I have heard a number of people say that solder joints in
transmittingloop antennas are *very bad*. I'm looking at the situation
and I can't see how it would matter.

If the joints are made well with small spacing between the metal tubes I
can't see how the difference in resistivity would amount to much.
Essentially the solder would be equivalent to about four times that
amount of copper which would be a tiny fraction of the resistance of the
rest of the loop. If a high silver content solder is used, it may well
be a lower resistance than the copper or aluminum of the loop.

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?

--

Rick

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.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

In article ,
Jeff Liebermann wrote:

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.

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

If a loop made from tubing and junctions is just "stuck together", it
may have the same resistance as a one-piece loop initially. The
tubing-to-joint connections typically involve enough scraping (an
interference fit) to create a fresh metal-to-metal junction... nice
low resistance.

I strongly doubt that it'd remain so, after a year or so of exposure
to air (and water and sun). Oxygen is going to infiltrate those
contact surfaces and oxidize the metal; any sulphur vapor in the air
will attack the surfaces as well. The quality of the connection will
deteriorate and the resistance will climb.

(Audiophiles who know their stuff will unplug and replug their signal
cables periodically, to "wipe" the contact surfaces free of oxide
and create a clean connection once again... RCA connectors are
quite awful and this is one of their failure modes. An unplug/replug
is far cheaper than buying the latest new overpriced "magic" cables
at the boutique audio store, and works just as well!)

I've seen plenty of antennas fail due to oxidizing and corroding
connections. We had to completely rebuild the grounding-and-radials
base of a Hustler G7-144 when the original interference-fit
connections deteriorated after a decade up in the sun.

A big benefit to soldering the joints, is that you'll seal the metal
contact regions away from oxygen, and fill the gaps with a metal which
is also a reasonably good conductor. Even if oxygen starts affecting
the outer surface of the tube, there will still be solid metal beneath
it.

A similar problem with oxidation affects screwed-together "American
Legion" J-pole antennas... they start generating intermod interference
and broadband hash when transmitting. A dab of TIG-weld bead to bond
the elements to the base eliminates the problem.

You might get some of the same benefit by using an antioxidant paste
on the joints when connecting them, but it wouldn't provide quite the
same protection, and it wouldn't provide the mechanical strength you
quite rightly point to as a benefit.

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

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

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

On Sun, 1 Nov 2015 14:57:36 -0500, rickman wrote:


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?

Sorry. Temp stability should not be included.

silver plated antennas

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?

Hams have spend considerably more money of smaller improvements. I
keep seeing HF antennas fed by 7/8" Heliax and wondering about the
size of the owners bank account. Anything for that last tenth of a
decibel. Considering the cost of monster towers, lowest possible loss
coax, digital everything, and strange looking antennas, I see silver
plating as a trivial expense.

Larger diameter tubing is probably a good alternative. I initially
played with a loop made from a length of RG-8/u coax cable. When I
found that the resistance of the braid was a problem, I switched to
RG-214/u (silver plated double braid). That worked much better, but
the higher Q now made mechanical rigidity an issue. So, I tried a
length of 9913, which was stiffer, but lacked the improved
conductivity of the silver plated double shielded braid. After that,
I tried a chunk of unlabeled CATV 75 ohm semi-rigid coax about 3/4"
diameter. The conductivity of the aluminum was lousy, but the
mechanical rigidity was great. There were also some marginal efforts
using Hula Hoops, aluminized duct tape, and Litz wire antennas (25
pair of #24 awg insulated telco wires). Incidentally, besides the
usual OTA (over the air) testing, I used the measured Q as a figure of
merit on the assumption that higher Q means lower losses.

Really? You want to design a copper antenna with series capacitors
scattered in your loop?

Nope. The press fit copper overlap is sufficient to make a tolerable
connection. The problem is that it's not a perfect connection, so
anything that moves will affect the tuning. If overlapping copper
didn't work, then all the coaxial connectors also wouldn't work.

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.

Yep. I hadn't planned to sell the antenna in that condition. I was
doing the same thing as the author of the article. I was testing the
antenna before soldering everything permanently in place to allow for
easier tweaking and adjusting.

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.

It's not the conductor. It's the plating thickness. I get into that
quite a bit with PIM (passive intermodulation) where the two
dissimilar metals create a diode junction and produce a mixing action.
Anyway, the idea is to make the plating thickness thick enough so that
all the RF is concentrated in the plating, and not distributed into
the base metal. As I recall, the calculated skin depth is roughly
where 63% of the RF is concentrated in the outer part of the conductor
down to the skin depth. In order to get that closer to something like
90%, you need 3 skin depth thickness. If you want to take advantage
of silver plating, it needs to be plated where the RF is moving.

So even if the solder if four times more
resistive it will be swamped by the 100's of times greater length of
copper.
(...)
The solder inside the
overlap would be inconsequential other than mechanical support.

Agreed. The only place where the solder might have an effect is on
mechanical rigidity. The small amounts used, even for a square loop
assembled from sections, it trivial compared to the losses in the
areas affected by skin effect. However trivial, it's not zero. I
suggest that you run the spreadsheet at:
http://www.aa5tb.com/aa5tb_loop_v1.22a.xls
and plug in various numbers for added resistance of the solder. The
numbers are tiny, but they will produce a noticeable change in Q and
therefore efficiency.

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

Because solder is soft compared to copper pipe. I don't know the
mechanism involved, but when I assembled a loop from pieces and used
it as a receive antenna, I found myself constantly retuning the loops.
I later put it on a sweeper and a return loss bridge and noticed that
the tuning was changing a little as the antenna was tapped with a
stick. I couldn't tell exactly what was causing the tuning change
because everything was moving. When I later soldered the antenna
together (using 60/40 solder), it was much more mechanically stable in
receive but still not perfect. The only thing loose was the coax
connector. At that point, I stopped tinkering and tried it on the air
at 5 and later 50 watts. Even if I did nothing to the antenna, it had
to retune it every 15 minutes or so. A similar loop (but somewhat
larger) where I had the local mechanical contractor bend into a loop,
didn't have these problems and only required retuning when the
temperature changed, or when I changed in frequency. I might have
missed some factor, but it would seem to me that the use of solder was
the only major difference between the loops. (Incidentally, the
various coax cable loops were worse than the soldered pipe section
loop and were only useable because the Q was lower and therefore had a
wider operating bandwidth).

Why would I want the silver to be as thick as the skin depth?

Because the RF goes through the outside of the conductor. Better to
have it silver, with its slightly better conductivity, than ordinary
solder. If I could silver plate the solder, just like the copper, I
probably would.

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

With copper plating on the ends of the aluminum pipes, you can solder
them together. Of course, you could also weld aluminum pipes
together, so that's not a big advantage except to attach coax
connectors and tuning caps.

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.

Fair enough. Permit me to offer a suggestion. Please state your
objective when you begin asking questions. Most of your postings
appear to be target practice aimed at the comments of the poster. It
often feels like a duck shoot, where you take shots at anything that
fly by. Your points are usually well taken, but totally aimless
unless you state what you are trying to accomplish. For example, you
haven't indicated if you plan to actually build an antenna, have built
an antenna, are having problems with an antenna, or simply want to
understand the technology from an academic point of view. The type of
replies vary with the intent. I was very interested in your LTspice
model of a loop antenna, but on which I do not consider myself
qualified to comment. However, when it comes to construction and
testing, I can supply some help.

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.

Have you considered copper or brass butterfly capacitors?
http://files.qrz.com/a/ab1pa/IMG_2964.JPG
I suspect that there are kits available in brass. The benefits of
soldering the rotor plates to the center shaft is well worth trying
copper. In an ordinary variable cap, any series resistance between
the plates and their connecting rod is going to cause problems. One
reason why butterfly caps are preferred is because they eliminate any
losses in the rotor plate to center shaft (at the price of half the
capacitance).




--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558