In article , I wrote:
The ARRL Handbook, and the Antenna Handbook, will both have
sections on antenna couplers. A simple L network is basically
only two components, with at least one of them being tunable.
The trick is choosing the components and their configuration,
and that's where the commercial product comes in: they've already
worked out compromise values.

There is an external antenna specified in the manual, and an aBay search
for completed listings, worldwide, for a 278-1374, gives this item
(hope it's usable):

https://www.ebay.com/itm/Radio-Shack...p2047675.l2557

Note that elsewhere in the listing it gives another part number, I think
one I saw listed somewhere as a "similar item". Looks like it might just
be a clip-on, retractable, extension wire?

Price was $8.88 plus shipping from Plano, TX, a Dallas suburb near Texas
Instruments headquarters.

Also, I wrote that you might need an antenna tuner for an external
antenna, but this just seems to clip on directly, so that's a good sign.
Otherwise I was going to guess that the receiver had inductive
antenna impedance compensation, varying by band, and building just a
series capacitor substitution box that went by decade from 50pf to
50nf might compensate for that in order to work with an antenna of
design impedance of, say, 50-300 ohms.

Also, the manual I found on the web states that below 7100 Mhz, the 375
(has gap from 6250-7100), and the 396 as well, use a combination of the
whip and the internal loopstick.

George