On Sun, 8 Nov 2015 19:12:38 -0500, rickman wrote:

On 11/8/2015 5:14 PM, Jeff Liebermann wrote:
You were asking about using a better dielectric than air. I found
this article:
http://www.qrpbuilder.com/downloads/loop%20antenna%20110310.pdf
which includes construction details for a piston capacitor arrangement
using various dielectrics. On Pg 8 is a chart of various plastics,
with dielectric constant, dielectric strength, and dissipation factor.
For cheap, the author recommends UHMW (polypropylene), which is one
tenth the cost of PTFE (Teflon).

Yeah, but he doesn't address the issue of temperature dependance of Er.
I don't even see it in his table.

True, but I don't think tempco is critical or required. A practical
loop antenna, with sufficiently high Q and narrow bandwidth, will
require an automatic tuning arrangement. I managed to built one where
the operating bandwidth on 80 meters was less than the occupied
bandwidth of a SSB signal. With that critical a frequency tuning,
manual or fixed tuning isn't going to work. Once you have an
automatic tuner, compensating for thermal drift is easy.

Incidentally, one of the problems I'm fighting is that the loop tuning
is different between TX and RX because the impedance of the receiver
and transmitter are slightly different and enought to detune the loop.
That's another problem that an automagic tuner should fix.

The text says he used PEX (cross linked polyethylene) for the capacitor,
but I can't find much info on the electrical characteristics. The one
that is hardest to find is the temperature dependence of Er. I don't
see PEX in his table at all. Very odd.

I use PEX plumbing pipe for coil forms, spacers, standoffs, and
spreaders. Works well, but seems a bit expensive. This might help:
http://www.smeter.net/daily-facts/11/fact21.php
Er = 2.3 and 60-90 kV/mm

Here's something on building a trombone capacitor:
http://www.eham.net/ehamforum/smf/index.php?topic=70784.0;wap2
Er = 2.25

This looks a bit more authoritative:
http://www.comfortprosystems.com/sites/comfortprosystems.com/files/cps_aquaheat_pex_pressdrop_tec-04.pdf
Er = 2.3

The info on PEX that I can find on the web indicates it may have
problems with use outdoors, but maybe this antenna isn't intended to be
used outdoors.

Just about everything plastic has problems with UV embrittlement. The
best fix I've found is Krylon clear acrylic spray.
http://yarchive.net/electr/plastic_uv_resistances.html
Hint: Search Yarchive and Google for posting by Dr Barry L. Ornitz
WA4VZQ. Lots of really good info on materials, chemicals, processes,
and antennas.

Incidentally, if you dive into the Yahoo magloop files sections, there
are some photos of the insides of the MFJ-1786 mag loop.
http://www.mfjenterprises.com/Product.php?productid=MFJ-1786
https://groups.yahoo.com/neo/groups/MagLoop/files/MFJ-1786/MFJ-1786%20coupling%20loop/
Two things worthy of notice. All the aluminum parts are brazed or
welded together and the matching(?) coil inside the box appears to be
silver plated. If MFJ's reputation for cheap construction is to be
believed, they would not silver plate anything if a cheaper
alternative would work.


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