freeview uk in sandyford/leopardstown signal update

Oh god, confusing stuff. As you confirm, it's all about getting as perfect a signal as possible before you start messing with masthead amps. Do NTL use RG11 or CT100 or what ? I have a big long length of black well screened with a foam dielectric which was used by NTL for an internal run in a building but cannot see any markings on it.

My Triax 100 (Divis H) is currently connected with a length of RG59 (old Wang Dual Coax cable - second cable to the contract 10 type aerial for Claremont Cairn V)). It's very well screened but the core looks like it's less than 1mm whereas the NTL stuff looks much thicker. The RG59 has a solid plastic dielectric.

Should I replace my RG59 with this other cable ? The Cable runs from the aerial into the attic (where it meets the masthead amp) so would be 10m at most.

Many thanks for your patience and advice


C

Does the size of the centre conducting core matter hugely ? If the RG59 cable is maybe 0.8mm as opposed to the other stuff which is maybe 1.0mm.


C

yes. Skinnier cable usually higher loss than fatter cable.
as per the link above in my post.

I looked in Maplin today.

The "92" element aerial is 23 element. Nice size but poor shape reflector. Bad design of directors. Instead of -- screwed to boom via spacer or proper X directors it uses > < with large spacing of plastic. This means it's likely not much better than a conventional 21 element yagi and very likely to shed directors as the very large plastic flanges are UVed, Iced and flex in wind. Nearly 90 Euro, waste of money Complete junk compared to a UNIX100, which I wouldn't buy, I'd use 2 x Blake SR18

They also had a quite well made "Contract" style (i.e. small reflector) 18 Element aerial for 29 Euro. The cap for cable didn't look big enough to have a Balun inside it. However it will last probably 20 years.

They also had a copy of the Televis DAT75 triple boom, which is about a 13 Element aerial. Maybe the best aerial they have, but overpriced. The 6 Euro Indoor Aerial is now nearly 14 Euros.

Ballinteer, Sth Dublin. Hope to change the RG59 over to RG11 some weekend soon


C

There are NO real 92+ elements.

The biggest aerials are called 100 Element (i.e. Unix100). They are massive and are actually about 25 elements. A pair of Blake SR18 is more signal and less weight and less size. I have compared both on same pole.

All you need is a 1"/ 25mm pipe about 1m / 3ft long minimum. 1.2m might be better. Mount middle on the pole and an SR18 at each end.
use 2 x indentical 1m cables to wire the aerials to a Splitter used as a combiner. use 0.25m cable from that to the mast amp lower on pole.

All that costs less than one Maplin "92" element or "UNIX100". More robust.

It's an absolute physical fact that as you make the aerial longer and add more aerials you get to a point where extra elements and more length makes very little difference. This is at about "two wavelengths" which at 600MHz UHF means there is little value in making an aerial much more than 1m long. The actual number of rods is nearly irrelevant. It's the overall length as 300/(Frequency in MHz) = metres wavelength.

But TWO identical aerials mounted far enough apart will obviously give twice the signal = 3dB more gain. Mount too far apart and you can't "match" the signals to add them together. That's why I recommend about 1m to 1.2m for UHF TV and IDENTICAL lengths of identical coax. Too close and they detune each other and the radio waves are shared so not so much gain.

To get more worth while gain, obviously you have to double the number of aerials each time.

This is absolutely basic physics and mathematics. Any particular amount of space only has a fixed amount of radio wave energy in it.

More than four aerials a dish is better. But at UHF it's a very big dish

Why does satellite use a Dish and TV doesn't?

The simplest answer is the amount of "space" the radio energy takes is related to wavelength about 0.5m for 600MHz UHF and about 0.025m for Satellite. So in the same space as one UHF TV wavelength you can fit nearly 20 Satellite TV wavelengths. The amount a dish captures is its area. PI x R squared. So a Dish of a particular size can catch almost 20 x 20 = 400 times more "RF signal" at 12GHz as at 600MHz. A UHF dish has to be very large...


This is also why Saorsat can use a smaller dish than Sky. It's nearly twice the frequency. The other reason will be that the centre of the beam will be on Ireland. We are at very edge of six of the seven beams used for Sky @28E.

I've simplified and ignored free space path loss. So a Sky dish doesn't really have 400 times more gain at 12GHz than 600MHz.

For Horizontal polaraisation:

All you need is a 1"/ 25mm pipe about 1m / 3ft long minimum. 1.2m might be better. Mount middle on the pole and an SR18 at each end.
use 2 x indentical 1m cables to wire the aerials to a Splitter used as a combiner. use 0.25m cable from that to the mast amp lower on pole.

use a Shelly Clamp to mount the cross bar on the top of the pole.

Mount the aerials on their holder on the cross bar.

If vertical polarised, then mount one aerial 15cm below of top pole and next aerial 1.2m lower. That aerial should be minimum 2m above anything else. Offset aerials from pole on their supply holder/arms.

GKP1, did you ever try pointing that setup at Kilkeel. My cousin near the IMI has Kilkeel OK.


C

Kilkeel would be BBC and ITV NI rather than Wales. However, you wouldn't get any Freeview until 31/12/2012.

Sorry for misleading you somewhat. I'm not sure as to what will happen to the Kilkeel transmitter post DSO or what strength it will be.


C

I'm sure the rule of thumb will need to be much higher if the signal from Divis is to reach around to Kilkeel. But yet it would seem crazy for Kilkeel to then have it's own mast and all that goes with it for just 100w or less. It's a great pity that we have to wait all the way until 31/12/2012 for DSO in NI. I am amazed that they didn't bring it (and more so Greater London) forward because of the Olympics. There again, maybe that's why they left it there until after then.


C

championc wrote: »

I'm sure the rule of thumb will need to be much higher if the signal from Divis is to reach around to Kilkeel.

Kilkeel does not receive its signal directly from Divis, but from a different reflector!

The signal is bounced twice before its beamed along the Irish Sea

From: http://www.mds975.co.uk/masts/kilkeel.html

The RX feed for this is recieved from a site at Killowen Mountain, NGR J207174, which is a purpose built active reflector (output channels, the same as the input channels) to give a reliable feed to Kilkeel.