Aerial orientation for UK channels - advice?

bazza

Hi all,

Bit of advice needed - I'm in North Dublin about 5 miles north of the airport and would like to pick up the UK terrestrial channels on an aerial. I got my hands on a 43 x-element wideband UHF antenna and a little masthead amp (23dB I think, will have to check). I'm on the north side of a hill, and ideally I'd like to attic mount the antenna if I can thus avoiding a mast. But I'm not adverse to putting on the chimney if required. Mast would be my last resort (I'm doing this on the cheap here and don't particularly like getting up on the roof!).

Any advice on best orientation for the antenna? From playing with it, horizontal and more or less pointing northward seems to give the best results. I presume I'm picking up Divis here?

Is a 43 x-type element enough - some of the antenna around my area seem to be a good deal bigger (most are longer and have a supporting arm underneath the boom). Any advice folks?

You could also get Brougher in Co Tyrone,I've seen aerials around Ashbourne pointed for that.

There is the possibility of the Kilkeel relay in Co Down vertically polarised, if you have a sea view north, northeast in that direction, it gives fair results down as far as Dalkey which is amazing for such a low power tx.
Have a look at the BBC Website for polarity aerial groups etc and channel numbers, which will give you an idea which of the three you are finding.
mm

tvman

Hi bazza

Having lived close to your area for a while, I can say that the Kilkeel Tx was first for new installations followed by Divis.
You could try for Camlough which is one of the latest Tx to come on air.
Camlough has a higher ERP (630W) than Kilkeel (500W) but it may be configured so it hasa null point in a southerly direction. For that reason I would be very interested to knowwhat signal strength it has at your location.
I am not so sure about Brougher Mountain giving a useable signal.

With a wideband aerial and amp used you can try for all txs and which ever is best youcould get a grouped aerial (and amp) which would probably improve things.
It may not be worth the extra expense though - keep using the wideband.
Therefore the following are your options.
Possible aerial groups are - WB, A, B & C/D
(V), (H) = vertical or horizontal polarisation.

Using WB, V
Kilkeel or Camlough would also give you RTE etc from Clermont Carn (RTE Quality - ???)
Therefore you would probably need a small WB,H for RTE from Three Rock.

Using WB or A, H
Divis (or Brougher Mtn???) but you would probably need a wideband for RTE from Three Rock, although they just might come in on the back end when using the WB as main aerial.

Using WB or B, V
Arfon in North Wales may be possible (or even Llanddona C/D H).
Again you would need a small WB for RTE from Three Rock.

As with a lot of things you will have to experiment as the local terrain etc will have abearing on final results.
Also you can sometimes get good reception from a long distance Tx because you are in linewith a strong signal beam, someone else quite close may get little or nothing.
Perhaps someone else could advise you about the Welsh Tx.
Finally the following URL will give you access to the BBC Transmitter info and all sorts of stuff about reception, aerials etc.

www.bbc.co.uk/reception/

From tvman

bazza

Thanks chaps.... I'll do a bit of experimenting and see how I get on.

Right now my best result is C4 in colour with a fair bit of noise and perfect sound, with a horizontal aerial more or less bang-on northward. The Irish terrestrial stuff is very good with this setup even with the WB pointed the way it is right now.

watty

As a former "Comms" engineer here is a tip..
An aerial gets progressively less gain more as it gets longer. The useful limit is 2.5 times the wavelength.
600MHz UHF = 0.5 metres wavelength, so more than 1.5 m long aerial not much point. After that to get more signal you need multiple aerials (Called an Array). Connecting more than four aerials in an array is problematic. Thats why the flat wide band aerials have four sets of aerials, and the big long stacked Yagi aerials (more gain but narrower channel range) are only 2 high and 2 wide.

If you have a LOT of patience you could connect two arrays with EXACTLY the same length of aerial flex into a ferrite bead type Y -Splitter / combiner and then in to head amp. Any much more than 20Db gain on amp is not any use, the Noise Figure is more important for Distance Work. You then have to move one array slightly up / down, left/right, forward / backward to peak the signal. The Arrays need to be stacked vertical about 1m apart.

Below 50ft, increasing height makes nearly more difference than an extra aerial array. Once you get to 40 to 50 ft adding more aerial gives better result than more height.

Unless you are an isolated house on a hilltop (then aerial at shoulder height on wall is fine!), you need at least a 4ft pole on the Chimney. Make sure your aerial won't get sooted.
Even for Analog, using Satellite grade double screen cable helps.

The Mast head amp should be at least 4ft away from aerial to avoid "feed back" (like mic and loudspeakers).
Don't tighten clamps on Coax outer so that cable deforms (causes signal loss and even ghosting).

I've erm only ever thought of aerial height as a solution to get over obstacles such as trees etc.
I don't think theres an aerial installer in the country/anywhere at all that thinks you rise the mast to get a better signal unless it's to get over an obstacle.
On the high ground above me for instance Presely comes in on a portable tv with just it's own telescopic aerial attached and no amp from a tx 120 miles or more away, albeit with a view out over half the Irish sea.
A quad array in that case sitting on the lawn two inches off the ground would give as good a result, if not better than 500 feet in the air.
mm

watty

Read the RSGB VHF / UHF handbook.
Only if your lawn is a steep slope dropping toward transmitter.

There is all kind of ground effects. Also as frequency is lower, the nearness of aerial to ground "detunes" impedance matching.

The effect is NOT linear. I.e. twice the hieght if you are at 36ft does not double the signal, but it will increase it.
6dB increase = Twice the signal

Thus as you move the aerial higher, there is less and less advantage.

Obviously you get a *HUGE* improvement moving the aerial above a near by obsruction.

Due to ground effects, multipath and earth curvature, for a path more than 30 miles on flat terrian and less distance on rough or hilly terrain, more hieght does give more signal (on average).

Moving from 4ft to 30ft will likely more than double signal (200%= add 6dB to signal strength reading in dB).
Moving from 30ft to 60ft maybe only slight increase (50%).

If your signal is "noisy" as a rough approximation for analog each doubling of signal is significant, a 50% increase would not.

For Digital there is a "threshold" a bit below it you get nothing, around it you get "freezing" and "crashing" of MPEG decoder, a bit above it you get OK picture, so for digital a 20% even if you are at the "threshold" means stable "perfect" pictures instead of "freezing" etc. Or intermittant pictures with freezing instead of nothing!



Due to reflections off the ground between TX and RX, as you vary the hieght you can experince signal "nulls" on one channel so it is always a good idea to vary the height up AND down to maximise the signal on all channels.

If there is a tall building or tree or Hill to "side" of path, adjusting the left / right position will find peaks / troughs in signal, as will rotating the Yagi (Yagi particularly suffer from "spiky" side lobes so it is always worth moving aerial around and rotating it for best signal.

Each reflection (or even a refraction on a long path with different layers of air temperature) will change to polarisation, so on a long path it is worth slowly rotatinng the aerail on axis as horizontal may not give the "best" signal for signal leaving the transmitter horizontally.

On a path that goes overland near ground that heats up you may get refraction during part of the day so the signal will fade as the day wares on and return after dark. A Circularly polarised aerial will pickup Horizontal and Vertical polaristation (and all inbetween) at about 1/2 the strength of circular polarised transmission or a normal aerial adjusted to match the received polarisation. The advantage is that you won't get signal fade due to sun or frost induced refraction at different times of the day.

I don't need to read the hand book now:D
As I am about to go on holidays, you have saved me money,I was going to read it on the plane;)

From practical experience, your height above sealevel if it is well above the hasl of the far away tx, accounts for a huge improvement in reception as it counteracts the effects of the Earths Curvature.
mm

watty

Yes. Thats why living on top of Three Rock or Keeper's Hill is so much better for UK TV than Tallaght or Limerick City

Lennoxschips

Great, I'm six meters below sea level, I guess I haven't a hope in hell of receiving anything but Dutch TV.

watty

I thought thats what (undersea) Cable was for.

You need a serious mast.

I want a nice lattice Mast. Who sells them?

Greenman

Its a pity you're not in Belgium they're 10 a penny.ie the masts.
Regards JD!!!! Belgium