On 11/1/2015 2:02 PM, Jeff Liebermann wrote:
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.

That's a lot more clear, but why do you say the "temp" stability of the
loop was improved? But first, you mean compared to doing *nothing* at
the joints to actually join the materials, right? I can't see how "no
connection" is an option. I would expect the durn thing to fall down
and if it didn't fall down the joints would oxidize to the point of not
working.


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.

That would be a lot of work for a 12 foot tall antenna even if not
terribly expensive, for very little gain, about 5% conductivity which is
cut in half by the skin effect. Do you really want to go to all that
trouble for a 2.5% improvement when you can just use copper with a 2.5%
larger OD to achieve the same benefit?


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.

Really? You want to design a copper antenna with series capacitors
scattered in your loop? Yes, it would not just affect the tuning, but
the tuning range and vary with all sorts of changes like temperature and
humidity. That strikes me as a crazy way to build an antenna.


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.

I don't know what "required more skin depth" implies. I have yet to
find a conductor that wasn't thick enough to provide 95% of the max
potential conductivity down to 70 kHz. The skin depth goes by the
square root of the resistivity, so there is minimal difference because
of 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.

BINGO!

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.

Again, no one cares about the DC resistance. The issue is not the
volume of solder in the overlap which would only be useful for
mechanical support, but in the *length* of the solder path at the outer
skin. Instead of an over lapped case, if two pipes were butted with a
tiny gap between them (and a pipe inside for mechanical support) the
length of the gap filled with solder would be tiny compared to the
length of the copper pipe. So even if the solder if four times more
resistive it will be swamped by the 100's of times greater length of
copper.

To then consider the case of the overlapped joints, the RF current will
only flow in the outer 3 skin depths (roughly) and see only the solder
making the fillet at the end of the overlap. If high resistance solder
is used you would want to remove as much of this fillet as possible and
sand off any solder on the surface of the tube. The solder inside the
overlap would be inconsequential other than mechanical support.


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.

I don't see any reason for a difference mechanically. We aren't talking
about a supporting structure for a house, it only has to hold itself up
and usually is supported at two points.

I have no idea why you think soldered joint would have poor 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.

Why would I want the silver to be as thick as the skin depth? The
silver is just at the contact areas to provide a solderable surface, not
for the conductivity. The aluminum is plenty good for that. The point
is to use a large diameter aluminum tube to get a low resistance. Then
to connect the sections the silver plating allows soldering.


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).

I have no interest in plating copper on zinc on aluminum. If I can do
silver that seems like the way to go.


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.

I'm trying to identify significant issues and the easy steps to mitigate
them. If it is not clear how significant an issue is, but the step to
mitigate it is easy, then why not do it.

I can't see *not* soldering the connections. The tuning capacitor will
be aluminum. To avoid connections between different metals the entire
unit will be aluminum. So I will need to solder the aluminum unless it
is easier to weld which I'm pretty sure is not the case. I think the
silver plating and silver solder is the short and easy path to an
optimum solution with low cost. But the jury is still out.

--

Rick