I've retuned my TV and I now have a choice it would seem

[dothawkes31]

have 13 channels including ITV but not the BBC channels or 38 channels including the BBC Channels but not the ITV channels. will this sort itsself out if i just keep retuning the set?

[Buenchico]

Hi Dot:

The signal strengths received by your aerial don't remain constant. They're affected by lots of factors, including weather conditions, the time of year (because the reflective layer of the ionosphere is at different heights depending upon the number of daylight hours), the 11 year sunspot cycle and solar flares on the surface of the sun. There are, of course, simpler things which can reduce the efficiency of your aerial, such as a tree growing in front of it or the wind knocking it out of alignment with the transmitter.

In theory a TV aerial should be designed so that the lengths of its elements are exactly matched with the frequency of the incoming signal. Since each channel is on a different frequency you theoretically need a different aerial for each channel. (Imagine that lot on your roof!). Since that's impractical, all aerials are only a 'rough match' to incoming signals, so they'll always be better at receiving some channels than others.

When you tune your TV it will only 'lock onto' signals with a certain minimum strength (even though a slightly lower strength might be acceptable for day to day viewing of the relevant channels). When you originally tuned your TV all of the signals met the minimum standard, so your TV stored all of the channels. Now that conditions are currently less favourable, your TV can only lock onto the strongest signals.

If you keep retuning the set you might well find that reception conditions have improved, so that your TV can again lock onto all available channels. If not, it's time to upgrade your aerial system. However (since your aerial is clearly almost good enough for reliable reception) that shouldn't mean paying an aerial fitter to install a new aerial and cabling. A cheap signal booster should fix the problem.

Chris