CAPT OF AN MLG CPR JET ACFT INADVERTENTLY SWITCHED OFF OF DEP FREQ RESULTING IN MOMENTARY LOSS OF COM.

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ON DEP FROM RWY 22L AT MIDWAY; THE VORS WERE SET TO MOLINE; THE FIRST OUTBOUND FIX; AND THE #2 DME WAS ON HOLD FOR THE 31C LOC. THIS WAS TO CHK THAT THE INITIAL TURN WAS MADE WITHIN 4 NM; AS REQUIRED BY THE MIDWAY 4 SID. AFTER OUR INITIAL TURN TO 250 DEGS; WE CONTACTED CHICAGO DEP AND WERE ASSIGNED A HDG OF 290 DEGS AND CLRED TO CLB FROM 2000 FT TO 4000 FT. AT SOME POINT DURING THE CLB; OR PERHAPS AFTER REACHING 4000 FT; I REACHED DOWN TO DE-SELECT THE DME HOLD. THINGS WERE PRETTY BUSY AT THIS POINT; AS THERE WERE A LOT OF ACFT ON THE FREQ; AND WE WERE IN SOLID IMC AND ICING CONDITIONS. SHORTLY AFTERWARD; I NOTICED THAT THE CTLING AGENCY ON THE FREQ WAS MIDWAY TWR. I LOOKED AT THE RADIO CTL HEADS AND DISCOVERED THAT THE COM HAD BEEN TRANSFERRED BACK TO THE #2 RADIO; WHICH STILL HAD THE TWR FREQ SET IN IT. I RESELECTED DEP AND A MOMENT LATER THE CTLR CALLED US WITH A CLB CLRNC. HIS TONE INDICATED THAT PERHAPS HE HAD ALREADY TRIED TO CONTACT US; ALTHOUGH HE DIDN'T SOUND UPSET. WHILE PUZZLING OVER HOW WE HAD GOTTEN OFF FREQ; I NOTICED THAT THE DME HOLD ANNUNCIATOR ON THE INST PANEL WAS STILL ILLUMINATED. LOOKING DOWN AT THE VOR CTL HEAD; I REALIZED THAT WHEN I WENT TO PUSH THE DME HOLD BUTTON TO DE-SELECT HOLD; I HAD ACTUALLY PUSHED THE NEXT ONE TO THE L OF IT; WHICH LOOKS JUST LIKE IT; BUT IS THE BUTTON THAT TRANSFERS COM FROM THE #3 TO THE #2. I WAS SHOCKED TO FIND THAT THROUGH A MOMENTARY LAPSE OF ATTN; I HAD RENDERED MYSELF INCOMMUNICADO IN THE CHICAGO CLASS B AIRSPACE ON A BUSY IFR DAY. THE COCKPIT WORKLOAD WAS HIGH AT THE TIME; AND THERE WAS NO URGENCY TO GET THE DME ON MOLINE. I COULD HAVE WAITED TILL THINGS SETTLED DOWN (THE WORD 'PRIORITIZE' ECHOES FROM PAST INSTRUCTORS). ALSO; I REALIZED LATER; THIS WAS MY FIRST FLT IN THE R SEAT OF THIS TYPE OF ACFT IN 2 MONTHS. I'M SURE MOMENTARY UNFAMILIARITY WITH THE CTLS DUE TO A LACK OF RECENT EXPERIENCE WITH THEM WAS A MAJOR CONTRIBUTOR TO THIS INCIDENT. (MOST OF MY RECENT FLYING HAS BEEN IN A SIMILAR ACFT; ALBEIT WITH DIFFERENT RADIO CTLS.)