On Sun, 1 Nov 2015 03:02:44 -0500, rickman wrote:
I'm not certain what you are saying.
Short summary:
1. DC resistance does not change with soldering.
2. Temp and mechanical stability of the loop is greatly improved by
soldering and welding. The stronger the joint, the more stable.
3. Soldering did not seem to affect the Q of the loop, although my
method was rather sloppy and results uncertain.
The part you quote sounds like he measured the DC resistance of the loop
which has little to do with the AC resistance at RF.
Correct. We're dealing with skin effect in a transmit loop.
I guess this begs the question of why are magnetic loop builders NOT
silver plating their loops? If skin effect is so important, then why
are many home made loops using unplated and often unprotected copper?
Electroless silver is easy to do and fairly inexpensive. I vaguely
recall that it's difficult to plate electroless silver thicker than
the RF skin depth on the lower bands, but I don't recall.
In particular the
solder joints end up being literally undetectable with DC because there
is a large parallel surface between the 45° unions I assume he used and
the pipe. Solder in this space joins the two copper parts with a much
larger cross section reducing the resistance of a path through a more
limited area of contact.
Hardly. Even if there was an air gap between the overlapping copper
sections, there would be enough capacitance in between for the antenna
to operate normally. Of course, the tuning would change, and it might
arc over, but it would still have roughly the same Q.
But with RF currents the path will only be on the outside surface of the
conductor. So without the solder the connection will be through a
limited amount of area but the same is true for the solder joint since
only the outer few mils of the pipe are used depending on the frequency
involved.
If the overlapping copper connections is really deemed a labyrinth,
which increases the effective length of the loop, it would produce a
rather drastic change in tuning. I've noticed a tuning change as the
loop is moved before soldering but not much. The lengths involved are
quite short when compared to the overall length of the loop.
The skin effect of different materials seems to be current issue:
http://owenduffy.net/calc/SkinDepth.htm
Looks like the higher resistivity of 63/37 solder, compared to copper,
required more skin depth. Adding some silver to the solder should fix
that.
In any case, the tiny amount of solder area, compared to the area of
the loop, isn't going to dramatically increase the loop resistance.
Let's try by example. I take two copper overlapping fittings and
grind off some copper so that when stuck together, they have an air
gap in between. I then fill the gap with solder. The added DC
resistance will be the bulk resistivity of the solder times the
surface area, which is small, but potentially significant when we're
dealing with milliohms. However, the RF path only has to bridge the
solder filled gap between the copper pipes. The increased RF path is
just the difference in areas between the inner tube OD, and the outer
tube ID. In other words, not much added RF path length from
soldering.
However, I literally can't imagine why the joints would not
be soldered.
Perhaps an analogy might help. If you assemble the parts of a ladder
but don't tighten any of the bolts holding it together, it would still
function as a ladder, but just wouldn't be particularly stable. Same
with the loop antenna. It's customary to assemble the sections
together, to see if they fit together, before soldering. It's also a
good idea to test the tuning of the antenna, which might require some
mechanical adjustments before soldering.
The issue I was addressing is the difference between a
solid tube and soldered joints.
My explanation, admittedly a guess(tm), is that there's little
difference in DC and RF resistance, but a substantial difference in
mechanical and electrical stability.
As to the strength issue and temperature effects, the entire loop would
expand evenly and so no real stress would be on the solder other than
the differential expansion of the two metals.
I ran my IR thermometer around a copper loop to see if there was any
unexpected heating. It was tricky, because the RF drove my IR
thermometer nuts. So, I had to xmit 10 mins, turn off the
transmitter, and then quickly take measurements. Hot spots were
difficult to see because the thermally conductive copper would
distribute the heat very quickly. Still, I managed to see tiny
increases in temperature around some soldered joints, and a rather
large jump where I had dissimilar metals (stainless hose clamps in the
T-match). I think the hot spots in the joints were caused by the
lower thermal conductivity of the solder compared to copper.
In a loop I was thinking of using tin-lead solder for the overlap area
of the joints and then finishing off the visible portion of the joint
with silver solder. I wasn't aware silver solder is stronger than other
solder. If so, I might just use it for the entire soldering process.
I forgot to include a link to the strength of various solder
compositions.
http://alasir.com/reference/solder_alloys/
On the top table, not the approximately 80% increase in tensile
strength for solder compositions that include silver. Although I do
it often, I'm not a big fan of mixing solders.
I'm actually thinking of using aluminum tubing and silver plating the
joint areas. I've seen a video on doing this, although they used copper
in the video they say it works with aluminum and allows it to be
soldered easily if plated thick enough.
If you build the loop in sections, such as in the original article I
cited:
http://www.nonstopsystems.com/radio/frank_radio_antenna_magloop.htm
plating the loop in sections is easy. Plating a single piece loop,
made on a tubing bender, is not so easy.
I've copper plated aluminum tubing, but haven't had a need to try
silver. No advice, but I suggest you calculate the skin depth and
make sure your plating is thick enough.
Tuning capacitors are usually
aluminum so I'm thinking it would be better with all the same material
as long as there aren't any chemical reactions between the aluminum and
the solder. I've been told aluminum likes to mess with other metals.
Visit your local hardware store and you'll find all kinds of bonded
copper to aluminum lugs, adapters, crimps, corrosion inhibitors etc.
Al to Cu transitions are common problem in house wiring.
You can plate copper on aluminum yourself, but it usually requires an
initial zinc coating:
http://www.finishing.com/0400-0599/555.shtml
I've copper plated aluminum foil, but nothing heavy or large. Again,
I suggest you want your skin depth (plating to 3 times the skin depth
is good enough).
Personally, I think you're overdoing it and are hung up on minutiae
and detail. Optimizing the loop resistance to the last remaining
decimal point might be useful after you have a reproducible initial
design, or if you're trying to build the ultimate magnetic loop
antenna. However, the various dimensional aspects of the design are
far more important. How big a loop? How to match it to 50 ohms?
What's the takeoff angle? Tuning range and bandwidth? Start he
http://www.aa5tb.com/loop.html
http://www.aa5tb.com/aa5tb_loop_v1.22a.xls
See Note 2.
Good luck.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558