Do I need a preamplifier on low-bands ?
Very often heard question, but not straightforward answer....
Some facts
1- When receiving on low-bands, gain is not the main thing. Signal Over Noise (/SNR) dictates all.
- I can not decode a S5 station if my noise is S7 (SNR = -12dB), but I can decode a S2 station if my noise is S1 (SNR = +6dB)
- Human brain is able to decode with some habit a signal with SNR = -6dB (S2 station in a S3 noise), not below;
2- RX antenna generally have low gain (-30dBi on 160m ~ -18dBi on 80m), but bring SNR improvment thanks to their directional pattern
- A 12dB SNR improvment can be considered as a starting point;
3- Low band preamplifiers have gain (18dB) which increases both signal and noise by the same amount
- If preceded by a well-designed bandpass filter, we can expect a 6dB SNR improvment, thanks to filtering.
4- On my TX vertical, I have typically a S7 background noise on 160m, and a S5 background noise on 80m
- My antenna is not very efficient on low bands. If you have lower noise, either you are lucky, or you have an issue on your TX antenna :-)
Some facts
1- When receiving on low-bands, gain is not the main thing. Signal Over Noise (/SNR) dictates all.
- I can not decode a S5 station if my noise is S7 (SNR = -12dB), but I can decode a S2 station if my noise is S1 (SNR = +6dB)
- Human brain is able to decode with some habit a signal with SNR = -6dB (S2 station in a S3 noise), not below;
2- RX antenna generally have low gain (-30dBi on 160m ~ -18dBi on 80m), but bring SNR improvment thanks to their directional pattern
- A 12dB SNR improvment can be considered as a starting point;
3- Low band preamplifiers have gain (18dB) which increases both signal and noise by the same amount
- If preceded by a well-designed bandpass filter, we can expect a 6dB SNR improvment, thanks to filtering.
4- On my TX vertical, I have typically a S7 background noise on 160m, and a S5 background noise on 80m
- My antenna is not very efficient on low bands. If you have lower noise, either you are lucky, or you have an issue on your TX antenna :-)
Minimum decodable DX signal versus band, noise conditions and RX capabilities
Results indicates the minimum decodable DX signal, as measured on the TX antenna.
160m |
Noise
TX vertical Preamp only RX ant only RX + preamp |
S0 S1 S2 S3 S4 S5 S6 S7 S8 S9
S0 S0 S1 S2 S3 S4 S5 S6 S7 S8 S0 S0 S0 S1 S2 S3 S4 S5 S6 S7 S5 S5 S5 S5 S5 S5 S5 S5 S5 S6 S2 S2 S2 S2 S2 S2 S2 S3 S4 S5 |
For a S7 noise on my TX vertical, I can copy
- a S6 signal on my TX vertical - a S5 signal with either preamp or RX antenna - a S3 signal if using RX antenna + preamp |
80m |
Noise
TX vertical Preamp only RX ant only RX + Preamp |
S0 S1 S2 S3 S4 S5 S6 S7 S8 S9
S0 S0 S1 S2 S3 S4 S5 S6 S7 S8 S0 S0 S0 S1 S2 S3 S4 S5 S6 S7 S3 S3 S3 S3 S3 S3 S3 S4 S5 S6 S0 S0 S0 S0 S0 S1 S2 S3 S4 S5 |
For a S5 noise on my TX vertical, I can copy
- a S4 signal on my TX vertical - a S3 signal with either preamp or RX antenna - a S1 signal if using RX antenna + preamp |
Typical example : Can I decode a S3 DX signal in S7 noise (160m) and S5 noise (80m) ?
160m example
- With preamplifier only, - DX signal goes up to S6, and noise goes up to S9 - S/N = -3 ==> Still impossible to hear the DX signal - With -30dBi RX antenna only, - DX signal goes down to "S-2", and noise goes down to S0. - S/N = -2 ==> Still impossible to hear the DX signal - With both preamp + RX antenna, - DX signal would go down to S1, and noise would go down to S2. - S/N = -1 ==> I can possibly decode something !!! |
80m example
- With preamplifier only, - DX signal goes up to S6, and noise goes up to S7 - S/N = -1 ==> I can possibly decode something !!!! - With -18dBi RX antenna only, - DX signal goes down to S0 and noise goes down to S0 - S/N = 0 ==> I can hear and decode the DX signal !!! - With both preamp + RX antenna - DX signal would go up to S3 and noise would go up to S2 - S/N = +1 ==> Almost an armchair copy, preamp not needed !! |
==> So do I need an external preamplifier ?
- Above -20dBi as my RX antenna gain, I don't need an external preamp.
- Below -20dBi as my RX antenna gain, I do need an external preamp.
- When using an external preamplifier, there must be a band-pass filter at its input ; Else, preamp is useless !!!!
Receiving capabilities summary (by increasing order of performances)
TX vertical ==> Filtered preamp ==> RX antenna ==> RX antenna + filtered preamp
- Below -20dBi as my RX antenna gain, I do need an external preamp.
- When using an external preamplifier, there must be a band-pass filter at its input ; Else, preamp is useless !!!!
Receiving capabilities summary (by increasing order of performances)
TX vertical ==> Filtered preamp ==> RX antenna ==> RX antenna + filtered preamp