Date: 201606
Local time of day: 1201-1800
Location: ZTL.ARTCC, GA
Aircraft: Small Aircraft; Low Wing; 2 Eng; Retractable Gear
Flight plan: IFR
Flight phase: Cruise
Airspace: Class E ZTL
ATC / advisory: Center ZTL
Second aircraft: Any Unknown or Unlisted Aircraft Manufacturer
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Troubleshooting; Workload; Distraction; Communication Breakdown; Time Pressure
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Separated Traffic; Air Traffic Control Provided Assistance; Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance
Contributing factors: ATC Equipment / Nav Facility / Buildings
Primary problem: ATC Equipment / Nav Facility / Buildings

Narrative: I was working the R-side control position at ZTL-R49/18 Logen/Commerce Sectors. Weather was VMC. Traffic and complexity were moderate. I was transmitting and receiving on frequencies 121.35 for Logen and 134.8 for Commerce. I was on main transmitters/receivers. I took the handoff on Aircraft X from AVL Tracon; enroute in level flight at 10;000 from AVL direct [destination]. The aircraft checked in on 134.8. The aircraft was transiting the far north eastern portion of the combined sectors and on initial contact the aircraft was 5x5. His Mode-C was not broadcasting so I verified that he was level at FL100. There were multiple other things happening in the sector as I was also sequencing air carrier aircraft into ATL from the north east. 	I detected a traffic confliction for Aircraft X. An unknown VFR aircraft was approximately 10 miles west north west opposite direction with Mode C indicating 9;800 climbing. The targets appeared likely to merge. I began to issue traffic to Aircraft X including my intention to initiate a traffic alert vector within 90 seconds if he did not see the other aircraft. I received no reply. I made two more transmissions without a response as the aircraft began to approach what I considered very unsafe proximity. Unfortunately; due to documented random frequency coverage issues on 134.8; I was unable to communicate any longer with Aircraft X using the main transmitter. I made two calls and then switched to Back Up Emergency Communications (BUEC); issued a traffic alert and a last ditch vector to the north away from the unknown traffic just as the traffic climbed through the altitude of Aircraft X. Aircraft X executed his vector and missed the traffic by what appeared to be less than a mile; co-altitude.If I could judge the proximity and if there was such a term in ATC land; I would have called this a near mid-air collision. Fortunately the BUEC worked (today). Note-the BUEC often does not cover this piece of the sector's airspace. We have complained over and over and over and over about 134.8. The frequency does not cover the ZTL-18 airspace in all portions of the sector. This is a known and on-going issue that the FAA seems unable or unwilling to spend the resources on mitigating. Someday soon; two aircraft are going to collide in ZTL-18 because the frequency 134.8 coverage / quality is so unsatisfactory.I recommend that the FAA either relocates 134.8 to a more appropriately sited Remote Center Air to Ground (RCAG) site; or else provide a new frequency for ZTL sector 18. It is merely a matter of [time] before the sheer volume of low-altitude traffic transiting ZTL-18 with flaky and unpredictable radio coverage results in a major midair collision with casualties; press coverage; and extreme liability for the Federal Aviation Administration.




Synopsis: Atlanta Center (ZTL) Controller reported a Near Mid Air Collision he attributed to lack of sufficient radio coverage. Controller had to use a backup emergency radio and transmit into the blind. Aircraft heard transmission and turned away from traffic.