Fairfax City and Falls Church last week signed cable television contracts with Media General Cable of Fairfax County Inc., leaving the town of Herndon the only jurisdiction bordering the county without a cable contract.

Herndon Town Clerk Mary Ingram said last week that two companies, Media General and Cable Communications Corp. of Loudoun County, have asked for copies of Herndon's guidelines for firms who want to file a proposal for cable service. Ingram said town officials expect both companies will file bids for the cable franchise before the Sept. 6 closing date and that the town council is expected to award a contract in the fall.

CCC, which won the Loudoun County franchise a few months ago, is considering purchasing the Leesburg franchise from Storer Communications, which is reportedly trying to sell the 2-year-old system for $2.4 million. The Loudoun system proposed by CCC will serve primarily eastern Loudoun County and, when finished, be adjacent to the town of Herndon.

Alexandria and Arlington residents already have cable, as do sections of Prince William and Prince George's counties. Montgomery County recently signed a contract and expects to have service within the next year.

The District of Columbia is the only major jurisdiction here with no cable system pending, although the City Council hopes to request bids for a system this fall.

Media General president Sam Jernigan said last week that the Media General system will be available to more than 200,000 homes in Fairfax County, and that the company was expecting 50 to 60 percent of the homes to subscribe. Jernigan said cable service in Fairfax City and Falls Church will be identical to that in Fairfax County even though the incorporated towns and cities are signing separate agreements.

The Fairfax City cable franchise is expected to bring the city about $37,000 a year in revenue, with at least half the city wired for service in 14 months.

Falls Church officials said they do not have projections on revenue from the cable franchise and may not have cable service available until early 1986.