Those of you involved with domestic TV systems in any way may find this story a familiar one. Make yourselves comfortable......

A guy I've done work for before seems to have gotten into whole-house TV in a big way. A couple of years ago I installed a satellite system for him using an older analog receiver and dish that he'd been given. Since then, he's swapped over to a digital receiver and run his own distribution system around the house. The feeder from the regular UHF roof antenna was looped through the satellite unit, then through the VCR, back up into the attic to a 4-way distribution amplifier, and from there back down to the main TV in the living room and to three other sets around the house. He also had direct SCART (audio-video) connections between the satellite/VCR/main TV.

All normal broadcast TV in the U.K. is on UHF. The four main networks on the local transmitter are up near the top end of the band on channels 55, 59, 62, and 65. By the time I looked at this set-up, the four off-air UHF signals viewed directly on any of the sets around the house were poor, to say the least. Apparently it hadn't bothered him too much, as all are available on the satellite anyway, so they only watched one direct if the satellite receiver was being used to watch/record something else at the same time. They could also switch the VCR to one of the networks and watch the VCR output around the the house, which resulted in much improved picture quality. So the set-up was far from perfect, but it had satisfied them for a while.

Now, came the problem. He'd gone out and bought a new digital terrestrial receiver which he wanted to add to the system. This needed a feed from the UHF roof antenna, and obviously its RF output would also need to be distributed to everything so it was yet another box for the line to loop through.

The result was that the four analog UHF signals were now varying in quality from very poor to terrible when viewed directly. He'd connected the new digi-box to the main TV with a SCART lead, but had run into big problems with trying to tune in the new signal on that set's tuner and around the rest of the house. He'd also got a pre-amplifier to insert in the attic on the incoming feeder from the antenna to try to boost the signals. That had worked (I should hope so, as it provided 20dB gain!) and the four analog UHF signals were now excellent throughout the house, but the satellite signal now had bad patterning. It was at this point he became totally confused and called yours truly (probably at the behest of his wife, who didn't understand why he wanted yet another box anyway ).

It's already too late to cut a long story short, but I found that the reason he couldn't properly tune in the signal from the new digi-box was that its output was set to channel 36. The VCR was already feeding its signal out on ch. 35. Retuning the digi-box output to a clear spot at ch. 48 fixed that.

The patterning on the satellite signal around the house was caused by the now very strong analog broadcast signals coming down from the attic. The digital satellite receiver he'd acquired had its RF output tuned to channel 68, and with the now 20dB stronger off-air signals the distribution amplifier in the attic was cross-modulating and throwing out all sorts of spurious signals across the band. One of them happened to be right on ch. 68. I retuned the satellite output down to the bottom of the band at ch. 21, and also found several bad joints at coax plugs. Fixing them resulted in good picture from the satellite receiver throughout the house.

Now one of the problems this guy faced with setting up all this equipment is the complicated on-screen menus used on all this equipment. Obviously we had to go around the house and re-tune every TV to the new frequencies, but every set was different, and a couple of them had such complex menus (and no instruction books!) that it took me at least ten minutes on each to figure out how to even get into the tuning menus.

The digital satellite box has a whole batch of set-up menus, but anyone looking for a way to change the RF output channel would be out of luck. You can go through every menu you can see and not find it, because it's on a hidden "installer" menu which isn't even mentioned in the user's manual. You just have to know to press 0, 1, Select. I can see how they might want to prevent Mr. Average from messing around with the LNB IF offset, 22kHz switching signal, etc., but why hide the option to change the output channel? More and more people are getting complex set-ups like this these days, so they're bound to need to change the output from the default at some point to avoid clashes.

Anyway, both the lady and gentleman of the house were satisfied with the eventual quality of all pictures, and I left them with consistent settings on all TVs: 1 thru 4 = normal networks, 7 = digital terrestrial, 8 = VCR, 9 = satellite.

I'm not sure they've quite mastered how to switch to direct A-V inputs on the main TV in the living room or quite figured out just exactly what combinations of channels they can watch and/or record at the same time, so I wouldn't be surprised to get another call sometime soon.

Did I mention that he also bought a new widescreen TV for the living room? I'll save my comments on that for next time!


[This message has been edited by pauluk (edited 07-31-2003).]

Paul:

Sounds as though you have been quite busy. I know that poor connector connections can be blamed for a whole lot of interference problems, poor signals, etc. Multiply that by 3 connectors and they're lucky to have had much of a signal at all.

BTW, there was an excellent article on spectrum analyzers and coaxial cable (diagnosing problems on the coax) in this month's S&VC.

The online version doesn't have the photos; let me know if you are interested and I can get a copy to you.
http://svconline.com/ar/avinstall_no_hit_miss/index.htm

Paul,
That's a great story.
In all the times that I have repaired TV systems like the type that you mentioned, the problem has been caused by the homeowner, who has used the original coax (normally single-screened R-206) or twin lead (ribbon) cable and fitted the highest gain amp in the land, to the system, in the hope that there will be MORE signal at the other end of the cable.
With respect to Output channels on Decoders, the Sky UHF Decoders over here, have a pair of 10 position (0-9) Rotary switches on the rear panel and these are used to set the Output frequency.
On the Digital decoders, the presets can only be accessed by inputting an installer code.
One thing I do know about Digital TV recievers, is that if there is a small fault in the input signal quality, you don't get a picture at all, as opposed to the snowy picture on an Analouge TV.
How do the Minimum Signal levels required for a Digital TV compare with that of an Analouge TV?.
Were F-connector type plugs and splitters used in this installation?.

Thanks for the link. I'll read it tomorrow over my coffee break (getting rather late now) -- Looks interesting.

That's one of the problems with these sort of high frequencies: It takes only one bad joint to cause an impedance mismatch, set up standing waves on the coax, and cause all sorts of interference problems.

Trumpy,
The F-connectors are only really used here on the coax from satellite dish to satellite receiver. All the regular UHF antenna connections are the Belling-Lee coax plugs (U.S.-style 300-ohm balanced feeders never caught on here for TV).

There were actually a lot more plug and socket connections even than my description might have led you to believe. The drop from the roof antenna had a splice in the attic on its way down to the living room. Each coax cable entered a coax socket in the corner of the room with a fly lead to the various equipments. B-L coax plugs were used to connect to the input and outputs of the distribution amplifier.

So with all that lot plus the connections at each box, the signal was actually going through no less than 13 plug/socket combinations on route from antenna to TV. With several bad joints you can see why the signal was poor, and that before you even include the attenuation through the threediplexer/combiners in the digi-box, satellite receiver, and VCR.

If you think that TV installation's complex don't look into an attic over here in Cork, Ireland!

In the days before BBC 1 & 2 were carried on SKY digital
Co. Cork was exclusively served by a few systems (operated by a monopoly cable company):
Cable in the city and larger towns.
MMDS in the non-urban areas.

(ITV/C4 still are not on the Republic of Ireland's SKY package..the irish terestrial channels are on 101-104 and BBC 1/2 on 214/215 so cable/mmds are still the only legal ways of receiving ITV or C4)

However, MMDS coverage isn't exactly universal and the company who operates the cable and mmds systems tends to be pricy and seem to have employed ex-gestapo agents as customer service people!

The result was that during the 1980s a community owned and operated UHF rebroadcasting system was established (without a licence) to rebroadcast BBC 1, BBC 2, ITV and C4 from overspill signal picked up on the east coast (130 miles away). The system picked up the UK broadcasts on a huge tower antenna and beamed them back to a network of distribution TX's around the county. Since cutting off people's favourite TV channels goes down like a lead balloon in an election, not to mention the fact that the TV community group inserted help messages after popular programmes like coronation street to rally the "troops" (community tv campaigners) to come out and march if anyone dared to raid their facilities , the system has been allowed to continue to operate as long as it respects the frequency allocations used by RTE and other broadcasters. However, I suspect that the advent of full "freeview" digital terrestrial type TV here will be the end of the line for the rebroadcasting system.

Anyway, back to the point:

RTE 1 / Network 2 broadcasts on a mix of VHF and UHF
(along with TV3 and TG4 only on UHF)

Southcoast community television broadcasts 4-5 channels on UHF
(for years this included sky one and eurosport!!!)

So the average house has 2 flat panel wideband (UHF/VHF) antennas on the roof pointing at different transmitters.

Using a completely weird mix of diplexers, filters and attenuators tuned to filter out unwanted signals the feeds from the two antennas are merged into one cable and usually fed from there to a distribution amplifier feeding at least 3-4 (if not all) rooms in the house. It would be quite common to add a door security camera on UHF, maybe the feed from the VCR/SAT box too !!

Our antenna installers tend to have to be able to resolve all sorts of ghosting, crosstalk, patterning, weak signals, etc etc..

Basically you have:

2 X antennas (with inline amplifiers on the pole)

feeding through all the attenuators etc..

the internal stuff (sat boxes, door cameras etc) is added in.. possibily more attenuators if there are problems with interference / crosstalk.

then on to the distribution amp..

it can get as complicated as a mini-cable tv system!

Generally analogue reception is excellent though!

ITS COMPLICATED


[This message has been edited by djk (edited 07-31-2003).]

[This message has been edited by djk (edited 07-31-2003).]

...which then amplifies the noise...

Ah yes...... I wish I had a nickel for every time I've tried to explain that to somebody. A wideband amplifier does just what it says and amplifies everything you feed into it, noise and spurious signals included.

The dual-use of VHF/UHF in Ireland (and most other countries) certainly makes the situation more complicated.

In the days of 405-line VHF television in the U.K., people often needed two antennas. In all but a few places, BBC and ITA (Independent TV) had their transmitter sites in different locations, so rooftops often had a band I (BBC) aerial pointing one direction and a band III (ITA) pointing in another.

By the time the UHF 625-line network was planned in the 1960s, the BBC and ITA had agreed to co-site their transmitters to use shared facilities, and the band was planned so that an eventual four networks could be broadcast from every site, the channels assigned in groups to allow higher-gain aerials to be employed (hence the four channels in this area being on 55, 59, 62, and 65).

It was only BBC2 on UHF at first, so many rooftops then added a third antenna for that, but when BBC1 and ITA went onto UHF from the late 1960s onward, it was possible to abandon the two tons of rooftop art and receive all three (later four) stations on a single UHF antenna.

This is one area where British planning was actually excellent, for once.

It's just a pity they've made such a mess of it in recent years with the fifth network and digital.

Paul:

Does each TV set in the UK still need to be licensed? Did radios (receivers) ever need to be licensed?

Yes, TV licenses still exist, at a cost which is now over £100 ($160) per year. A separate license for each individual set is not required -- Just a single license per household.

Radio licenses were introduced back in the 1920s when the BBC came into being. WHen TV arrived, if you had a TV license then you were also covered for radio and didn't need a separate radio license.

The radio license was abolished in 1971.

ThinkGood,
You're on to it!.
Like Paul said, most people don't understand that RF amplifiers amplify EVERYTHING that is fed into them, noise included.
All a masthead-type amp, does, is maintains the signal at a given level, to remove some of the loss in the downlead, a bit like having your TV on the roof!.
Customers that I've spoken to in the past, seem to think that an amp will make up for a poor Terrestrial signal or a shoddy installation job and man have I had some funny looks when I have told them the truth!.

Ahhh, the fun of MATV setups gone bad!

Albiet, nowadays (since 2001 to be exact), equipment is easier for end users to obtain, install and configure (plus plan out!) - resulting in a record low in panic calls to "come fix it!!!"
Haven't dealt with any calls for almost 2 years!

Now prior to that (<2001), things were much different, and had many odd mismatches and incorrect connections to deal with.

Problems ranged from the very simple, to the very extreme, as:

• Incorrectly setting TV/VCR to a "Dead" VHF channel - typically Ch 03 (60-66 MHz),
• Inputs/Outputs swapped,
• RCA type patchcords for line level patching either in bad shape, or patched thru incorrectly,
• No AC power to preamps, dist. amps or MUX(es),
• Really crappy make-up on "F" connectors, loose and / or incomplete connections,
• Using RG-59u Ethernet type coax (10 base 2) with RG-6 coax and connectors, or RG59 connectors on RG-6 cable,
• Using Ethernet 10 base 2 coax - with / instead of - RG-6 cable,
• Connecting any type of 75 ohm Z coaxial cable directly to 300 ohm Z terminals, without going thru a Z matching transformer,
• Long unamplified looped runs, using a bunch of 3-way splitters,
• Driving VCR output, or CATV input, directly into a VHF/UHF Antenna,
• Poor connections thru RF switching items - such as Video Game Interfaces,
• Broken RF in/out connections on TV or VCR,
• VHF / UHF Antenna setup and aiming problems,
• DBS Antenna setup and aiming problems,
• Kinked feeders (causing standing waves),
• Being stepped on by other broadcasted stuff (herringbone effects from HAM setups, etc.).

Ran into all sorts of issues with VHF / UHF Antenna setups.

The typical Antenna for multiband TV here is the Parasitic Yagi.

The larger VHF section consists of a Log-Periodic Dipole array, which the driven element is located near the UHF section at the front, and the reflecting elements can be "swept forward" (angled to 60°) to increase gain of lower VHF band, or not swept (straight / 90°) for normal overall gain.

The UHF section is typically a multi element Dipole arrangement - sometimes using a "Bow Tie" - and normally surrounded by a corner reflector array (reflectors above and below are 45° + and -).

Antenna problems include Off-alignment, 180° alignment (facing backwards), ½ the elements swept back, swept elements laying on unswept ones, damaged, missing or broken rear reflecting elements, other elements missing or damaged, reversed UHF reflectors (seriously!), damaged, missing or "tweeked on" driven elements and/or connections to it,
no Z matching transformer (mentioned before), bad mounting location or height, and use of 300 ohm Z lead in wire which is in bad condition, plus is strapped directly to Antenna mast, other cables, metal pipes, etc. (run without stand-offs).

Some of the location problems result in "Ghost" reflections from trees and buildings.

Some "Fringe Areas" have low power problems, which usually result in channels being stepped on, or just turn out to have a solid "Ant Race" over a snowy picture.

The Hz's here cover:

<OL TYPE=1>

[*]Lower VHF TV Bands (Ch. 02 thru 06): 54 MHz thru 88 MHz,

[*]VHF FM commercial Radio: 88 MHz thru 108 MHz,

[*]Upper VHF TV Bands (Ch. 07 thru 13): 176 MHz thru 216 MHz,

[*]UHF TV Bands (Ch. 14 thru 83): 470 MHz thru 890 MHz.
</OL>
TV channel occupies 6 MHz, whereas the FM Radio channel occupies 200 KHz.

Just some FYI

Scott35

p.s. Thanks, Paul, for the thread!

The combination VHF/UHF TV antenna never caught on here, due to the nature of our separate 405 and 625-line services.

Basically, up until the 1960s TV was on VHF only using the 405-line system. In 1964 the second BBC service was opened, using the new 625-line UHF allocations. Dual-standards receivers appeared at that time, with separate VHF and UHF tuner inputs, and anyone in the BBC2 service areas thus needed both UHF and VHF to receive all three networks.

BBC2 started regular color broadcasts in 1967, and in 1968/69 the other two networks also started opening color transmitters on 625/UHF. The 405/VHF services continued to simulcast signals (in black-&-white only) until 1985, but of course anyone wanting all three stations in color had to switch to UHF. It was only a few years before all new sets in the U.K. came equipped with just a UHF tuner for the 625 system.

This is our typical UHF antenna.

We have a number of narrow band UHF yagi type antennas in use exclusively for picking up RTE/NET2/TV3 & TG4 on UHF only.

These are colour coded depending on the channels a particular transmitter or relay is using.

This type covers everything, including rebroadcast UK channels on "deflector" systems.



End of an era:
VHF is gradually being switched off as its bandwidth is being swollowed up for various digital broadcast services etc.
here's how its done:
Taken from a Cork City VHF PAL 625 line broadcast of RTE 1



[This message has been edited by djk (edited 08-02-2003).]

I think you use the same UF aerial grouping as we have in the U.K.:

Group A (red), ch. 21 - 37
Group B (yellow), ch. 35 - 53
Group C/D (green), ch. 48 - 68
Group E (brown), ch. 39 - 68

Most installations use a group A, B, or C/D antenna, but group E is specified in a few areas where the channels couldn't be kept all together within one of the other
groups.

Wideband aerials (group W, black) are also widely available.

Just to complete the picture for anyone not familiar with our channels, the UHF band runs 470 - 854 MHz with 8MHz channel spacing.

djk,
That's a good pic of a Phased Array UHF Aerial, we get really good results over here in NZ with these aerials, but boy are they expensive, so we normally end up using a Yagi-type Aerial arrangement, for most customers.
Paul,
I went to do a Fault-finding job on a customers set-up the other day.
And I have to say, that it was the tidiest, most well thought-out TV/Radio/DVD/VCR and Audio Reticulation System that I have EVER
seen put into a house by anyone!.
Everything was marked correctly and at ALL plugs, he had a system diagram with coded outlets and he had a "Central Systems Hub" in his roof void, where he had all the system amplifiers, connectors, etc, all neatly laid out in a way that would make a TV systems tech shed tears(Why aren't they all like this?).
We got to talking and I found out that he was a Labourer that had scraped and saved for the last 10 years to make up and install this system and I commended him on his eye for detail.
The reason I was called there though, was because he had no Terrestrial TV (VHF/UHF) reception, which I traced(by use of my trusty Field Strength Meter) to a cut down-lead as it went into the house under the steel flashing(another guy had installed the aerials, due to the owner being scared of hieghts), so I pulled in a new D/Lead and sealed the area where the cable entered, with RTV to prevent the cable being cut again, too.
We checked the picture on his Big TV and he was happy!.

Wow! All neat and tidy, AND everything labeled -- Now that's a rarity!

I haven't expanded upon this part yet:
Like many TVs these days, this set was obviously designed for use in different world markets and had the typical modern array of on-screen menus to set up everything. The manufacturer often uses the same basic programming for every set in the range but masks out menu options which are not needed for a particular country/area or for a particular model.

This set looked as though somebody hadn't quite decided where it was going to be sold!

The manual stated PAL system I (U.K./Ireland) and B/G (most of Europe, except France). Like many new sets, it has auto-tune and pre-programmed lists of stations for each country so that they can be put on the presets that most people use (e.g. 1=BBC1, 2=BBC2, 3=ITV, etc.). The channel listing options had U.K., Germany, and a couple of other European countries which I forget, but also included the French listings. But the set had no facility for switching to SECAM system L for France!

Despite the manual indicating that the model was also sold for Ireland, it had no Irish station lists, nor was there an option to select Irish VHF channels (which are different to European).

It did, however, have the option to switch to OIRT (Eastern Europe) channels, and the menu allowed switching to NTSC as well! (Though there was no way to select American VHF/UHF channel allocations.)

The illustrations in the book (which clearly stated PAL-only in the specs for this model) showed brightness, contrast, color, andhue adjustments. The owner assumed something was wrong, because his menu didn't include the hue adjustment (I verified that it did appear when the set was switched to NTSC mode).

The average buyer is confused enough by all these different settings without being confronted with this sort of inconsistency as well.

As I already mentioned earlier, Asutria has 2 (now 3) terrestrial networks. ORF 1 is VHF ch. 1, ORF 2 is something like UHF ch. 34 (lotsa interference with nearly all VCRs being UHF 36) and the new private ATV+ is somewhere in the higher UHF area. Many people here just have a small VHF/UHF antenna on top of their TV set. 2 rods for VHF and a Dipol for UHF, or even just the 2 rods sticking out of the top of the TV set. VHF reception is usually quite good, ORF 2 is poor and ATV is terrible (mostly black&white and lots of ants running), so some people start resorting to amplifier antennae. These usually boost up ORF 1 to more or less perfect and ORF 2 to viewable (at least in our area of town, somewhere else it might be very well the other way round). The only way to get decent ORF reception is cable TV.
TV and radio licenses are still needed here and not too cheap, I think something like 22 Euro per month. The license specifies how many TV sets and radios you're allowed to run. A license is even needed if you only have satellite TV and no antenna.

Paul,
With respect to a "Wide-Screen" TV.
I realise the size of a screen is related to the Aspect Ratio, but does a wide-screen TV have an Aspect Ratio of 16:9?, or some other figure?.

[This message has been edited by Trumpy (edited 08-16-2003).]

I've noticed that with TVs here too.

I think it's a little more than just software to save cost they have one basic TV unit with different tuner moduels that can be plugged in for different markets during manufacture. The TV itself will be exactly the same, the software will be the same but certain functions may be different. A French version will suport SECAM and French channel system. An Irish TV will incorporate a VHF and "Cable Hyperband" tuner as well as UHF. The UK version will simply have UHF etc etc.

As for the Manuals they're often very generic and badly written so may just include absolutely everything with **
** = Not applicable in the UK, Germany, Ireland etc..

Most TVs here auto tune the channel names too although get them slightly out of date.

They should read:
RTE 1, NET 2 (since 1988), TV3 and TG4

But you always seem to get:
RTE1, RTE2, TV3 and TnaG

Our sony TV seems to support just about everything in Europe.

It gives a list of countries and you just pick which one you're in. Seems to have no problems supporting SECAM or PAL I/B/G etc..

Trumpy:

We usually have a pair of those phased array aerials

Yes, some models leave out particular sections of circuitry which are not applicable to the intended market. But these days so many parts of the set are in large-scale integrated circuits that the functionality is still there. For example, many modern sets use a multi-mode color decoder IC which supports NTSC, PAL, and SECAM by applying a selection voltage to the appropriate pin, so they just mask out the options that don't want you to see in the menu programming.

Trumpy,
The widescreen sets are a 16:9 aspect ratio. This is actually another thing which is bugging me somewhat. 16:9 mode is fine for watching widescreen movies broadcast in that format, but "regular" TV is 4:3.

So many of these sets have a default setting which stretches the normal 4:3 broadcast picture to fit a 16:9 screen, resulting in lots of fat-faced people and flattened circles and squares. Considering the huge amount of work in the earlier days of TV devoted to obtaining the best linearity and picture geometry possible, I find it ironic that many people now put up with this horrible distortion just to fill the whole screen.

Some don't actually realize that there's an option to switch to 4:3 mode for normal broadcasts.

*An aside: Early British broadcasts were actually 5:4 aspect ratio. The 4:3 ratio was standardized in the late 1940s.

The more expensive Widescreens, e.g. the top of the range Sonys etc do a much nicer job of "stretching" standard broadcasts to widescreen. I'm not 100% sure what goes on but I think they use software to apply the stretching to different parts of the screen so you don't just end up with noticable distortion.

It's often better to leave the TV in switching mode so when it's receiving widescreen it'll fill the screen and when it's normal broadcast it will remain square.

The problem is that applying digital trickery to stretch only parts of the picture still means that some portion of the image has to be grossly distorted.

AS you say, I think it's much better to just have a black space at the left and right of the screen when running in normal 4:3 mode.

Guys,
Check out this link!.
http://www.xtramsn.co.nz/technology/0,,7004-2649263,00.html

I've seen the plasma screens at some of the "Big Box" stores over here. I think a lot of people are getting interested in the idea of a flat-screen on the wall as being a neat idea (and the stuff of futuristic settings only a few years ago!).

I've not been at all impressed with the LCD screens for TV. I find the rendition to be very artificial-looking.

Or have I been conditioned by years of watching a regualr CRT?

cheaper ones produce that harsh look. Also very big screens fed from a digital source can look rather harsh. PAL and CRT often makes colours look more natural thanks to good old fashioned analogue technology

I think duplication of CRT-beam/phosphor image rendition is a ways off yet.

Yeah,
I'd agree with you there, Paul about the Plasma screen TV's.
I went to have a look at a TV reception problem yesterday and found that the signal level on VHF was too low and installed a new Log-Periodic Yagi aerial.
But, long story short, I could'nt get over how strange the picture on this TV screen looked, it looked more like a computer monitor than a TV set.

I had to go back to the house I mentioned at the top of this thread today.

The owner had moved the antenna (as I discovered later!) and rerouted a lot of the cables, running them down trunking to the sockets in the living room.

Then he discovered a problem. The strong RF signals from the VCR, DT (digital terrestrial) box and satellite receiver were fine all around the house. Two of the four off-air analog stations were of acceptable quality but not perfect, one was quite snowy, and the fourth very bad.

That was without the extra pre-amplifier in the attic on the downlead from the UHF roof antenna. When he'd connected the amplifier in line, the whole band was awash with spurious cross-modulation products, rendering even the strong locally-generated RF signals unwatchable. Not surprising considering there was 20dB gain on that amp, then a route through three diplexers in the various boxes, then into a distribution amp with another 16dB gain to each output.

Simple fact is that the signal strengths aren't too bad here, and with a 13-element Yagi array on the roof, a 20db pre-amp is just not necessary! Out it went.....

I checked that the signal coming in off the roof antenna was good -- It was.

Next step was to fix several bad joints -- Again! (Why do some people find it so difficult to install a coax plug properly?)

That improved the signal strengths no end. I then tossed out the cheap and nasty RF patch cords supplied with each box and installed replacements with decent coax cable. By this time I had all four UHF stations to acceptable quality around the house, along with all the VCR, DT, and satellite signals.

All seemed fine, until I'd began checking out the satellite receiver, then switched back to analog UHF and noticed that one station was now very poor again.

Cut a long story short (Too late! ), disconnecting the satellite dish feeder resulted in perfect UHF reception. Turning off LNB power from the receiver gave the same result.

It was only then then the subject of the mvoed antenna came to light, and when I went out and looked up on the roof, there was the UHF antenna sitting about 18 inches away from the LNB on the satellite antenna! Not surprisingly, the UHF antenna was picking up signals from the LNB which were desensitizing the receivers, especially with the 16dB gain provided by the distribution amplifier.

But why had it been all right earlier?

It was only as I was switching back and forth and turning off the LNB power from the satellite receiver menu that the penny dropped.

The universal LNBs used with most modern Sky Digi-boxes in the U.K. are dual-band. A 22kHz command signal sent up the feeder from the receiver switches the local oscillator in the LNB on the dish from low-band to high-band.

A few tests confirmed it: The UHF reception was poor only when the satellite receiver was on a channel which resulted in the LNB being switched to high band.

Moving the UHF antenna away cleared the problem.