Birderman wrote:
As a beginner in SWL I hope that somdebaody can give me some advice:

(welcome!)

From what I have already read a longwire is the solution, but some
references refer to connecting the wire directly to the receiver and others
talk about a Balun. Since my receiver has two antenna connections (1 hi
impedance and 1 low impedance) I assume that if I connect just the wire I
connect it to the high impedance but if I use a Balun I can connect to the
low impedance. But what If I do not use a Balun and connect to an ATU
instead , the output from it is PL259 coax connector do I just connect this
to the low impedance input or to the high impedance one. Or do I connect the
balun close to the antenna and use a coax feed between it and the ATU low
impedance input.

Both the balun and ATU have (relatively) low-impedance outputs - should
be connected through coax to the low-impedance input of the receiver.
If you use both, then yes, the balun connects to the ATU's low-impedance
input.

To get the required length if I was to run the antenna in my attic is it
reasonable to loop it around the attic several times or should it be a
single loop.

Additional loops beyond the first won't make a huge difference but if
you already have the wire - and don't have the space to spread it
straight out - then it won't hurt to loop it more than once.

When calculating the required length I assume that full wave is preferable
but if its not possible then it should be calculated to be 1/2 or 1/4 is
that a correct assumption. Is there any drawback in having it shorter than
fullwave ?

For receive applications there is no "required length". The preferable
value is "as long as possible". The longer the antenna, the more
effective it is.

"Magic" lengths like full-wave, 1/2 wave, 1/4 wave are mostly for
transmitting applications. These magic lengths have predictable
impedances that can be connected to transmitters without risk of
overloading high-powered parts. If you don't intend to transmit, then
you don't care.

"Magic" lengths are also somewhat directional. However, if you have no
way of aiming your antenna at the desired stations (it looks like you
don't) then there's no point in using an intentionally-directional
antenna. In any case, once you place it in an attic (and not outside in
the clear) the directional characteristics are unpredictable.

Point being, just make the wire as long as possible and don't worry
about any specific length.

Should the antenna wire be insulated or non-insulated type ?

If it isn't going to come in contact with conductive objects (electrical
conduits, heating ducts, etc.) then it doesn't matter.
--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com