In uk.d-i-y, The Natural Philosopher wrote:

We are not I believe talking about splitting it or tapping into it. we
are talking about joining it. A solder joint as I described is usually
better than a plug and socket.

Agreed. (The wording of the original post did suggest a 3-way "tap"
rather than a simple join, but I think that was an oopsie of phrasing).
A *neat* solder joint, as taught to radio hammer-chewers, is indeed more
likely to perform well at RF than introducing a plug-n-socket; although
a crappy soldered joint would be worse than competently-attached plug and
socket.

So, to repeat in Stefek-speak the "how to solder coax" instructions: the
goal is to create as neat and just-like-the-rest-of-the-cable join as possible,
definetely including using the same inner insulation ('dielectric' if you
like the fancy words) and resulting cable dimensions, not great gobs of
insulating tape, stupidly sharp turns, wierd lumps, odd transition
materials, or whatever. So you strip back both ends, carefully: strip
outer insulation for say 2 cm, slitting it neatly up to the stripping point
and not chucking it away. Push back the outer braiding into a nice bell
shape. Cut off the inner insulation/dielectric for say 1cm on both cables;
slit one of those slugs down its length. Now lay the inner 1cms side by
side, and holding in place using your third and fourth hands solder
quickly together (don't melt the insulation higher up) - practice this
step on scrap bits. If you don't have a third and fourth hand, you can
wrap the two inners firmly together with some thin wire before you solder.

Now put the slit slug of dielectric back over the soldered-together inners;
a single turn of thin insulating tape can help hold it in place, but don't
overdo it with lots of turns. Now it's time to join the outer braids together.
As others have written, if it's copper it's eminently solderable: tease
the end of the braid apart a bit, weave the two ends together to create
a nice not-noticeably-thicker interlacing of the two braids, spot-solder
(don't overdo the soldering - again, the idea is not to melt the inner
dielectric). If aliminium or copper foil, soldering won't work: smooth
into place, maybe add the bit of ali foil another poster suggested.
Cover with the previously slit slug, or a scrap bit of the outer
insulation, holding down with insulating tape or the previously-
mentioned heatshrink (which you didn't forget to slip on first ;-). For
a joint to be subjected to sun and rain, it's worth hunting down the
adhesive-lined heatshrink, which will make a much more waterproof joint.

If you think you're too cackhanded to do this neatly, then a plug-n-socket
might give better results. BUT how are you going to attach those? If
soldering, it's somewhat easier than a cable join, but still susceptible
to oopsies; if you get crimp-on fittings, you'd better know someone who
will lend you the 100-quid-plus proper crimping tool - you're unlikely to
get a quality joint with your second-best pair of slipjoint plumber's
pliers ;-) If you really want to louse up the RF properties of the coax,
separate out the inner and outer conductors for a good few inches, join
inner-to-inner, outer-to-outer in a bit of oversize (30 amp) choccie block,
wrap with gobs of self-amalgamating tape, bang a masonry nail through
it all to 'secure' it to the wall, and call yourself a registered $ky
installer ;-)

HTH, Stefek