"JEFF UK" wrote.

I am now planning to put up my first dipole for SWL.

.............. let me ask first, why you especially want a dipole? Dipoles
are great for directional
broadcasting. Shortwaves come in from random reflection points only in the
general direction
of ..... there-to-there. :) Different frequencies will resonate best at
certain angles from the
antenna. 90-degree broadside is only for transmitted frequencies specific to
the cut dipole
lengths.

1) A sensible length for general SWL.?

............... for the range you specified, you might be well satisfied with
50-100 feet of wire.
The idea is to get it as high as possible off the ground, and not threaded
through trees.

2) I would like a diagram of a 1:1 balun construction.

.............. Construction of a Balun for impedence matching is a matter of
some patient
engineering. Better to purchase one already made. A good one for use with
52 ohm coax
will best be mounted at the top where the legs of the dipole meet, before
signal travels down
the coax to your RX.

3) I will be using 75 ohm tv coax to its feed point which is 90 feet from
the house.
Will this be ok ?

................. 75 ohm coax will be fine. Too much emphasis is placed on
use of 52 ohm coax.
Both are considered low-impedance. But, get 100% insulated shield. Braid,
not aluminum
sheath TV cable. Better, however, would be 600-ohm TV track wire for that
distance.
Cheap stuff. 600 Ohm is considered hi-impedence, and should enter the RX
using that
connection instead of the PL-259 Lo-Z input.... or feed it thru a passive
tuner. See below.

4) I will need to bend the legs of the antenna to fit them in the garden.

.................. Uh Oh! Yes, this will indeed affect impedence, freq
response, as well as
Mu factor and shape of Q. Consider, frequency response may be enhanced one
way
or the other by passing the signal through chokes or coils. Bending your
antenna around
corners "to make it fit" is essentially starting such a "coil" and the
frequency response will
be affected according to how much wire length you are using.

Bottom line: Keep it straight, high, well insulated with propers hardware
mountings. Feed
your downlead end through a passive tuner such as a TUN-3 or TUN-4 then to
the RX.
GROUND the receiver chassis with THICK cabling, perhaps several strands of
THICK
twisted TV aluminum grounding wires. Go the shortest, most direct route to a
real earth
grounding rod, or bury a piece of concrete reinforcement grid underground to
hook onto.