Introduction This is a practical, safety-first guide to building and operating low-power electronic warfare toolkits for portable use. I assume the goal is lawful spectrum awareness, signal detection, direction finding, and controlled lab testing of receiver-side countermeasures. I do not cover or endorse unlawful jamming or interference. Read and obey your local regulations before you transmit anything over the air.

Design goals and constraints When you design a portable EW kit you balance three constraints: observable RF capability, battery and power budget, and legal safety. Low-power in this context means staying inside the output capability of small SDRs and passive front ends so you can do effective monitoring and close-range experiments without bulky amplifiers. Typical hobby/entry SDRs produce on the order of single digit to low double digit milliwatts of RF, which is plenty for local over-the-air experiments when combined with good antennas and sensible test procedures.

Legal and safety baseline Federal law in the United States forbids the operation, marketing, or sale of devices intended to deliberately block or jam authorized communications. That prohibition includes small portable jammers, GPS blockers, and cellphone jammers. If your EW work requires transmitting, confine it to licensed channels, sanctioned test ranges, or fully shielded test enclosures. If you need to test an active countermeasure that would otherwise cause interference, use a Faraday enclosure or RF absorber lined chamber. Report suspected real-world jamming incidents to authorities rather than attempting to counter them in the open.

Choosing an SDR platform and RF front end For portable low-power EW you want a compact, well supported SDR with a predictable transmit capability and a decent RX chain.

  • HackRF One is a low-cost, wideband half-duplex transceiver used in many portable builds. Its typical maximum transmit power varies with frequency but is roughly in the single digit to low tens of milliwatts across much of the HF to low GHz bands. It is commonly used for close-range experiments or to drive an external amplifier if higher EIRP is required in a regulated environment. Know its RX input limits and protect the front end from high signals.
  • LimeSDR boards provide higher fidelity transceivers with broader bandwidth options. The vendor lists CW power up to about 10 dBm for some boards, making them useful for portable testbeds where USB 3 power and host processing are available. Choose the variant that matches your size, bandwidth, and power constraints.

Use cases where low-power is sufficient

  • Spectrum awareness and occupancy mapping in congested bands. - Protocol sniffing and demodulation for short-range links. - Close-in direction finding and signal localization with portable directional antennas. - Receiver performance testing using signal substitution inside an RF enclosure. All of these can be done without high EIRP if you accept short ranges and careful antenna placement.

Antenna selection and EIRP basics A small transmitter plus a high-gain antenna equals higher EIRP. If you want to maximize detection or localization range without raising transmitter power, optimize antenna gain on the receive side, and use directional elements for DF work. Remember to calculate EIRP and ERP correctly when you do any on-air testing. The FCC provides guidance for determining ERP and EIRP from measured power and antenna gain. An elemental calculation: EIRP (dBm) = Tx output (dBm) + antenna gain (dBi) - feedline loss (dB). Use that to ensure you remain within any regulatory limits for the band you plan to use.

Example: power vs battery budget Use realistic battery math to size a portable kit. A representative 18650 cell has nominal voltage near 3.6 V and typical usable capacity in the 3200 to 3400 mAh range depending on vendor. Using the Panasonic NCR18650B typical rating as an example, a single cell stores roughly 3.35 Ah at 3.6 V, which is about 12 Wh of energy on paper. If your SDR and host draw 2.5 W while transmitting or processing heavily, a single cell could in theory supply that load for around 4 to 5 hours. In practice you must account for DC-DC conversion losses, regulator overhead, and battery discharge inefficiencies, so assume something closer to two thirds of the ideal runtime. If you need longer runtime carry additional cells in a proper pack or use a USB PD power bank that can supply higher sustained current.

Power budgeting checklist

  • Measure idle and transmit current on your SDR and host during representative tasks. Many SDRs quote typical USB bus current draw in their documentation.
  • Decide if you will run the SDR from a laptop, a single-board computer, or a standalone embedded host. Each adds computational load and power overhead. - Account for peripherals such as directional rotators, active antennas, displays, and external filters. - For long duration field ops, use multi-cell packs with BMS and proper connectors rather than loose cells.

Minimizing radiated footprint tactically If your objective is low-probability-of-detection monitoring rather than intentional interference, reduce emissions in these ways:

  • Lower TX duty cycle. Transmit only when necessary. For monitoring you often do not need continuous tones. - Use the lowest practical RF output setting of the SDR and rely on the antenna system to shape the radiated field. - Narrow the transmitted bandwidth and occupy the smallest spectral footprint consistent with your task. - Time-synchronize scans to avoid lengthy on-air dwell. These are fundamental trade offs between detection probability and experimental effectiveness.

Receiver-side countermeasure experiments Many EW lessons are learned on the receive side. Low-power experiments can validate detection algorithms, spectral classifiers, protocol fingerprinting, and short-range DF techniques. When running such experiments on live spectrum prefer passive monitoring or use a shielded enclosure for any active testing that would otherwise radiate. Passive monitoring also avoids the legal risk of harmful interference.

Practical portable build example (safe, lab-first)

  • SDR: HackRF One or LimeSDR Mini for compact TX/RX capability.
  • Host: Raspberry Pi 4 or an M.2 equipped laptop depending on software needs. - Power: 2x 18650 with a regulated boost to 5 V for USB bus power, or a USB PD power bank if sustained higher current is required.
  • Antennas: small omni for general sweep plus a compact Yagi for direction finding. - Filters: bandpass filters ahead of RX and TX to prevent out of band emissions and protect front ends. - Software: GNU Radio, SDRangel, and specialized DF or signal classification scripts. - Test environment: start in a shielded box or anechoic enclosure before any over-the-air demonstration.

Measurement and validation tips

  • Calibrate receive chains using a known signal generator and substitution method to determine EIRP and RX sensitivity. The FCC guidance documents explain substitution and radiated test methods.
  • Log RF power draw and compute average power using measured duty cycle. That gives realistic battery estimates. - Use external attenuators and limiters during bench work to protect SDR front ends from accidental high-level signals. Community reports show front-end components can be damaged by ESD or high input power events, so add protection when connecting unknown sources.

Wrap up and best practices Low-power portable EW is about doing more with less: good antennas, careful measurement, conservative power budgeting, and a strict legal and safety posture. Focus on passive spectrum awareness, receiver-side experiments, and lab-contained active tests. If you must test an active technique on the air, arrange for a licensed test range, explicit authorization, or a fully shielded chamber. Stay within documented device limits, keep logs, and prioritize safety for yourself and the public.