16-05-2023, 01:09 AM
The strongest & longest bursts from meteors result from the overdense situation; a reflection.
I have attached a drawing that shows the overdense return to 70seconds (66Mhz data?).
The photo is from the '50s and shows common underdense return. If you watch these waterfalls,
unless you have a wideband system, you are missing half the picture; see the other attachment
(143Mhz, QRO).
Here's some more:
The relative length of meteor burst is related to a square.
The relative strength of meteor burst is related to a cube.
So since 144/50 is 2.9, the burst on 6m is 2.9*2.9 or 8 times longer on 6m than 2m, and
since 144/50 is 2.9, the burst on 6m is 2.9*2.9*2.9 or 24 times stronger on 6m than 2m.
There are also about 2.9 times as many bursts on 6m than 2m.
A table, with initial 2m burst length 50mS and strength 0db (a 6db increase perhaps one "S" unit)
frequency length strength
144Mhz 50mS 0dB <= reference levels
50Mhz 410mS +13.7db +2 "S"
40Mhz 648mS +16.7db +2½ "S"
29Mhz 1230mS +20.8db +3 "S"
This is all relative to same power level; if the transmitted power* is 6dB greater on 50Mhz than
144Mhz, then the 50Mhz level would be 13.7+6=19.7dB better than the lower power 144Mhz signal.
clearly, lower frequencies are best, so how about 40m?
7Mhz 20sec +40dB +6 "S"
The "regular" E layer is present on 40m during daylight hours, so to be sure that your 20second
burst really is MS, best to try at night for a contact within the skip zone...
Now, take off angle is an important thing here;
600km 18°
800km 15°
1200km 8°
1600km 4°
take off angle is related to the height of antenna above ground (unless you live on the side
of a hill, in which case it is the height of the hill that is important).
But wait, there's more. And that is background noise level. This background noise level consists
of galactic noise, atmospheric noise (QRN) , solar noise and the background QRM (electrical noise) level.
So considering background QRM only:
frequency strength background dB above noise
144Mhz 0dB 0dB 0dB <= reference levels
50Mhz +13.7db 10dB 3.7dB
40Mhz +16.7db 12dB 4.7dB
29Mhz +20.8db 20dB 0.8dB
7Mhz +40dB 40db 0dB (for 40m the summer QRN level can exceed background QRM)
This is an approximation; the actual background noise level depends on your situation; the rural
noise level is more than 20db lower than the urban noise level. That means that a MS signal (or
any) received at S3 in a rural area can be below the noise when received in the city. If you
are in a rural area, this improved signal to noise allows direct FSK for MS data. FSK allows
for faster data rates and rejects amplitude variations (most noise). Effective FSK matches the
transmit modulation index with the receive IF bandwidth.
*adding more elements is not the same thing because going beyond 5 elements reduces the meteor
'capture area' too much.
overdense.jpg underdense.jpg echoes.jpg
I have attached a drawing that shows the overdense return to 70seconds (66Mhz data?).
The photo is from the '50s and shows common underdense return. If you watch these waterfalls,
unless you have a wideband system, you are missing half the picture; see the other attachment
(143Mhz, QRO).
Here's some more:
The relative length of meteor burst is related to a square.
The relative strength of meteor burst is related to a cube.
So since 144/50 is 2.9, the burst on 6m is 2.9*2.9 or 8 times longer on 6m than 2m, and
since 144/50 is 2.9, the burst on 6m is 2.9*2.9*2.9 or 24 times stronger on 6m than 2m.
There are also about 2.9 times as many bursts on 6m than 2m.
A table, with initial 2m burst length 50mS and strength 0db (a 6db increase perhaps one "S" unit)
frequency length strength
144Mhz 50mS 0dB <= reference levels
50Mhz 410mS +13.7db +2 "S"
40Mhz 648mS +16.7db +2½ "S"
29Mhz 1230mS +20.8db +3 "S"
This is all relative to same power level; if the transmitted power* is 6dB greater on 50Mhz than
144Mhz, then the 50Mhz level would be 13.7+6=19.7dB better than the lower power 144Mhz signal.
clearly, lower frequencies are best, so how about 40m?
7Mhz 20sec +40dB +6 "S"
The "regular" E layer is present on 40m during daylight hours, so to be sure that your 20second
burst really is MS, best to try at night for a contact within the skip zone...
Now, take off angle is an important thing here;
600km 18°
800km 15°
1200km 8°
1600km 4°
take off angle is related to the height of antenna above ground (unless you live on the side
of a hill, in which case it is the height of the hill that is important).
But wait, there's more. And that is background noise level. This background noise level consists
of galactic noise, atmospheric noise (QRN) , solar noise and the background QRM (electrical noise) level.
So considering background QRM only:
frequency strength background dB above noise
144Mhz 0dB 0dB 0dB <= reference levels
50Mhz +13.7db 10dB 3.7dB
40Mhz +16.7db 12dB 4.7dB
29Mhz +20.8db 20dB 0.8dB
7Mhz +40dB 40db 0dB (for 40m the summer QRN level can exceed background QRM)
This is an approximation; the actual background noise level depends on your situation; the rural
noise level is more than 20db lower than the urban noise level. That means that a MS signal (or
any) received at S3 in a rural area can be below the noise when received in the city. If you
are in a rural area, this improved signal to noise allows direct FSK for MS data. FSK allows
for faster data rates and rejects amplitude variations (most noise). Effective FSK matches the
transmit modulation index with the receive IF bandwidth.
*adding more elements is not the same thing because going beyond 5 elements reduces the meteor
'capture area' too much.
overdense.jpg underdense.jpg echoes.jpg