General aviation pilot reported an altitude excursion during cruise in IMC weather as the pilot entered a downdraft and was unable to control the aircraft. The pilot regained control and a loss of separation was reported by the controller.

Narrative

I was flying from ZZZ to my home base; JWN; at 4000 feet; on an IFR flight plan; and in contact with Memphis Center. Conditions in IMC were smooth and my autopilot with altitude hold was engaged. Communications with Memphis Center were 5 by 5 and the frequency congestion was light.I was told to contact Nashville Approach on XXX.X and communications deteriorated immediately to static and frequency congestion. The Nashville Controller immediately advised me of weather to my left and ahead. My iPad was connected to a Stratus ADS-B In device. No weather was painting on my iPad. No other weather was in the area; when I briefed the flight; I acknowledged ATC and advised I was IMC. I was alert for any change in conditions; and expecting rain.Seconds later my left wing dropped and I began descending in a no rain downdraft. I had to disconnect my autopilot; I leveled the wings and then applied full power. Even so; I was still descending within the downdraft. But for the descent; conditions were smooth. Maximum descent rate was 769 FPM; and averaged 335 FPM. The downdraft conditions lasted about three minutes; and I lost about 1;000 feet in total. During this period I advised ATC I was 'descending'. In retrospect I should have said 'Unable to maintain altitude.'; but I had my hands full and my first priority was to get out of the downdraft with my wings level. Static still prevailed on the frequency and it was difficult to listen to calls. I did try to contact ATC several times during the descent; but did not receive a reply. I learned later the controller was coordinating my loss of altitude with Memphis Center.Once I excited the downdraft and I contacted ATC again. I was chastised; on frequency; for an extended period of time for violating separation. I offered to continue this conversation offline; and the controller cleared me direct to JWN at 3;000 feet; but then began explaining his side of the story.I have only guessed at the separation loss with the other aircraft in this report.Lesson learned: I should have said 'Unable to maintain altitude' instead of saying 'I'm descending'. My phrasing caused confusion in my communications with the controller. I also experienced a true to life lesson on the latency of ADS-B weather.An additional safety factor; affecting this flight and others; is the constant static on Nashville Approach frequencies caused my numerous radio antennas in the area.