RE: Attic Ant.-Closed Loop vs. Open Ends?

[Todd Fonstad]

Greetings,

It is interesting that this topic has arisen now, as I am wrestling with
some of the same challenges previously posted. I recently remarried and
moved across town to ‘her house’ I bought half of it … the north half :). I
put up a 110' long attic loop at about 25' above ground which is fed with a
short length of 300 ohm ladder line down through the ceiling to my shack. It
works very well from 40M on up, but although the K2's ATU will 'tune' itself
for 80M and even 160M duty, the loop is essentially a dummy load on those
low bands.

I would like to try to cut the loop at its center so as to have a
'convoluted doublet' (for lack of a better term) 55' long per side. I think
it would do relatively well on 80M but I have no idea what effect doing so
would have on noise levels and efficiency on the other bands.

I'm trying to avoid extensive experimentation. The low head room and the
thick insulation (I have to put plywood sheets down to avoid going down
through the ceiling) make negotiating the attic very difficult.

The main challenge is finding the exact center of the loop opposite the
feedpoint. There is no way this 63-year-old body is going to measure every
inch of the antenna to find this point where I would make the cut. Does
anyone have an 'elegantly-crafted' method of finding this point, perhaps
electronically?

Best regards,
Todd
N9NE
Oshkosh, WI



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[Ron D'Eau Claire-2]

Can't help you with an "after the fact" method of measuring the center, but
my approach wherever I've had to string up a doublet that wasn't in a
straight line is to first cut it over-length with equal lengths on each side
of the feed point.

Then I install it working where I'll put the shortest side first. Install
it, cut it to length, and measure what I cut off. Then I cut that off of the
other end and install that side.

Truth to tell, where you feed it has little bearing on the operation,
particularly in an attic doublet, as long as it's not at one end (unless
it's 1/2 wave long <G>). The building will unbalance it and it doesn't
really need to be balanced. That's why leaving the balun out of the system
usually has no effect on the signal; sometimes it's an improvement! Balance
helps reduce the feeder radiation, but such radiation usually doesn't hurt.
Besides in an attic installation, the feeder length is pretty short anyway.

Ron AC7AC


-----Original Message-----
Greetings,

It is interesting that this topic has arisen now, as I am wrestling with
some of the same challenges previously posted. I recently remarried and
moved across town to 'her house' I bought half of it . the north half :). I
put up a 110' long attic loop at about 25' above ground which is fed with a
short length of 300 ohm ladder line down through the ceiling to my shack. It
works very well from 40M on up, but although the K2's ATU will 'tune' itself
for 80M and even 160M duty, the loop is essentially a dummy load on those
low bands.

I would like to try to cut the loop at its center so as to have a
'convoluted doublet' (for lack of a better term) 55' long per side. I think
it would do relatively well on 80M but I have no idea what effect doing so
would have on noise levels and efficiency on the other bands.

I'm trying to avoid extensive experimentation. The low head room and the
thick insulation (I have to put plywood sheets down to avoid going down
through the ceiling) make negotiating the attic very difficult.

The main challenge is finding the exact center of the loop opposite the
feedpoint. There is no way this 63-year-old body is going to measure every
inch of the antenna to find this point where I would make the cut. Does
anyone have an 'elegantly-crafted' method of finding this point, perhaps
electronically?

Best regards,
Todd
N9NE
Oshkosh, WI

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[Stuart Rohre]

Now this method is a bit convoluted, but you could put extra long leads on
an ohm meter that will measure low ohms.  Start at the feedpoint with one
lead, and experimentally tap onto the antenna where you think the 55 foot
point is.   If you have found the center, you will have the resistance of 55
feet of wire indicated on your meter.  You likely know the gauge of your
wire, thus, you can use a wire table to predict what 55 feet of that wires
should measure on the resistance scale.  You will have to zero out the extra
long test leads you make to reach to the far side of the loop.  The easiest
way is to make a test lead of 55 feet and then have short lead and the meter
with you as you seek the center on the far side.

Other means of accomplishing the same thing, are to find equal capacitance
point on far side to each side of the feed point insulator, or equal
inductance.  However both of those methods may suffer from incidental
coupling within the attic.  The low ohms method is done at DC to overcome
those disadvantages.

-Stuart
K5KVH


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