LZ1AQ / pa0nhc wide band loop
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Update : 20190828
Update : 20190716
|Overvoltage protection detail||Amplifier housing||Construction
Frequency characteristic of the whole system.
Measured with NanoVNA.
Loop replaced by a 1.1 uH choke.
C5/10 = 150 pF.
NanoVNA port "ch0" connected with 2x 470 Ohms in balance to "Loop".
NanoVNA port "ch1" connected to splitter RFout.
Blue = Fchar.
In practice is 16 kHz still well readable.
Reception using Airspy HF+ and SDRconsole in a VERY noisy city location.
30dB "Visual Gain", resulting in signal indications equivalent to a full sized dipole antenna in equal field strength.
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.
General properties :
- Pure H-field (magnetic) antenna.
- Antenna and transmission line are fully balanced.
- Power supply and signal transport by means of one standard "straight" CAT5 network cable.
- Insensitive to E-fields.
- No loop screening needed
=> Simple construction.
=> Simple installation.
- Loop and splitter are not grounded
- Only the connected receiver should be connected to a safety ground.
- Very effective internal common mode chokes and balanced transformers.
=> Less transmission line de-noising measures needed.
- Useable at very low height above ground with little signal loss.
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.
External noise free power supply : 13.5 Vdc to
17Vdc / 135 mA.
The LZ1AQ / pa0nhc amplifier unit.
- Fully balanced.
- Two stages for ample amplification (48 dB) and large bandwidth.
- This amplifier is stable, and can cope with very strong signals.
- No signs of oscillations were noted.
- No IMD occurred even when receiving VERY strong signals on
LW, MW and SW at the same time.
- Input impedance abt. 3 Ohms balanced.
- The loop runs therefore in "short circuit mode".
- Output impedance 100 Ohm balanced to match a CAT5 twisted wire pair.
- Amplifier bandwidth (measured with nanoVNA) :
50 kHz -20 dB. REM : reception of 22.5 kHz still very good.
500 kHz to 23 MHz +0/-3 dB => WIDEband.
30 MHz -5 dB. Amplifier noise floor is noticeable.
70 MHz -45 dB. Good suppression of FM local broadcast.
- Loop bandwidth (circumference 2 m) 15 MHz.
Splitter RF output impedance is 50 Ohms asymmetrical.
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 blooded to receiver ground via one free CAT5 wire.
I deliberately chose for easy installable wired components and wide PCB component holes as :
- IMD free film and NP0 capacitors are mostly available as wired.
They are needed for low IMD, and temperature stability (outside temperatures can vary between -20C and
- Common mode chokes wound on optimal #31 ferrite cores for better cable noise rejection above 1 MHz.
- Added Tr2,, a 100 ohms alance to 50 Ohms unbalance transmission line transformer with separated windings.
- Transformers Tr1 and TR2 are both wound on #77 ferrite cores for wide
bandwidth down to 25 kHz.
- Fine tuned the values of C5 and C10 for optimal bandwidth, together with
good FM broadcast suppression.
REM: 5 pF loop capacitance and 7 pF PCB+wiring capacitance are
- Designed two double sided PCB's with :
RJ45 busses and
a large number if ground plane via's.
- Made a component ordering list with ordering details.
Output, bandwidth and noise floor :
In a field strength : 0,02 V/m, a correctly loaded 40m long 3.65 MHz dipole at 20m height, will give 0dBm output.
A 60cm diameter loop antenna output is then 0,014 mA RF current.
Calculated with LTspice :
This 60cm dia. 50 Ohm loop output will then be -43 dBv or -30 dBm (S9+43 dB). The output difference between this loop and an 40m long dipole is therefore -30 dB. To have practical. signal strength indications, i suggest to set your software's gain to + 30 dB, in order to compensate for the differences between dipole en small loop outputs.
Measured with the loop replaced by a 2.2 uH choke, B=10 kHz, 50 Ohms output.
MHz : 1.8 3.65 7.1 14.175
-dBm : 112 115 115 117
Loop construction :
I used easy available 2m long 15x2mm aluminum strip for the construction of the loop. This loop is connected to a special, by LZ1AQ designed two stage loop amplifier. It is fully balanced, and has a very low input impedance of abt. 2x2 Ohm. The loop therefore behaves like a current source with the
loop self inductance of 1.6 uH in series with it. If such a lowZ loaded loop is located in a magnetic H-field of constant strength, it generates a constant RF current for all wavelengths larger than 10 loop circumferences.
This loop is in its near field only sensitive to
E-field noises sourced from within its near-field, will be capacitive coupled. They are high impedance of nature. And will cause weak noise currents into the loop. These weak currents will cause across both lowZ amplifier inputs very weak voltages (U=IxR). Due to the small loop dimensions, these noise voltages will have at both loop connections (nearly) equal amplitude and phase. As the amplifier has a balanced input and output, these noises will further be attenuated, resulting in a nearly pure H-field antenna.
The influence of the loop self
inductance to the amplifier noise floor.
Up to 14 MHz, this 60cm loop self noise is weaker than the atmospheric noise.
In city environments, having a higher noise level than in the country site, the antenna self noise is not important.
According to LZ1AQ, a loop
diameter of abt. 1m is optimal.
- Low noise input transistors will NOT lower the noise floor.
- A better conducting loop (copper and/or silver plated) will NOT lower the noise floor, as it does not lower the loop inductance.
- A larger loop circumference makes the S/N WORSE, as it results in a higher loop inductance.
- A loop with more than one winding connected in series will WORSEN THE S/N, as it has a far higher self inductance.
- A thicker loop (f.i..36mm) WILL IMPROVE the S/N, as a thicker loop has a lower loop inductance.
- A loop with more than one winding connected in parallel WILL IMPROVE the S/N, as it has a lower loop self inductance.
Transmission line and splitter.
For the connection between splitter and antenna a cheap (weather resist) "straight / standard" 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 remaining wire pairs are only connected to the
ground surface of the splitter.
One of them is for bleeding static voltages connected to the amplifier "ground" through a 100 k series resistor.
mode noises at the CAT5 network cable in the amplifier blocked by the secondary of Tr1, and
Common Mode Choke L7.
In the splitter Common Mode Currents they are blocked by CMC's L10, L12.
The specially developed transmission line output transformer Tr2 ensures very good separation between the RF transmission line and the 50 Ohms output. The RF transmission line is effectively floating and balanced, as both transmission line transformers Tr1 and Tr2 have no galvanic connection between primary and secondary windings. Ensuring very good wideband decoupling of common mode signals on the CAT5 cable. Two extra #43 + #31 CMCs onto the CAT5 cable showed to make no difference on noise levels in my very noisy antenna location.
With power to the antenna switched off, resulted in signal or noise levels to drop by 53 dB (MW) to 32 dB (SW).
Z1 and F1 in the splitter prevent damage from to high or wrong
Fuse : 5 x 20 mm, 200mA T (slow).
The needed external power supply.
- MUST be a LINEAR LOW NOISE type, using a 50/60Hz mains transformer.
- Check if the rectifier diodes in it are each paralleled by a
0.1 uF capacitor.
If not, install these capacitors.
- Its DC output must be floating, and may NOT be grounded.
- This power supply may ONLY be connected to the splitter, NOT to other equipment.
TIP : On the Internet some small cheap
noise HiFi audio power supply" are available.
For instance : "Breeze Audio", 12V / 15 W : $56.
Their output voltage is internally adjustable
Minimum needed supply voltage.
The LM2940SX-10 low drop stabilizer on the amplifier PCB needs at least 11 Vdc to work well.
With the supply current of 135 mA, every 10m CAT5 cable causes abt. 0.5V voltage supply loss.
The minimum to the splitter connected external supply voltage therefore depends on the length of the CAT5 line.
With the power supply adjusted for 12.5 Vdc, the max. length of the network cable is 30 m.
Even with a short network cable connected, it is safe to supply up to 16 Vdc.
Grounding, connecting and de-noising.
The splitter nor the power supply may be grounded.
The connected receiver or PC MUST be grounded.
If you experience a high noise level, this noise
can be injected into the system in various ways :
a. Through the mains power supply grounding wire.
This applies both to the power supply for the loop antenna as well as the power supply for the receiver.
When you use a power cord with a three-pole equipment plug, you probably have a "Pin1 problem".
One way to cure this "NOISE IMPORTING" problem is to modify the power cord as follows (see photo) :
- At a distance of abt.10cm
from the equipment power plug, cut the power cord into two
- Shift a piece of crimp hose over the longest piece of power cord.
- Remove over a distance of 3cm the outer insulation on both pieces of power cord.
- At the longest piece of the power cord, shift a piece of crimp hose over each of the three wire ends.
- Remove 3mm insulation of all six wire ends.
- Solder both blue wire ends (neutral) together, and insulate the solder connection using crimp hose..
- Solder both brown wire ends (phase) together, and insulate the solder connection using crimp hose..
- At the shortest piece of the power cord, cut off the yellow/green wire end (disconnecting the safety ground from the
inside of the apparatus) .
- Solder a 15cm long yellow/green wire to the yellow/green wire end (safety ground) at the longest piece of power cord,
and insulate the solder connection using crimp hose..
- Shift the piece of crimp hose already present at the longest piece of the power cord over the combined connections,
and crimp it. Let cool for a few minutes.
- Connect the 15cm long yellow/green ground wire to the OUTSIDE of the power supply cabinet, using a cable eye
(reconnecting the safety ground to the outside of the apparatus).
Through remaining cabling, try the following :
1. Change the routing of the network cable.
Installing a common mode choke :
2. On the CAT5 network cable, as close as possible near the splitter.
3. If apply able, on the USB cable, as close as possible near the SDR unit.
4. On the 12Vdc power cable, near the splitter.
5. On the 240V~ power cord, near the power supply.
6. On the 50 Ohm coax, close to the (SDR) receiver antenna input.
Reception through the loop antenna itself, try a combination of the following :
- Rotate the loop.
- Change the location of the loop.
- Change the vertical position (height above surroundings) of the loop.
For vertically polarized low incident DX or near field signals,
try to minimize the most annoying noise by turning the loop antenna.
For NVIS signals (1.8 MHz - 7.2 MHz from 30 km to 300 km) a vertical loop shows no directivity.
Signals coming from above (NVIS), or from below the antenna (mains and network cabling), cannot be attenuated by turning the loop.
The CAT5 network cable is very good decoupled from the antenna and the splitter. But if you cannot avoid that it is running close to noise sources (like mains cables), you could try ferrite material over it.
to install CommonModeChokes :
Wind as much turns as possible THIN (1/10 ") cable through the hole of one FairRite mix #31 core.
Use a - Ring core 2631801202
- "Snap-It" core 0431173551
IMPORTANT for good performance :
The following is also valid for winding the transformers and the common mode chokes on both PCBs.
- Always wind in ONE direction around the core circumference. Never wind back.
- Every turn MUST lay beside another. Never crossing another.
- When the core circumference is fully filled, it is allowed to wind further in the same direction
(see the coil on right).
One incorrect wound turn will cancel one other turn, resulting in TWO inactive turns.
Using a split core :
When both halves of a split ferrite core do not close in perfect mechanical contact with each other, the CMC does not work at all,
- To be sure both core halves close perfectly, the cable turns
should fit loosely into the hole of the split-core. Check it.
- Apply extra pressure to the core halves by installing a black ty-wrap around the core housing. Pull it tight.