[HCRA] Re: [YCCC] Attic Antennas

Sat, 16 Nov 2002 19:37:12 -0500

Greetings!

Marc has made some excellent points. I'd like to offer some additional
thoughts. Remember, however, that *anything* you put up will
radiate--even a light bulb on the end of a coax feedline (which
essentially describes some of the antennas still available on the
amateur market). How well and in what direction the antenna radiates are
the main considerations, and the more physically contorted it is, the
more unpredictable its pattern--which will vary from band to band
anyway. That said, it's awfully difficult to avoid RFI with an indoor
antenna.

The primary reason to avoid a trap dipole is that traps can be lossy,
but if you can find good one that fits the diagonal, it might be worth a
try, and you might get away with not having to do a lot of tuning.
Additionally, coax feedlines are physically easier to route and should
not radiate.

You also can install a center-fed antenna of any length (ie, as long as
you can make it; it need not necessarily be cut to frequency on any
band, and it probably would not be, since you're contemplating multiband
operation) and feed it with 450-ohm (or, for 100 W, even 300-ohm
TV-type) balanced feedline. (The 300-ohm stuff, although it's not as
available as it once was, can handle 100 W with no problems, and it's
also easier to route indoors.)

The length of the balanced feedline will affect its tuning--this kind of
system will require a bit of experimentation. While balanced feeders are
not *supposed* to radiate in a conventional (ie, open-air) setting, you
could have radiation problems if the feedline's routed near wiring,
flashing, gutters or other metallic objects (even if these are on the
structure's exterior). 

Speaking of centerfeds, a "G5RV" is a specific kind of center-fed
antenna for 20 meters--a 102-foot flattop with a balanced feedline of
any practical length. The designer found it worked well on 20 meters and
provided a reasonable match for the typical tuner. I have one
(outdoors), and it works especially well on 20 (no surprise there).
While not designed to work on other bands, it also can be made to work
on 7 MHz and higher with a tuner. I'm unaware that there's anything
especially magic about the 102-foot length (or any fraction thereof),
however. Some antenna manufacturers now sell "G5RV" antennas as well as
so-called "Windom" (off-center-fed) antennas that claim to provide
reasonably flat SWRs and employ coax feedlines to the shack. I'm wary of
some of their claims, however.

If you're going to try some kind of "horizontal" dipole arrangement,
it's OK, as Marc says, to bend the ends. You can even to coil them up or
otherwise "load" the antenna, including by the use of coils or--better
yet IMHO, with capacity hats--to make it electrically longer. In fact,
you might be able to get on 80 this way and have a reasonably efficient
antenna. 

In any event, with a horizontal antenna, try to take advantage of the
peak of the roof (assuming it has one). This is mainly to keep the
antenna as far as absolutely possible from your ac, telephone and cable
wiring, but it also puts the center up as high as possible. 

My first reaction to your e-mail, however, was that a loop (fed with
some kind of balanced feeders although a relatively short length of coax
should be OK) might be the best option of all. Again, with a tuner you
should be able to make it work on several bands. Quieter? Dunno about
that. This is not the typical application. You can design a loaded loop,
too.

A "random wire" can be made to work, but you need to work it against
something (an RF ground), so my suggestion would be to stick with dipole
or loop-style antennas. (When I lived in an apartment house a couple of
decades or so ago in Virginia, I loaded up the downspout and worked it
against ground. It was an especially good antenna on 20 meters, but it
didn't work worth a flip on 40.)

Incidentally, I have a Force-12 ZR-3 sitting in my basement that covers
20, 15 and 10 meters. That antenna is considered by its manufacturer to
be a vertical. See the "Product Review" I wrote on "compact" antennas in
the March 1998 QST
http://www.arrl.org/members-only/prodrev/pdf/pr9803.pdf. You might
discover some other ideas there as well. Anyway, a ZR-3 could be made to
stand in an attic (that way your neighbors wouldn be neither puzzled nor
affronted by its, uhhh, odd appearance). Let me know if you're
interested in cutting a deal on it, because I'm not using it (it has had
very little use, in fact). 

BTW, I operate a lot of mobile on 40-meter CW using 100 W to a
Ham-Stick-style antenna (about 7.5 feet tall), which, antenna experts
will agree, is pretty lossy and not terribly efficient. It works just
great, and I even work lots of DX too. 

73, Rick, N1RL
Feeding Hills MA

-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf Of Marc
Sent: Saturday, November 16, 2002 11:04 AM
To: Daniel J. Sullivan
Cc: [email protected]; [email protected]
Subject: [HCRA] Re: [YCCC] Attic Antennas

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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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