Mobile-Whip: open-source active HF receive antenna for handhelds

A pocket-sized active receive antenna built for portable HF listening has been released as an open-source design. The Mobile-Whip, published on GitHub by a developer who goes by skuep, is based on the well-known PA0RDT Mini-Whip. It is only 200 mm long, small enough to mount directly on a handheld receiver or a dongle SDR. The project came to wider attention through Ernie, NH7L, who passed it along to RTL-SDR.com.

An active antenna like this pairs a very short pickup element with a built-in amplifier, so it can deliver usable HF signals from far less metal than a resonant wire. It is a receive-only device. The original Mini-Whip has been popular with SWLs and hams for years as a compact way to listen on the low bands and below.

This version can run from a single 18650 lithium cell with an onboard USB charger. It can also be powered from 5 V USB, the 4.5 V bias tee on the RTL-SDR Blog V4, or a standard 12-15 V Mini-Whip bias tee. Instead of the usual JFET, it uses an LTC6268 op-amp with ultra-low input capacitance, and it draws about 20 mA. The design targets 100 kHz to 30 MHz, though the author says it also performs reasonably well on FM broadcast and 2 meters. Suggested hosts include an HF-modded UV-K5, portable receivers and SDR dongles.

A Polish ham compared the Mobile-Whip with a 27 MHz telescopic whip in two YouTube videos, using a Radtel RT-950 Pro, a UV-K5, an ATS Mini and a Malahit DSP2. The small PCB antenna generally did better. On the RT-950 it picked up Polish Radio on 225 kHz longwave, which the telescopic whip could not receive. He also cautioned that its high gain can overload receivers with weak front-end filtering.

The author describes the current board as a first-revision preview with some known issues. A revision B with better VHF/UHF performance is reportedly in progress. There is no protection against transmitted RF yet, so keying a transceiver with the antenna attached could destroy it. Anyone using it on a handheld that can transmit should be careful never to press PTT.

The KiCad files, simulations and JLCPCB-ready assembly files are on GitHub under the CERN-OHL-P-2.0 license. Assembled boards come to roughly 25 euros each.