No signal on your TV? Which channels you lost tells you why

Sydney is unusual among Australian capitals in that all five free-to-air networks are broadcast on VHF Band III rather than UHF, from three towers clustered within about a kilometre of each other on the lower North Shore. For aiming purposes that is effectively one direction. For fault-finding it is five separate frequencies, and that is the part worth knowing.

Seven, 177.5 MHz

Transmitted from Willoughby. The lowest frequency of the five.

SBS, 184.5 MHz

From Artarmon.

Nine, 191.6 MHz

From Artarmon. The top of the lower group.

Ten, 219.5 MHz

From the ABC tower at Gore Hill, and the start of the upper group.

ABC, 226.5 MHz

Also Gore Hill, and the highest frequency in the Sydney line-up. All five run at the same 50 kW, so when one network drops and another holds it is not because one is transmitted more weakly.

Note the gap. Seven, SBS and Nine sit between 177 and 192 MHz. Ten and ABC sit nearly thirty megahertz higher, at 219 and 226. Almost every loss mechanism in a domestic antenna system — corrosion, water in a connector, a long or tired cable run, an antenna losing elements, an amplifier past its life — costs you more at the high end than the low end. Which is why the failure so often looks like this: ABC and Ten go first, and everything else carries on as though nothing has happened.

What the pattern of loss tells you

ABC and Ten gone, the rest fine

The most common presentation in Sydney and the most informative. Something in the path is rolling off at the top of the band, and the usual suspects are a corroded connector, water that has tracked into the cable, a badly weathered antenna, or simply too much cable and too many splitters for the signal you started with. It is very rarely the television, and re-tuning will not bring them back for long because the margin is gone rather than the memory.

Seven gone, the rest fine

Seven is the odd one out twice over: lowest frequency, and the only one from Willoughby rather than Artarmon or Gore Hill. The towers are close enough that aiming rarely singles it out, so a Seven-only fault more often points at something frequency-selective, and the usual answer is an antenna that is not well matched at the bottom of the band. Worth metering rather than guessing.

Everything gone at once, no picture at all

That is a break rather than a degradation. Somewhere the path is open or shorted: a fly lead pulled out behind the television, a wall plate with a broken tail, a cable chewed through in the roof space, a masthead amplifier whose power injector has died, or an antenna that has come off its mast in a southerly. The power injector is worth checking first because it is indoors, it has a light on it, and it fails more often than people expect.

It breaks up when it rains, then recovers

Water is getting somewhere it should not, and the classic location is an F-connector at the antenna or at a joint in the roof space that was never weatherproofed. Rain bridges the gap, the connection degrades, then it dries out and the picture returns. The tell is that it recovers rather than staying broken, and it will get worse each season until the corrosion becomes permanent.

It is worse on warm evenings, especially in summer

That is usually not your system at all. Temperature inversions over the coast let distant transmitters carry much further than normal, and a co-channel signal from outside Sydney arrives to compete with the local one. It affects whole streets, it lasts hours rather than weeks, and no amount of work on your roof will prevent it. If your neighbours have it at the same time, wait it out.

It started when you added another television

Then the house has run out of margin rather than developed a fault. Every split costs signal on all legs, so the third or fourth outlet is often the one that pushes the weakest channels below the threshold. Note what this means: the new outlet is not faulty, and neither is the old one. The distribution needs rethinking for the number of points, which is a different job from replacing an antenna.

Three things worth trying before you call anyone

• Re-tune the television, and do it as a full re-scan rather than an update. If a channel has moved or the set has kept a stale entry, this is the free fix, and it takes two minutes.
• Swap the fly lead between the wall plate and the television, and push both ends home properly. It is the cheapest component in the whole chain, it is handled more than anything else, and a loose or damaged one produces exactly the symptoms of a much more serious fault.
• If the house has more than one outlet, try the television on a different one. If the second outlet is clean, the problem is downstream of the splitter and the roof is fine. If every outlet is equally bad, it is upstream, which means the antenna, the cable from it, or the amplifier.

Those three cost nothing and they genuinely resolve a fair share of call-outs. What they also do, if they fail, is hand a useful answer to whoever comes next.

What you cannot settle from inside the house

Everything above narrows the fault. None of it measures anything. The number that decides the job is how much signal is actually arriving at your roof and how clean it is once it reaches the wall plate, and that needs a meter at both ends. It is also the reason a picture that looks fine is not proof the system is healthy: digital reception holds a perfect picture right up until it does not, so a system running on almost no margin looks identical to one running on plenty until the first bad weather of the season.

Where the checks above do not clear it, that is TV antenna repairs and reception fault-finding, metered at the antenna and at the outlet so the cause is found rather than replaced around.