diff --git a/doc/user_guide/en/protocols.adoc b/doc/user_guide/en/protocols.adoc
index f52b67cb9..35ea1b871 100644
--- a/doc/user_guide/en/protocols.adoc
+++ b/doc/user_guide/en/protocols.adoc
@@ -274,6 +274,15 @@ with constant envelope, equivalent to a Minimum Shift Keying (MSK)
 waveform.  Frame duration is 72 ms, so the effective character
 transmission rate for standard messages is up to 250 cps.
 
+Contest Mode in MSK144 conveys an additional acknowledgment bit (the
+"`R`" in a message of the form `W9XYZ K1ABC R FN42`) by using the fact
+that meteor scatter and other propagation modes usable with MSK144 are
+generally effective only out to distances of order 2500 km.  To convey
+the message fragment `R FN42`, WSJT-X encodes the locator as that of
+its antipodes.  The receiving program recognizes a locator with
+distance greater than 10,000 km, does the reverse transformation, and
+inserts the implied "`R`".
+
 MSK144 also supports short-form messages that can be used after QSO
 partners have exchanged both callsigns.  Short messages consist of 4
 bits encoding a signal report, R+report, RRR, or 73, together with a
diff --git a/doc/user_guide/en/vhf-features.adoc b/doc/user_guide/en/vhf-features.adoc
index f3e359423..a34f7a6db 100644
--- a/doc/user_guide/en/vhf-features.adoc
+++ b/doc/user_guide/en/vhf-features.adoc
@@ -271,6 +271,21 @@ messages at 50 or 70 MHz.  At these frequencies, most pings are long
 enough to support standard messages -- which have the advantage of
 being readable by anyone listening in.
 
+- A special *Contest Mode* for MSK144 can be activated by checking a
+box on the *Settings | Advanced* tab.  This mode is configured
+especially for VHF contests in which four-character grid locators are
+the required exchange.  When *Contest Mode* is active, the standard QSO
+sequence looks like this:
+
+ CQ K1ABC FN42
+                      K1ABC W9XYZ EN37
+ W9XYZ K1ABC R FN42
+                      K1ABC W9XYZ RRR
+ W9XYZ K1ABC 73
+
+In contest circumstances K1ABC might choose to call CQ again rather
+than sending 73 for his third transmission.
+
 === Echo Mode
 
 *Echo* mode allows you to make sensitive measurements of your own
diff --git a/lib/fix_contest_msg.f90 b/lib/fix_contest_msg.f90
index 2374f8fb0..411ef282a 100644
--- a/lib/fix_contest_msg.f90
+++ b/lib/fix_contest_msg.f90
@@ -5,25 +5,21 @@ subroutine fix_contest_msg(mycall,mygrid,hiscall,msg)
 ! of grid1.
 
   character*6 mycall,mygrid,hiscall
-  character*22 msg,t
+  character*22 msg
   character*6 g1,g2
   logical isgrid
 
-  t=trim(mycall)//' '//trim(hiscall)
-  i0=index(msg,trim(t))
-  if(i0.eq.1) then
-     i1=len(trim(t))+2
-     g1=msg(i1:i1+3)
-     if(isgrid(g1)) then
-        call azdist(mygrid,g1,0.d0,nAz,nEl,nDmiles,nDkm,nHotAz,nHotABetter)
-        if(ndkm.gt.10000) then
-           call grid2deg(g1,dlong,dlat)
-           dlong=dlong+180.0
-           if(dlong.gt.180.0) dlong=dlong-360.0
-           dlat=-dlat
-           call deg2grid(dlong,dlat,g2)
-           msg=msg(1:i1-1)//'R '//g2(1:4)
-        endif
+  n=len(trim(msg))
+  g1=msg(n-3:n)//'  '
+  if(isgrid(g1)) then
+     call azdist(mygrid,g1,0.d0,nAz,nEl,nDmiles,nDkm,nHotAz,nHotABetter)
+     if(ndkm.gt.10000) then
+        call grid2deg(g1,dlong,dlat)
+        dlong=dlong+180.0
+        if(dlong.gt.180.0) dlong=dlong-360.0
+        dlat=-dlat
+        call deg2grid(dlong,dlat,g2)
+        msg=msg(1:n-4)//'R '//g2(1:4)
      endif
   endif