LZ1AQ / pa0nhc wide band loop
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Update 20190912 (Tr1,2-C1.4,20)
Update : 20190716-p92
Update : 20190916
|Overvoltage protection circuit detail||Antenna
PCB V90 housing sketch
75mm PVC pipe
Reception in a VERY noisy city location.
Noise levels on my larger 10m circumference TXloop are up to S9+8 dB.
SDRconsole "visual gain" is here adjusted for 30dB extra gain, to compensate for output difference with a 40m long dipole antenna.
During evening times VERY strong signals are recieved (Romania S9+56 dB)
20190605 VLF output, using new #77 transformers.
Frequency characteristic roll-of only begins below 25 kHz.
High man-made noise level : S9+10 and more.
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.
When all other antennas are useless due to a high man-made noise level, this loop proves to be the only useable antenna.
General 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.
You need an external noise free power supply : 135 mA @ 13.5 Vdc to 17Vdc.
The LZ1AQ / pa0nhc amplifier unit.
- Fully balanced.
- Two stages for ample amplification (48 dB) and large bandwidth : 25 kHz - 36 MHz +1 / -3 dB,
- Input impedance abt. 3 Ohms balanced.
- Output impedance 100 Ohm balanced to match a CAT5 twisted wire pair.
- Constant loop sensitivity up to (lambda = 10 x loop circumference, 25 kHz - 15 MHz).
This amplifier is stable, no signs of oscillations were noted. Due to the heavy loading by the amplifier, the loop runs in "short circuit mode".
In the splitter, a second output transformer matches from symmetrical 100 Ohms to asymmetrical 50 Ohms output.
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
- Wound the common mode chokes on #31 ferrite cores for better cable noise rejection above 1 MHz.
- Added Tr2 100 : 50 Ohms output transformer, a balance to unbalance transformer, vastly improving common mode cable noise rejection between 10 kHz and 800 kHz.
- Transformers Tr1 and TR2 are both wound on #77 ferrite cores to widen VLF 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 calculated in.
- 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.
Amplifier bandwidth with connected loop : 24 kHz - 36 MHz +1 / -3dB. -30dB > 65 MHz.
Measured with the antenna input loaded with 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 loops 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 (magnetic) H-fields.
E-field noises sourced from within its near-field, will be capacitively 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.
Common mode noises at the CAT5 network cable in the amplifier blocked by the secondary of Tr1, and Common Mode Choke L7. In the splitter they are blocked by CMC's L10, L12, and 100 Ohms balance -to- 50 Ohms unbalance transformer Tr2.
Z1 and F1 in the splitter prevent damage from to high or wrong
Fuse : 5 x 20 mm, 200mA T (slow).
Including a mains filter,
a 12V output filter,
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.
- A version including netfilter is preferred.
- 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.
A $75 25W version includes a mains filters.
Their output voltage is internally adjustable
Minimum needed supply voltage.
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.
A UA7810CKTTR stabilizer on the amplifier PCB needs at least 12.5 V to work well.
An LM2940SX-10 low drop stabilizer on the amplifier PCB needs at only 11 Vdc or more to work well.
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.
- LM2940-10 is : 11 + (3 x 0.5 V) = 12.5 V.
Connections and grounding.
If you connect the
splitter to a receiver via a 50 ohms coax cable :
This loop amplifier and its splitter may not