[HCRA] Re: [YCCC] Attic Antennas

[Marc]

Dan,

Antennas are loads of fun.  :>] Here are a few
comments:

> I am an apartment dweller. I have an attic app.: 45'
> x 25' ---51.5' diagonal--- give or take a few inches
> here or there. It is located 3 stories up and
> begging for an HF antenna of some kind allowing me
> to operate. I am trying to get 10-80m (I am insane
> but not fooling myself about 160m) operation using a
> Kenwood TS 820S and/or a Drake TR4CW thru a Dentron
> MT2000a Tuner. I would like to use at least do 100
> watts although I am afraid of RFI. I think I may
> have gotten lucky with some lattice in the walls
> protecting the few brain cells I have left after
> some of my other stunts.

I'm not sure I understand your dimensions..  Is the
shack going to be in the attic space, or one floor
down with you having access to the attic?  Also, given
the dimensions, 80 might be a bit tough.

You will need to check on the RF exposure levels being
that close to the antenna.  On 10 meters you might
have an issue at 100 watts.  I forget the numbers...
Check out www.arrl.org for more info.

> 1) Trap Dipole: I have seen a decent article on one
> design but I am wary of all the crud they emit for
> RFI. http://www.nerc.com/~jdegood/coaxtrap/

What do you mean "crud"?  Why would a trap emit crud? 
Not sure what you are refering to here...
> 
> 2) G5RV: I can contort one into the attic, but G5RV
> seems to argue against twisting his antenna too
> much.

If you want to stick to a G5RV type design, you can
divide the dimensions by two and work only the upper
bands.  Also, bending down the ends of a dipole do not
adversely effect dipole operation due to the current
distribution, so you could get an extra 5 feet or so
on each side.

> 3) Loop: I could whip up a 40m loop easily in that
> attic. It wouldn't be a perfect square but it would
> accomplish what I want. It is also a damned quiet
> antenna to operate in a city setting. I would still
> have RFI problems on 75 and 80 though. *As I sit and
> think I am wondering if I would have much RFI on
> properly balanced and fed loop.*

First off, I'm going to assume that you are one floor
down from the attic.  If you are not, my comments are
even more applicable.  One common thread to your note
is about RFI or radiation from the feedline. Remember,
you are only a hand full of feet away from the
radiator which is hopefully radiating a majority of
your input power.  The vast majority of RFI problems
will come from the fact that the antenna (radiator) is
close to those things suseptable to RFI, not due to
tthe feedline, "bad" traps, or a non-balanced antenna.
 

By the way, yes a loop can be quiet, when placed in
the clear...  It is worth a shot in the attic.

> 
> 4) Random Wire: I have done this with a friend once
> before. The only worry I found was we needed to stay
> away from roofing nails. The buggers load up wrong
> and/or ground you out.
> 
 
Wht does "ground you out" mean?  To ground out some
thing you must have a low impendance path back to the
source.  Roofing nails, unless touching each other all
the way to the earth or back to your radio cannot
ground out the antenna.  They may provide intermittant
connections causing noise or (rectification into RFI I
suppose).  Also, do not use bare wire.  Try THHN wire
(available where you work I believe).  This will
eliminate you (or roofing nails) from being in direct
contact with the radiator.  

As for loading up wrong, simply attach a 1/4
wavelength wire (for each band) on the back of the
antenna tuner (ground lug).  (Remember that at the end
of the wires is a high voltage node, so keep the end
insulated so you do not get RF burned) These will
provide a low impedance path for the station (at RF)
and will make the tuner much happier (and easier to
tune).  A good RF ground can help to solve many "shack
RFI problems".

> I would like some feedback about which antennas
> people have tried and how well they performed. Have
> you had big problems with RFI or QRM? Do any of
> these antennas perform better then the rest? Also, I
> am looking almost exclusively at feeding these
> antennas with some form of twin lead. I think I
> would a much wider resonant frequency response, and
> I want to hook it up to an old short-wave receiver
> sometime.

Attic antennas can be finicky, but they will radiate
and net you some contacts.  The best antenna [for 40
to 10 meter coverage](in the attic) I've tried is a
simple 44 ft long dipole fed with open ladder line. 
No traps (losses...), low feedline losses due to open
line construction, and pretty cheap.  I've tried loops
(which worked ok), G5RV types (less performance than
44 ft dipole), and assorted wire beams for 10 meters. 


I've had very limited luck on 80 meters.  40 through
10 are ok considering the circumstances.  I've worked
all around the world (UA, 4X4, PY (5 watts), etc.)
with the 44 ft long dipole (but under good
conditions).  As the sunspot cycle declines, your
mileage will vary!!!  Try some out.  Wire is
relatively inexpensive and you might learn something
(oh, I mean have fun...)!!  

Having your room, I would try out the 44 ft dipole
first (with 1/4 wavelength wires off of the tuner). 
Get a feel for how it plays and maybe try out a loop
fed with ladderline.  Let me know what happens.

Besides the ARRL antenna boook, try www.cebik.com for
some great antenna ideas / information including the
44 ft dipole...

If I can help, you know where to find me!!

73,
Marc
K1TS

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