Lee Creamer targeted areas such as Portsdown Hill and the Rock Gardens in Southsea, where men are known to meet.
Some of his victims were married men who were so desperate not to be outed they handed over hundreds of pounds.
Creamer has admitted blackmailing three people.
But police say he could have struck many times and are trying to find other men who may have fallen victim to him.
Detective Constable Graham Gray, of Portsmouth CID, said: 'He had a lot of gall to impersonate a police officer because someone could have called his bluff.
'It was calculated. He hasn't gone round the corner and stolen something from a shop. He has targeted areas and it's quite a thought-out offence.
'If you've got a wife and children then the last thing you want is to get arrested by the police for having sex with men in public.'
Creamer gave his victims a choice – they could go to court and face their families finding out what they had been doing or pay an on-the-spot fine. Desperate not to be 'outed' in public, the men handed over hundreds of pounds.
In truth, Creamer was a jobless 26-year-old who had been in and out of trouble for years.
The police say he used his experience of being on the wrong side of the law to pull off a convincing act as an undercover officer.
Creamer, of Prospect Lane, Havant, would drive his victims, to a cash machine where they handed over money for an on-the-spot fine and a course he said they had to attend, before writing them a receipt which he signed using a false name.
On June 7 he took £420 from a man, before doing the same thing to another seven days later.
And on June 15 he took £720 from a third victim.
Creamer pleaded guilty to three charges of blackmail and one of inpersonating a police officer when he appeared in court.
At Portsmouth Crown Court Judge Richard Price told Creamer: 'You need to understand the very strong likelihood is you will receive a very substantial period of imprisonment for these offences.'
He was remanded in custody.
A receipt Creamer gave one of his victims
Police believe there maybe others who have fallen foul of Lee Creamer's scam but are too afraid to come forward.
Det Con Gray said: 'If you have been approached by this man make contact with us and we can protect your identity.
'We know that he would go to Portsdown Hill and the Rock Gardens but there could be numerous other spots that he went to.'
Anyone with information should contact Det Con Graham Gray at Portsmouth CID on 101 or 08450 45 45 45.
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