"Jeff Liebermann" wrote in message
...
On Thu, 23 Jan 2014 15:52:40 -0700, "Irv Finkleman"
wrote:
From all the reading I have done, the Magnetic Loop Antenna seems to be
a good performer, and ideally suited to locations where large or long
antennas are out of the question.
Irv VE6BP
I agree. A magnetic loop will both fit and work best at your
location. No grounding system or ground radials required.
I don't have a magnetic loop of my own but I've helped build 3 of them
(all different) with local hams.
Some things to think about before blundering forward:
1. Where are you and the loop going to live? The Q of the loop is
very high which means you're going to be retuning the loop every time
you change frequency more than a few KHz. If the loop is outside on
the balcony, and you're inside near the radio, you're going to be
running back and forth all day long. The only saving grace is if
you're doing PSK31, which lives on a single frequency per band, and
therefore doesn't require constant retuning. Think about motor driven
capacitor tuning.
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!
2. Magnetic loops generate rather high voltage across the tuning
capacitor. You'll probably need either a wide spaced cheese cutter
type capacitor, a Jennings vacuum variable capacitor ($150 to $350 on
eBay), or a butterfly type variable capacitor. I prefer the
butterfly, but they're difficult to find. So, build your own:
http://www.alexloop.com/artigo21.html arrangements:
http://www.alexloop.com
I will be operating QRP and have a good selection of all types of
capacitors, some fairly high voltage although many of the QRP
designs I have seen will allow a simple broadcast radio capacitor
(not those new plastic jobbies) to operate -- sometimes as high as
20W1
3. I've seen problems with the loop tuning drifting with temperature.
It's not the transmit power but heating from the sun. Even if you're
planning on sitting on one frequency all day, you should check for
tuning drift.
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!
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.
4. Magnetic loops are somewhat directional. You may need some method
of spinning the loop for maximum signal. When mounted vertically, the
loop is horizontally polarized. When hung over the balcony rail
horizontally, it's vertically polarized. See photos of mounting
I plan to mount it vertically using a piece of 2X4 sitting in a big pot or
tub
filled with gravel. 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.
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.
5. Magnetic loop calculators:
http://www.66pacific.com/calculators/small_tx_loop_calc.aspx
http://www.aa5tb.com/aa5tb_loop_v1.22a.xls
6. Losses. If you plug the numbers into an antenna simulator, you'll
find that the losses are HUGE. If you go to the bottom of the page
at:
http://www.aa5tb.com/loop.html
there's a graph of losses versus different loop material sizes. Notice
the 1" and 1" (0.5 ohms) plots. Only 0.5 ohms of added resistance and
the losses increase by about 16dB at 3.5 MHz. That's a nice way of
saying you can't just throw it together. Everything has to be
soldered carefully and no sliding contacts on the caps. Your MFJ
antenna analyzer will be handy for testing the construction.
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? 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.
A few of the magloops I've looked at on the net were not all
soldered -- many just used nuts, bolts and starwashers. 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.
One of the Manufactured loops is made of aluminim strips bolted
together, and it seems to get good ratings on EHam.
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. Did I mention two Slinky's that I want to
try -- first as a short vertical, and then as a helical magnetic loop!
I can hardly wait to get going -- and I'm getting closer every day!
Thanks for the tips Jeff -- your contributions to the group are
always great.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558