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Very wideband, constant sensitivity, active RX loop with 2x OPA847ID .
pa0nhc 2090514 s16. Development data, not tested.
The use, copy and modification of all info on this site is only permitted for non-commercial purposes and thereby explicitly mentioning my radio amateur call sign "PA0NHC" as the original writer / designer / photographer / publisher.

This is my version of a loop amplifier, thereby using data from PA3ANG and LZ1AQ.
Optimized using LTspice. The best of two worlds ?



Very flat and wide frequency characteristic from
optimized LTspice simulation.
0.1 -27 MHz +2 / -3 dB.
C4/5 are parasitic circuit capacitances.
Input from loop = 1mA.
Full Line = RF out in dBv.
Dashed line = phase.

This loop needs no screening, this amplifier needs no grounding. The RF transmission line is balanced, and terminated by effective build-in  CommonModeChokes. No ferrite chokes over the transmission line are therefore needed. This all makes installation of the cable, and the antenna on a heavy 25 kg garden umbrella foot, very simple and fast. The only grounding point is at the receiver. Installing some ferrite cores directly near the splitter and the receiver is recommended.

For frequencies below 30 MHz, the optimal loop diameter is 1m. With my wideband amplifier, the loop is heavy loaded, therefore non-resonating. Its skin losses are far lower than caused by the loading of the amplifier circuit. The metal type of the loop (stainless steel, aluminum or copper) therefore has nealy no influence on the antenna performance. A thin wire loop like from 3mm ALU wire already works well. A thicker loop results in a lower self inductance, and concequently in a lower high frequency noise floor.  If the antenna noise floor at the highest SW bands is most important, a very thick (60 mm ALU) and smaller diameter (40 cm) loop could be tested. The loop output current will be a bit lower, but will be compensated due to the use of the special negative feedback circuit in my amplifier.

Easy and practical is to use a 2mm thick, 25mm (or 15mm) wide and 2m long ALU strip, which results into a sturdy, weatherproof 60cm diameter loop.
See the photos below for suggested construction details.

Equal RF-field strengths on different amateur bands will give equal signal strength indication.

The nice flat total (antenna + amplifier) with LTspice optimized system frequency characteristic is between 100 kHz and 27 MHz about +2dB / -3 dB. This {loop+amplifier} combination is really wideband. The systems low cut-off frequency will only be determent by the inductances of line matching transformers Tr1 and Tr2.

Overload due to FM broadcast, VHF communication, UHF TV or GSM networks is prevented by very steep HF roll-off : 80 MHz : -17dB, 100 MHz -21 db, 300 MHz -39 dB.

            Amplifier details.
 Diodes D1-8 limit the input voltages to a safe value. The amplifier should not be damaged from the induction of a lightning strike at 100m distance, nor from 1 kW radiated power at 10m distance. R9 / 10 leak static charges from the loop to PCB GND surfaces.

The self inductance of the by me used 60 cm diameter loop is calculated to 1,22 uH. Its reactance (inductive impedance) therefore rises by nature 6dB/octave with rising frequency. With an amplifier which would have a constant voltage gain over its full frequency range, the sensitivity of the total antenna system at 30 MHz would be 10 times (20 dB) lower than on 3 MHz.

The used OPA847 low noise SMD RF opamps have a very large gain/bandwidth product. Negative feedback can therefore be used to both :
-   Set its stage gain to a practical level,
-   Flatten the total [loop+amplifier] sensitivity response.

My amplifier version therefore use OPA847 as very lowZ input amplifier, with frequency dependant negative feedback. The amount of negative feedback decreases at a rate of 6 dB/octave towards higher frequencies (R12/C17), to compensate for the effects of the rising loop impedance. C5 and R3 were added, and the values of R1,3,12,C5,17 were LTspice optimized, resulting in a theoretically nearly flat overall [loop+amplifier] sensitivity characteristic.

The amplifier input impedance is very low, and the loop is loaded into short-circuit mode. The loop functions as a current signal source. With constant magnetic field strength, its current output is theoretically nearly constant up to 1/10 lambda loop circumference. The upper limit for a 1m diameter loop (having 3.14m circumference) is therefore 30 MHz.

The amplifier noise floor is mostly determent by the thermal noise of the input circuit. The ICs noise figure has very little influence on it. Below 20 MHz the system noise floor is lower than the noise level of a very quit shortwave band in a very quit location.

            PCBs.
The PCBs are designed for use with easy installable wired components.
On the amplifier PCB only two SMD components are used, as there is no DIL version available for the OPA847 ICs. Impedances are sometimes very lowZ. Stray inductances of circuit connections and part inductances need therefore extra attention. The PCBs are carefully RF designed, and are two-layered with large component holes for easy installing and removing of components. To ensure lowest mass plane inductances, best stability and screening, the amplifier PCB contains a large number of wide via's, which interconnect top and bottom ground surfaces. Interconnecting traces are as short as practically possible.

      Balanced feed lines connect antenna with receiver.
A straight CAT5 network cable is used for the connections between the amplifier and the splitter. All four wire pairs in the CAT5 cable are twisted, therefore balanced in nature. They have an impedance of abt.100 Ohms. The DC power to the amplifier (RJ1,2), and the RF signal to the splitter (RJ7,8), are transported via two of those twisted wire pairs. Common mode currents are blocked by optimal constructed Common Mode Chokes (L2,101,102). These balanced lines are therefore very in-sensitive to E- and H- noise fields. Via spare wire (RJ4) static charges and limited common mode signals are leaked to the splitter (and receiver ground). The three remaining spare wires are left floating at the amplifier PCB, but are grounded at the splitter PCB. They could be used for connecting an anti-condensation heater or a relay. Remember : In  the CAT5 DC wire pair, with 132 mAdc current, about 0.5Vdc power loss exists in every 10m cable length.

Not only the CAT5 signal cable, but also the antenna and amplifier circuits are fully balanced. The loop amplifier input is very low-Z. If noises from surroundings will be (capacitive and hiZ) coupled to the very lowZ and relative small loop, the noise currents in the loop will be very weak. These noise currents are also nearly equal in strength and in phase at both amplifier inputs. The balanced amplifier will therefore cancel them. This is the reason why this loop/amplifier combination needs no screening. A grounding of the antenna even could introduce a ground loop with noise injection as the result.

            Transformers.
"Balance to balance" output transformer Tr1 at the amplifier PCB matches the amplifier output to the 100 Ohms impedance of twisted CAT5 line RJ7,8. Tr1 is wound on a multi-aperture core MIX61. This ferrite is (according to FaiRite specs) the optimal material for low loss inductive applications up to 25MHz. L102 (bifilar wound on #31 ferrite) acts as a very wideband balun. 

TR101 in the splitter is a 100:50 Ohms unbalance to unbalance matching transformer, identical to Tr1, but connected as an Un-Un auto transformer 3:2.

If available, only use multi-aperture (pigs nose) cores for best results.
Wind tha maximal number of turns possible.
Always twist wires.
Wind CLOSE to the core, to get minimal wire length around the core.
When winding ring cores, wind directly aside each other in one direction, not over each other nor back.
Keep wire connections short.
BEFORE connecting and soldering, MEASURE winding connections.

If frequencies below 4MHz are more of interest, you could wind both transformers on multi-aperture core's MIX43 or MIX75. 
If frequencies below 500 kHz are most important, materials MIX77 and MIX78 are optimal. 
Performance on higher frequencies is than questionable.

            Power.
According to the data sheets for the low drop LM2940 stabilizer, the input capacitor (C15) for VR1 should be 0.47 uF.
Its output capacitor (C13) should have a value of at least 22 uF, and an ESR between 0.1 Ohm and 1 Ohm over wide frequency- and temperature ranges.  C15 (47 uF / 16V low ESR) is loacted as close as possible near VR1. A further buffering is done by C14 (470uF/16V). Capacitor C12 on the amplifier should also best be a LOW ESR type, as any noise on the IC + inputs will be amplified by the OPA's. Luckely should be canceled due to the balancing action of the amplifier circuit.

            Splitter and power supply.
On the amplifier PCB, the applied DC power is lowered to a stable 10Vdc. Low drop stabilizer VR1 needs more than 10.5Vdc to work properly. As the voltage loss over a CAT5 cable pair is up to 0.5 V / 10 m, the to the splitter connected supply voltage must be higher than 10.5 Vdc. 

The splitter contains power RF filtering, very effective balancing CommonModeChokes, and a wide band 100 : 50 Ohms matching transformer. The DC power input is fused against wrong polarity, over-voltage and a short-circuit in the CAT5 cable. 

A generally useable value extrenal supply voltage is 12.5 Vdc to 17 Vdc at 132 mAdc. 

The external DC power supply MUST be totally noise free, and must operate using an old fashioned noiseless 50/60 Hz mains transformer

An Switched Mode Power Supply is NOT useable.

Amplifier SCHEMA
Update 201905140-16

PARTSLIST
Update 20190514_s16

Splitter SCHEMA
Update 20190507

 


Amplifier PCB with wired components. Only two SMD ICs.


Splitter PCB