pa0nhc / lz1aq wideband
receiving loop antenna system.
System noise floor (SDRconsole)
50kHz -135dBm, 1MHz -129dBm, 10MHz -136dBm, 20MHz -139dBm.
Noise floor including 30dB software gain :
50kHz -105dBm, 1MHz -99dBm, 10MHz -106dBm, 20MHz -109dBm,
REM : Airspy HF+ SDR RX unit noise floor including 30dB software gain : -122dBm @ 20MHz.
The large insensitivity for coupling E-field noises into this antenna system is
accomplished by : :
Measures against "Pin1 problems" ground loops and feeder common mode coupling :
1. Amplifier and splitter are installed in shielding closed metal boxes.
2. Three water resist N-busses are installed onto the amplifier box.
3. Three BNC-busses are installed onto the splitter box (Ant, RX and 12Vdc too).
4. All busses are fully metal and installed in good electrical contact with the OUTSIDE of the boxes.
5. Mass surfaces of both PCBs are at one point grounded to the INSIDE of the boxes.
6. Effective Common Mode Chokes (CMCs) are installed on the coax feeder every 3m..
7. The whole antenna system is grounded at ONE point : the receiver ONLY.
See "Construction details" and "De-noising the external powersupply"
Optimal settings for SDR software.
The output of this active antenna system is 30dB lower than the output of a 1/2 wave dipole antenna. By setting 30dB extra software gain in the SDR software,
this antenna system will give the same S-meter readings compared with a dipole antenna.
The relation between loop inductance, amplifier noise floor and loop thickness.
According to LZ1AQ, a loop diameter of abt. 1m is optimal.
The noise floor of such an antenna system is mainly determined by the loop inductance.
A lower loop loop inductance will lower the amplifier noise floor.
The kind of loop conductor metal is not of influence to the inductance of a loop.
A lower inductance can be achieved by using a thicker loop conductor.
Therefore, only the thickness of the central conductor is of importance.
Below 15MHz, the by the loop
received noise is always dominant.
The "man made noise" is in most locations above 15MHz still stronger than the system noise.
Only in very quiet locations is the amplifier noise above 15MHz stronger than the galactic noise received.
Then a low inductance loop can enhance the antenna sensitivity.
A screened low inductance loop could be made using very thick 50mm Cellflex LCF158-50JA coax.
LZ1QA designed the amplifier input circuit to be protected for :
1. Induction of a lightning strike at 100m distance.
2. Up to 1,5 kW radiated power at 10m distance.
The splitter is guarded against :
Wrong polarized power, over-voltage and short circuit.
Screened balanced loop, circumference 2m to 3m.
The screening of the loop is ONLY connected to the OUTSIDE of the metallic antenna box.
Nominal loop capacitance : 2x125pF tot 2x 150pF balanced.
Used maximal loop circumference of coaxes :
- 2,7mm RG316 : 2,8 m.
- 11mm RG213 : 3,15 m.
- 50mm LCF158-50JA : 4 m.
A to low loop capacitance must be compensated by placing C5 and C10 onto the amplifier board,
until a total input capacitance of 2x125 to 2x150pF is accomplished.
- Fully balanced.
- The input circuit is designed for a loop capacitance of 2x125pF to 2x150pF.
- Stable, no oscillations.
- Floating screening metal cabinet, not grounded.
- Input impedance 2x1.4 Ohms.
- Amplification factor 42dB.
- Bandwidth +/- 3 db :125 kHz to 27 MHz (still ample reception down to 15 kHz).
- Including 30MHz LPF input filter to protect the connected receiver against overload from local FM broadcast stations.
- Noise floor (including +30dB software gain) : -120dBm @ 20MHz.
- Output : 50 Ohms. Ground of coax will be connected to the metal box.
- Nominal output power -30 dBm at a field strength of 0,02 V/m.
Thin coax RG316 / RG174 with every 3m a CMC. See "Construction Details".
Experiments learned me, that an electrically floating coax feeder can work better than an UTP feeder. When using coax the prevention of "PIN1 Problems" and common mode current problems is much easier. Thin coax cable can be used (2.5mm coax in stead of 6mm UTP). Adding good Common Mode Chokes is much easier.
Output impedance : 50 Ohm asymmetric.
Filtered BNC power bus : 12.5 to 16Vdc, 150mA.
Protected against over-voltage, wrong polarity and short circuits.
The power connection cable should be screened.
See : "De-noising the external powersupply."
When i switch the antenna power to off, all signals and noises drop -30dB to -70dB in strength.
This is an indication of good decoupling of feeder common mode currents from antenna and receiver.
these measures, you will practically be only receiving with the LOOP, not with the feeder.
Prevent ground loops !
ONE point grounding : Ground the receiver only (or only the antenna housing).
DO construct a SCREENED loop.
DO use screening metal PCB housings.
DO mount all coax busses in good electrical contact with the OUTSIDE of the metal housings.
DO use full metal busses AND plugs of good quality for RF and for power.
REM : cheap bad quality BNC plugs can cause noise reception problems.
DO install Common Mode Chokes on the feeder.
You will be rewarded with the best Signal To Noise Ratio possible in your active loop antenna location.