On 11/2/2015 12:17 PM, Jeff Liebermann wrote:
On Mon, 2 Nov 2015 00:16:42 -0500, rickman wrote:

Short replies... It's Monday and the phone is ringing.

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.

I'm using the shield connection, not the center conductor. The center
pin will probably be destroyed by the high currents and from arcing
due to high voltages. If crimped, the shield will probably survive.
If I wanted to prove it, I would calculate the square mils of surface
contact area for the shield in the connector. Overlapping CLEAN
copper 3/4" tubing makes a tolerable coax connector with the addition
of slots and a hose clamp for compression. I've seen Cu plumbing
parts used as welding cable connectors.

I don't know why you keep shifting gears. DC current is nothing like RF
current. DC will use every molecule of conduction path. The skin
effect hugely influences AC conduction making much of the connection
between two concentric conductors unavailable for conduction.

I looked up the coax connector shield connection and they are rated for
0.2 mohm outer contact and 0.1 mohm braid to body, so maybe they could
pass the large currents seen in these antenna. But that does not relate
to the concentric copper tube because the coax connector is specifically
designed for this. The copper tube is just the opposite.


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

The lengths of various sections so that the tuning range of the
capacitor works as planned. My first plumbing loop was calculated for
a loop circumference based on the center line of the plumbing. I had
forgotten to include the length of the capacitor stator frame in the
loop length. I also found that the location where I attached my
tuning capacitor was important. I ended up too low in frequency and
had to trim back a few Cu pipe sections.

You have to do that exactly once. After that there is no reason to
leave the joints unsoldered.


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.

Yes, I want to silver plate the entire tube, any hardware that carries
RF, and possibly the tuning capacitor. The silver isn't what costs
money, it's the setup and plating labor. If all the copper parts are
plated individually, it's much easier, but then the solder doesn't get
plated. Plating the finished antenna is probably impractical. So, I
guess the solder doesn't get plated.

I don't see any useful value to silver plating. It gains you 2.5% in
improved conductivity. Really? You are the one telling me *I'm*
overdoing this. Also, I'm not planning to use copper, rather aluminum.
I found 20 foot lengths of aluminum 3 inch Al tubing for $3 a foot,
much cheaper and as good a conductor as 2.5 inch copper.

Why silver plate when you can get a bigger improvement by going up in
tube diameter?

The solder, properly done, will only cover a tiny fraction of the total
loop. Pointless to even consider plating it, especially when it can be
a silver compound as well.


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.

Agreed. However, I know what every home building will do. They'll go
to the hardware store, buy the plumbing parts, buy plumbers flux and
Sn-Cu solder, and solder it exactly like a plumber. Using silver
solder will probably be limited to the fanatics and those that have an
inventory of silver bearing solder.

So? If people can't follow instructions they get what they get.


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.

Nice drawing. I like it.

---------,./.
| | \ .
| | \ . 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.

Ok, I yield. That's a much smaller resistance than I would have
expected. Since the other resistive losses are 3 orders of magnitude
larger, I guess we can discount the resistance of the solder.

Unexplained issues are not really proof.

Agreed. I just thought my observations might be of interest. I think
I made it clear that I don't have a complete understanding of what
happened, only a guess(tm).

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???

Good point. A few minutes with a heat gun should demonstrate the
cause of the drift. If he has an MFJ-259/269 antenna analyzer, it can
be used to measure resonance. White knuckle tuning is the only
problem:
https://www.youtube.com/watch?v=0CgO5ThFsQs (3:19)
I haven't tried this yet because I just bought a very used MFJ-269,
fixed it, and now the RF connector is intermittent. That's what I
should have been doing this weekend instead of ranting on usenet.

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.

Good question. I don't have an answer. Something moved the tuning,
but I couldn't tell what it was. I might have soldered it together
under tension, which was somehow relieved by heating in transmit.
Dunno.

What about other effects. What happens to the inductance if the loop is
a bit out of plane? Any idea if your loop flexes around in wind or
whatever?


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.

No problem, as long as you don't expect my unrelated experiences to
directly answer your question. I was working on a completely
different problem (minimum practical size of a loop) and not working
so much on the effects of soldering and plating. I apologize if my
experiences and speculation don't neatly dovetail with your questions
and seem unrelated. I had hoped that you would accept them as clues
or partial answer, not rigorous proofs.

As to the minimum size of the antenna... the formula that surprised me
and made me realize there is a nearly brick wall is for radiation
resistance. It's proportional to the 4th power of the ratio of loop
radius to wavelength... the *4th* power! That is hard to overcome by
any small effect or even moderately large ones. Push just a little bit
and you see huge results, like making your loop 33% larger increasing
the radiation resistance by 3x! (or making your loop 25% smaller
reducing the radiation resistance 3x Makes it hard to get anything
like acceptable efficiency if the loop is even a little too small.


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.

Once upon a time, I worked with an engineer who refused to build
anything until he completely understood the design. I was the exact
opposite, and would rush to build a prototype even if I had some
unanswered questions. The results were predictable. His final design
was usually good, took forever to deliver, and blew multiple
deadlines. Mine were a series of failures eventually leading to
something that worked. The total elapsed times were about the same. I
still don't know which method is better, but today I still prefer a
series of tweaked prototypes to a pile of calculations and a detailed
model. That might explain some of my recommendations and choice of
methods.

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.

You posted it to S.E.D. I looked it over but there were runtime
errors that I didn't want to fix. The title is Antenna_trans_loop.asc
dated 2013-02-27. If you have something later, I would be interested.
However, my abilities to use LTspice for RF design seems to have hit a
roadblock. About a month ago in S.E.D., I was involved in a
discussion about the operation of a common CATV splitter/combiner. I
decided to model the device with LTspice and ran into an odd problem.
The graphs produced by LTspice are in dB(volts) rather than dBm or
watts. I'm stuck trying to figure out how to produce dBm so that
graphs of filters, loops, and such look sane.

Are you saying the version I posted didn't even run? Odd. It is late
now, but I'll try to dig it out tomorrow.


I want to build this from scratch if I do it. I don't see a problem
with aluminum.

No problem electrically. Big PITA mechanically because aluminum is
difficult to solder without Cu or Ag plating.

That's why I want to silver plate it. The plating looks to be easy.
Others have talked about being able to solder aluminum by using
something to block the air, but I don't recall the details. It sounds
much more difficult.


I can't see the benefit of soldering the rotor plates.

If you use an ordinary non-butterfly capacitor, the loop current goes
through the capacitor. That means it goes from the stator mounting
rod, through the plates, through the air, through the rotor plates,
though a bearing/bushing, and finally through the rotor shaft. Just
follow the RF path. Most of that path is fairly solidly built or
welded, but not the connections between the plates and the shafts.
Often, they're crimped together, resulting in a minimal point contact.
Better is on a threaded shaft, with compression making the connection.
Best is soldered, welded, or machined from a solid piece of metal.

But you still have the bearing contact which makes it impractical for a
transmitter from what I hear. No point in welding a rotor if you have
such a joint carrying the RF.


A butterfly capacitor eliminates the worst culprit by removing the
rotating shaft from the RF path. There are two sets of stator plates
that need to be secured to two mounting shafts, but these are fairly
simple to build, compared to the rotor shaft found in the common
variable capacitor. The only problem is cost and the half the
capacitance from stator to stator.

Not really an issue if the difference is that it works and the brushed
or bushed rotor doesn't work at high RF power levels.


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.

Well, the easy way would be to discount my observations and move
onward. The worst that can happen is that you'll repeat my
observations, my mistakes, or both.

Yes, but this will be a *lot* of work to assemble a large antenna like
this. The cost won't be small either.


As I previously concluded, the only real benefits of silver solder is
mechanical strength and rigidity. If your method of construction
requires these, such as if the tuning capacitor mounting is such that
movement of the loop will cause a movement in the capacitor, then
silver solder might help.

Yeah. I should stick by my guns and believe that standard tin-lead
solder just won't impact the function of the loop to any detectable level.

--

Rick