The analogue signal will be gradually switched off
Retailers have been criticised for giving "shocking" advice about the digital switchover in Dorset.
Trading standards secretly contacted electrical stores and aerial fitters and found many gave wrong advice.
Officers found some fitters recommended new aerials while one store said customers would need a new £1,000 television despite there being no need.
The analogue signal will be gradually switched off region, by region and completed by 2012.
Officers called 14 aerial installers and visited 15 electrical retailers as part of the operation.
They said nine of the installers gave bad or misleading advice, suggesting that people are likely to need a new aerial for switchover.
Ofcom believes only 5% of homes will need to upgrade their aerial systems.
Five installers told the mystery shopper they would need a "digital aerial" to receive digital TV, when in fact there is no such thing.
Only two pointed to the test card which is a good indication whether an aerial upgrade will be needed.
Out of the electrical stores, one wrongly recommended a new television costing £1,000 - a Freeview box costing about £20 will convert an old set, trading standards said.
Bill Taylor, South West Regional Manager of Digital UK, said: "These are shocking figures.
"It's clear that only a tiny minority of retailers and aerial installers manage to give completely accurate advice about digital TV switchover.
"It's absolutely essential that no one is pressured into buying equipment they do not need."
Bill Jaggs, Dorset council's head of regulatory services, said: "This survey makes depressing reading.
"We will be working with retailers and installers to improve their service."