On 10/31/2015 10:18 PM, Jeff Liebermann wrote:
On Sat, 31 Oct 2015 18:58:38 -0400, rickman wrote:

So what is the big deal of insisting solid metal loops are a "must" for
a high efficiency transmitting loop antenna rather than soldered sections?

Paranoia. See:
http://www.nonstopsystems.com/radio/frank_radio_antenna_magloop.htm
There's quite a bit of useful info on building magnetic loops. One
interesting comment was:
UPDATE 24-Feb-2012: one and a half year after construction,
I have used a professional milliohm meter (HP 4328A) to measure
the DC-resistance of the copper parts of my loop (i.e., octagon
+ wires to the clamps): 3.2 milliohm. I also measured the resistance
of a brand new round loop - without solder joints: also 3.2
milliohms. My joints are pretty good! However: I measured a
DC-resistance of over 4 milliohm between the copper wires and
the stainless steel clamps (i.e., 2 x 4 milliohms total). This
kills the efficiency of the antenna - which I had already noticed
over time. Must use a different method in my next design!

I've seen the same thing with a loop I built (using an ESR meter to
measure resistance). The DC resistance of mating parts is the same
whether it's soldered, or just stuck together. In other words, the
loop resistance doesn't change with soldering.

So, what does the soldering do? One possibility is that it helps
produce an unbroken surface area, which is useful when all the RF
conduction is via skin effect. Unless there's some kind of dramatic
change in element diameter, I don't see skin effect as a problem
worthy of soldering.

There's also the theory that different construction techniques produce
a different loop Q. That's easy enough to measure in receive with a
VSWR guesser, or in my case, a return loss bridge. However, I wanted
to do it in transmit. So, I fed the transmitter with a sweep
generator (with markers)[1] and looked at the return with a ferrite
toroid directional coupler. A loop, before and after soldering,
looked the same which I suspect means the Q is also the same.

My guess(tm) is that soldering improves the mechanical stability of
the loop so that things do not change when the TX power is applied. I
somewhat verified this guess(tm) when I found myself constantly
retuning a press fit and duct tape loop made from overpriced copper.
The copper would expand when hot while transmitting, which would put
some stress on the solder. The solder is soft, so it moves, thus
changing the tuning. Had I braze or weld the connection instead of
soldering, it would have been somewhat stronger, moved less, and
probably require less retuning.

I'm not sure about silver solder versus ordinary unleaded solder. I
haven't done any testing here. Silver solder is much strong than
regular Sn-Cu solder which produces a stiffer joing.

In short, all soldering and welding does is add some desperately
needed mechanical rigidity.


[1] Hint: Don't do that on a weekend during a contest.

I'm not certain what you are saying.

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. 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.

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. However, I literally can't imagine why the joints would not
be soldered. The issue I was addressing is the difference between a
solid tube and soldered joints.

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.

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'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. 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.

--

Rick