Improved biastee for active loop / whip

Created: Sep. 2025.

Introduction

Biastees are required to energize active whips (e.g. PA0RDT) and active loops (e.g. LZ1AQ / PA0FRI / MLA-30). Previously, we posted a simple & cheap biastee based on an axial (wire-ended) inductor [1-2]. Although the axial inductor, e.g. Taiyo Yuden LAL04 or Fastron SMCC, is cheap and eliminates the need for coil-winding, the component parasitics limits the highest frequency to ~80 MHz. To extend the bandwidth, we investigate replacing the axial inductor with a cheap ferrite ring inductor.

Material & method

The ferrite ring is available from China-based sellers with the description "8-4-3 mm" [3]. Those familiar with toroids will immediately recognize that the description refers to the OD-ID-Height. Unfortunately, neither permeability nor inductance factor is provided by the seller. Despite the absence of these critical information, the cheap price (~USD0.01 each) makes it very tempting. Through experiment, we discover that a 20 turn winding (32 swg) on the ring yielded a value of 38 uH. Each biastee requires two of these rings in positions L4 & L5 (fig. 1).

Fig. 1: circuit

Fig. 2: part-list

Fig. 3: PCB & photo of assembled prototype

Fig. 4: Biastees after packaging inside boxes

Result

The prototype performs well over 0.2 - 150 MHz. Over this frequency range, the insertion loss is better than 1 dB and the Return Loss is better than -15 dB (fig.5). Isolation exceeds 30 dB in this range.

Fig. 5: Over 0.2 - 150 MHz, the insertion loss is better than 1 dB and the Return Loss is better than -15 dB

Conclusion

Replacing the axial inductor in a biastee with a cheap ferrite ring approximately doubles the bandwidth.

References

[1] "A cheap biastee for active loop / whip". https://sites.google.com/site/randomwok/Home/electronic-projects/test-equipment-accessories/a-cheap-biastee-for-active-loop-whip

[2] Penang Radio Lab facebook post, 19 May 2024. https://www.facebook.com/share/p/1NowD7qwSh/

[3] https://www.lazada.com.my/products/550100pcs-ferrite-bead-toroide-cores-coil-inductor-ring-cables-filter-kit-i3121238628-s15662090927.html?

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