On 11/1/2015 10:59 PM, Jeff Liebermann wrote:
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.

I didn't say anything about cost. I said trouble.


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.

A larger diameter loop is a great way to go. Radiation resistance goes
up with the 4th power of radius while the dissipative resistance goes up
by linearly, greatly increasing the efficiency.

Larger diameter tubing is good way to reduce the dissipative resistance,
as long as you can afford it or can work with it. The tubing isn't so
pricy, but I've seen that joints cost as much as a 10 foot piece. I
haven't checked that myself. The perception of many is that joints are
bad, so they will want to bend the tubing to form a circle, this gets to
be hard with larger diameter tubing. I'm ok with an octagon (only
loosing 5% of the area of a circle with the same circumference) or maybe
even a square Which looses about 21% of the area. In terms of radiation
resistance these impacts should be squared to give 90% and 62% of the
radiation resistance. The square is easier to make, but the octagon in
addition to being half again more efficient, has a certain panache
compared to a square. I need to find out just how much unions are for
large diameter tubing.


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.

There is a *big* difference between a precision machined connector and
concentric copper tubes. Heck, there is a big difference between
quality connectors and cheap ones!!! Besides, a coax connector isn't
designed to pass such high currents as a tuned loop antenna. Try
putting those in your loop and I bet it fails very quickly.


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.

When constructing a loop antenna of copper or aluminum tubing, what is
there to tweak that would be easier with unsoldered joints?


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.

3 skin depths gets you 95% of the conductivity. But the context isn't
making any sense. Copper tubing and solder joints. What are you
planning to plate to get 3 skin depths, the entire copper tube? I'm lost.

In my thinking you need to minimize the use of solder and keep it to as
small an area as possible. Because of the skin effect it will impact
any surface it is on the outside of. So get rid of it or don't use it
in the first place. Or use a very high silver content solder.


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 think that is a pretty bogus statement. Using the default numbers in
the spreadsheet I could add up to 0.1 mohms before it even changed the Q
factor in the 4th significant digit. The formulas seem to be locked, so
I can't tell what is being done, but I assume the "added loss" is just
added to the loss resistance formula shown on the "formulas" sheet.

Tube thickness of 40 mils. Resistivity (rho) around 1.5 * 10^-7 ohm-m.
Tube diameter of 2 inches. Assume the solder forms a triangular fillet
in the L at the end of the overlap. Length of the hypotenuse is 56
mils. So change the triangle into a rectangle of half that length 28
mil and 28 mil high (max thickness from hypotenuse to right angle
corner). So the resistance will be...

I'm not sure this ascii art will help, lol.

---------,./.
| | \ .
| | \ . 56mil
40mil | \ .
| | \ .
| | \ .
---------' \ /
--------------------------
|-40mil-|

R = rho * L / ( W * H ) = 1.5e-7 ohm-m * 0.712 mm / (0.712 mm * 50.8
mm) = 3 micro-ohms. Yes, MICRO ohms.


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).

Unexplained issues are not really proof. Someone in another group has a
coax antenna that detunes with temperature. I should ask him if it
detunes with time or just temperature. His frequency drift is some 20
times larger than I can explain with the expansion of the materials in
the capacitor and the loop. Since he is using the coax which is very
flexible, maybe the plastics involved are causing a dimensional change
large than would be seen for solid metal???

Solder may be softer than copper, but it is hard to explain how a solder
joint would change the length of the tubing by enough to cause a detune.


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've lost context here. Silver only gives a 2.5% improvement in
conductivity when accounting for skin depth. This is pointless really.
I don't know where this silver is supposed to be, but the sliver
plating I am talking about will be the surface the solder adheres to, so
it would be *under* the solder, not on top of it. I am only considering
using it because it is very hard to solder aluminum unless it is plated.
I'm not trying to improve conductivity at all. If I want better
conduction, I'll use a larger diameter aluminum tube.


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.

I can solder the silver plating which is why I'm doing it.


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.

Sorry if my comments feel like pot shots. That is not my goal. I am
trying to understand what is being said. To be honest, a lot of your
comments seem to wander and not connect to what I have posted or even to
what you have stated elsewhere in the post or thread. This is probably
because I'm not picturing fully the ideas you have.

To respond to your request, initially my interest was basically
academic, but as I hear more seat of the pants info from experienced
people I am more interested in finding out what really works and what
doesn't which means I'll have to build my own.

Did I ever send you my spice model? I haven't done anything with it in
a long time. It was a receiving antenna. One point I understand better
now is the radiation resistance which I could add in a calculation for.
Initially someone gave me a number I used. But for the small loop I
was looking at and the very low frequency (60 kHz) the radiation
resistance would be very tiny and so not really a factor.


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).

I want to build this from scratch if I do it. I don't see a problem
with aluminum. I can't see the benefit of soldering the rotor plates.
So far no one has been able to explain how there would be any difference
in voltage except for very small values. If I felt the need to connect
them I would likely silver plate and solder rather than weld. But your
findings above with the lack of stability concern me with soldering, at
least in the main loop.

--

Rick