Hi Warren,
I assume you have a personal field called "antenna" and active in your Log Main Windows.
At first you must add the new value typing Tools>Setup>List of valid value and clicking twice on "antenna" field name.
Add with "+" the new antenna value and his description and SAVE.
Goto Logmain window and type Log>Setup>Default.
In the Field column choose antenna(user) and into Expression field using the filter on the right you can built the same description you recorded before and that you want auto fill into the qso.
Hope this help for you.
73's de IKØCHU Mauro
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IKØCHU Mauro