Co channel interference?

[tntfr]

I'm getting a really poor signal from Rowridge on the BBCA mux. I'm using an omni-directional amplified aerial, it's an SLX Digidome. I realise a lot of people think these are carp, but I'm getting good results on all other MUXes with the exception of com7, which I can't see at all.

I am wondering if I could be suffering from co-channel interference from another TX, possibly Mendip or Midhurst. I realise that these other 2 are using CH59 and 60, which shouldn't conflict with anything on Rowridge but I was wondering if this type of interference is still possible.

What I do find confusing is that I'm able to receive the 50Kw Arqiva MUXes with no trouble, so the 200Kw BBCA mux should be no problem, right?

Any ideas would be gratefully received.

Many thanks.

PS: I have a Televes 4G filter inline, so I have ruled out 4G interference.

 

[Analoguesat]

You may well be saturating & overloading the tuner with the 200kw mux!

 

[tntfr]

Hi AS. Thanks for coming back so quick. Would you be able to explain this a little? I'm even more of a DTT than a Satellite TV novice!!!

 

[Analoguesat]

Overloading the tuner can cause it to become "deaf". At the very least - you can get all sorts of odd effects as well.

Bypass / remove the amp and see if the BBC signal appears.

There is an over amplified monstrosity of an aerial on top of my workplace. Its pointing at the Galashiels Langlee transmitter (although its set H instead of V) Not only does it pick up Langlee, (half a mile away) but its so over amplified it also gets all the muxes from Selkirk 90 degrees off beam and behind a hill!

 

[tntfr]

Unfortunately, removing the amp isn't an option. The Digidome amp is integrated and doesn't seem to work at all without the power. I've attached a picture.

I am wondering if my options are therefore limited to playing with the location and orientation of the thing?

 

[tntfr]

I forgot to mention that results for COM5 channels aren't great either, and both BBCA and COM5 seem to be worse in the evening.

Detailed results attached.

 

[Terryl]

It may not be co-channel but something call "Inter-mod" this is where two (or more) strong frequency's combine in an amp or second stage mixer in a tuner and create a third channel.

This third channel can be right on top or very close to the channels you want to receive.

Take the two strongest channels frequency's an add them together, then divide the sum by two, this would give you the inter-mod frequency. (or frequency's)

You may have to do this several times to find the right combinations.

 

[Archived-1]

I am wondering if I could be suffering from co-channel interference from another TX

Looking at the below and checking your location from your IP I would say you could be correct on this.

http://www.ukfree.tv/transmitters/tv/Rowridge

 

[Lazarus]

Don't forget French DTT Transmitters might also be in play.

 

[Analoguesat]

It may not be co-channel but something call "Inter-mod" this is where two (or more) strong frequency's combine in an amp or second stage mixer in a tuner and create a third channel.

This third channel can be right on top or very close to the channels you want to receive.

Take the two strongest channels frequency's an add them together, then divide the sum by two, this would give you the inter-mod frequency. (or frequency's)

You may have to do this several times to find the right combinations.

Bingo - Mux B & Mux 2


474+522 divide by 2 = 498 (Mux 1)

 

[satelliteman]

Intermodulation distortion, third order harmonic on said freq.
It's a shame the Digidome doesn't work in passive mode. Not a great lover of these. Looks good but not directive enough for every day rigs.

 

[Terryl]

Solution would be to use a directional antenna, one that can put the offending channel(s) off to one side or at it's back side.

Or an old trick is aluminum foil on one side of the antenna.

But if the amp built inside the antenna is at fault then the only fix is a new antenna.

 

[tntfr]

Not normally a problem with digital, more likely a problem with the antenna. How far are you from Rowridge and what direction is it in relation to your existing antenna? Have you tried with a simple antenna in the loft?

Hi,

Many thanks to all for the help and advice. I've always found this forum to be a pretty awesome source of learning.

To answers Rolfw's question, I am 39k from Rowridge according to the digitaluk site. I haven't tried an antenna in the loft yet, I don't own one and was hoping to solve this problem without any additional spend.

The digidome has 2 directional arrows on it (one on apposite sides), one of which is supposed to be pointed towards the desired TX (Rowridge in my case). I've used a compass and looked at neighbouring aerials to do this as well as I can.

Interestingly, I would say there is a 50/50 split in my street to those with aerials pointing at Rowridge and those pointing at Midhurst. I'm guessing Rowridge might have been a bit of a stretch pre-DSO before the power was boosted. (it's a bit hilly around here so true LoS isn't really achievable)

From what I've read, it seems like my options with this piece of kit are quite limited. I may try moving it to other locations and perhaps experiment with the "tinfoil" idea to deflect unwanted signals away. I'm guessing this will do nothing for the intermodulation problem though?

Thanks again. All other thoughts and suggestions are most welcome and appreciated.

 

[tntfr]

Yeah. Could be worth a try I guess. According to digitaluk, Midhurst is just 23km away. Power levels are a lot lower though: 20kW instead of 200kW on the PSB MUXes and 10kW instead of 50kW on the Arq MUXes.

 

[tntfr]

Well the plot thickens....

The weather outside is foul tonight here: windy and plenty of rain. I did a quick test to see what affect this would have on my already crappy BBCA/COM5 signals. Bizarrely, it is working perfectly for the first time since I put the Digidome up. I had a look at the signal quality being reported by the TV and it had increased and isn't fluctuating like before.

Could the rain be attenuating whatever was interfering with my TV signal?

 

[2cvbloke]

Rain tends to attenuate most signals, so, yes it could be weakening the signal enough to the point where that toy aerial isn't being overloaded with signal...

Time for a Log Periodic aerial...

 

[PaulR]

If you have to keep the Digidome then you could try the trick that Terry suggested above to put large pieces of aluminium foil in strategic places to reduce the signal strength.

But really and truly a smaller, more directional aerial is what is needed.

 

[2cvbloke]

But really and truly a smaller, more directional aerial is what is needed.

Given my proximity to Pontop Pike (about 2 miles LOS), mine's a tiny aerial, but trimmed to the right frequencies and is very stable...

 

[tntfr]

Would a bandpass filter help any? I've seen this on ebay:

http://www.ebay.co.uk/itm/Triax-470...Signal_Amplifiers_Filters&hash=item35e04265a9

(Rowridge is obviously group A)

If not, looks like I'm making a tin foil hat at the weekend

PS: If I got one of these, it would presumably negate the need for my current 4G filter as this looks like its supposed to apply <30db attenuation to everything outside of Band A?

 

[2cvbloke]

The issue is not with filtering, it's with amplifying what doesn't need to be amplified, you need a passive aerial, go passive first, then filter out the other stuff...