[aprssig] APRS Messenger PSK-63 On The Air Reports -- Magnetic Loop Antenna Now Up

Rudy Benner benner at vianet.ca
Tue Mar 9 20:15:08 EST 2010

That is a keeper. Perfect for WSPR too.



> I'd do a web page on it now, but I'm getting ready for the IWCE
> (International Wireless Communications Expo) in Las Vegas this week.
> Basically, a "magnetic loop" antenna is a very small (relative to
> wavelength) single-turn loop tuned to resonance with a series
> capacitor.   A second, much smaller loop connected directly to a coax
> feed line is placed inside the larger loop, nearly tangent to the first
> loop at the side opposite the capacitor.   This small loop forms the
> primary winding of an air-core RF transformer with the larger loop
> forming the secondary winding
> Such a device can be nearly as efficient on transmit as a full-sized
> dipole -IF- the loop and capacitor are very efficient and low loss.
> The MFJ "Super-HI-Q Loop" antenna and the old AEA "IsoLoop" are
> commercial versions of such an antenna that are tunable between 10 to 30
> MHz.   With a 100 watt transmitter, HUNDREDS of RF amps circulate in the
> loop and 4,000-to-10,000 volts can appear across the capacitor.
> My loop is constructed of 3/8" soft copper "refrigeration tubing" from
> Home Depot.   Ten feet of this are bent into a circular loop about 1
> yard in diameter.    The support is a vertical mast  made from a piece
> of 1" Sched 40 PVC water pipe. with two 3/8" holes drilled through it's
> diameter about 37" apart.
> The trick was the capacitor.  Normally, mag loop ants use motorized
> high-voltage butterfly, split-stator variable caps or variable vacuum
> caps to tune the loop to resonance while withstanding very high RF
> voltages.  (I.e. the kind of variable caps you see in high power antenna
> tuners.)  Since I didn't need the antenna to be tunable (it's going to
> set to a single spot frequency (10.149 MHz) permanently, I calculated
> the required capacitance and set out to create a cheap fixed-value cap
> with 4-5 KV breakdown.
> The inside diameter of the 3/8" tubing is an EXACT fit for the center
> conductor and dielectric of RG-8 or RG-213 coax.   I stripped the outer
> jacket and braid off 37" of RG-213 cable and then stripped the
> dielectric off half an inch of the remaining insulated center
> conductor.   I soldered the exposed half-inch of the cable to the INSIDE
> of one end of the  loop tubing.   (You'll need a old-fashioned  150
> soldering iron  or a Berz-O-Matic torch to heat up the copper tubing
> (not a wimpy electronics pencil or temp-controlled soldering station)
> since the copper is such a good conductor of heat!    I then jammed the
> remaining 36 1/2" of dielectric-covered center conductor into the open
> other end of the copper tubing.  Push it in until the loop is nearly
> closed with only a 1/4" so so gap between the two ends.
> This construction creates a coaxial capacitor with the coax cable center
> conductor being one plate and the INSIDE of the copper tubing being the
> other plate.  The 36" or so of cable inside the tubing creates a
> capacitance of about 75pF required to resonate the loop.  Normal 50-ohm
> coax has a capacitance of about 30-33 pF/foot.   The snug fit between
> the center conductor/dielectric, and the inside diameter of the tubing,
> very closely duplicates the geometry, and thus capacitance of the
> original coax .   (You could literally make your own copper hardline
> from this tubing by pushing yards and yards of center
> conductor/dielectric from RG/8, RG/213 or RG/214 into it.)
> You tune the loop to exact resonance by pulling the loop apart, or
> pushing the loop together exposing more or less of the coax cable.
> (Only the portion of the coax opposite the tubing's inside diameter
> contributes to the capacitor value.)  In my model, I had to expose about
> 3/8" of the dielectric to get to 10.149 MHz.  The tuning is VERY sharp
> (indicating hi Q and high efficiency).  A change of 1/8th inch moves the
> resonance about 100Kz.  The bandwidth for 2:1 SWR is only about 12
> KHz.    This is actually an advantage for single frequency operation -
> the antenna is so selective that transmitting on other HF bands
> simultaneously won't desensitize the 30M receiver at all.
> Finally, create the coupling loop. I cut 22" of normal  #14 THHN copper
> house wire, formed it into a loop approximately 7-1/2" diameter and
> soldered the ends to the center conductor and braid of any convenient
> length of 50-ohm coax to reach to the radio.   This loop is then placed
> so that it is about 1/4" away from the inside circumference of the large
> loop on the side opposite the open-ends/capacitor.  I drilled a couple
> of additional holes in the PVC pipe to support the small loop in the
> proper orientation. By bending (distorting this circle) so that more or
> less of it is closely parallel to the big loop, you fine-tune the SWR at
> resonance.  Mine  has an SWR of under 1.5:1 at resonance.
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