I'd start your search on the 18th
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Here's my list from the 2016 Inauguration. Some of it changes and some deleted but still a good place to start.
The Coast Guard is tasked with airborne homeland security patrols and intercepting unauthorized aircraft entering restricted airspace over D.C. These helicopters use scrolling banner signs that tell pilots to contact them on 121.5 (122.75 has been used for training exercises), where they are given further instructions. Coast Guard helos assigned to D.C. are known as “Blackjack” because they originate from Air Station Atlantic City. They operate locally from Hangar #7 at National Airport.
Coast Guard
156.8000 s Marine Ch. 16 (Hailing, Distress)
156.8500 s Marine Ch. 17 (D.C. harbor patrol coord.)
157.0500 s Marine Ch. 21 (CG, Blackjack helos)
157.1500 s Marine Ch. 23 (CG Sta. Washington)
162.0500 s [ $293] CG-108
165.3125 s [ $293] CG-121 (CG Sta. Washington)
412.9750 s [ $293] CG-409 (CG aircraft and cutters)
345.0000 s Coast Guard Aircraft (primary)
237.9000 s Coast Guard Aircraft (secondary)
326.1500 s Coast Guard Aircraft (working primary)
379.0500 s Coast Guard Aircraft (working secondary)
The USCG has more than 40 non-marine channels available for use. On the water, the agency will establish this security zone on the Potomac and Anacostia during the inaugural period.
Joint Base Andrews (KADW) Aviation Freqs
ANDREWS TOWER: 118.4 349.0
ANDREWS GROUND: 121.8 275.8
POTOMAC APPROACH: 128.0 269.0
POTOMAC APPROACH: 118.675 323.175
POTOMAC DEPARTURE: 118.95(WEST) 125.65(EAST) 257.2(WEST) 348.725(EAST)
POTOMAC APPROACH/DEPARTURE 323.175
CLEARANCE DELIVERY: 127.55 285.475
AFRC 756th ARS OPS: 351.2
DC ANG 201st AS Ops/CP 314.25
US Navy VR-53 Ops/CP 328.4
EMERG: 121.5 243.0
PMSV METRO: 344.6
ANDREWS COMMAND POST (c/ss Griffin): 378.1/141.55 (AM)
RDR: 128.35 257.2 286.6 301.5 316.7
PTD: 139.3 (AM) 372.2
SPISY STAR: 133.75 254.35
GCA: 125.35 335.5 360.8 379.2
TIKEE STAR: 120.45 306.925
VUDOO STAR: 126.4 282.375
DC ANG 121st FS Supervisor of Flying (SOF) 139.9 (AM) 234.8
Air Force 1/SAM A-A 292.8
1st Helo Squad, 89th A/W, (c/ss "Musel") 141.700 (AM) / 292.200
DC ANG 121st FS A-A discretes (all AM) 138.025/139.15/139.45/143.15/143.6/143.85
Fort Belvoir/Davison Army Airfield (KDAA)
Davison Army Airfield provides "shuttle" services around the Military District of Washington for military VIPs.
118.850 Davison AAF GCA/P
119.950 Davison AAF RADAR
121.900 Ground Control
126.300 Tower
139.400 Davison AAF Metro “Davey Ops”
229.400 Tower
241.000 US Army common
245.200 Clearance Delivery/Ground Control
246.800 Army National Guard, Air to Air
255.700 Army National Guard, Air to Air
Pentagon Heliport (KJPN)
143.100 Tower
231.300 Tower
268.000 Wheelhouse Pentagon switchboard
287.600 Foghorn Control
The Marine Corps presidential and vice presidential executive flight squadron is known to the military as HMX-1, to the Secret Service as Nighthawk, and to the public as Marine 1 or 2. HMX-1 helos & V-22 Ospreys are based at MCAF Quantico VA, but often stage at the old Naval Air Station in Anacostia.
USMC Executive Flight Squadron (HMX-1), MCAF Quantico VA (KNYG)
Quantico ATIS 263.150
Quantico BASE OPS 355.3
Quantico GND 340.200
Quantico APP 127.05/290.375
Quantico TWR 118.600/360.2
Quantico Common 265.8
Quantico Helipad 375.0
HMX-1 CONTROL 277.175
HMX-1 discrete 273.950
HMX-1 discrete 143.825 (AM)
HMX-1 discrete (c/ss NIGHTHAWK) 142.75 (AM)
HMX-1 Ops 276.4
HMX-1 Base 318.9
HMX-1 Maintenance 320.4
Combat Air Patrols
139.700 (AM) Huntress, Coast Guard Helos
260.900 Huntress, CAPs
228.900 Huntress
135.525 Guard Dog CAP Ctrl
350.250 Guard Dog
Here are other important aviation frequencies including 134.35 which is especially useful to identify what helicopters are flying over D.C.
National Airport (Mount Vernon Sector)
118.300 / 306.300 Final East DCAFE
118.950 / 257.200 Departure West (loiterers over D.C.)
119.100 / 257.600 Reagan Tower
119.300 / 335.500 Approach ADWAR
119.850 / 322.300 Approach West QJAAY
134.350 / 257.600 Wash Helo Control
121.050 / 343.700 West 10,000' up to FL 230 FLUKY
121.500 / 243.000 Aircraft Emergency
124.000 / 279.575 Andrews for Arl Flyovers, ADWFR
124.200 / 269.000 Approach East ENSUE
124.700 / 338.200 Final West DCAFW
125.650 / 348.725 East 9500' & below KRANT
126.550 / 269.500 East 10,000' up to FL 190 DAILY
126.550 / 269.500 Dep. East 10K ft up to FL 190 DAILY
128.350 / 270.275 App. SE 19K ft down to 3K ft DEALE
Civil Air Patrol (CAP)
Don’t confuse Combat Air Patrols with the Civil Air Patrol, both CAP! The Civil Air Patrol radio nets are typically activated to support the inauguration. See Radio Reference for other simplex channels that could also be used.
143.900 Maryland Civil Air Patrol (FM Mode)
148.1250 r/s [varies] (143.55 in)
148.1500 r/s [136.5] (143.7 in)
148.1375 r/s [varies] (143.625 in)
150.5625 s [151.4] Falcon Virgo intercept exercises
Department of Homeland Security (DHS) aviation assests
DHS discrete 136.275 (AM) (FBI)
DHS discrete 136.375 (AM) (Customs)
DHS discrete 136.475 (AM)