Sunday, October 27, 2013

Active Ferrite Rod Antenna by OH7SV / OH2NLT

I built an active ferrite rod antenna designed by OH7SV, modified by OH2NLT. The circuit provides ample of signal from 80m to 20m. The efficiency of the ferrite rod used is questionable above 30m. The antenna is a perfect accessory for spending lonely evenings on a hotel with my ATS-3b transceiver. The antenna has sharp nulls on vertically polarized magnetic fields and is quite insensitive to near electric fields. Contrary to the common belief , its directionality is poor for signals reflected by the ionosphere.

Original source:
Active Ferrite Rod Antenna for HF, 08.10.2005 OH7SV
Active Ferrite Rod Antenna OH2NLT

The ferrite rod made by Pramet Sumperk during the Iron Curtain era may be sourced cheaply from
http://ferity.cz/e-shop/en/ferity/548-n2101x144-0000000038430.html
The rod is not as good as the Amidon 61 material counterpart, namely the Pramet N2 material has 2x higher permeability than the Amidon 61 material and it has lower Q and higher losses on HF than the Amidon material. But you cannot beat the price, the rod is about $1.50. If there is enough interest, I may organize a group buy. I may also source the boxes and pieces of the PVC tube.

Tuning control, power off switch, power indicator LED and BNC output on the front side. The ferrite rod is fixed in the PVC tube by injecting hot melt glue at both sides.
Old Russian made polyvaricon placed above the ferrite rod, because the box standoffs would stay in way if the rod was positioned at the top. The box has a convenient 9V battery compartment.

Tiny amplifier board made on a double sided blank board by cutting traces with a needle file. Using two BFR93 low noise transistors as OH2NLT. The output is coupled to the BNC connector by a voltage balun wound on a tiny Amidon 43 material binocular core.
The antenna works well as an accessory to a QRP radio for direction radio finding. It would be cool to add the whip antenna input and 8/cardioid switch. For a single band, I could imagine wiring the amplifier on a additional plug-in board for the ATS-3b.