Tony wrote:
"Looking to build a decent 6 mtr antenna... probably a loop... Space no
problem indoor or out..."

I suppose vertical polarization is used in the repeater you would like
to reach 50 miles away, so that it is compatible with mobiles.

Hopefully your repeater antenna is at high altitude on a TV tower or
some such lofty perch. You need about 1250 feet of elevation to provide
a horizon 50 miles away.

Height of the station antenna on your end reduces the repeater antenna
height required. A vertical loop antenna is fed in the middle of a
vertical side and makes an array of two 1/4-wave vertical dipoles,
separated in space by 1/4-wave. It has slightly more gain in the
direction of the plane of the vertical sides. The ground quality
surrounding the antenna affects its performance but radials are not
worthwhile.
There is a slight gain versus a groundplane antenna.

In free space, the radiation resistance of a square quad loop of one
wavelength circumference is about 120 ohms. It would work best with
balanced feed.

ON4UN`s "Low-Band DXing" has a chapter "Large Loop Antennas" which would
likely be worth a look.

Best regards, Richard Harrison, KB5WZI