UHF HDTV reception (Freeview) in the UK

[sophiecentaur]

Hi
This is probably only suitable for answers from UK members because it's about the specific problem of DTV in the London area. Freeview channels in London cover pretty much the whole of the UHF band now.

Three years ago we moved into the house and I noticed that the range of Freeview channels limited the HD programmes to progs 101,102,103,104 and 105. I misguidedly (but after looking on line for advice) bought a Crystal Palace Group A aerial and split (distribution amp) the antenna output to feed two receivers. We could then get a range of HD programmes but things soon fell back to the lower group of progs. I put this down to the antenna having been pointed upwards during a high wind episode. I didn't bother to attempt a fix until yesterday. It seems that there had been some re-assignment of prog frequencies round about the time of the blow.

I re- pointed the antenna but no more HD channels were available. I wound up the DA gain and more channels appeared (on one set only). On line sources tell me that those HD progs are way up at 750MHz and that transmitter power is nearly 6dB below 'max'. I guess I am now marginal, because of the antenna bandwidth and also the low Tx Power.

Apparently, there are plans for re-allocating some frequencies to allow coexistence with 4G. My question is simply, should I buy a Group W antenna or a Group T antenna, which would handle all the multiplexes up to COMM7. What is the significance of the 4G issue? Is it about possible interference? Won't there be filters available to deal with that?

I am looking for an 'expert' who's in the know about this issue so that I only need to buy one more aerial at this house. Can someone help me please.

 

[sophiecentaur]

I just managed to open a door in the internet adventure game and found what I think is the answer. The high UHF channels will move to 4G 'after 2020' and Comm 7 and Comm 8 will not be used by Freeview. Channel space will, apparently be found in the existing bands. It seems that London is favoured at this time because other areas do not have the full array of HD services.This is causing some resentment in other regions; the word "Postcode" seems to be a major feature of the complaints. I imagine, if there was any money to be made, someone would be opening up those multiplexes in other places.
I think another aerial is called for. No shortage of field strength on the main multiplexes so the lower gain of a wide band aerial is not a worry.

But, post the Brexit Event, we will all be too miserable and penniless to be watching TV, in any case.

 

[CWatters]

Not sure if the offer of free filters is still valid but...

https://at800.tv/guide/4g-filters-guide/

 

[CWatters]

This companies website is usually good for tech info.

http://www.aerialsandtv.com/crystalpalacetx.html

They say if you want com 7 and 8 you need a wide band but point out these have reduced gain compared to a band A. Think they are suggesting adding a second band E aerial if you need the gain.

 

[sophiecentaur]

I'm a fair way from Crystal Palace but signal strength seems to be fine. The reduced power from Com 7 is not a problem - it was marginally there before, in any case.
I eventually decided to buy a Labgear Band T aerial with a nominal 15dB gain. Bottom line is that all signals are now available and I have not seen any drop outs. The aerial has a built in filter to deal with 4G. There is a short extra element just behind the first director and the driven element is a bit fancy, too. It is wide with slots at each end to give it an H shape. As for 4G, I can barely get 4G at all on my phone so I don't think it will be a problem until they put in another mobile base station.
I only paid about £25 for the aerial and the download is only a year old.
RESULT!

 

[CWatters]

We're on Sandy Heath. When we moved in I realized I'd forgotten to sort out an aerial so I rushed out and grabbed the first wideband I could find in Maplin intending to replace it later. Stuck it in the loft on a temporary cable. It's worked so well I haven't bothered to replace it. It's pointing through a roof, a tree and 20 miles to Sandy Heath. Far from ideal but it's worked fine for 10 years.

 

[sophiecentaur]

I remember my Dad put an X aerial up in our loft (in Plymouth) with a bell wire down lead to receive early VHF telly from the newly erected North Hessary Tor. I couldn't see anything wrong with the picture - but I was only nine years old and I was seriously chuffed to be able to watch just one channel in low res monochrome. And VHF goes round corners better than UHF.

 

[tech99]

I remember my Dad put an X aerial up in our loft (in Plymouth) with a bell wire down lead to receive early VHF telly from the newly erected North Hessary Tor. I couldn't see anything wrong with the picture - but I was only nine years old and I was seriously chuffed to be able to watch just one channel in low res monochrome. And VHF goes round corners better than UHF.

When ITV started from London (189 MHz) my neighbor made his own aerial using bicycle spokes and used a single wire as a feeder. He did not know that the elements had to be a certain length, and that coaxial cable was necessary. But it fired my curiosity about antennas.

 

[sophiecentaur]

did not know that the elements had to be a certain length, and that coaxial cable was necessary.

Judging by the number of people who do fine with a coat hanger stuck in the back of the set, I would say "Where ignorance is bliss, 'tis folly to be wise."
Everyone has heard of the Sketchly Dipole, at least in the UK, surely.

 

[gneill]

When I was (much) younger my family would go on camping holidays (well, not so much camping as dragging a small travel trailer from national park to national park). We had a small black-and white TV with us (about a 12 inch screen, I think) that had a built-in extendable whip antenna. Quite often distance and local terrain would make for poor reception (if there was any!).

The trailer had a tent that fitted to its side to create a nice sized room to lounge in when weather was poor or sleep in at night. Its "bones" were made from aluminum struts that fit together with nylon connectors (think it was nylon) under spring tension. When the reception was poor and the whip antenna proved next to useless, I'd pull out a bunch of alligator clip test leads and clip the antenna to a strut. Then bridged the nylon connectors between various other struts to find a combination that worked. Or didn't. Sometimes there's no saving a situation with bubble gum and baling wire.

 

[anorlunda]

I have a related question. Is there a difference in the design of antenna to receive analog TV or digital TV?

 

[sophiecentaur]

I have a related question. Is there a difference in the design of antenna to receive analog TV or digital TV?

Despite the recommendations of aerial salespersons and fitters, there is no essential difference. A TV aerial is a pretty wide band device which brings in a wide range of Analogue channels.
The environment is a bit different in the UHF bands but afaik, the DTV multiplexes are all assigned to groups of the previous analogue rf channels, largely within the 'old' aerial groups. It's only when you want to pick up the multiplexes outside the original channel group that problems arise - as when I wanted 700MHz transmissions on my original low band antenna. Plenty of political agro if you read the complaints from people who don't live in the big conurbations and who want all the HDTV signals.
The way the digital channels are assigned to the UHF band in UK is shown in this link.
The protection ratio for co-channel DTV to DTV is around 19dB and that's less stringent than for the old Analogue to Analogue so directivity won't be a problem. Also, the digital multiplexing takes care of multipath effects.
I don't know how a boat owner could go about receiving the whole range of DTV signals unless they are moored near a handy transmitter. But it could well be OK on a boat because any likely TV aerial on a boat would be small and wide band in any case. Tides could be a problem!

 

[CWatters]

I have a related question. Is there a difference in the design of antenna to receive analog TV or digital TV?

Short answer is no.

In the UK there were multiple analogue transmitters scattered about the country side. To prevent interference between adjacent transmitters the available band was divided into groups..A, B, C etc. Where possible transmitters physically close to each other broadcast their TV channels in different groups. Transmitters a long way apart could use the same group as there was less chance of interference. It is/was a lot more complicated than that but that will do for now.

Another advantage of this approach is that in many cases the receiving aerial only needs to have a bandwidth wide enough to receive a group, not the whole TV band. That helps reduce noise.

When digital TV was introduced they had to find a way to broadcast both for a few years. Many people thought it would be impossible and it was very complicated. In brief... They typically allocated some digital channels to a second group. That meant some houses, but not all, had to switch to a wideband aerial to get the new digital channels/group. For marketing reasons theses wideband aerials were called Digital Aerials (at least they were here in the UK).

The Digital TV network in the UK requires less bandwidth than the old analogue network and bandwidth is valuable. The government soon realized they could sell off part of the old TV band to the mobile phone companies. For this reason the channel/group allocation on some transmitters continued to evolve for some time to free up space and unwind the complications caused by the need to broadcast both.

As a result of the ongoing changes I think a few houses that had to switch to a wideband aerial to get digital early on might benefit by switching back to a narrow band group specific aerial in the future.

Filters may also be necessary to prevent interference from the new mobile phone channels.

Edit: looks like sophiecentaur beat me to it, and probably explained it better.

 

[jim hardy]

Is there a difference in the design of antenna to receive analog TV or digital TV?

No, that's marketing hype to sell antennas.
Not to say that a newfangled active (amplified) antenna couldn't take advantage of new IC technologies

but what FCC did is keep the old frequencies ,
and let stations swap to a new frequency but still refer to themselves by their prior call letters and channel number
meaning your local station that claims to be channel 3, which you'd expect to be at 63 megahz, is actually transmitting on RF channel 36 way up at 605 megahz. My local Idaho Falls channel 3 did just that.

So an old VHF only antenna would no longer see that channel 3 after it moved to UHF band..
But an old UHF only antenna will suddenly "find" new stations sporting VHF channel numbers.

Search on US TV frequencies turns up
http://otadtv.com/frequency/index.html

and this FCC site will find your actual house location and predict your reception including effects of elevation and terrain
https://www.fcc.gov/media/engineering/dtvmaps

here's FCC's prediction for my house and the two stations i stand a chance of receiving.

note they give you the actual RF channel for a station so you can look up its frequency and calculate a half wavelength antenna element.

I'm down in a valley so had to make a ten element Yagi to get enough signal at back of my set.
@dlgoff kindly loaned me a RF microvolt meter
i got 60 microvolts on the UHF channel(20) and 180 on the VHF(8)
I tried out my prototype Yagi on my neighbor Harry's porch at top of the hilll, a hundred feet higher
and it delivered 5X more voltage than at my house.
and 20 log(5) = 14db
so elevation IS important.

it was a fun adventure, described in https://www.physicsforums.com/threads/can-i-parallel-yagis.806936/

ARRL's Antenna Handbook still rocks !

old jim

 

[sophiecentaur]

@jim hardy Never knowingly under-engineered!
I just went out and bought one on the grounds that an off the shelf antenna will cost less than the bits (unless you have a workshop with a critical mass of off-cuts and then things are different). Also, I could guarantee that mine would be three or four times heavier!

 

[jim hardy]

I've never 'crosssed the pond'.
Does "Her Majesty's Communication Commission" have a similar resource for helping individuals select and aim their antennas ?

 

[CWatters]

Some info here...

http://www.digitaluk.co.uk

If you enter a postcode and drill down you can get info on the recommended transmitter its groups, MUX frequecies distance etc.

 

[sophiecentaur]

I've never 'crosssed the pond'.
Does "Her Majesty's Communication Commission" have a similar resource for helping individuals select and aim their antennas ?

HM doesn't care so much about her subjects (lol) as to help them with that problem but the figures are available and there's always a friendly company who will offer to sell you the most appropriate aerial.
At one time, the BBC had an Engineering Information Department (BBCEID) which would actually talk to individual members of the Public and give them advice. That facility was judged too expensive and they stopped it.

 

[Klystron]

Apropos antenna placement I learned by helping my father install and tune neighbors' TV antennas around Silicon Valley (Santa Clara Valley), a shallow wide valley near the Pacific coast. The marketing craze back then was for tall vertical extension poles complete with guy wires. Deluxe models had electric drive motors with preset stops supposedly optimized to favourite channels with multiple antennas. Father emphatically demonstrated a short antenna pole strapped securely to a chimney supporting a single correctly tuned array received adequate signal as the expensive spinning tall rigs. Help was free though friends had to listen to dad's lectures on commerce and physics.

 

[sophiecentaur]

Deluxe models had electric drive motors

In the UK, rotators have never been required - except by enthusiasts and Hams - because the service was planned centrally so that a vast proportion of viewers (95+%) have been catered for from a single Tx site. A possible down side was that only four or five channels were available over air. It's only since DBS and terrestrial DTV that people expected to get more than that. A different environment for the broadcasters and audience than in the US.