RF Link & Antenna Calculators

How RF Link & Antenna Calculators Close a Link

A radio link is one long sum in decibels. Transmit power, antenna gains, cable losses, and path loss add and subtract until you reach the received level. RF Link & Antenna Calculators run that ledger, then compare the result with what the receiver needs.

Work in dB and the maths stays simple. Gains add, losses subtract, and the difference between received power and sensitivity is your margin.

EIRP and Path Loss

The EIRP Calculator gives the power actually radiated in the main beam. It is transmit power plus antenna gain, minus cable and connector loss.

A 20 dBm radio with a 12 dBi antenna and 2 dB of feedline loss puts out 30 dBm EIRP. Regulatory limits are written in EIRP, not transmit power, so that total is the number that has to comply.

The Free Space Path Loss Calculator and Friis Transmission Calculator handle the space between the antennas. Loss rises with both distance and frequency, and doubling either adds 6 dB.

Free space is the best case, with nothing in the way. Real paths lose more. Published propagation models add roughly 10 to 30 dB in urban areas, 6 to 15 dB in suburbs, and 3 to 6 dB in rural terrain with vegetation.

Link Budget and Margin

The Link Budget Calculator puts the whole chain together: EIRP, path loss, receive antenna gain, and receive-side losses.

Follow the example through. That 30 dBm EIRP over a 2.4 GHz kilometre loses 100 dB, gains 12 dB at the receive antenna, and drops 1 dB in the receive cable. Received power lands near −59 dBm.

Sensitivity comes from the noise floor. Thermal noise starts at −174 dBm per hertz, so a 20 MHz channel has a floor near −101 dBm. Add a 4 dB noise figure and 20 dB of required signal-to-noise, and sensitivity is about −77 dBm.

Margin is the gap: −59 dBm against −77 dBm leaves 18 dB. As a guide, 10 dB is the usual minimum for a fixed outdoor link, while critical links are designed at 20 to 30 dB to cover rain, multipath, and ageing hardware.

Cable loss is the term most often left out. A 30 m run of LMR-400 at 2.4 GHz costs about 3.5 dB, and four connectors add another 0.6 dB.

Receiver Noise and G/T

The Cascaded Noise Figure Calculator applies the Friis cascade formula to a receive chain. The first stage dominates, because later stages are divided down by the gain ahead of them.

That is the whole argument for a low-noise amplifier at the antenna. Put the LNA first and its noise figure sets the system. Put a lossy cable first and its loss is added to the system noise figure directly.

The Antenna Noise Temperature Calculator works in kelvins instead of decibels, which is how satellite work states the same idea. A noise figure converts to temperature as 290 times the quantity 10 to the power of NF over 10, minus 1, so a 0.7 dB LNB is about 51 K.

G/T is the receive figure of merit: antenna gain in dB minus ten times the log of system noise temperature. A 41 dBi dish on a 100 K system gives 21 dB/K. Raising gain or cutting noise temperature both improve it.

Antenna temperature depends on where the beam points. Cold sky reads a few kelvins, while any spillover onto warm ground pushes it up and pulls G/T down.

Antenna Geometry and Coverage

The Microstrip Patch Antenna Calculator sizes a patch for a target frequency. It returns patch length and width, the effective dielectric constant, the fringing extension, and a minimum ground plane size.

A patch is about half a wavelength long in the substrate, so a higher dielectric constant shrinks it. Fringing fields make it act slightly longer than its physical length, which is why the extension term exists.

The Antenna Downtilt and Coverage Calculator turns antenna height, receiver height, and beamwidth into a tilt angle, coverage distance, beam edges, and radio horizon.

Tilt trades coverage for reach. More downtilt fills in close to the tower and reduces interference with neighbouring cells, while less tilt reaches further out and raises overlap.

Radar Range

The Radar Maximum Range Calculator uses the radar equation, where the signal travels out and back. Transmit power, antenna gain, frequency, target radar cross-section, and receiver sensitivity all feed the result.

The two-way path is what makes radar hard. Received power falls with the fourth power of range, so doubling detection distance needs sixteen times the transmit power, all else equal.

That is also why real radars lean on processing instead of raw power. Pulse compression and integrating several returns add tens of decibels of effective gain, which buys range far more cheaply than a bigger transmitter.

Common Mistakes

Confusing antenna gain with EIRP. Transmit power plus gain is EIRP, and regulations limit the total.

Using free space loss on a terrestrial link. Buildings and terrain add 10 to 30 dB that the model does not include.

Forgetting feedline and connectors. Four decibels of cable and connector loss is easy to miss and hard to recover.

Designing to zero margin. A link that just closes on paper fails in rain or fading.

RF Link FAQs

How do I calculate free space path loss?

Add 20 times the log of distance in kilometres, 20 times the log of frequency in megahertz, and 32.44. At 2.4 GHz over 1 km that gives about 100 dB. Every doubling of distance or frequency adds 6 dB, so the same kilometre at 5.8 GHz costs about 108 dB.

What link margin is enough?

Aim for at least 10 dB on a fixed outdoor link, and 20 to 30 dB where downtime is unacceptable. Margin is received power minus receiver sensitivity. Multipath and fading alone can swing signal levels by 20 dB, so a link with 3 dB of margin works in clear weather and drops out the first time conditions change.

How do I find receiver sensitivity?

Start at −174 dBm per hertz of thermal noise, add ten times the log of bandwidth in hertz, then add noise figure and the required signal-to-noise ratio. For a 20 MHz channel with a 4 dB noise figure and 20 dB SNR, that is about −77 dBm. Datasheet sensitivity figures should land close to this.

Why does the first amplifier set system noise?

In the Friis cascade, each stage’s noise contribution is divided by the total gain before it. A 1 dB LNA with 20 dB of gain makes the second stage’s noise almost irrelevant. Reverse the order, putting 3 dB of cable ahead of the LNA, and that 3 dB is added to system noise figure with nothing to offset it.

What is G/T and what is a good value?

G/T is antenna gain minus ten times the log of system noise temperature, in dB/K, and it summarises how well a receive station hears a weak signal. A 41 dBi dish with a 100 K system gives 21 dB/K. Typical VSAT terminals fall in the teens to low twenties, and large earth stations go far higher.

Why does doubling radar range cost so much power?

The signal spreads on the way out and again on the way back, so received power falls with the fourth power of range. Twice the range needs sixteen times the power. Bigger antennas, pulse compression, and integrating multiple returns are the practical routes to more range.