TV aerial

[AlanH]

I've seen several boats with an Omnimax aerial mounted on a mast. That's a good idea I thought so I bought a mast, made and fitted some brackets to hold it and mounted the aerial on it so that the aerial head is about 2.5m above the roof.

On experimenting, I found that the thing works better when it is lying on the roof than it does when erected. Why is that then?

[chris w]

I've seen several boats with an Omnimax aerial mounted on a mast. That's a good idea I thought so I bought a mast, made and fitted some brackets to hold it and mounted the aerial on it so that the aerial head is about 2.5m above the roof.

On experimenting, I found that the thing works better when it is lying on the roof than it does when erected. Why is that then?

 

This is just a guess (I hope an educated one) but it may be that the TV signals in your area are vertically polarised and I believe the Omnimax is for horizontally polarised signals. It could be that when the aerial is close to the roof, the signal is being reflected off the roof and that reflection is changing its polarisation somewhat. Of course, this may all be b*ll*cks

 

Chris

[Ray]

Might be worth checking the height difference with the same length of antenna cable if that is possible. You might have a dodgy connection in any extension that you are using.

[churchward]

My experience with Omnimax or similar ariel types is that they are not much cop no matter what you do with them. I prefer to use a log periodic ariel on a pole. The upsideis that normally I get a clear picture and good freeview the downside is that it is directional and needs adjusting to face the signal direction.

 

one like this is similar to what I have.

 

http://www.roadpro.co.uk/retail/product_de...BLE&id=1084

[AlanH]

Might be worth checking the height difference with the same length of antenna cable if that is possible. You might have a dodgy connection in any extension that you are using.

It is the same cable. I have the bottom section of the mast attached to the front of the cabin and I just lift the rest of mast off and lay it on the roof without disconnecting the cable. It doesn't just happen at my home mooring but it seems to be universal.

[Keeping Up]

... I believe the Omnimax is for horizontally polarised signals ...

I don't think so, it seems to work equally well for horizontal or vertical signals.

 

It does seem to significantly affected by reflections from the roof of the boat; for example, we found that in the north-west midlands this year we were getting perfect digital signals on BBC but nothing on ITV, but raising the Omnimax by just 10 cm made a huge difference; in other digital areas this did not apply.

 

Interesting to note that this year on all our travels from Milton Keynes to Llangollen, Gloucester, Boston, and the Chesterfield, with the exception of the signal-free corridor to the southwest from Birmingham to Gloucester there have only been a half-dozen days when the Omnimax did not receive a good (usually digital) signal.

[alan_fincher]

.............and I believe the Omnimax is for horizontally polarised signals.

You would think it would be easy to find out, wouldn't you ?

 

Neither Maxview's site, nor a bit of further Googling can turn up an answer for me.

 

I have to say that all "omni's" are a big compromise, and our Omnimax consistently fails to deliver in all but the best signal areas.

 

It could be because it's an early one, and maybe newer ones are better, but at circa £80, I certainly will not be paying to find out.

 

We bought a Maplin Satellite Suitcase for appreciably less than a new Omnimax "egg whisk", and when we bother to watch TV, (which wasn't at all, on our last 3 weeks away), it gives far more consistency than bothering with terrestrial TV.

 

If I did want land based TV, I'd use a directional antenna.

[Nickhlx]

When any ( terrestrial) aerial is sitting on the (metal?) roof its (reception) performance will be affected. It may be "better" or "worse", depending on a few things including from which direction is it trying to receive the signal, which will of course vary each time you move the boat more than a few 10's of centimeters.

 

The best way to get the intended performance is to remove the ground effect of the roof and mount it as high above the roof as reasonably possible ( more than approx 18 inches / 50 cms) to escape the majority of the effect. Height gain is also useful ( with diminishing returns) but if you could swing the aerial up on say a 10 foot pole, it should invariably be better than when it is just above the deck.

 

If you are near / next to a transmitter then almost anything will do, but the weaker the signal is in the area the better the aerial and its position will need to be ( assuming it is a terrestrial transmission) - Satellite reception will generally be better if you have a clear view to the satellite.

 

Nick

Edited October 15, 2008 by Nickhlx

[RobinR]

These are rubish! Their unamplified gain is -3dBd (yes, minus 3!!) compared with 7dBd for log periodics and up to 12dBd for a yagi aerial - but these are BIG. Every 3dB represent halving or doubling power depending on variation from the base of measurement. So for a 250kw transmitter the signal to an amplifier from a log periodic will be equivalent to 2500kw transmitter when compared with the amplifier input from an Omnimax. The omnimax incorporates an amplifier and one can similaraly be attached to a log periodic. There is however a minimum input required for any amplifier and if the output from the omnimax is below this threashold then there will be insufficient input to the TV tuner regardless. The additional 7dB will often be sufficient to pass this minimum threshold.

 

I also have a satellite suitcase and have frequently commented elsewhere on this site.

 

 

Have a good read at this http://www.aerialsandtv.com/touringaerials...avanBoatAerials and explore the many links highlighted in blue. I hope this is useful

Edited October 16, 2008 by RobinR

[Keeping Up]

These are rubish!

Like I said above, "this year on all our travels from Milton Keynes to Llangollen, Gloucester, Boston, and the Chesterfield, with the exception of the signal-free corridor to the southwest from Birmingham to Gloucester there have only been a half-dozen days when the Omnimax did not receive a good (usually digital) signal". I don't call that rubbish!

[alan_fincher]

Like I said above, "this year on all our travels from Milton Keynes to Llangollen, Gloucester, Boston, and the Chesterfield, with the exception of the signal-free corridor to the southwest from Birmingham to Gloucester there have only been a half-dozen days when the Omnimax did not receive a good (usually digital) signal". I don't call that rubbish!

Allan,

 

Out of interest, how old is yours ?

 

Is it the type where all the electrics are at the "inside" end, or does the antenna have some amplification.

 

Ours is pretty crap, but came with the boat, and may, I suspect, be fairly old.

 

I can only assume they have improved them considerably in later production ?

 

Alan

[Keeping Up]

Allan,

 

Out of interest, how old is yours ?

 

Is it the type where all the electrics are at the "inside" end, or does the antenna have some amplification.

 

Ours is pretty crap, but came with the boat, and may, I suspect, be fairly old.

 

I can only assume they have improved them considerably in later production ?

 

Alan

Alan (with one)

 

It's about 10 years old, and its amplifier is inside the boat. Our earlier one removed itself from the roof under the railway bridge at Stoke on Trent.

 

And yes the amplifier does also make a significant difference on digital signals; many theorists will tell you that an amplifier doesn't help on digital signals, but I can definitively state that in practice it does help.

 

On the very few occasions when we can't get a decent signal with the Omnimax, we tune the TV using the Omni so that it gets something however weak and then swap it for an old Labgear indoor set-top aerial which I've modified to fit the Omnimax mount on the roof, pointing it for the best result before retuning the TV again.

 

Allan (with two)

[chris w]

Alan (with one)

 

It's about 10 years old, and its amplifier is inside the boat. Our earlier one removed itself from the roof under the railway bridge at Stoke on Trent.

 

And yes the amplifier does also make a significant difference on digital signals; many theorists will tell you that an amplifier doesn't help on digital signals, but I can definitively state that in practice it does help.

 

On the very few occasions when we can't get a decent signal with the Omnimax, we tune the TV using the Omni so that it gets something however weak and then swap it for an old Labgear indoor set-top aerial which I've modified to fit the Omnimax mount on the roof, pointing it for the best result before retuning the TV again.

 

Allan (with two)

 

Allan

 

The amplifier may not be helping through amplication per se but because it will greatly improve the VSWR by an amount equal to twice the nominal dB gain of the amplifier (I know that you know what this means) and so may help by providing a far better match to the aerial.

 

Aerial amplifiers are really only any use for amplification if they are put at the aerial end to compensate for cable loss. They are amplifying the signal and the noise equally and so cannot improve the S/N ratio.

 

Chris

[Keeping Up]

Allan

 

The amplifier may not be helping through amplication per se but because it will greatly improve the VSWR by an amount equal to twice the nominal dB gain of the amplifier (I know that you know what this means) and so may help by providing a far better match to the aerial.

 

Aerial amplifiers are really only any use for amplification if they are put at the aerial end to compensate for cable loss. They are amplifying the signal and the noise equally and so cannot improve the S/N ratio.

 

Chris

Chris

 

You could be right about the VSWR, I'd be very surprised if the Omnimax is an accurate match on its own, but I've got no way of measuring it so I'll just have to accept the results.

 

You are of course perfectly correct that the amplifier will not improve the S/N ratio, so it would make no difference if it were then feeding a 'perfect' receiver. But the receiver is of course not perfect; the effect of receiver's own front-end noise will be reduced if the incoming signal is stronger, so there will be an improvement if the amplifier has a lower noise factor than the TV (which I hope it would do, being specifically intended for amplifying low-level signals); similarly I find that real demodulators tend to work slightly better with stronger signals even though with theoretical ones it makes no difference.

 

Allan

[RobinR]

This is just a guess (I hope an educated one) but it may be that the TV signals in your area are vertically polarised and I believe the Omnimax is for horizontally polarised signals. It could be that when the aerial is close to the roof, the signal is being reflected off the roof and that reflection is changing its polarisation somewhat. Of course, this may all be b*ll*cks

 

Chris

 

The Omnimax has nominally the same (lack of) gain for both horizontally and vertically polarised signals. The field strength of all radio signals varies with location - more so at UHV thus small movements of an aerial will be more noticeable under marginal reception conditions!

 

I don't think so, it seems to work equally well for horizontal or vertical signals.

 

It does seem to significantly affected by reflections from the roof of the boat; for example, we found that in the north-west midlands this year we were getting perfect digital signals on BBC but nothing on ITV, but raising the Omnimax by just 10 cm made a huge difference; in other digital areas this did not apply.

 

Interesting to note that this year on all our travels from Milton Keynes to Llangollen, Gloucester, Boston, and the Chesterfield, with the exception of the signal-free corridor to the southwest from Birmingham to Gloucester there have only been a half-dozen days when the Omnimax did not receive a good (usually digital) signal.

 

And a log periodic or yagi may well have received a usable signal in one or more of these "exception" locations.

 

Allan

 

The amplifier may not be helping through amplication per se but because it will greatly improve the VSWR by an amount equal to twice the nominal dB gain of the amplifier (I know that you know what this means) and so may help by providing a far better match to the aerial.

 

Aerial amplifiers are really only any use for amplification if they are put at the aerial end to compensate for cable loss. They are amplifying the signal and the noise equally and so cannot improve the S/N ratio.

 

Chris

 

An amplifier can not change the voltage standing-wave ratio (VSWR) in a feeder however a transformer / balun circuit at the input and/or output can enable it to "see" a more acceptable value and thus be more effective in its operation. When an amplifier is located close to the aerial, interference (noise) picked up by the feeder will not be amplified and thus the signal to noise ratio at the input to the receiver will, for an otherwise identical set-up, be better than for an amplifier located at the TV input.

[john the duck]

The Omnimax has nominally the same (lack of) gain for both horizontally and vertically polarised signals. The field strength of all radio signals varies with location - more so at UHV thus small movements of an aerial will be more noticeable under marginal reception conditions!

 

 

 

And a log periodic or yagi may well have received a usable signal in one or more of these "exception" locations.

 

 

 

An amplifier can not change the voltage standing-wave ratio (VSWR) in a feeder however a transformer / balun circuit at the input and/or output can enable it to "see" a more acceptable value and thus be more effective in its operation. When an amplifier is located close to the aerial, interference (noise) picked up by the feeder will not be amplified and thus the signal to noise ratio at the input to the receiver will, for an otherwise identical set-up, be better than for an amplifier located at the TV input.

 

I use a image 64 which does perform better on its side on the roof its agreat ariel thats good enough for free view anywhere

[bottle]

I use a image 64 which does perform better on its side on the roof its agreat ariel thats good enough for free view anywhere

 

Living, boating in London within the transmission area of the largest (power) transmitter, a piece of wire would probably work.

 

By the way welcome to the forum.

[chris w]

An amplifier can not change the voltage standing-wave ratio (VSWR)

 

Oh yes it can.................

Edited October 19, 2008 by chris w

[Grasshopper]

When people report different results with the Omnimax, I wouldn't discount the difference being the actual digital receiver.

 

I have an Omnimax and it gives fairly indifferent results on analogue but good enough since I only holiday on the boat and the most that I want to do is catch the news & weather. I could usually get one channel or another to achieve that.

 

But as switch off loomed, I thought that I'd try a RoadPro digital box. I tested it at home and it worked perfectly. Tried it at the boat. Nothing. Oh well I thought, when the time comes, it will have to be a dish.

 

Then my old analogue TV went kapput and I ended up replacing it with a Sony one with both digital & analogue receivers built in. Purely because it was refurbished & I got a good deal. Of course I didn't expect the digital to work. But blow me down, it pulled in every available digital channel. A few break up late at night but all the main channels are rock solid.

 

So it may not be the age of the Omnimax that makes a difference (mine was secondhand from someone's caravan) but the quality of the receiver itself.

[Cinimod]

Would this little compact directional thing be any good do you think - MAPLINS

[chris w]

Would this little compact directional thing be any good do you think - MAPLINS

 

Any aerial of this type "swordfish" (known as a "Yagi" array) will be immensely better than an omnidirectional aerial like the Omnimax. The only slight downside of Yagis is that you have to point them at the transmitter (ie: rotate them until you get the best signal strength). You need to know too whether the local transmitter is vertically or horizontally polarised. That means, do you use the aerial as per the Maplin's photo (with the swordfish bit horizontal) or rotated through 90 degress so that the teeth would be pointing vertically.

 

In my expererience, most seem to be horizontally polarised. Despite the fact that I use a very high gain Yagi array, I usually end up using the satellite dish because canals sit in valleys and are surrounded by hedges and trees which will effectively block the terrestrial signal the majority of times.

 

Chris

[RobinR]

Oh yes it can.................

 

Amplifier circuits can only change the value at the input/output to/from the feeder and thus the apparent value at the far end of the feeder (Coaxial cable). The actual values of the peak voltage etc whilst moved to a different relative position along the feeder have otherwise unaltered characteristic values and thus inerrant loss characteristics. Whilst at LF and HF these are generally negligible, at VHF and especially UHF thy can be significant especially with lossy, cheap feeders.

[Cinimod]

I usually end up using the satellite dish because canals sit in valleys and are surrounded by hedges and trees which will effectively block the terrestrial signal the majority of times.

OK - which then brings me to the question that I did buy this sat kit - MAPLINS - and a friend managed to help me get it working once, but I can not manage to tune the thing in myself - not sure what I am doing wrong... I have a finder box that I plug into the LNB and the signal goes way up and it screeches, but when plugged into the decoder, there is nothing coming through...

[chris w]

Amplifier circuits can only change the value at the input/output to/from the feeder and thus the apparent value at the far end of the feeder (Coaxial cable). The actual values of the peak voltage etc whilst moved to a different relative position along the feeder have otherwise unaltered characteristic values and thus inerrant loss characteristics. Whilst at LF and HF these are generally negligible, at VHF and especially UHF thy can be significant especially with lossy, cheap feeders.

 

Amplifiers will improve the VSWR by the fact that they have far greater loss one way than the other and so act as an effective isolator between the source and the load.

 

As a simple analogy, if you stuck a 10dB pad in the line it would improve the VSWR by 20dB as it will have a 20dB return loss (wouldn't help the signal level though of course)

 

Chris

 

OK - which then brings me to the question that I did buy this sat kit - MAPLINS - and a friend managed to help me get it working once, but I can not manage to tune the thing in myself - not sure what I am doing wrong... I have a finder box that I plug into the LNB and the signal goes way up and it screeches, but when plugged into the decoder, there is nothing coming through...

If you're getting a strong signal but no picture, you're pointing at the wrong satellite. Don't bother using your finder box, the Maplin unit has a built in audio signal which goes from low to high pitch when it's on a strong signal. (Still has to be the correct satellite though). The advantage is that there is then nothing to have to plug in or out of the cable.

 

Chris

Edited October 20, 2008 by chris w

[Cinimod]

If you're getting a strong signal but no picture, you're pointing at the wrong satellite. Don't bother using your finder box, the Maplin unit has a built in audio signal which goes from low to high pitch when it's on a strong signal. (Still has to be the correct satellite though). The advantage is that there is then nothing to have to plug in or out of the cable.

Thanks

 

I downloaded their 'idiots' guide to setting it up... find south point your right arm there, then your left arm at 90 degree, halve it with you left arm, and point the dish half way between where you arms are now, then move the dish VERY slowly to the left!

 

See if it works!