A cheap biastee for active loop / whip

Created: May 2024

Introduction

Active loops (e.g. LZ1AQ / PA0FRI / MLA-30) and active whips (e.g. PA0RDT) require biastees to power their pre-amplifiers. The aforementioned active aerials are intended for HF and possibly 50 MHz. However, commercial biastees have bandwidths in excess of the aforementioned frequencies.

There are three reasons for building your own instead of buying:

1. Biastees sold online may have performances that are "hit & miss" affairs [1].

2. They come with SMA connectors, but I prefer BNC.

3. Building my own allows incorporation of safety features like protection against reverse supply polarity and short-circuit in the coax cable.

This design is intended to suit the active loop / whip's limited frequency range of HF-50 MHz. So, if you need a higher maximum frequency, please look at this alternative design [2].

Material & method

The biastee circuit is conventional except for the addition of D2 and F1 for protecting against reverse polarity and coax's short-circuit (fig 1). For user's convenience, an LED, D1 is included to indicate the power status.

Fig. 1: Biastee complete circuit, part-list and PCB layout

The FR4 PCB measures 50 x 25 x 1.5 mm (fig 2-3). It is populated with leaded components, except for C8 which is a retrofitted chip cap because I have forgotten to design it in. The L4 position is bypassed with a jumper. During the PCB layout, I planned for the extraneous L4 just in case if there is a future need to improve isolation between RF and DC.

Fig. 2: Photos of assembled biastees with two different DC connectors. The top one is equipped with Molex KK.100 headers, while the bottom one uses Faston 0.25" blades

Photos of the unpopulated PCBs

This design's maximum frequency is primarily limited by the two right-angle BNC bulkhead female connectors J4-5 (fig. 4). These plastic molded connectors are intended for low frequency Audio-Visual equipment and the two pins at the rear have characteristic impedance Zo deviating significantly from 50 ohm. These 0.7mm diameter pins are spaced ~2.5 mm apart and an analysis in Appcad [3] reveals that the pins have a Zo ~218 ohm. Due to these pins significant length (~12 mm), they must be accounted for in circuit modelling.

Fig. 4: The right-angle BNC bulkhead female connector is intended for low frequency Audio-Visual equipment and the two pins exiting the rear have characteristic impedance Zo deviating significantly from 50 ohm. This discontinuity must be accounted for in the connector model.

Conventionally, the biastee's inductor is implemented with a ferrite ring because of its wide bandwidth and self-shielding. To reduce cost, we opted for a 10x4mm axial lead inductor, e.g. Taiyo Yuden LAL04 / Fastron SMCC / China clone, because it costs a fraction (~1/100) of the ferrite ring inductor. Aside from lower cost, the axial inductor is more convenient than the ferrite ring which requires wire winding. On the flip-side, the axial inductor trades off bandwidth and shielding, but these compromises are deemed acceptable because the target application is below 50 MHz. Another feasible substitute is a ferrite rod inductor, e.g. Fastron SMSC, which costs halfway between ferrite ring and axial inductor.

To understand how component values influence performances, we modeled the biastee in a circuit simulator (fig. 5). The BNC's pins are represented by the ideal transmission lines, TL1-2. The PCB traces connecting the two BNC connectors are TL3/4/6. Inductor L5 is modeled in two parts: the upper section representing the main parameters, while the bottom section, the 2nd order parasitics.

Fig. 5: An equivalent circuit model of the biastee

The L5 model parameters are obtained by curvefitting to measured data (fig. 6). The measurement data was obtained by measuring the S21 of the inductor over 1-250 MHz in a gapped microstrip test fixture.

Fig. 6: The L5 model was created by curvefitting (red dotted trace) to the measured data (black solid trace). The inductor measurement uses a gapped microstrip test fixture (bottom photo).

Simulation shows that L5 inductance value determines the biastee's low frequency response (fig. 7). Larger inductances will extend the minimum usable frequency. A value of 12 uH is chosen as a compromise between minimum and maximum usable frequencies.

Fig. 7: Simulation shows that L5 inductance determines the biastee's low frequency response.

Results

The prototype achieves better than -1 dB loss over 1-90 MHz (fig.8). The simulation agrees with the measurement within 0.3 dB in the target 1-50 MHz range. The simulated notch is erroneously ~30 MHz higher than the actual. The notch's frequency discrepancy is most probably caused by errors in the model parameters.

Fig. 8: The prototype achieves better than -1 dB loss over 1-90 MHz (solid black). The simulation (blue dots) agrees with the measurement within 0.3 dB in the target 1-50 MHz range.

The prototype's Return Loss is better than -10 dB over 1-50 MHz (fig. 9). The simulation error is ≤ 3 dB over the same range.

Fig. 9: Over 1-50 MHz, the experimental Return Loss (black trace) is better than -10 dB. The simulation (blue dots) agrees well with measurement and the error is ≤ 3 dB.

Conclusion

A low cost biastee for active loop / whip has been designed around a 10 x 4 mm axial lead inductor. The prototype meets the target 1-50 MHz operating range, and is even capable of reaching 90 MHz.

Reference

[1] Penang Radio Lab facebook, 11 May 2024, www.facebook.com/permalink.php?story_fbid=pfbid032Y32wpxGpiuhY285k7CSxp4TubpDB5zZJKyEAr3ifuvBwRG1mwTPXxrurAcSzoFgl&id=100077880194698

[2] "Simple 10-2000 MHz biastee", https://sites.google.com/site/randomwok/Home/electronic-projects/test-equipment-accessories/10mhz-1-5ghz-biastee

[3] http://www.hp.woodshot.com/

Page updated

Google Sites

Report abuse