September 2000 QST

Figure 4—Schematic diagram of the control unit. All parts are available fromRadioShack unless otherwise indicated. Layout is not critical. Simply mount the five“capacitor switches” on the front of the enclosure along with the tuning potentiometer(R3). By using various combinations of switches and tuning, you can tweak the loop formaximum received signal strength from the comfort of your shack.C1—2200µF, 35-V electrolytic capacitor(272-1020)C2—0.1µF ceramic disk capacitor(272-135)F1—0.5A, 120 V fuseR1—240 Ω resistorR2—4.7 kΩ trimmer potentiometer(271-281)elegant way to match and interconnect theloop to the coax, and it works perfectly. Iran about 60 feet of RG-58 coax from theSO-239 connector on the capacitor box tothe shack-mounted preamp.The control unit is equally simple (seeFigure 4). Because the relays had 24-Vcoils, I used a RadioShack 24-Vtransformer and a full-wave bridge for therelay power supply. This pulled in the relaysjust fine.For the varactor tuning voltage, Iregulated the raw output of the relay supplywith an LM-317 three-pin voltage regulator.I set the regulated voltage to 20 V dc andused a 50-kΩ potentiometer to provide 0 to20 V to the diodes. One side of thepotentiometer is connected to ground whilethe other connects to the 20-V regulatedoutput. A wire from the wiper arm connectsto the 200-kΩ resistor that feeds the tuningdiodes. A set of five SPST toggle switchesactivate the relays to select the variouscapacitor combinations.To the Roof!A lot of Old-Timers say loops don’t haveto be mounted high, but I wanted to get thisparticular loop as high as possible.Remember that this is a large, unwieldyantenna. I first tried to hoist it to the roofmyself, with almost disastrous results.Get help erecting this antenna!If the loop tilts more than 30°, one personprobably can’t handle it. It’s also somewhat40 September 2000R3—50 kΩ potentiometer, linear taper(271-1716)S1-S5—SPST toggle switches (275-624)S6—SPST switch (275-603)T1—120/25.2-Vac transformer (273-1366)U1—Bridge rectifier, 4A, 100 PIV(276-1171)U2—LM317T adjustable voltage regulator(276-1778)heavy and has noticeable wind resistance.I hoped to be able to rotate the antennawith a small RadioShack rotator, but thesheer size of the loop made it impossibleto mount the rotator near the antenna, so Imounted it at the bottom of the antennamast support assembly.I took the completed loop assembly,with a five-foot mast sticking out of thebottom, and carefully laid it down on theground and attached a 10-foot section ofmast to the existing five-footer. I slid a ninefootsection of 1 1 /4-inch schedule 40 PVCover the 10-foot mast section, making surethat the PVC pipe was fully contacting thepipe from the completed loop assembly.Next, I drilled a small hole through thePVC pipe and internal mast about two feetfrom the bottom of the entire assembly. Iran a small bolt through this hole andfastened it with a nut. Now the PVC pipecouldn’t slip off. The whole assembly canbe carefully raised by at least two peopleand bracketed to the side of a structureusing four TV mast brackets. Make sure themast brackets are securely anchored to theside of the structure. Once the assembly isbracketed in place, the bolt you installedshould be removed, allowing the loop to berotated from the bottom.How Does it Work?I had my doubts about loops before I builtthis one. The others were poor performersand I had to tweak them incessantly toachieve only marginal performance. After allthe adjustments, my short 160-meter sloperwould always outperform the loops.When I finished the Monster Loop, Itemporarily strapped it to the deck of ourpool (which drew an interesting stare frommy wife). I promised her it was onlytemporary and proceeded to connect thecontrol wires and coax. Back in the shack, Ipowered up the RX-320, which was tuned tothe AMRAD LF beacon frequency on136.745 kHz. The receiver came to life andDCU (a commercial data station somewherein Nova Scotia) blared from the speaker!The Canadian signal was quite strong. Iswitched to the 160-meter sloper. Yes! Thesignal was there, but nowhere near as strongas the loop. I flipped the switch on the controlbox that inserted a 1640-pF capacitor acrossthe loop. The signal from DCU increasedmarkedly. With that particular capacitor inparallel with the loop, the Varactor diodesallowed me to tune the loop to resonance.Later that day, WA2XTF/6 and /12 movedmy S-meter for the first time! And every timeI compared the loop to the sloper, the loopwon hands down. By rotating the loop I couldeffectively eliminate about 80% of the linenoise that was giving me trouble. The bigloop has an incredibly deep null. Being ableto null unwanted noise sometimes makes thedifference between hearing a signal well andnot hearing it at all.I experimented with the loop’s switchablecapacitors and found that the loop can bemade to resonate anywhere from slightlybelow 90 kHz to just above 450 kHz.Remote tuning diodes are the only wayto go. The resonance peaks are quite sharp,and you have to retune every couple of kHz,but the incredible performance makes it allworthwhile.If you build a version of this loop foryourself, please remember to be carefulduring installation. Although it performswell, it’s awkward to install. I also have myfinger crossed as to its survivability. We’vehad a few strong winds since I’ve installedthe “Monster.” It swayed back and forth,but no harm was done.Last but not least—building this big loopwas a lot of fun. It had been a while sinceI’d brought back a load of hardware andturned it into something useful andattractive.You can contact the author at 199 MapleAve, Mechanicsburg, PA 17055; wa3usg@compuserve.com.