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