On Thu, 23 Jan 2014 21:06:12 -0700, "Irv Finkleman"
wrote:
I already have a small 12v motor and have tested it with a number of
different capacitors. With a simple 10k wirewound pot I can slow it
down to almost nothing, or bring it back up to some value, and
that beautiful pot just gets lukewarm. Using a DPDT toggle switch
I can control the direction as well. Remote tuning on a shoestring!
Loose the potentiometer and replace is with a gearbox reducer. You
want to retain all the torque and power from the drive motor. Also,
don't forget to hi voltage insulate the motor, so that you don't arc
over to the motor in transmit.
I plan to cut a big cloth circular case to drop over the loop. It will be
some nice flowery design, and a plan to put it over the back of a
lawnchair so that anyone who walks by and looks up to my
3rd floor balcony will see what appears to be a nice comfortable
highback chair! That's mainly for subterfuge!
If the lawn chair has an aluminum frame, it will detune the magnetic
loop antenna. All plastic lawn chair is required. Also, don't fire
up the transmitter when someone is sitting in the lawn chair.
At the same time, having been experimenting with assorted antennas
over the years (before I began to deteriorate) I had a big yard full
of wire, ground rods, and a tower with all kinds of things hanging
off of it. I always kept an eye on the SWR bridge for any
variations, so any of that sun heating drift will be watched
carefully. Thanks for the tip though! For all my reading that is
one thing I hadn't encountered before.
It's only a problem with high Q antennas. Figure on a Q of 100 to 200
for the lower bands. On 80 meters, that's a 2:1 VSWR bandwidth of
about 25 to 35 KHz. It doesn't take much to drift the antenna off
frequency. Most magnetic loop users don't notice because they're
constantly tweaking the tuning capacitor.
I plan to mount it vertically using a piece of 2X4 sitting in a big pot or
tub filled with gravel.
So much for the disguise antenna. Don't forget to pound a ground rod
into the gravel. It won't do anything useful, but will make an
interesting conversation starter as to whether a ground is necessary.
Anticipating lots of experimentation once I get
on the air, I can drill it, nail it, or whatever for various forms of
support, and even clamp a whip on it. I also anticipate that I will
eventually turn it into something akin to swiss cheese, and when that
comes about, I can replace my 'cheapo balcony tower' with ease.
Let me know when you're ready to build my inflatable antenna tower.
I will also constuct a small platform to hold the tub/pot and
by using some of the small plastic swivel casters I'll make the
whole works rotatable.
Lazy Susan works well. I use that for my direction finder hacks. Use
lots of grease if left outside as the bearings tend to rust.
I'm counting on the MFJ Analyzer for lots of help. I'm very away
of the losses, and where they occur and will be careful. As far as losses
are concerned, when running QRP why worry?
If you only have a little power to work with, then losses become even
more important than if you had hundreds of watts to waste.
Under good band
conditions like we had in the good old days (with propagation ever again
improve?) you can work the world with only a few watts. some guys do it
with milliwatts! I only want 75M for very local work within about
200Km, but I know I'll be quite efficient on 20 which is where I have
always had the most fun.
They still do it with milliwatts.
http://hflink.com/jt65/
I'm not sure what's wrong with propagation these days. Probably
global warming, or a government conspiracy.
A few of the magloops I've looked at on the net were not all
soldered -- many just used nuts, bolts and starwashers.
They don't work well. One of those I rescued was initially thrown
together but not soldered on the assumption that future modifications
would be easier without soldering. It didn't work until after it was
soldered.
Whenever
I ran across them, I'd get back to the builder/operator and ask
how they worked, and had they taken the resistance losses into
consideration. In all cases they were happy with the way things
were and making lots of contacts.
You can also make a fair number of contacts with a dummy load. I
know, I've done it by accident. I also made a fair number of contacts
on one Field Day using a coax cable that was not connected to any
antenna. In some cases, a dummy load or no antenna would be an
improvement over some of the home brew antennas I've seen.
One of the Manufactured loops is made of aluminim strips bolted
together, and it seems to get good ratings on EHam.
You can test that yourself with your MFJ antenna analyzer. Build a
loop that way. Measure the VSWR curve and impedance at resonance.
Then, rattle the loop a bit to move the bolts a little. Measure
again. My guess is you'll see substantial changes every time you bang
on the antenna. It might also be interesting to use an ESR
(equivalent series resistance) meter to measure the DC resistance.
That's also going to vary.
On hand, I also have a yoyo antenna, a Miracle Whip antenna, and the
MP1 which will be on the air as soon as I can measure the radials and
see how it works indoors.
Anything with amazing, magic, miracle, ultimate, or other superlatives
is usually over-rated.
Did I mention two Slinky's that I want to
try -- first as a short vertical, and then as a helical magnetic loop!
A what? Never mind. I don't have time to model a pretzel made from a
Slinky.
I can hardly wait to get going -- and I'm getting closer every day!
Oh-oh. Slow down. It might be more fun doing cut-n-try, but it's
much easier if you plan your antenna carefully, and build it once, not
20 times.
Thanks for the tips Jeff -- your contributions to the group are
always great.
Y'er welcome. Good luck.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558