Hi Mike,
On 15 Feb 97 20:12:26 EST William Hansgen <102762.1311@compuserve.com>
writes:
>... wire in your attic ... Use heavy gauge wire
> I am soon to replace my smaller gauge wire with #8 speaker wire.
Whoa, that's expensive stuff! I have a suggestion: try aluminum
foil. At a few pennies per foot, and 12 or 18 inches wide, it is not only a "big" conductor, but really cheap. Attics are friendly places to
antennas- no wind or ice or rain, so the fragile nature of aluminum foil
is no drawback. The tough part is connecting to it. Try soldering
your feeder to a strip of pc board a few inches wide and as long as
your aluminum foil. Wrap the foil around it twice, scrunch it down,
and put a bunch of clothespins on it to keep the contact. Yes,
eventually you may get problems with the dissimilar metals corroding,
but you'll still have capacitive coupling. If your dissimilar metals oxidize and make a little rectifier there, why, you know right where to go
when you start getting strange interferrence problems. (My antennas are
never up long enough for that to happen.) The dry wood of the attic
rafters is a reasonable insulator, but little slings of nylon fish line
could be used to isolate the antenna if you wish. Probably more of a concern at high voltage points- that is, away from the feeder.
If you can't bring yourself to use anything so mundane as foil,
there's always 1/4" copper tubing, which is not as flexible as that #8
speaker wire, but still about 1/2 the price, and readily available at
hardware stores. And as I mentioned in a previous post, solid conductors supposedly work better as HF conductors than stranded wire- although
I still have trouble understanding why. "Stranded wire is much easier
to use...The greater propagation resistance than that of a solid
conductor can be minimized by design." - on p.10 of Wirebook III, by
Press Jones, N8UG.
However, all this is academic. Conductor size only begins to play
a significant role when your antenna impedance becomes small compared to
your wire's rf resistance, as happens in a small magnetic loop. By
small, we mean less than a quarter wavelength in circumference, like the
MFJ and AEA loops, where the radiation resistance is less than 1 ohm
(more like 0.08 ohms at their lowest frequency of operation). I doubt
anyone could measure any difference in efficiency of your system were you
to switch to #4 wire.
Just for fun I modeled your 95' attic wire on EZNEC. I neglected all that other stuff you've got up there in your attic. I made it a rectangle 16 x 32 (96' around), 20' up (2 story house?), and fed it at a
corner. I started with 18 gauge wire.
At 1.8 MHz: 4.995 + j 657 (zero-resistance wire: 0.81 + j 653)
At 3.6 MHz: 21.5 + j 1937 (zero resistance wire: 8 + j 1950)
At 7.1 MHz: 103.47 - j 1880 (zero-resistance wire: 93.4 -
1889)...
so on 40 that 18 gauge wire adds about 10% resistance (103/93) = 10%
increase in loss, or .46 db. You'll never miss it! But if you really
want to do better... Also at 7.1 MHz, but using:
#16 gauge Cu wire: 101 - j 1843
#14 gauge Cu wire: 99.6 - j 1798
#4 gauge Cu wire: 94.8 - j 1568
..25 inch copper tube: 94.4 - j 1528
4 inch aluminum: 89.3 - j 960
Finally I tried tying two 18 gauge wires in parallel, running them a
foot apart (two 16 x 32 rectangles- one at 19.5', one at 20.5'). At
one corner I brought them together into a single wire for feeding. I got 66.6 - j 973. Note how two small paralleled wires simulate a MUCH
larger conductor.
I bet you are losing more in your tuner than the 0.46 db you would
gain by going to a thicker conductor. And you could easily be losing
that much in your feedline, too, depending on what hidden metal stuff
you had to run it past. I think I'd try to arrange to get rid of some
of that reactance- by making the thing larger.
I tried pushing out over your spare bedroom (garage/toolshed/porch), making the rectangle into an L. The pooched-out portion is 8 x 8,
which adds 16' to the 96' we had, for a total of 112'. EZNEC says 69
- j 899. This might be easier to feed than the 96' wire, but is no
better than the 96' "foil" model, or the paralleled wires. What you
really need is about 140' up there. Sorry, no time to model more.
For me no trip to the attic is complete without a good conk on the head by some ill-mannered rafter, and a nice splinter or two, so I only
change my attic antenna around about every year. I'm overdue. I
think I'll try foil this year.
GL es 72
; D DWink@Juno.com Dan Winkler N7IVR Seattle, WA
----------whom the gods would destroy, they first make proud ----------
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