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LZ1AQ / pa0nhc wide band loop receiving antenna.
Publication granted by LZ1AQ, Chavdar Levkov.
pa0nhc 20190416

These pages are translated from Dutch to English.

The original article by LZ1AQ is 


Version 1.
Cross : 32mm / 19mm PVC tube.
Loop : 15x2mm ALU strip, circumference 2m.


The ALU antenna strip fits in three places in slits, and is fixed using black tywraps.

Antenna amplifier.

This PCB needs no screening.


Splitter.

This PCB must be installed into a closed metal box.

REM: The RJ45 busses are installed onto the bottom of the PCBs.


Version 2. Amplifier in 75mm dia. PVC pipe.
Use stainless steel hardware.

          LZ1AQ :
This antenna acts almost as a pure magnetic transducer. The input impedance of the amplifier is so low that any currents induced by electric field become very small compared to the currents induced by magnetic filed. This antenna does not need shield or any type  of grounding. For vertically polarized low elevation angle signals the antenna has very sharp null. The directivity for the sky wave signals is not determined since their polarization is stochastic. The influence of nearby non-resonant conductive object is negligible. The differential circuit also reduces the influence of common mode currents. It works from height almost zero above the ground (there is almost no change in signal levels when the loop side is placed several  centimeters above the ground in field environment). The wideband properties are excellent  - from LW to upper HF even 50 MHz band can be included. The dynamic range obtained from on the air tests on the bands  is good and no apparent non-linear distortions are found.  The circuit is very simple, stable and cheap and there is nothing critical for adjustment.  The antenna can be mounted outdoor and connected with FTP cable to RX and PS parts. The FTP cable  is widely available  and the associated connectors are very reliable and cheap.  This is my favorite antenna for my city office where nothing else can survive the EMC pollution. The only drawback of this active antenna is  its relatively higher  noise floor specially for frequencies above 10 MHz  which is several dB above the atmospheric noise levels for quiet rural locations at some frequencies and times of the day (for single loop 1m diam.) . The antenna noise floor is acceptable  and suitable for all locations where the man made noise is moderate and above. The noise floor limit of these types of WSM loops is essential  - see the Appendix section for more details. The noise floor can be reduced by using “fat” , parallel or parallel crossed loops especially for places where the electromagnetic noise is very low. 

            Specifications for 1m dia LZ1AQ loop :
Loop : 1 turn, circle shaped, diameter 1m.
Antenna material : 3.4mm aluminum.
Antenna factor : 6dB meter-1 @ 10 MHz. 1 uV/m input results in 0.5 uV output.
Dynamic range : 1 dB compression point >= 130 dBuV/m.
Freq. characteristic : within 3 dB meter-1  0,5 - 30 MHz.
Noise floor : <= 0,7 uV/m.
 

See LZ1AQ for other loop shapes, which improve S/N above 10 MHz.

                LZ1AQ :  
When all other antennas are useless due to a high man-made noise level, this loop proves to be the only useable antenna. 

            Properties :
Pure H-field antenna.
-  The whole system is very insensitive for E-fields.
-  No loop screening needed => simple construction.
-  Balanced power supply and signal transport by means of one standard "straight" CAT5 network cable. 
       => Insensitive to E-fields. => Simple connection and - installation.

-  Internal very effective common mode chokes => No transmission line de-noising measures needed.

       => Simple installation.

Loop and splitter are not grounded => Simple installation. 
            Only the connected receiver should be connected to a safety ground (for bleeding static charges).
-  Useable at very low height above ground with little signal  loss.

-  I recommend a rotate able installation for optimal S/N.
        Then one signal (noise) source can be up to 30 dB suppressed (depending of the vertical angle of incidence).

        By means of internal safety measures this system should withstand :
1. Induction from lighting at 100m distance,
2. Up to 1,5 kW radiated power at 10m distance.
3. Static charges and common mode voltages.
4. Supply over-voltage and wrong polarization.

Needed external supply : 136 mA @ 13.5 Vdc to 17Vdc.
REM : The minimum supply voltage is influenced by the length of the CAT5 connection cable.


            My version :
For the construction of the loop, i used easy available 15x2mm aluminum strip. As the max. available length was 2m, the diameter of the loop is smaller than recommended : abt. 62 cm. This loop is loaded by a special, by LZ1AQ designed loop two stage amplifier. It is fully balanced, has a very low input impedance of between 3 Ohm and 9 Ohm, and a bandwidth of at lest 500 kHz to 50 MHz. Due to the heavy loading, the loop runs in "short circuit mode". It behaves like a current source with (depending on its diameter and thickness) a self inductance of 2 to 5 uH in series. Over a large frequency range, it gives a nearly constant signal output when in a constant H-field strength.

Weak (capacitively with high impedance) coupled E-field noises will cause very weak noise currents in the loop. These weak noise currents will cause very weak noise voltages (U=IxR) over both lowZ amplifier inputs. The amplifier balancing could further weaken them.

Therefore is this combination of small loop and low-Z balanced amplifier a nearly pure H-field antenna.

This loop needs no screening, nor grounding. 
This loop amplifier and its splitter may not be grounded.
The connected PC or receiver should be grounded for bleeding possible static charges.

        I made the following changes :
Improved the baluns L7/8/9/10/11/12.
-  Designed two PCB's with RJ45 busses for easy connection and installation.
-  Added a 100 : 50 Ohms output transformer.
-  Made an component ordering list.

            Noise.
Up to 14 MHz the antenna self noise is weaker than the atmospheric band noise. 


In city environments, having a higher noise level, the antenna self noise is not important. Important is : the antenna in-sensitivity for E-filed noise.

The influence of the loop self inductance.

Due to the very low amplifier input impedance :
Less loop self inductance => lower antenna noise floor. 

-   The noise floor of the loop amplifier is mainly determent by the terminal noise generated in the amplifier input impedance.
-   Low noise input transistors will NOT lower the noise floor.
-   The total system bandwidth is mainly determent by the circumference of the loop (1/10 lambda rule).
-   Very high Ft (2-5 GHz) transistors will NOT widen the system bandwidth. The loop bandwidth is not influenced.
-  A thicker loop (f.i..36mm) WILL improve the S/N, as a thick loop will have a lower loop self inductance.
-  Two loop windings connected in parallel WILL IMPROVE the S/N, as they will result in a lower loop self inductance.
-  Two or more loop windings connected in series make the S/N WORSE, as as they will result in a higher value loop self inductance.
-   A better conducting loop (copper and/or silver  plated) will NOT lower the noise floor, as the reactance of the loop self inductance is dominant over its series resistance.
-   A larger loop circumference makes the S/N WORSE, as it will result in a higher value loop self inductance. And it will lower the upper limit frequency range.

According to LZ1AQ, a loop diameter if 1m is optimal.

             The LZ1AQ amplifier.

Fully balanced.
-  Two stages for large bandwidth and ample amplification.
-  Input impedance 3 to 9 Ohm.
-  Output impedance 100 Ohm.
-  Output matches a CAT5 twisted wire pair. 

In the splitter a 100 Ohm to 50 Ohm output transformer matches to asymmetrical 50 Ohm coax. 

        For best IMD properties : 
-  Transistor pair Q1 and Q2  MUST be selected for equal Hfe.
-  Transistor pair Q3 and Q4  MUST be selected for equal Hfe.
-  C5 and C10 MUST be NP0 capacitors.
-  C1 and C4 MUST be film capacitors, preferably of equal value (keeping balance at very low frequencies). 

To prevent overload from strong local FM broadcast transmitters, C5/L1 and C12/L2 attenuate the FM broadcast band. 

The minimal 13.5 Vdc to maximal 17Vdc power is lowered to 10Vdc on the amplifier PCB. The used UA7810CKTTR stabilizer is cooled by soldering it onto the PCB surface.

        Coils.
Wide band transformers Tr1 (L4/5/6) and Tr2 (L21/22/23) are wound tri-filar (three wires in parallel), first twisted every 10mm. Seven turns 0.3mm enamel wire, on 12.5 mm Ui=850 (mix 43) ferrite ring cores. The by LZ1AQ specified inductance is 20 uH. Output transformer Tr1 is connected as balanced. Matching transformer Tr2 is connected as an auto transformer.

My version of the baluns (common mode chokes) L8, L10 and L12, are wound with 18 turns 0.3mm twisted enamel  wire on #31 cores

This #31 material is especially developed for common mode chokes use. It ensures best performance over a wide frequency range, and over a wide range of transmission line lengths. And this material is relative cheap.

        Winding :
-  Different colored wires make connections more easy.
-  Bend it to bi- or tri-filar with exact equal lengths.
-  Twist pairs to 1 turn/cm while gently pulling it straight (using an electrical hand drill).
-  Wind twisted the wires very close against the body of the core .
-  Wind continuously in one direction.
-  Avoid turns on top of each other. 
-  Divide all turns over at least 300 degr. with equal spaces between them.

        Connecting :
REM : Once a transformer is soldered, you cannot check the correct phase of the windings. Therefore iI suggest to install as follows :
=  Keep wire ends as short as practical.
=  Pre-tin all wire ends using a very hot solder iron.
=  Solder one winding at a time.

=  First solder ONE wire end to a numbered connection which is marked with a star (*).
=  Measure which other end is connected to that soldered wire.
=  Solder that end to the PCB hole without a star but same number.

Go on with the next wire.

RESPECT the + and - signs for all power connections !

Fix all coils to the PCB with glue. 

Damage prevention against strong electromagnetic fields.
Fast switching diodes D1-8 limit to high RF signal voltages on both amplifier inputs. 
To high common mode and static voltages are limited to +-15V by
R25, D9, D10, Z2 and Z3, and are bleeded via one free CAT5 wire to the GND connection of the connected receiver .

            Transmission line and splitter.
In the splitter Z1 and F1 prevent damage from to high or wrong connected power.
Fuse : 5 x 20 mm, 200mA T (slow).

For the connection between splitter and antenna a cheap (weather resist) "straight" CAT5 network cable is used. This makes the system "plug and play", and easily deployable for temporary use.

As the used wire pairs are twisted, they are balanced, and insensitive to surrounding noise fields.
A vast plus over coax use.

One wire pair is used for signal transport.
Another wire pair is used for power.

Both free wire pairs are only connected to the ground surface of the splitter.
One wire is HiZ connected to the amplifier for bleeding static voltages.

Common mode noises are blocked in the amplifier by L6 and choke L8, and in the splitter by L10 and L12.

           The external power supply :
-  MUST be a LINEAR LOW NOISE type, using a 50/60Hz mains transformer.
-  Check that the transformer rectifier diodes in it, are each paralleled by a 0.1 uF capacitor. If not, install these capacitors.
-  May ONLY be connected to the splitter, NOT to other equipment.
-  Its output may NOT be grounded.

        Needed minimum supply voltage.
An LM2940SX-10 low drop stabilizer on the amplifier PCB needs at least 10.5 V to work well. 
A UA7810CKTTR stabilizer on the amplifier PCB needs at least 12.5 V to work well. 
With the supply current of 136 mA, every 10m CAT5 cable causes 0.5V supply loss.
The minimum external supply voltage therefore depends on the length of the CAT5 line. 

        Example :
When 30m CAT5 cable should be used, the minimum external supply voltage for LM2940-10 is : 10.5 + (3 x 0.5 V) = 12.0 V.
When 30m CAT5 cable should be used, the minimum external supply voltage for UA7810       is : 12.5 + (3 x 0.5 V) = 14.5 V. 

Connections and grounding.

If you connect the splitter directly to a receiver via a 50 ohms coax cable : 
Use thin 1/10" 50 ohms coax for connection between receiver and splitter.
Wind as much as possible coax turns on a 1 1/4" mix31 ferrite core.

Connect a safety ground ONLY to that receiver.
All other equipment must be left floating from ground.

If you use a PC, USB connected to an SDR receiver :
Use a thin 1/10" USB cable for this connection.
Wind it with as much as possible turns on a 1 1/4" mix31 ferrite core.
Connect a safety ground ONLY to that PC.
All other equipment must be left floating from ground.
See block diagram below.

 

 

PDF parts list
Update 20190416-17
PDF
amplifier schema
Update : 20190406-14
PDF
splitter
schema

Update : 20190406-23
PDF
overvoltage circuit.