Satpal Chahal left his 72-year-old victim, who suffers from a heart condition, "absolutely traumatised" and living in fear that he could be attacked again.
Maidstone Crown Court heard Chahal jabbed at John Whiteford with the Stanley knife-like weapon while repeatedly slapping him around the head and eventually robbing him of his £350 iPhone.
Chahal, who was wearing a hoodie and a scarf across his lower face, was said to have "invaded" the pensioner's home in a hunt for "easy-pickings" on January 24.
But despite his fear, Mr Whiteford - who lives alone in Gravesend with his two dogs - stood up to the 29-year-old, ordering him out of his home and warning he would call the police.
Chahal, who has two previous convictions for robbery, was arrested after police found his DNA on a sock he wore over his hand.
Jailing him for seven years and four months, Judge Jeremy Carey said the sentence would have been 11 years had it not been for his guilty pleas to robbery and having a bladed article.
He added that while Mr Whiteford had not suffered physical injury, "the scars of this traumatic incident are likely to run deep".
Judge Carey also said the victim's vulnerability and the use of a potentially deadly weapon were just two of "at least" eight aggravating features of the case.
He said: "It was not an instrument for work, it was a weapon you had to frighten the life out of him, which you did.
"But, not content with threatening this elderly man, vulnerable and weaker than you, you brutally slapped him about the head, jabbing the knife towards him and robbing him of his mobile phone."
Judge Carey said it was to Mr Whiteford's "great credit" that even though he was scared he told Chahal to get out of his home.
He said: "Unsurprisingly, his victim impact statement speaks of the dread that he has of this happening again; no longer will he feel safe in his own home - something that is so precious to most people that its loss should not be underestimated by anyone, least of all those who have the duty of sentencing offenders such as you."
James Ross, defending, said the best mitigation he could put forward on Chahal's behalf was his guilty pleas.
He told the court that despite having 10 previous convictions for 13 offences, Chahal, of Rose Avenue, Gravesend, had not committed any violent crime since 2004.
Mr Ross said having made a determined effort to put his offending behind him, to stop taking drugs and get a job as a machine operator with a printing firm, he had behaved "wholly out of character" on the day of the robbery as a result of alcohol.
"He would like me to express in open court his remorse and repentance and pass on his best wishes to the victim and wish that he doesn't suffer anymore," added Mr Ross.
"It was not an instrument for work, it was a weapon you had to frighten the life out of him, which you did..." - Judge Jeremy Carey
Prosecutor Vivian Walters said the pensioner answered and was confronted by Chahal, who swore at him and asked where his sister was.
Mr Whiteford tried to explain that he did not know what he was referring to and suggested he may have the wrong address.
However, Chahal continued to ask after the whereabouts of his sister – which Judge Carey later branded as a "nonsense pretext" – while moving towards Mr Whiteford as he backed along his hallway.
Chahal then produced the knife and slapped Mr Whiteford four times, causing his glasses to fall off, before demanding money and patting down the victim's pockets.
Jabbing at Mr Whiteford with the knife, Chahal demanded his mobile phone.
Having handed it over, the pensioner told him to "get out" and warned he would call the police. But the court heard due to the loss of his glasses he was unable to see the numbers on his landline and had to shout for help.
Mr Whiteford later told police he could feel his heart pounding and was so shaken that he felt "absolutely traumatised".
His phone has not been recovered.
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