How big is your window? A loop of wire 15" tall and 29" wide, fed with
coax in the middle of one vertical side, gives a pretty good match to
50 ohms and has a little bit of gain over a dipole. You can stick it to
the inside of the window using suction cups of the kind they sell for
hanging Christmas lights (or just tape.) You may have to adjust the
dimensions to account for the effects of the glass. Or you can hang
it on the back of a curtain in front of the window.
Antenna have gain because they focus more energy in some directions
than in others. A beam of some sort sticking out the window can be
arranged to minimize radiation back into the room, but to some
extent that will depend on the antenna design and the direction you
want to point it in. Some high-gain yagis can actually radiate more
power in the rear lobe than a dipole, while with others it can be less
by a factor of 10 or more. If you really are concerned about radiation,
coat the back of your curtains with aluminum foil and use it as a reflector -
that's probably the best you can do for shielding in such a short space.
Certainly use only the amount of power necessary to have a clear signal
into a repeater. I use 50 watts only in fringe areas. I leave my home
rig on the 5 watt setting and I can hit plenty of repeaters.
There are lots of possible beam designs, for yagis and quads, plus some
other quirky varieties. If the antenna has too much gain it will have
to be rotated, though fortunately the beamwidth is wider for vertical
polarization than for horizontal with a yagi. A short 3-element yagi
can be made easily from a piece of wood or plastic pipe and some copper
wire - all you have to do is to figure out how to mount it.
As you look around on the web you will find a lot of different designs:
don't feel you have to build any of them exact in all details. There are
some that have good construction techniques but a poor electrical
design. Some over emphasize design parameters that realy aren't
of much importance. In fact, the whole process of choosing a beam
is a matter of trading off a lot of different factors including size, cost,
gain, front-to-back ratio, beamwidth, SWR, SWR bandwidth and feed
method, along with mechanical considerations such as weight, material
availablility, and ruggedness. So the first step is to consider what
of those are most important to you: what beamwidth do you need to
cover all (or most) the repeaters and simplex directions of interest?
How practical is it to reach out the window to adjust the direction of the
antenna, vs. leaving it fixed in one place? What construction methods
are you comfortable with? What test equipment do you have to get
an antenna tuned up properly? How can you get the coax back inside
the window? And what is the building made of?
You may find that a simple dipole spaced out from the side of the
building is adequate for most operation. Or you may need some sort
of beam hinged against the building at the back to allow you to
aim it over 180 degrees to get the repeaters you want (or your friend's
house for simplex.)
But the best advice is try it and see. We can't know everything about
your building and operating interests - and as a newcomer you may not
know them yet either. Start with something simple - even the mag
mount on the filing cabinet - and see how well it works. Then try a
simple vertical hanging in front of the window and see if that is better.
You may find that moving the antenna around inside the room makes
a huge difference - though the optimum location may be different
for different repeaters. (I knew one family who had an X of masking
tape on their living room carpet - that was where you had to stand
to work a particular repeater with a hand held.) If that does everything
you want for the moment, fine. If not, make a list of the places that
need improvement: perhaps there is one repeater on the opposite
side of the building, in which case you may end up using one antenna
sticking out each side of the building.
One of the things to learn as a ham is that antennas are never permanent.
They aways can change to better suit your needs. Don't worry about
having to have everything perfect to start with - until you get some
operating experience you may not even know what type of operating
will interest you most next year. So start simple and improve things
as you see the need.
Oh, and do keep your power down - good antennas will make far more
difference than raw power output. And remember that you can't work
a station if you can't hear them (which a good antenna also helps with)
regardless of how much power you run.
If you want some idea how difficult it is to build a beam antenna, check
out the Clear Lake Amateur Radio Club site at www.clarc.org
on "articles of interest", then choose "V/UHF Antennas - CHEAP". That
is one of the best feed designs I have found - simple to build and
works well without needing much adjustment. (I use PVC pipe for the
boom rather than wood, but that is a minor point.)