Little active noise sampling antenna.
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This little active "Noise antenna" is developed in conjunction with the "Local-noise filter", to suppress on 80m and 40m bands the rattle noises from a local plasma TV. For use on higher frequency bands both circuits possibly have to modified slightly.
This noise antenna is tested together with my "3m circumference tuned transmitting loop". On 80m the bandwidth of the loop is only 15kHz. Tuning the noise filter therefor is critical. On 40m the loop bandwidth is 200kHz as it there represents a full size 1/4wave loop. Filter tuning is far less critical.
When using a resonating dipole antenna or a wideband active receiving antenna i expect less critical tuning on 80m also.
A small un-buffered whip antenna, directly connected to a coax cable, only supplies very small signal levels. The cable capacity attenuates the signals. If a ferrite antenna is used, the cable capacitance detunes the coil to a very low frequency. The high Q of such a ferrite coil attenuates the signals on the wanted frequencies too.
This leads to a severe drawback: P1 in the filter unit has to attenuate the wanted signals from the main antenna severely, in order to achieve the needed amplitude balance between the noises from the main- and the noise antennas.
This little active magnetic antenna generates a relative strong noise sample, possibly stronger than the noise level on the main antenna. Then not P1 has to attenuate the main antenna signal, instead the sample from the noise antenna can now be attenuated using P2/3.
P1 in the filter unit can remain in the max. position. Switching the filter unit on/off now not influences the S-meter reading from the wanted signals. And the adjustment of P2/3 in de filter unit is easier
This circuit is not critical. It also can be constructed as "dead bugs" on a piece of copper clad. But keep the wiring short.
De (single sided) PCB given below fits in a small plastic box (Hammond 1591MBK).
Fets T1 and T2 are soldered onto the underside. All other components on top. Glue the ferrite antenna. M3 is a 3.2mm screw hole.
I had good results with the noise antenna taped at the center of the back of the TV, with the ferrite antenna in vertical position.
Connect this little antenna to the filter unit using thin 50Ohms coax (RG174). DO NOT FORGET to put a 5mm ferrite clamp at the beginning and the end of this coax with a few turns coax through it.
Advised: also put ferrite clamps near the TV over all connected cables there.
Xant schematic 600DPI
How does it work?
This circuit is powered from the noise filter unit. In the local noise filter unit, L2/C18/TR1 combine the supply voltage to the noise antenna with the noise signals coming from the noise antenna.
L1 is not loaded by the coax cable, and now resonates halfway the used frequency range. It is damped by R1 so the output voltage is fairly constant over the 80m and 40m bands. L1 is 50 turns 0.7mm over a cheap small ferrite antenna rod 8mm x 50mm met Ui=300 (Conrad orderNr 535575).
T1 / 2 are also the modern BF999 mosFETs, chosen for high steepness (20mS) at low drain current (10mA) and 0 gate-source voltage. The output impedance of T2 therefore is (Z=1/S) 50 Ohms, nicely matching the output cable impedance. Phase transformer Tr1 in the filter unit now sees en neat resistive source, resulting in easy adjustments of P2/3.
IMD in the noise antenna circuit will be heard in the connected receiver. R3 introduces strong feedback in T1, reducing chances of IMD. As Rs(T1) : Zd(T1) = ca. 1:10, T1 amplifies 20dB (10x). T1 only draws 3mA, the total current drawn is only 10mA.
FB1 and FB2 are 3x3.5mm ferrite beads. They suppress parasitic oscillations, and keep the antennas self noise level low. "Stop" resistors here made the self noise level higher. R8 not only is a RFfilter, but lowers the drain voltage of T2 to a safe level. C4 HAS to be a foil capacitor in order to prevent IMD (as most ceramic capacitors do make IMD).
Phase adjuster P3 also can compensate for differences in cable lengths from both antennas. If P3 is in an end-position, try :
- turning the noise antenna box by 180 degr.
- make one of the antenna cables a few meters longer.