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.
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.
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.
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.
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.
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'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).
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 magloop nightmare come true:
http://www.mixw.co.uk/MagLoop/magloopF.htm
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558