On Sun, 18 Jan 2015 13:00:01 -0700, Irv Finkleman VE6BP
wrote:

I intend to order the MFJ-1780 Box Fan Portable Loop.
I'd have it on order already but they are closed on the weekend.

You can view an image at
https://farm4.staticflickr.com/3913/...596a70d0_d.jpg

http://www.mfjenterprises.com/Product.php?productid=MFJ-1780
You're going to pay $330 for that? Surely you can find a cheaper mag
loop antenna?

One chap has had his hanging in a tree, with weather
protection provided by a plastic garbage bag.

Great idea. However, the reason is not so obvious. See below.

This antenna was obviously not designed for permanent outdoor
installation. It says so in the manual:
http://www.mfjenterprises.com/Downloads/index.php?productid=MFJ-1780&filename=MFJ-1780.pdf&company=mfj
OUTDOOR USE
Note : The MFJ Box Fan Loop can
be used outdoor only in good
weather conditions ( not in windy or
stormy weather).

Mounted horizontally any water that collects inside will end up on the
insulated side plates, where the water will not do any major damage.
However, mounted vertically puts the plates of the tuning capacitor in
a horizontal position, where they can collect water. This is bad. If
you want to hang it vertically, please mount it with the capacitor
plates vertical. This is NOT the ideal position for vertical
polarization, so there may be some signal loss. Also, drill some weep
holes in the bottom of the box so that water will drain out.

The trash bag cover has a unique advantage. The black low-density
polyethylene (LDPE) trash bags are transparent to long wave infra-red
radiation, also known as heat. Hang a trashbag full of water in the
sun, and it will soon deliver hot or warm water. That's how these
work:
http://www.coleman.com/product/5-gallon-camp-shower/2000014865?contextCategory=28170
If some water gets into the bag (e.g. condensation) and lands on the
tuner, as soon as the sun comes out, the bag will get hot enough to
evaporate the water. Be sure to leave the bottom of the trash bag
partly open, so the water vapor has a way to get out of the bag. Even
in the dim light of dawn, there will be enough heat to prevent the
temperature from staying below the dew point (unless it's really
cold). I don't know exactly which style or thickness of garbage bag
to use. My guess is as thin as possible and only one layer, so that
the most IR gets into the bag.

If you want to lube the capacitor bushings or bearing, just use some
heavy waterproof grease. I tend to use lithium white grease for
everything, but almost any other grease will work. Since you're
pushing RF through the grease, I guess dielectric grease might be a
better choice. Forget about WD40.


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

Jeff Liebermann wrote:
On Sun, 18 Jan 2015 13:00:01 -0700, Irv Finkleman VE6BP
wrote:

I intend to order the MFJ-1780 Box Fan Portable Loop.
I'd have it on order already but they are closed on the weekend.

You can view an image at
https://farm4.staticflickr.com/3913/...596a70d0_d.jpg

http://www.mfjenterprises.com/Product.php?productid=MFJ-1780
You're going to pay $330 for that? Surely you can find a cheaper mag
loop antenna?

One chap has had his hanging in a tree, with weather
protection provided by a plastic garbage bag.

Great idea. However, the reason is not so obvious. See below.

This antenna was obviously not designed for permanent outdoor
installation. It says so in the manual:
http://www.mfjenterprises.com/Downloads/index.php?productid=MFJ-1780&filename=MFJ-1780.pdf&company=mfj
OUTDOOR USE
Note : The MFJ Box Fan Loop can
be used outdoor only in good
weather conditions ( not in windy or
stormy weather).

Thanks for another of your always good and informative replies Jeff. We
are aware of the outdoor use warning -- but we are both experimenters
and as he has his already, and I just ordered mine this morning, we are
watching carefully to see the effects of his outdoor setup.


Mounted horizontally any water that collects inside will end up on the
insulated side plates, where the water will not do any major damage.
However, mounted vertically puts the plates of the tuning capacitor in
a horizontal position, where they can collect water. This is bad. If
you want to hang it vertically, please mount it with the capacitor
plates vertical. This is NOT the ideal position for vertical
polarization, so there may be some signal loss. Also, drill some weep
holes in the bottom of the box so that water will drain out.

Weep holes -- a great idea, Thanks! hanging it so that the plates are
vertical -- another great idea! Thanks even more. Since it is a
magnetic loop antenna, rotating it to place the capacitor plates
vertical should not make any difference.


The trash bag cover has a unique advantage. The black low-density
polyethylene (LDPE) trash bags are transparent to long wave infra-red
radiation, also known as heat. Hang a trashbag full of water in the
sun, and it will soon deliver hot or warm water. That's how these
work:
http://www.coleman.com/product/5-gallon-camp-shower/2000014865?contextCategory=28170
If some water gets into the bag (e.g. condensation) and lands on the
tuner, as soon as the sun comes out, the bag will get hot enough to
evaporate the water. Be sure to leave the bottom of the trash bag
partly open, so the water vapor has a way to get out of the bag. Even
in the dim light of dawn, there will be enough heat to prevent the
temperature from staying below the dew point (unless it's really
cold). I don't know exactly which style or thickness of garbage bag
to use. My guess is as thin as possible and only one layer, so that
the most IR gets into the bag.

Well, I even learned about trash bag long-wave transparency! I like
your comments on the evaporation of condensation!

If you want to lube the capacitor bushings or bearing, just use some
heavy waterproof grease. I tend to use lithium white grease for
everything, but almost any other grease will work. Since you're
pushing RF through the grease, I guess dielectric grease might be a
better choice. Forget about WD40.

WD-40 is out -- lithium or similar grease is in!

Thanks a lot Jeff -- your posts are always helpful and informative. In
particular, you don't just post instructions on what to do, but you
provide good supportive reasoning which I am sure has been of value
to many on the group.

73

de Irv VE6BP

On Mon, 19 Jan 2015 11:09:21 -0700, Irv Finkleman VE6BP
wrote:

Thanks a lot Jeff -- your posts are always helpful and informative. In
particular, you don't just post instructions on what to do, but you
provide good supportive reasoning which I am sure has been of value
to many on the group.

Thanks. That's what I try to do. One line "do this" type answers are
useful, but without the explanation and logic behind the
recommendation, it's of little use. Besides, if I just supply the
logic, and let you make the decision, and it doesn't work, it's YOUR
fault for accepting my logic, and drawing the wrong conclusion. It's
much like getting an opinion from an attorney. You'll never get just
one option or recommendation. Instead you get a fair number of
"options" among which you're expected to choose. If you pick the
wrong one, it's your fault, not the attorney's.

I'll do a little reading and testing later this week on the different
types and thickness of LDPE sheets:
http://www.bagbarn.com/all_purpose_poly_bags/garbage_bags.php
Some are probably better than others. If you have an incandescent
bulb flashlight (heat source) and digital IR thermometer gun, you can
probably do your own testing. However, I don't want to get it too hot
or it might melt the loop circuitry. That's also another reason to
keep the bottom open, so that it won't overheat. I suggest testing
the garbage bag over a thermometer in a cardboard box simulation of
the loop, before trying it on the real thing.

Note: I used to design marine radios and therefore know something
about waterproofing.

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

Jeff Liebermann wrote:
On Mon, 19 Jan 2015 11:09:21 -0700, Irv Finkleman VE6BP
wrote:

Thanks a lot Jeff -- your posts are always helpful and informative. In
particular, you don't just post instructions on what to do, but you
provide good supportive reasoning which I am sure has been of value
to many on the group.

Thanks. That's what I try to do. One line "do this" type answers are
useful, but without the explanation and logic behind the
recommendation, it's of little use. Besides, if I just supply the
logic, and let you make the decision, and it doesn't work, it's YOUR
fault for accepting my logic, and drawing the wrong conclusion. It's
much like getting an opinion from an attorney. You'll never get just
one option or recommendation. Instead you get a fair number of
"options" among which you're expected to choose. If you pick the
wrong one, it's your fault, not the attorney's.

I'll do a little reading and testing later this week on the different
types and thickness of LDPE sheets:
http://www.bagbarn.com/all_purpose_poly_bags/garbage_bags.php
Some are probably better than others. If you have an incandescent
bulb flashlight (heat source) and digital IR thermometer gun, you can
probably do your own testing. However, I don't want to get it too hot
or it might melt the loop circuitry. That's also another reason to
keep the bottom open, so that it won't overheat. I suggest testing
the garbage bag over a thermometer in a cardboard box simulation of
the loop, before trying it on the real thing.

Note: I used to design marine radios and therefore know something
about waterproofing.

I was in the Navy for 26 years and just learned to tolerate the moisture.
Besides, all our outside electronic equipment was pretty well taken
care of with nitrogen gas! A little more than a ham needs to deal with!

Irv