On Tue, 23 Mar 2004 02:28:32 GMT, loopfan wrote:
Where do I begin?
After reading this, that is what I asked myself too.
Let me say that I need help analyzing a dual-balun problem with my short
loop.
Small loop would be a better description. Also, loops are described
in diameters (or radius) not conductor length. Yes, there is a
correlation, and yes it is important to note the wavelength dimension
of that conductor length, but it is uncommon and most references stick
with diameter/radius for many and good reasons (such as radiation
resistance formulae).
I have a configuration that is working great, but other than that
it shows that I know next-to-nothing about why it is working ...
Which proves a little knowledge is dangerous I suppose, but we proceed
in good faith.
I have a short wavelength (about 0.10 wavelength long) wire loop
attached to the balanced port eyelets of a 1:1 voltage balun from W2AU.
Things were OK to this point.
I am using twin-lead feedline but get this: the feedline is ALSO
attached to the the balanced port eyelets of the voltage balun. The
unbalanced port with the SO-239 jack is unused and unterminated. THIS
is driving me crazy since it seems to work.
Yup, it caused me a whiplash too. I can't ask "why" you would want to
do this because you already admit you know next-to-nothing.
At the end of my feedline I am connected to my tuner's 4:1 internal
voltage balun. (An old Swan/Cubic Tee-type tuner).
That much makes sense - in isolation of the other statements.
I definitely need the loop balun
Considering you have described two BalUns, this reference is vague.
, otherwise my loop becomes a noisy
omnidirectional random wire.
You confirm one of the qualities of the BalUn. Obviously it
reinforces your hunch of doing something right (now if we can only
differentiate what that something is).
This wacky configuration seems to be the
only thing that keeps the whole system balanced, and I don't know why.
That is the same quality that rejected the noise: balance. This
balance through choking has eliminated common mode currents that blows
away the nulls by filling them in.
(I have also tried using a current balun (W2DU), but it also turns my
short directional loop into a random wire.)
How you did this is missing. If it was done in parallel with this
twin-lead, then you have what is called a ground loop, you shorted out
the choke. Again (with that "if") I won't ask why.
It just plain looks wrong even though it works great. I'd appreciate
some feedback on my funky configuration.
Fun-Kay!
Since the antenna covers many
bands with the tuner, I make no attempt at matching the feedline
impedance to the loop, and use a tuner to resonate the whole system.
All perfectly reasonable - in isolation of the other statements.
I've made a web-page about my projects in case you want to look at it
and laugh. My project site has info about my experiences with
coax-loops, but I have since moved on to plain wire loops since they
seem to perform as well or better than the coax-made ones.
htt://www.greertech.com/gpmag/gpmagloop.html
You (and everyone else) should cut and paste links from a browser
opened to the page that is referenced so we don't have the mistake
above. Typing URLs from memory often reveals IQ problems.
You can find the remote configuration at the bottom of the page.
I just can't explain what's going on here and could use your help. Many
thanks in advance!
73 de LoopFan
Hi OM,
The W2DU BalUn is an unnecessary item, dangling there like an
appendix. At most it is additional capacitance at the feed point.
Now, why you chose to switch from coax to twin-lead is a personal
issue. The distances involved do not demand it of necessity, that
much is certain. It cannot have anything to do with balance, because
your loop worked well with the coax and W2DU already. It works, which
is true enough, and enough to say it is simply a different way of
accomplishing the same thing.
As for the other material on your page about wire size, you need some
real data instead of those qualitative statements. I will repeat some
that I posted he
Let's look at small loops' Rr for various sizes in tabulated form:
Fo 1M diameter Efficiency with 1 mOhm loss
160M 29 µOhms 2.8%
80M 500 µOhms 33%
60M 1.5 mOhms 60%
40M 7.5 mOhms 88%
30M 24 mOhms 96%
20M 120 mOhms 99%
Let's examine the validity of that generous assignment of 1 mOhm loss
and see if it is reasonably warranted. Skin effect is the single
largest contributor to this loss as a source (aside from poor
construction techniques). Using the 1M diameter loop as being a
practically sized construction, and if we're using 2.54cM diameter
copper wire/tubing we find:
Fo skin effect loss
160M 13.8 mOhms
80M 20 mOhms
60M 23 mOhms
40M 28 mOhms
30M 33 mOhms
20M 39 mOhms
Well, 1 mOhm was too generous and if we look at those loops' Rr once
again against a robust, thick loop element:
Fo 1M diameter Efficiency with skin effect loss
160M 29 µOhms 0.2%
80M 500 µOhms 2.4%
60M 1.5 mOhms 6%
40M 7.5 mOhms 21%
30M 24 mOhms 42%
20M 120 mOhms 75%
Now, admittedly, this work above has a different diameter. but not
much different. It also reveals mediocre efficiency for 40M, but you
steer your readers away from that towards a much lossier system.
You might want to reflect on where the efficiency goes in your
suggested designs. Anecdotal reports such as yours suggest the #12
wire is fine, but the statement about large conductors being poor
sensitivity is suspect (you are suffering different issues).
Basically there is a conflict here you need to resolve.
By the way, flat strips are NOT better as substitutes for conductor
mass. If you must use aluminum-foil, wrap it around a hula hoop for
the round conductor shape that is much MORE efficient. You might want
to reflect how you are going to insure such aluminum-foil
constructions are going to maintain low mOhm conductivities - it
doesn't automatically come by wishing it so. Such foil oxidizes quite
rapidly on the unwaxed side (did you know there are two different
sides?). Needless to say, the waxed side isn't particularly
conductive either. Trying to solder to seams or make leads brings
even more issues spelled Ohm.
With small loops, the name of the game is resistance: pure and lossy.
73's
Richard Clark, KB7QHC
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