EME path budget

Last Updated on May 18, 2026 by John Berry

Using either the bistatic radar method or the billboard method, I calculate the round-trip EME path loss at 252dBi at 144MHz. My calculations show a 7dB margin, making EME at 144MHz possible between two well equipped stations. Below I calculate an EME path budget for the three ‘beginner’ bands. As I show, the margin degrades slightly to 6dB at 432MHz, and to 6dB at 1296MHz (with a 2.4m dish).

I’ve set out my calculations below, and there’s a description of the path loss calculation on an adjacent page. The 432MHz default values in this calculator mirrors my installation. I typically exchange signal reports of around -21 with similarly equipped stations.

Simply scan down the table for each frequency and focus on the Margin result at the bottom. Then read on.

The EME path budget

Regulators may demand a larger dish size in the future as a condition of SECONDARY use of the 23cm band. This is to control inter-service interference. They may demand a minimum of 3m (30dBi) gain as agreed at RA/WRC23.

Bibliography from 2005 suggests that the value for antenna system threshold degradation might be a bit pessimistic. Typically ham rigs have relatively poor front end noise figures. Use of a mast-head mounted LNA with a noise figure of around 0.5dB will possibly give a 4-5dB improvement on each band.

Data system enhancement

The point about the modern rash of data modes is that they can be decoded significantly below the 2.5kHz ‘normal’ SSB bandwidth. Here’s a diagram that explains the relative bandwidths, specifically of JT65/Q65. Together with coding and a priori decoding, this gives the data system enhancement.

The JT65/Q65 threshold is represented as a single value for discussion. They may be different. It indicates the level at which data will be recovered with an ‘arbitrarily low’ [good] bit error rate. The indicated signal report (on WSJT-X or other application) is calculated from the ratio of the received signal level and the 2.5kHz bandwidth threshold. That’s typically in the range -10 to -28, and sometimes as low as -32dB.

Importantly, in the calculator, if the margin is positive, communication will be possible.

Mathematical Logic

The calculator was built in an Apple Numbers spreadsheet and the arithmetic checked. I then used Google Gemini AI to build the calculator. My HTML is not that good! Here’s the logic from the spreadsheet.

Transmission Loss [5] = TX Gain [1] + Path Loss [2] + [3] +[4] – other losses [A to E]

Effective JT65/Q65 Threshold [9] = Receiver threshold [6} – Coding gain [7] – System degradation [8]

System value [11] = Effective threshold [9] – TX power [10]

Fade margin [12] = Transmission loss [5] – System value [11]

Margin for viable comms

So, given a margin of 7dB, EME will work at 144MHz for a decent percentage of time. However, those stations must run 500W power at the antenna, four-stack arrays and JT65. Things will, of course, improve at perigee. See elsewhere for a discussion on how variables aggregate to yield a chance of communication. And a low noise amplifier at the mast head is always needed on receive to overcome feeder losses.

This EME path budget calculation illustrates that reliable communication at 144MHz is possible. If operators allow losses to creep in by using poor feeder, if a lesser data system (such as CW) is used, or if lower RF power is used, the margin will go negative.

Conversely, at perigee the chance of communications improves. And bigger stations at 144MHz give more operational certainty. A marginal station (like that described here) will need to rely on bigger stations for QSOs when conditions are less than optimal.

The same discussion applies at 432MHz and 1296MHz.

Other possible losses in the EME path budget

There are several fading mechanisms yielding loss over optimum that may conspire to thwart success in all cases ([A], [B], [C], [E] and [F]). Some commentators call those collectively, degradation. Sprinkle a bit of any or all of these effects and the result is catastrophic for communications. I also assume that there’s no ground gain [D]. These variables are discussed separately on other pages. Here are the links to those.

Libration fading loss [A]:
Varies from -23dB (loss) to +10dB (gain). Error correction helps overcome libration fading.

Sky noise degradation [B]:
Varies as sky background. From 2dB to 12dB. Avoid pointing at a noisy sky region such as the Sun and Milky Way.

Amplitude scintillation fading [C]:
Small variations in the refractivity of the tropospheric part of the path.

Moonrise/moonset ground gain [D]:
Add up to 6dB when Moon is just above horizon. But quickly degrades as the Moon rises.

Geometric rotation polarisation loss [E]:
Since a horizontally polarised wave can be received vertically polarised. Polarisation discrimination loss can be high.

Faraday rotation loss [F]:
Polarisation rotation as wave transits the ionosphere. Polarisation discrimination loss can be high.

They’re all set to zero in the above budgets, illustrating the most optimistic state (most pessimistic for ground gain). For a viable path the margin should be significantly above zero. The fading mechanisms, and the fade margins needed, are discussed elsewhere on this site.

Try using the calculator to emulate your own installation, or intended installation. Try adding some additional losses and see how quickly the margin reduces below zero.

As a final note, I’ve not included anything for pointing loss – the loss resulting from imperfect pointing of the antenna (at the moon). The gain of all antenna types used falls away off the bore sight. Pointing will be more difficult at higher frequencies as beamwidths narrow, and hence loss through imperfect pointing will rise with frequency.