Power supply to TV aerial amplifier

[David Schweizer]

Based on the recommendation of a Forum member who fits TV aerials, I replaced the old useless TV aerial on our boat with a Pro Logic Aerial with masthead amplifier. The digital signal is often quite weak on the boat and I had hoped that the amplifier would improve reception, but although the power pack is described as a 12v DC power supply, it requires an 240v AC input, which is converted into 12v DC.

 

I could run it through the inverter but it seems a waste to convert from 12v DC to 240v AC and back again just for a few milliamps to power the aerial amplifier, so I choose to run it with the signal not being passed through the amplifier. I have wondered whether a direct 12v DC power supply is available, and if so where I might get one. Alternatively, is it worth me dismantling the existing power supply, removing the transformer and rectifier and wiring it for 12v DC?

 

One of the other problems seems to be that the signal does not like being put through several connectors, and whilst the TV will often work with the aerial lead connected directly between the Aerial and the TV (via an opened window!), it will not work when pushed through two more socket boxes, one of them is brand new, and I intend to replace the other one soon, but would too many connectors impede the signal?

 

Any advice from any experts would be greatly appreciated.

[bottle]

First no expert but that has never stopped me.

 

If the powerpack does truly give 12v dc it maybe ac (look at the label) then it is possible that the amplifier could not cope with the boats supply of nearer 13v dc.

 

Too many connections will indeed affect the signal.

Just one other thing, the amplifier, can only amplify not improve a signal, basically crap in, crap out.

[David Schweizer]

First no expert but that has never stopped me.

 

If the powerpack does truly give 12v dc it maybe ac (look at the label) then it is possible that the amplifier could not cope with the boats supply of nearer 13v dc.

 

Too many connections will indeed affect the signal.

Just one other thing, the amplifier, can only amplify not improve a signal, basically crap in, crap out.

I have heard that repeated numerous times but do not really understand it, if an amplifiwer will only amplify a poor quality signal what is the point of it in the first place? with the old analogue system you got anything from a perfect picture to fuzzy haze, but with digital you either get picture or do not get a picture, and very little in between

 

Is an amplifier useful for my type of setup, where the signal directly from the aerial to the TV is fine, but putting it through several connectors causes the TV to display a "No or poor signal" screen. Will the amplifier increase the strength of the signal so that it will pass through the several connectors and a produce a picture?

 

I understand what you are saying about the boat's power supply, but by the time it has travelled up a thin coaxial cable the 12.4v registering at the TV will struggle to be 12v by the time it gets to the amplifier on the masthead. As a matter of interest, I have tested quite a few "12v" power packs and without exception they have all produced more than 12volts DC, including the one supplied with the boat TV.

Edited July 13, 2012 by David Schweizer

[bottle]

Taking the crap in crap out.

 

If the signal is weak, ie you are a long way from the transmitter and there is no radio interference, then an amplifier will work

 

but

 

if the signal is weak and there is radio interference then the amplifier will amplify the interference as well, it cannot differentiate between the tv signal and the interference.

 

If you are getting a good TV picture with a direct connection, you do not need the amplifier.

 

The amplifier would help if you had a long run from the aerial to the TV

 

Now this bit I am not sure of but

 

if the signal goes through the amplifier but the amplifier is not powered (and it needs to be) then that is like having a bad signal.

[Guest]

i bought a cheapo amplifier from asda with a mains supply. On dismantling it turned out to be a 12V transformer inside. A few minutes with soldering iron and it is now a 12V unit. Can't say it will last forever, but it was only a few quid.

[Nickhlx]

I have heard that repeated numerous times but do not really understand it, if an amplifiwer will only amplify a poor quality signal what is the point of it in the first place? with the old analogue system you got anything from a perfect picture to fuzzy haze, but with digital you either get picture or do not get a picture, and very little in between

 

Is an amplifier useful for my type of setup, where the signal directly from the aerial to the TV is fine, but putting it through several connectors causes the TV to display a "No or poor signal" screen. Will the amplifier increase the strength of the signal so that it will pass through the several connectors and a produce a picture?

 

I understand what you are saying about the boat's power supply, but by the time it has travelled up a thin coaxial cable the 12.4v registering at the TV will struggle to be 12v by the time it gets to the amplifier on the masthead. As a matter of interest, I have tested quite a few "12v" power packs and without exception they have all produced more than 12volts DC, including the one supplied with the boat TV.

 

The power supply module will be superimposing the 12 volts onto the coax centre core and the surrounding braid.

 

As the current flowing is in the order of a few milliamps, there will be negligable volt drop, even though some co-ax cores are small.

 

I would be surprised if the voltage is critical as some runs are very long and some quite short, so there would need to be some headroom and some tolerance at the far end of these slightly differing voltages anyway... If in doubt arrange for a regulated 12 volts from a small DC-DC inverter (Ebay)

 

The point about having an amp and at the mast head (and before a long / lossy cable run down to the set) is that the signal level will be possibly too much "down in the noise" after it has suffered possibly a further 10 to 20dB loss, just in getting to the set in the first place...

 

You will need to inject a fused ( e.g. 1 Amp) 12 volt supply at the right point in the "power supply module's" circuit - if you just stick 12 volts on the coax you will slug the signal and possibly (probably) do bad things to the circuit at either or both ends.

 

The module is not just a power supply - it will at least have blocking inductors to keep the wanted signal out of the power supply part of the module..

 

You must ensure that if you decide to modify the module, that a boat fault doesn't try and use your negative supply as the boat system earth. This means fusing both the negative and positive of the 12 volt supply with as low a value fuse as will not blow in normal operating conditions ( probably less than 1 amp).

 

Depending on what inverter you have, the easy way is to just use the mains, and use the module as supplied... and accept the small extra drain ( 1 amp or less ?)

 

Nick

[jonathanA]

If you do the signal to noise ratio (SNR) calculations then an amplifier at the 'masthead' then a downlead is significantly better than the same amplifier at the end of the downlead, thats why an amplifier can make a difference.

 

Any connectors you have will affect the signal strength depending on the conenctor type and how well its connected to the cable. Use the best conenctors you can make sure all the connections are properly made, preferably soldered or crimp. these days there seems to be a suggestion that digital Tv needs a better grade of co-ax - i don't know if thats true or just hype but the better the cable the better the signals going to be, but on a boat the lengths should all be short anyway (compared to going up to a roof aerial)

 

"You must ensure that if you decide to modify the module, that a boat fault doesn't try and use your negative supply as the boat system earth. This means fusing both the negative and positive of the 12 volt supply with as low a value fuse as will not blow in normal operating conditions ( probably less than 1 amp)."

 

I really can't see the sense in this unless you've connected the screen of the coax to the hull somewhere. Although it can't do any harm its a bit OTT IMHO - a small fuse in the 12v supply makes sense for sure. I think this used to be more of an issue with 'car aerial' used with car radios on boats.

 

I thought I'd seen a design for a dc power injector somewhere that was simple to make, but can't find it at the moment.

[Nickhlx]

"You must ensure that if you decide to modify the module, that a boat fault doesn't try and use your negative supply as the boat system earth. This means fusing both the negative and positive of the 12 volt supply with as low a value fuse as will not blow in normal operating conditions ( probably less than 1 amp)."

 

I really can't see the sense in this unless you've connected the screen of the coax to the hull somewhere. Although it can't do any harm its a bit OTT IMHO - a small fuse in the 12v supply makes sense for sure. I think this used to be more of an issue with 'car aerial' used with car radios on boats.

 

 

 

I guess it depends on your attitude to risk - as advice is being given to people who may not appreciate certain risks, it would be better to err on the side of caution and suggest fail-safe potential remedies - it is then up to the user to decide if a fuse is "needed" for his particular installation... His aerial may or may not be DC grounded at the fixing point..

 

Personally, I like to design out all possible potential problems - there are enough on a boat without adding more ....

 

Nick

Edited July 13, 2012 by Nickhlx

[David Schweizer]

Thanks for all the advice, I must admit that some of the technical detail is a bit outside my understanding, but I understand enough to realise that there is more to my suggestions than I initially thought. The additional connections and fusing suggested is really more than I wanted to do, quite apart from the untidy mass of cable I will be "required" to hide, so I am going to think again.

 

I have already replaced the cable from the aerial to the external socket with new top quality coaxial cable, and am proposing to replace the internal cable as soon as I get round to removing the cabin lining!! I have replaced the internal coaxial socket and have purchased a new one to replace the present external socket, but there will still be more connectors than I would wish, not helped by the fact that I have had to incorporate an F socket to F socket converter at the aerial end because, amongst the hundreds of alternative socket & plugs available, no one seems to sell an F socket to cable connector.

 

Reading through all the advice, I have realized that, somewhat stupidly, I have not actually tested the usefulness of the masthead amplifier using the inverter, so that will be my next job. If it makes no difference I have been wasting everyone's time, and will have to try and find a way in which I can reduce the nunber of connectors etc. Alternatively perhaps I should wait for Lidl/Aldi to have their Satelite kits on offer!!

[MikeV]

....these days there seems to be a suggestion that digital Tv needs a better grade of co-ax - i don't know if thats true or just hype.....

True! I replaced a 1 metre length of coax with moulded connectors (from the TV to the wall socket) with a 0.5 metre length made up using good quality co-ax - the type with both a braid and foil sheath.

 

At the location where we were at the time the TV was tuning in the ITV multiplex but not the BBC one. After replacing the cable it tuned in the BBC signal as well with a solid picture. I then went back to the old cable to see what the picture was like and it kept breaking up. Since replacing the cable we're having a much higher success of getting watchable TV. For reference the aerial is an Maxview Omnimax (not directional) mounted directly on the roof (no mast).

[jonathanA]

True! I replaced a 1 metre length of coax with moulded connectors (from the TV to the wall socket) with a 0.5 metre length made up using good quality co-ax - the type with both a braid and foil sheath.

 

At the location where we were at the time the TV was tuning in the ITV multiplex but not the BBC one. After replacing the cable it tuned in the BBC signal as well with a solid picture. I then went back to the old cable to see what the picture was like and it kept breaking up. Since replacing the cable we're having a much higher success of getting watchable TV. For reference the aerial is an Maxview Omnimax (not directional) mounted directly on the roof (no mast).

 

thanks mike,

 

I recently bought a new coil of co-ax and as it happens 'digital' cable was cheaper than the old style brown coax, it looks better engineered (foil plus better braid etc) but I've not actually compared the technical specs. real life experience is what counts though !

 

 

but there will still be more connectors than I would wish, not helped by the fact that I have had to incorporate an F socket to F socket converter at the aerial end because, amongst the hundreds of alternative socket & plugs available, no one seems to sell an F socket to cable connector.

 

not sure I've seen one either what I've used for connecting my sat dish to my external socket is a back to back F connector. The one I've used is designed for mounting in equipment so has a flange+nut+washer. use the usual f type connector on the lead to the inside and then its ready to take the lead from the sat dish/aerial. it cost a few pence from CPC but no doubt you can get them from other places too

 

http://cpc.farnell.com/jsp/search/productdetail.jsp?SKU=CN0701305