen to the customers complaining and bleeting that equipment does'nt work or pixillates or Audio drops out or certain muxes break up when they were lead to believe it was Plug and Play.

959 posts

Ultimate Geek


  #262189 7-Oct-2009 17:47
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Before shelling out for any more hardware,why dont you get a professional with a
Spectrum analyser that does BER,
to put it in the optimum position on you roof!
Siganls can vary massively if you are shaded form the Tx

25 posts


  #262196 7-Oct-2009 18:09
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Cause proffessionals with $6000 to $10,000 NZ spectrum analysers cost , and most people think that its only an aerial how hard can it be " DIY mentality. I agree i tote around about $18,000 in test equipment between Satellite and terrestrial every day. .

The other problem is ,you try relocating a UHF aerial which is a few years old.The back reflectors on UHF aerials some only last a few years look around at all the missing ones around Auckland . One i pass every day has been like it for 18months,Yes you can climb up on the roof with a test unit but the chances are the customer will want to be convinced that his trusty aerial array which has given him no problems over the last ten to fifteen years is really to blame for his brand new pride and joy dropping pictures and or audio on more than one of the freeview muxes.

Simpler to adopt the rule i previously outlined , that is unless you dont mind wasting hours and hours only to be told "We'll think About It" " Its really not that bad"

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