The Quantum QX Loop v2.0 is the latest version of the original Quantum Loop, first introduced in the mid-1980s.  It was the first "DX tool" offered by Radio Plus+ and continues to be our most popular product.  Hundreds have been sold and used around the world and are responsible for tons of exciting DX.

When I began designing the original Quantum Loop, I compiled complaints by DXers about  the ferrite rod loops on the market at the time. The principal complaints centered around:  (1) Poor sensitivity; (2) Noisy amplifier; (3) Poor nulling; and (4) Unattractive appearance.
    "Poor sensitivity" was addressed in the Quantum Loop v2.0 by designing a two-stage, double-balanced JFET/JFET amplifier yielding 30+ dB of amplification throughout the MW range.
    The "Noisy amplifier" complaint was dealt with by using the lowest noise semiconductors on the market and selecting bias values that provided the best balance between noise floor and gain.  Many have commented on how quiet the Quantum's amp is.
    "Poor nulling" is the result of an imbalance in either the physical or electrical construction of the loop.  The Quantum QX Loop v2.0's coil is wound with Litzt wire and is balanced to within a fraction of a uH for near perfect balance.
    The "Unattractive appearance" complaint was handled by constructing the Quantum QX Loop v2.0 from a semi-gloss black, sloped front (ergonomically friendly) metal base, a chrome tube pedestal, and a black Plexiglas loop housing.  In my opinion, and apparently in the opinion of many others, the Quantum QX Loops are pretty handsome.

   Construction quality is also a goal at Radio Plus+.  Glass epoxy circuit boards, variable capacitors (instead of varactor tuning diodes), name brand semiconductors, heavily chromed components, high quality acrylic, expensive Litzt coil wire, shock-mounted ferrite rod, Teflon bearing surface treatments, anti-scratchiness compounds on gain controls, high Mean Time Before Failure switches, etc. are all used in the Quantum QX Loops.  As a DXer, I use these DX tools myself and have made them the best I can while remaining within sight of a target price point.   As proof of this philosophy, each Quantum QX Loop has an unlimited one year warranty...should a loop fail within a year of purchase (unless it has obviously been abused), it will be repaired free of charge.  Failures beyond the one year date are repaired at the cost of materials (labor is free) to the original purchaser.


     --  TUNES 530-1700+ KHZ

     --  LED ON/OFF light




    LONGWAVE LOOPHEAD: (~160 to ~530 kHz); $99

    TROPICAL BANDS LOOPHEAD: (~1.8 to ~6.2 MHz); $99


 FRONT PANEL CONTROLS (New illustration under construction)


  POWER SELECTOR --  Switches between the AC adapter and a 9VDC battery (battery
          holder located on rear of cabinet).

GAIN CONTROL -- Allows optimal setting of gain to prevent receiver overloading.

    Q-MULTIPLICATION -- Controls the degree of positive feedback for increased signal
           level and sharper passband.

   TUNING CONTROL -- Peaks desired signal.

   TUNING RANGE EXTENDER -- H(igh) position tunes ~800-1700+ kHz; M(edium) position tunes ~600-800 kHz;
         L(ow) position tunes ~530-600 kHz.

   Q-SPOIL CONTROL -- Low, Medium, and High Q (tuning sharpness); used with
         Quantum Phaser for deep strong signal nulls; also used to control audio fidelity.


SIZE:    Overall height:   9-1/2"
               Base:   7" x  5" x  2"
               Loophead housing:    8-1/2" x  2-3/4"  x   1-3/4"
COLOR:    Black and chrome
WEIGHT:    < 2 lbs.

AMPLIFIER:   Two-stage, double-balanced JFET/JFET
GAIN:    30+ dB
POWER REQUIREMENTS:  9-15 VDC  (2.5mm miniplug; tip positive) or 9VDC battery
COIL:  Balanced, center-tapped, Litzt solenoid winding
FERRITE ROD:  Custom 7-1/2"  x  3/8"
TUNING RANGE:  530 - 1700+ kHz
NULL DEPTH:  >60 dB (depends on site/signal conditions)


"Can I use the Quantum QX Loop V2.0 with a portable radio?"
The Quantum Loops were designed for use with desktop communications receivers but you can use the Quantum Loops with a portable.  There are a couple of conditions to be aware of, however.  First, if your portable has external antenna connections, you will need an adapter or patch cord to match the external antenna connection (unless you use the Quantum Coupler, mentioned below).  Secondly, there are two popular methods of providing external antenna connections on portables:  (1) a jack (that usually leads to a filter circuit), or (2) screw terminals that end with several turns of wire around the internal ferrite rod.  In the first instance, it is usually the case that the external antenna connection was designed for SW use and manufacturers almost always introduce MW attenuation circuitry to protect the radio's front-end. (This is not good for the MW DXer.)  In addition, this jack sometimes disconnects the radio's internal ferrite rod antenna so that the radio only receives a signal from the external antenna.  (This is good for the MW DXer.)  In the second case (i.e., turns of wire around the radio's internal ferrite rod), no attenuation circuitry is introduced but the degree of coupling is usually pretty "light" and, of course, the internal ferrite rod remains in the circuit (and continues to intercept signals).  With both of these popular methods of connecting an external antenna to the radio, the reception of MW signals is almost always compromised.  Radio Plus+ offers a solution to this problem with the Quantum Coupler (see Products page).  In brief, the Quantum Coupler bypasses the attenuating circuitry for optimal transfer of signal to the portable.  NOTE, however, that Q-multiplication will be sub-optimal or nonexistent because the radio's internal ferrite rod (unless it has a disconnect jack/switch) will continue to be receiving the full bandwidth signal; increases in signal strength may be noticed, though. 

"Will I be able to null my strong, local pest stations?"
That depends on a couple of variables.  The Quantum QX Loop v2.0 is capable of providing nulls greater than 60 dB but the maximum null depth you obtain is dependent on the RF "purity" of your radio shack.  If you have several external antennas entering the shack, large bodies of metal in the general vicinity of the loop, or if the loop is next to the electrical wiring in an adjacent wall (all of which are capable of re-radiating RF signals with unusual phase distortions) , your null depths (with any loop) may be compromised.  Similarly, occasionally you may encounter signals at a particular distance from your shack or with weird phase irregularities that may blunt your loop's nulls.  With an RF tidy shack and the average MW signal, you should be able to attain nulls of at least 60 dB.  (I pretty routinely am able to attain nulls of 80 dB and more by carefully tilting and rotating the loop head in my RF clean shack.)

"What kind of DX can I expect to hear with the Quantum QX Loop v2.0?"
Whew, that's a tough one.  First of all, the more time you spend "at the dials," the more you will hear.  Secondly, what you hear will depend on the quality of your receiver, reception conditions, your skill at optimally tuning the receiver and loop, your knowledge of propagation principles, etc.  Dumb luck helps too.  I can say that I, using the Quantum Loop (original and QX versions) and a Drake R-8 receiver from my location in northwestern Florida, have heard MW stations across the U.S. as well as transatlantic stations in Europe, the Middle East,  and Africa, and hundreds of Latin stations in Central and South America and the Caribbean. The current sunspot cycle is heading toward a minimum in the coming years so MW DX can be expected to improve even more.

"Which loop should I buy?  The Quantum QX Loop v2.0 or the Quantum QX Loop v2.0+?"
There are two principal differences between the v2.0 and the v2.0+.  First, the v2.0+ uses a different amplifier than the v2.0.  The v2.0 uses a JFET/JFET design whereas the v2.0+ uses a JFET/MOSFET amp.  The noise floor of the JFET/JFET amp is a couple of dB lower than the JFET/MOSFET amp but the JFET/MOSFET amp provides several dB more gain.  In practice, the v2.0+ is capable of lifting a marginal signal to somewhat easier readability.  This improvement in readability is most useful, in my experience, during daytime DXing.  During typical nighttime DXing, especially domestic DXing, there is little practical difference.  The second principal difference is that the v2.0+ has a continuously variable Q-spoiling control.  This permits more precise Q control than with the 3-position switch on the v2.0.  Practically, a continuously variable control allows easier and more effective use with a phaser to widen the phaser null to include the passbands (and not just the carrier).  A second use of a continuously variable Q-spoil control is that it allows precise control of the signal reaching the tuning and amplifying circuitry (to prevent overload) when a long external antenna is connected to the loophead and the loop is being used as a preselector.  Combining either loop with the Quantum Phaser will permit a single null to be obtained, which is an advantage if you are located near the coast and DX transoceanic MW signals.

"What's the advantage of the Q-multiplication feature in the QX Loops?"
Q-multiplication is the process whereby a tuned signal is amplified and routed back through the tuning circuitry.  This circular process results in an increase in the signal level and a sharpening of the tuned passband.  In practice, this can result in an increase in S-meter reading of as much as three S-units and a sharpening of the passband so that adjacent channel slop can be reduced.  In some instances, this can result in logging a previously inaudible station.  For example, I have a strong, sloppy local station on 980 kHz that is in direct line with another station, located 350 miles away, on 970 kHz.  The station is simply inaudible regardless of which radio or antenna I am using; it's simply covered by the 980 kHz slop.  By switching the receiver to LSB and judiciously adjusting the QX (i.e., "Q-multiplication") control, I can tighten the passband of the loop to give a good, readable signal of the 970 kHz station.  Let me state again, this station is simply not detectable on any receiver (regardless of price) or antenna, including phased antennas.  This capability of extremely tightening the passband can be invaluable when DXing next to domestics, trying to separate a split-frequency transoceanic station from a domestic, or improving the signal-to-noise ratio on, for example, longwave non-directional beacons.  I also anticipate that this feature will become valuable in the future when the relatively broad-band high-definition AM stations start to appear in greater numbers.