On 11/9/2015 7:52 PM, Jeff Liebermann wrote:
On Mon, 9 Nov 2015 01:59:06 -0500, rickman wrote:

I'm not familiar with automatic tuners that can tune the antenna while
in use.

The basic benefits of having a remote controller a
1. RF safety and you're not part of the antenna system.
2. A controller is easier to umm.... control.
3. Automatic remote tuning reacts to changes when you're not looking
at the VSWR meter.
4. White knuckle tuning is difficult. Let the servos do the work.
5. It's the only effective way to tune a loop mounted on a tall pole,
tower, or roof.

A remote controller is not an automatic tuner. Automatic implies the
controller tunes the resonance while a remote controller is just that, a
control to adjust the tuning remotely. I don't see how this could work
while transmitting. The tuning needs to be done separately.


One of the issues someone pointed out was that the dielectric
can heat up from the energy absorbed during transmission. Is an antenna
tuner real time in this case?

Yes in receive. Probably not in transmit. In receive, you can tune
all you want and nothing will explode or catch fire. In transmit, you
can easily tune through full power and arc over the tuning capacitor.
Auto tuning also goes through the optimum VSWR point several times
during the tuning cycle. You transmitter may not like operating into
a high VSWR load during tuning.

How does an automatic tuner operate? When you mention VSWR, how is that
affected by tuning?


The solution is to tune at low tranmit power levels. Once the lowest
VSWR point is found, you can increase your power. However, that
usually prevents you from "tweaking" the tuning at full power, which
is what you need to compensate for thermal drift. It's also difficult
to tune with any modulation other than CW. I think (not sure) that
some controllers have this ability, probably with warnings and
disclaimers. I wouldn't trust it. Basically, to make it work
requires a VWSR sensor and calculator that works when there's
modulation, and a tuning capacitor that can tolerate moving while
passing high currents. Arcing and welding the bearing and bushings
might be a problem.

I crunched some numbers and found 100's of PPM change in tuned frequency
due to ambient temperature change over the course of a year for an
antenna with an air or vacuum tuning capacitor. I can find ceramic
dielectrics that would be lower than this and even in the opposite
direction to offset the natural drift. But I can't find this info for PEX.

I looked and also didn't find anything. The problem is that you don't
find tempco data for plumbing parts that were not intended to be used
for RF components.

Substances have properties regardless of usage. PEX is not just used
for plumbing. It is also used in electrical cables where the dielectric
properties are very important.


I'm not terribly concerned with the actual value of Er and even the
dielectric strength. What is important to me is the temperature
coefficient of Er.

Again, I think you might be trying to solve a problem that has already
been solved by automatic tuning (which you need anyway). Unless you
plan to transmit endlessly, you can simply press the tune button on
the controller a few times per hour, and be done with trying to
temperature stabilize the loop.

That's not how it was presented to me. The suggestion was that tuning
will change during transmission due to heating from the power being
transmitted. Of course that depends on not just the Er dependance with
temperature, but also the dissipation factor. So perhaps with material
of a sufficiently low DF the Er dependence on temperature is not so
important.


Interesting, but nearly every discussion I find on loop antennas has a
lot of fluff content.

RF is magic. It's difficult to explain some things.

You mean people don't understand it. Anything that is understood can be
explained. If you don't understand it, you can't explain it. Even so,
that's not fluff. Fluff is when things are described in non-rigorous
ways like, "solder joints result in sub-optimum performance". Hard to
prove or disprove. Clearly they will have some effect even if that
effect too small to be measurable. "Sub-optimal" is pretty meaningless
in general until you define the details of "optimal".


Here is some from this discussion, "By the way
PEX is cross linked polyethylene and is superior to using sheet Teflon
in this instance." Unless the reason is stated for considering PEX
superior to Teflon, I haven't learned anything. I'm certainly not going
to take an anonymous person's word for it.

The world is divided between practitioners of theory and of practice.
Those who favor can explain anything, but can't build anything that
actually works. Those that favor practice tend to build strange
contraptions that they can't explain. That's also probably the main
source of what you call "fluff". Sometimes, I run into a theoretician
that knows which end of the soldering iron to grab, but they are rare.
For the record, I'm a practitioner of practice, trial-n-error, and
magic. I have difficulty explaining some things, and I think you've
seen my horrid math.

I think the world is *not* divided at all, rather there is a range of
abilities on both theory and practice scales with independent values.
Fluff is fluff no mater what you are good at. If someone can't
recognize fluff, then they are missing a lot of understanding.

I'm used to seeing this sort of lack of rigor in many pursuits, cars,
sports, etc. I've also seen it in safety. Lots of people use seat of
the pants concepts in analyzing safety. It shows up very easily when
you simply ask questions about the source of the info. Same with the
amateur design of antennas. Lots of talk, but very little data in most
cases, like with solder joints.


I'm talking about water impacts. Humidity and rain soak into materials.
Some by absorption, others by infiltration into micro-cracks. I saw
some materials that talked about water trees in PEX. This is not a
universal problem in all plastics.

I come from the marine radio part of the business. Water and
corrosion are key parts of the marine radio problem. Many materials
are hygroscopic and will absorb moisture. Apply some RF and the water
boils out, as in a microwave oven. Do it too fast, and the material
can crack. Of course, the tuning will change. Lots of info on
plastics selection for minimal water absorption found online. For
example:
http://www.curbellplastics.com/technical-resources/pdf/water-absorption-plastics.pdf
Polypropylene would be my first choice for minimal water absorption.
However, it requires UV protection, which for RF applications means
some kind of conformal coating. (Adding carbon black is a bad idea as
it causes heating problems).

Plastic in a tuning capacitor should be protected from the elements in
other ways. Conformal coating is not really needed if the entire
capacitor is in a box, even a transparent plastic box as long as it
blocks the UV. Most plastics do.


There are simple facts about silver that make it only very
slightly better than copper for RF circuits. I know that you can
increase the size of the conductor by less than 5% as an alternative to
using silver plating if the electrical characteristics are the goal.
Can you explain why silver is required? The numbers don't show it.

Antennas are mounted outdoors where exposed copper is an invitation to
corrosion. If one must protect the copper with something, why not use
silver, which also improves its RF characteristics?

My solution would be to use aluminum instead. Copper is not really
superior in a meaningful way and costs a lot more.


My magloop nightmare come true:
http://www.mixw.co.uk/MagLoop/magloopF.htm

I don't get your point here.


--

Rick