The length of the antenna and the frequency of receiving the signal
What size should the antenna be when receiving with SDR? To calculate the wavelength and the length of the antenna, all you need to do is perform one simple math or use a smartphone app that includes an antenna length calculator .
Antenna length formula
Matching the antenna to your goals is key, so it's important to know how to calculate the wavelength and length of the antenna . Fortunately, there is a simple formula for this.
To calculate the effective length of the antenna , the formula λ= v/ f must be used . The Greek lowercase letter lambda ( λ) is used to denote wavelength . The wavelength is calculated by dividing the phase velocity of the wave (v) - the speed of light 300,000. km/s (299,792,458 meters per second) by the frequency expressed in MHz (f). The result is obtained in meters. For example, for a frequency of 145 MHz, the length of the antenna will be 2.06 m.
Antenna length calculator
We built the calculator below so you do not have to do the math by hand or install anything. Enter the frequency, set the velocity factor to match what you are building with, and read the length straight off the table.
Antenna Length Calculator
Enter a frequency and pick the antenna type. Lengths are given for a wire with a velocity factor you can adjust.
| Antenna type | Length | Inches |
|---|---|---|
| Full wave (λ) | ||
| Half wave (λ/2) — dipole total | ||
| Quarter wave (λ/4) — whip / ground plane | ||
| 5/8 wave | ||
| Dipole leg (λ/4 each side) |
Formula: λ [m] = 300 / f [MHz], then multiplied by the velocity factor.
A quarter-wave whip is 75 / f [MHz] × VF.
Worked example. For the 2 m amateur band at 145.500 MHz with bare wire (velocity factor 0.95), a quarter-wave whip comes out at 49.0 cm, and a half-wave dipole at 97.9 cm in total, so 49.0 cm per leg. Cut the element a few millimetres long and trim it while watching SWR: shortening is easy, adding metal back is not.
If you need something more advanced, for example several antenna types in one place, Yagi element spacing or impedance matching, a dedicated mobile app is worth having. A good example for radio amateurs is the Amateur Radio Toolkit.
It contains many antenna length calculators to help you design radios and find out what the effective length of the antenna will be . You can choose from the types of antennas, i.e. dipole antenna, vertical antenna, ground plane antenna . Here you do not need the formula for calculating the length of the antenna , because it is enough to enter the frequency you are interested in (in Hz or mHZ) into the calculator, and all calculations will be done automatically.
In the free version, some of the features of this application are not available, but the extended version of Amateur Radio Toolkit does not cost much. The pro version is currently available for PLN 11.99. The app is available for Android and IOS.
Antenna selection
The length of the antenna matters when receiving radio frequencies from a particular range. So when you're working on your project, you need to know how long your antenna's "arms" should be.
RTL-SDR Antenna Kit
RTL-SDR antenna kits are available in the Sapsan store , which can be adjusted in length. For example, such a universal set includes, among others:
- 2x telescopic dipole arm, which can be from 23 cm to 100 cm long,
- 2 x telescopic dipole arm 5 cm - 13 cm.
This is a great set to start your adventure with antennas. It additionally includes an adjustable suction cup for the glass and a base with three flexible supports and a cable from 65 cm long. There is also a cable extension (3 m) with SMA male / SMA female connectors.
Telescopic antenna with SMA connector to work with HackRF One
The ANT500 75MHz telescopic antenna receives signals in the 75 MHz to 1 GHz band. As for its length, you can adjust it from 20 cm to 88 cm. This antenna model is admittedly designed to work primarily with HackRF One . However, feel free to use it in other projects as well.
The HackRF transceiver covers 1 MHz to 6 GHz with up to 20 MHz of bandwidth. What is it useful for? Bluetooth sniffing and advanced RFID analysis (it talks to all RFID tags regardless of their frequency), remote control analysis, GPS manipulation, or IMSI interception.
One note on the hardware. The HackRF One has been discontinued by Great Scott Gadgets and replaced by the HackRF Pro. For antenna work the difference that matters is the lower end of the range: the Pro starts at 100 kHz instead of 1 MHz, so it reaches the long wave and the bands below it, where a full wavelength is measured in kilometres rather than metres. It also adds a TCXO for frequency stability and a 16-bit precision mode. The ANT500 fits both, since it works from 75 MHz upwards.
As for compatibility, HackFR One will be compatible with all major SDR packages across multiple platforms (GNU Radio, SDR#, SDR_Radio, Universal Radio Hacker, QSpectrumAnalyzer).
It is worth knowing that, despite everything, antenna installations can be periodically slightly out of tune. Why is this happening? Because the speed of light in a vacuum will never be constant. Similarly, the speed of the electric wave in the cables. It all depends on the current position of the Earth relative to the Sun and the position of the Moon relative to the Earth.
Getting the equipment right for your purposes is key. That is why we also suggest what are the best walkie -talkies for use in the field and at home. And if you have any questions, feel free to contact us.