Non-resonant attic loop

[Eugene Worth]

I'm needing to install an antenna in my new home. An attic loop is a
necessity ... can't run an outside wire easily.

I'm assuming that an 88-ft loop will be non-resonant in all bands 80-10,
and fed with ladder line to the tuner should work.

Other suggestions?

Thanks.

gene
KC0RXY soon /9


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[Ron D'Eau Claire-2]

Gene, KC0RXY wrote:

I'm needing to install an antenna in my new home. An attic loop is a
necessity ... can't run an outside wire easily.

I'm assuming that an 88-ft loop will be non-resonant in all bands 80-10,
and fed with ladder line to the tuner should work.

Other suggestions?

----------------------------------------

For a number of years I was an apartment dweller who only once had
permission to run an outdoor antenna. My antenna of choice in each situation
was a doublet using whatever length I could manage. The feeder was open wire
line made up of two small white wires (since apartment walls are invariably
white) that went straight from the ATU on the operating table up the wall
(held to it with a couple of small staples), through two small holes in the
ceiling where it meets the wall and up to the center of the doublet. Once
above the wall, a couple of lightweight insulators made from bits of scrap
plastic (cut from film cans) were used as spacers. The spacing is not
critical.

It worked very well on all bands where I could get at least 1/4 wavelength
of wire up into the attic. That is, with about 35 feet of wire (17 feet each
side of the feeder) hung in the attic just under the roof so it was sort of
inverted V, I could work on all bands 40 meters and up. The loss of
efficiency of a short doublet doesn't really show up until the radiator gets
to 1/4 wavelength or below.

Of course, it would also load on 80 and even radiate. Shoot, an 8-foot
mobile whip will radiate on 80. But I never seriously tried to operate on 80
(or 160) from an indoor antenna because of the electrical noise. Lamp
dimmers, blow-dryer motors, every sort of appliance seems to be designed to
emit maximum electrical noise on 80 and 160 meters!

Only one time did I find my performance rather disappointing from such an
attic antenna, considering its low height (usually about 20 feet since I was
usually in a two-story building). It worked okay on 40 meters but
performance on 20 meters and up was dismal compared to other QTH's. The roof
was tile. Up close it appeared to be a colored concrete with a lot of air in
it, so while the tiles were waterproof, they weren't as dense and heavy as
normal tile or concrete. I found a broken piece in the garden, evidently
left by someone who had done some repairs on the roof.

On a hunch I took the piece of roofing tile into the house and put it in my
microwave oven (with a glass of water to protect the magnetron). After a
short run the water was warm and the piece of roofing tile was blistering
hot! Minerals or something in the tile mix was definitely not "transparent"
to radio waves - and its ability to soak up RF probably went up with
frequency.

So the sort of roof you have really does matter!

The open wire feedline made of white wires attached to a white wall was
virtually invisible - a nice feature since I often had my operating desk in
the living room. Friends visiting would have to be shown the "feeder" before
they'd see it on the wall from a few feet away.  

Ron AC7AC


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[g4ilo-2]

My attic loop is probably a bit less than 88ft, and probably a bit too
close to 66ft, but it's the biggest I can fit in my attic. With an
Elecraft BL1 balun at the feed point, and a couple of metres of RG-213
down to the radio directly below it, the KAT2 can match it on all bands
160m to 10m, except 60m which I don't have a permit for and my K2 can't
do anyway. My new T1 will match it on all bands 80m to 6m.

Although I can match the loop on 80 and 160m it isn't very useful. I
think it radiates in a vertical direction on those bands. Mostly I just
pick up loads of computery noises. I've never made a contact on any of
those bands, which I've always considered closed to QRP operators
restricted to indoor antennas. (If anyone knows how to radiate a decent
signal on 80m from something that will fit in a 6m x 6m sq attic, I'd
like to know the secret.)

By the way, I used an LDG 4:1 balun previously, and the results were
much improved when the BL1 replaced it. Signals seemed stronger, the
KAT2 managed a lower SWR on 40m and it easn't possible to match the loop
at all on 80 and 160 with the LDG balun.

73,
--
Julian, G4ILO (K2 #392)
G4ILO's Shack: http://www.tech-pro.net/g4ilo

Gene Worth <[hidden email]> wrote:

I'm needing to install an antenna in my new home. An attic loop is a
necessity ... can't run an outside wire easily.

I'm assuming that an 88-ft loop will be non-resonant in all bands 80-10,
and fed with ladder line to the tuner should work.

Other suggestions?


Thanks.


gene
KC0RXY soon /9

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[John Magliacane]

> I'm needing to install an antenna in my new home. An attic loop is a
> necessity ... can't run an outside wire easily.
>
> I'm assuming that an 88-ft loop will be non-resonant in all bands 80-10,
> and fed with ladder line to the tuner should work.
>
> Other suggestions?


Gene,

A small, closed loop operated below resonance will have a very
low feedpoint impedance.  While it will radiate virtually all the RF
it is fed, its low impedance might be tough to efficiently match to a
50-ohm unbalanced transceiver.

So, instead of making just a single turn loop, how about making a two
(or more) turn loop, instead?  This would bring the natural resonant
frequency of the antenna significantly below that of a single turn loop.

If planned out correctly, you might be able to find the ideal dimensions
of a multi-turn loop that will provide self-resonance on the low end of
80-meters.  Anything above that in frequency should be easy to match
as others have already mentioned.

Just a thought...

Good luck!


73, de John, KD2BD


Visit John on the Web at:

        http://www.qsl.net/kd2bd/


               
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[n6wg]

Gene
Can you tell us what linear dimensions your attic would have
for potential dipoles?
They also work at nonresonant lengths, and work quite well.

At frequencies above their resonant length they can actually
contribute some gain.  Height above ground will be a bigger
factor, in that for freqs below the resonant freq, you will
find your major lobe pretty much straight up.  But it will
be a fat lob, and work out to a distance as well, just not
as well as a dipole up over a quarterwavelength on the band
in use.

73, Bob N6WG

-----Original Message-----
From: [hidden email]
[mailto:[hidden email]]On Behalf Of Gene Worth
Sent: Monday, April 11, 2005 6:52 AM
To: [hidden email]
Subject: [Elecraft] Non-resonant attic loop


I'm needing to install an antenna in my new home. An attic loop is a
necessity ... can't run an outside wire easily.

I'm assuming that an 88-ft loop will be non-resonant in all bands 80-10,
and fed with ladder line to the tuner should work.

Other suggestions?

Thanks.

gene
KC0RXY soon /9


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[Eugene Worth]

Good points, Bob. And ... errrrrrrrrrr ... no, not exactly .... I know
it is 20+ feet above the ground, so far so good. We are moving 500 miles
from our present QTH, and I didn't have a ladder to get to the attic
opening when we were shopping for a house. I do know that outside, I'll
have a flagpole antenna ... maybe a short wire, but it will have to be
modest.

The attic length less than 44 feet, that I do know, so the non-resonant
dipole isn't a great choice, unless I let the ends droop a bit. I do
have a "slinky" dipole that may find its way into the house attic or the
garage attic (12-15 feet AGL).

I know that part of the house has cathedral ceilings on the upper floor,
so I'll just have to have a look and see if I can snake some wire over
that area and keep it clear of electrical stuff.

I just wondered if there was a magic number for a non-resonant loop. My
thinking is that I could get enough wire up there for a 40-meter loop.
And, even if I had to double back in places, it would still work
reasonably well. I'll probably feed it with ladderline b/c it will be
non-resonant at a number of the frequencies I plan to work. Then, let
the tuner take care of the rest.

73, gene KC0RXY

Robert Tellefsen wrote:

> Gene
> Can you tell us what linear dimensions your attic would have
> for potential dipoles?
> They also work at nonresonant lengths, and work quite well.
>
> At frequencies above their resonant length they can actually
> contribute some gain.  Height above ground will be a bigger
> factor, in that for freqs below the resonant freq, you will
> find your major lobe pretty much straight up.  But it will
> be a fat lob, and work out to a distance as well, just not
> as well as a dipole up over a quarterwavelength on the band
> in use.
>
> 73, Bob N6WG

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[Stuart Rohre]

Those having low band noise using indoor or attic antennas, might make a
large copper pipe loop for low noise receiving.  It should be symmetric for
best effect.

-Stuart
K5KVH



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