Last Updated on October 3, 2024 by John Berry
In other pages on this site, I’ve calculated the two most important parameters – the free space path loss and the billboard or passive reflector gain of the trail. Now it’s time to use those two to compute the meteor scatter path budget. The path budget is key to understanding the reflection phenomenon and dimensioning a system for meteor burst communications.
The budget is set out below. Whilst meteor burst communications are effective from about 30MHz to about 200MHz, the most popular frequency is 50MHz and I’ve done the budget for that.
The conclusion is that for a 400Watt transmitter using the MSK144 transmission system, there’s around a 30dB margin above threshold for an optimum meteor trail. That suggests that at 6dB per S-point, the received signal would indicate S5 on the receiving rig – a strong signal.
Path budget
| Parameter | Value | Unit | Comment |
|---|---|---|---|
| Frequency | 50 | MHz | |
| Transmission losses | |||
| TX antenna gain [a] | 11 | dBi | Single 6-element Yagi |
| TX feeder loss [b] | -2 | dB | |
| Free space loss [c] | -191 | dBi | Assuming a trail at a height of 100km. As 32.4+20logf+20logd. |
| Billboard or passive reflector gain [d] | 9 | dBi | As 20log((πd)2/λ2) |
| RX antenna gain [e] | 11 | dBi | Single 6-element Yagi |
| RX feeder loss [f] | -2 | dB | |
| Total gains minus total losses [g] | -164 | dBi | As a+b+c+d+e+f. |
| System value | |||
| Receiver threshold [h] | -159 | dBW | The 2.5kHz SSB sensitivity of a typical ham rig. |
| Coding and bandwidth gain [i] | 8 | dB | MSK144 coding gain. No bandwidth gain. |
| LNA threshold improvement [j] | 2 | dB | Calculated considering cascaded noise figure with the low-noise amplifier in place. |
| Effective threshold [k] | -169 | dBW | As h=i-j. Gains improve the threshold. |
| TX output power [l] | 26 | dBW | As 400W RF from the transmitter |
| System value [m] | 195 | dB | As l-k. Total loss permissible between TX port and RX port. |
| Margin over threshold | 31 | dB | Or S5 on the meter assuming 6dB per S-point. As m+g. |
Optimum trail
An optimum trail is one that is a) overdense. b) travelling in line with the transit of the transmission from TX to RX, c) wide enough to give billboard gain, and d) occurring at the mid-point of a long path such that the angles of incidence and reflection are low. We also assume that there is no polarisation distortion along the path – true in most cases.
30dB is a high margin and would be supported from experience by many hams. It suggests that if only humans could talk fast enough, meteor scatter could support phone SSB communication. Actually, some have tried it, concluding that it’s impractical.
Non-optimal trails
Underdense meteor trails have significantly less billboard gain – in fact a loss. As discussed elsewhere, underdense trails tend to scatter rather than reflect. Underdense meteors tend also to be from sporadic meteors, lower in mass and energy. Their trails are therefore narrow, ranging from millimetres to a few centimetres. The combination gives rise to significant excess loss.
And typically, many meteors don’t travel along the transit between transmitter and receiver – they pass orthogonally, or at some random angle. This gives rise to a loss of up to 6dB over the ideal.
The result is that often underdense meteors perform as a poor billboard or passive reflector, and the result is that they do not support communications. Although there are plenty of them, they are not as useful as shower meteors.
Hams get excited when ‘big stones’ from showers are likely, approaching the ideal and avoiding the high losses from meteors more commonly available.
Other notes on path budget
I’ve assumed the use of a low noise amplifier. This requires that the noise level of environmental electrical noise at the receiver site is low to benefit from overcoming the receiver feeder losses. Whether at low VHF this is worth the cost is moot.
I’ve also used a figure of 8dB for coding gain for MSK144. This is as reported by Joe Taylor K1JT. There is, of course, no bandwidth factor to improve the threshold since MSK144 makes use of the whole 2.5kHz channel bandwidth.