Build a capable, reasonable quality, radio frequency (RF) laboratory on a budget.

Goals and Limitations

Impedance

Components should have 50 Ω impedance.

Power Range

Components should be rated for at least 25 W whenever possible. Most of what I want to do is 5 W and below; but I want to have the option of going up to 25 W.

Assuming components with 50 Ω impedance, the following table is a useful reference.

Frequency Range

Components should be rated for DC – 3.0 GHz whenever possible. I want to be able to handle at least the following: common Amateur radio bands (29, 51, 144, 222, 420, 902, and 1240 MHz); CBRS (27 MHz); MURS (151 and 154 MHz); FRS/GMRS (462 and 467 MHz); ADS-B (1090 MHz); Zigbee (900 MHz or 2.4 GHz); 802.11 b/g/n (2.4 GHz); and Bluetooth (2.4 GHz).

Connectors

Standardize on Type N connectors and SMA connectors. All non-N, non-SMA connectors should immediately connect to an adapter.

Recommended Equipment

Software Defined Radios (SDR)

• NooElec NESDR SMArt (Amazon): $24
  • Power range (RF): 0 – 10 mW RX.
  • Frequency range: 24 – 1766 MHz.
  • Sample rate: 3.2 MSPS.
  • 0.5 PPM temperature compensated crystal oscillator (TCXO).
  • Connector: SMA-female.
  • Interface: USB 2.0.
• LimeSDR Mini (Crowd Supply): $160
  • Power range (RF): 0 – 10 mW RX, 0 – 10 mW TX.
  • Frequency range: 10 – 3500 MHz.
  • Sample rate: 30.72 MSPS.
  • Connectors: SMA-female.
  • Interface: USB 3.0.

The R820T chip used in many low-cost SDRs has a maximum RF input power of around 10 mW (10 dBm), and the input power should generally be kept below 100 μW (-10 dBm) in order to avoid saturating the receiver.

According to the LMS7002M ADC electrical specification, the typical input amplitude is 0.8 V_pp (0.4 V_p ), which is a power level of about 1.6 mW (2 dBm). A maximum input amplitude is not listed in the table, but I’ll be keeping my input power levels under 1 mW (0 dBm) to be safe.

If you are working with stronger signals, you must attach appropriate external attenuators. For example, to produce a 100 μW (-10 dBm) signal, a 1 W (30 dBm) signal requires 40 dB of attenuation and a 5 W (37 dBm) signal requires 47 dB of attenuation.

Dummy Loads

Radio transmitters must be connected to an electrical load when transmitting. Without an electrical load, the transmitted power is reflected back into the transmitter, which can damage the transmitter. An antenna is an electrical load that radiates power as radio waves. A dummy load is an electrical load that dissipates power as heat. In any situations where you need to test a transmitter, but don’t want to radiate radio waves, a dummy load should be used instead of an antenna.

• BECEN 25 W dummy load (Amazon): $13
  • Power range: 0 – 25 W.
  • Impedance: 50 Ω.
  • Frequency range: DC – 3.0 GHz.
  • SWR: less than 1.2:1.
  • Connector: N-male.
• MFJ-262BN dummy load (MFJ): $75
  • Power range: 0 – 35 W continuous, up to 200 W for 5 seconds.
  • Impedance: 50 Ω.
  • Frequency range: DC – 3.0 GHz.
  • SWR: less than 1.2:1.
  • Connector: N-male.

Attenuators

An attenuator reduces the power of signals passing through the attenuator. While dummy loads dissipate all of the incoming power as heat, attenuators dissipate part of the incoming power as heat and pass the remaining power through to the output connector. An appropriate set of attenuators is required to attenuate various 0.5 – 25 W transmitters down to the approximately 100 μW needed by SDRs and various receivers.

Mini-Circuits has an affordable kit of 2 W attenuators with SMA connectors. If you only need 2 W attenuators, or if you want additional low-power attenuators for fine-tuning the attenuation into an SDR, the following attenuator kit is a good option.

• Mini-Circuits K1-VAT2+ Kit of 5 Attenuators (Mini-Circuits): $62
  • Power range: 0 – 2 W.
  • Impedance: 50 Ω.
  • Frequency range: DC – 6.0 GHz.
  • Attenuation:
    • VAT-3W2+: 3 dB.
    • VAT-6W2+: 6 dB.
    • VAT-10W2+: 10 dB.
    • VAT-20W2+: 20 dB.
    • VAT-30W2+: 30 dB.
  • SWR: less than 1.5:1.
  • Connectors: SMA-male to SMA-female.

However, since I need attenuators that are rated for at least 5 W, and preferably 25 W, the following attenuators are a better fit for many of my needs.

• BECEN 10 W, 40 dB Attenuator (Amazon): $22
  • Power range: 0 – 10 W.
  • Impedance: 50 Ω.
  • Frequency range: DC – 3.0 GHz.
  • Attenuation: 40 dB.
  • SWR: less than 1.2:1.
  • Connectors: N-male to N-female.
• Hestish 30 W, 40 dB Attenuator (Amazon): $47
  • Power range: 0 – 30 W.
  • Impedance: 50 Ω.
  • Frequency range: DC – 3.0 GHz.
  • Attenuation: 40 dB.
  • SWR: less than 1.2:1.
  • Connectors: N-male to N-female.

Attenuators can be connected in series and their attenuation values sum.

Splitters

• N-female to 2 N-female Splitter (2-pack) (Amazon): $10

Directional Couplers

• Mini-Circuits ZFDC-15-5-N Directional Coupler (Mini-Circuits): $77
  • Power range: 0 – 0.5 W (1 – 10 MHz), 0 – 2 W (10 – 2000 MHz).
  • Impedance: 50 Ω.
  • Frequency range: 1 – 2000 MHz.
  • Directivity: 25 dB.
  • Connectors: N-female.

Noise Generators

Spectrum Analyzers

Spectrum analyzers are extremely expensive. Even entry-level 1 GHz spectrum analyzers start at around $1000. Fortunately, affordable SDRs can be used as simple spectrum analyzers. While SDRs aren’t calibrated and don’t have the full capabilities of an actual spectrum analyzer, SDRs provide enough capabilities for hobbyist purposes.

Testing an RTL-SDR Spectrum Analyzer is a good article by Kerry Wong about using an RTL-SDR as a spectrum analyzer.

Coaxial Cables and Adapters

Buying premade cables is expensive, and the selection can be limited. Buying bulk coaxial cable, crimp connectors, and a crimping tool is more cost-effective and flexible, especially when you need specific cable lengths or combinations of connectors.

• CableWholesale RG-58A/U Coaxial Cable (1000 foot spool) (CableWholesale): $115
  • Impedance: 50 Ω.
  • Capacitance: 93.5 pF/m (28.5 pF/foot).
  • Velocity of propagation: 80%.
  • Conductor: 20 AWG, stranded, bare copper.
  • Insulation: PE.
  • Braid: 95% covering, bare copper.
  • Jacket: PVC, black.
• Amphenol 172100 Type N, Male, Crimp Connector (Digi-Key): $5.50
• Cinch 25-7307 PL-259, Male, Crimp Connector (Digi-Key): $3.00

Common exceptions to making your own coaxial cables are one-off adapters for converting from random connectors to either N or SMA. Especially when working with SDRs, I find the following adapters useful, and more cost-effective than making all of them myself.

• NooElec SMA Cable Connectivity Kit (Amazon): $25
  • SMA-male to N-female.
  • SMA-male to F-female.
  • SMA-male to SO-239 (female).
  • SMA-male to BNC-female.
  • SMA-male to MCX-female.
  • SMA-male to PAL-female.
  • SMA-male to RP-SMA-female.
  • SMA-male to SMA-male.

Optional Equipment

Filters

Filters pass certain frequencies (passband) and block certain frequencies (stopband). The following filters are useful in various SDR applications and can be used as test devices for filter analysis.

• RTL-SDR Broadcast FM Reject Filter (RTL-SDR, Amazon): $15
  • For receive use only.
  • Band-stop filter.
  • Stopband frequency range: 88 – 108 MHz.
  • Stopband attenuation: greater than 50 dB.
  • Connectors: SMA-female.
• RTL-SDR Broadcast AM Reject Filter (RTL-SDR, Amazon): $15
  • For receive use only.
  • High-pass filter.
  • Stopband frequency range: 0 – 2.6 MHz.
  • Stopband attenuation: greater than 50 dB.
  • Connectors: SMA-female.

Diplexers

• MFJ-916BN 1.8-225 MHZ, 350-540 MHZ Duplexer (MFJ): $35
  • Power range: 0 – 200 W.
  • Impedance: 50 Ω.
  • Frequency ranges: 1.8 – 225 MHz, 350 – 540 MHz.
  • Isolation: greater than 55 dB.
  • SWR: less than 1.2:1.
  • Connectors: N-female.

SWR and Watt Meters

The SDR, noise generator, and directional coupler listed above can be combined for Measuring Filter Characteristics and Antenna VSWR with an RTL-SDR.

However, it is also useful to have an in-line SWR/Watt meter for real-time feedback when transmitting.

• MFJ-884 Cross-Needle SWR/Watt Meter (MFJ): $120
  • Power range: 0 – 200 W in three ranges (2 W / 20 W / 200 W).
  • Impedance: 50 Ω.
  • Frequency range: 1.8 – 525 MHz in two overlapping ranges:
    • 1.8 – 200 MHz transceiver and antenna connectors for HF.
    • 125 – 525 MHz transceiver and antenna connectors for UHF/VHF.
  • Connectors: SO-239.