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

Stephen H. Smith wa8lmf2 at aol.com
Mon Mar 8 19:01:10 EST 2010

Lynn W. Deffenbaugh (Mr) wrote:
> Stephen H. Smith wrote:
>> Eagerly awaiting the stations from the midwest and east coast that 
>> normally start showing up about 16:00 local time. Details on this 
>> dirt-cheap easy-to-build compact antenna  for 30 meters (that could 
>> be stealthed in an attic) will follow in a week or so.
> And I'm eagerly awaiting the antenna details!  I just posted a request 
> for 30m antenna suggestions to another group and received some 
> suggestions, but I'm looking to go cheap, dirt-cheap is even better!  
> Any early hints or links so I can start to get a handle on what to 
> expect?  Even if it's a reference to a page number or article title at 
> arrl.net or the 2010 Handbook, I'm all eyes!
> Lynn (D) - KJ4ERJ-1 - IGating 10.147.600 both AX.25 and PSK-63 from 
> Palm Bay, FL  USA (EL97qx)
> PS. See http://tinyurl.com/IGATE-KJ4ERJ-1 for recently heard stations
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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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