Do You Really need a Preamp?
So, do you really need a pre-amp for vhf weak signal work? Well, the definitive answer is ... maybe. There are three main factors to consider.
- The signal/noise ratio at the antenna
- The noise figure of the first device in the receive path. (including the coax)
- The coax loss between antenna and the first amplifier (as above)
S/N ratio at the antenna.
Let’s face it, if the signal you want is buried in QRM and/or QRN at the antenna, there is no way a preamp will recover it. It is what it is and a preamp amplifies noise and signal without distinction. The answer of course is to get the antenna into as low noise location as possible. If you finish up with a long length of coax, it’s ok. A preamp can fix that!
Front end noise figure
The maximum sensitivity of a receiver is constrained by the noise generated internally, primarily by the first stage. Any signal weaker than this will be masked by the noise. This is termed the noise floor. To be "discernible", any signal should be 3dB above the noise floor.
Noise Floor = Noise figure + 10logBandwidth -174 dB
where 174 is the arbitrary environmental noise floor.
Clearly, any improvement in Noise Figure gives a corresponding improvement in the noise floor, hence the minimum discernible signal (MDS). Most modern transceivers have a noise figure typical 2 - 4 dB. Modern pre-amps, 0.5 to 1 dB., yielding a potential improvement of 1.5 to 3.0 dB in MDS. Having said that, the pre amp built into the FT-8x7 series is about 1.2dB. A misaligned preamp peaked for gain not noise could well have a NF of 2db...
Long runs of coax.
Would you put a 6dB attenuator in front of your 2M receiver? No I didn’t think so! 30metres of RG-59 is just that, and will increase the noise figure and drop the MDS by 6dB! Same, same 65 metres of RG 213.
So a tall tower and long run of coax needs a preamp at the top of the tower. Once the MDS is gone it cannot be recovered. A preamp with 16dB of gain will bring the signal (noise and signal, remember), well above the point of the receiver MDS.
The definitive answer
- Yes... If you have a very old transceiver (old land mobiles have 8db NF!) or a poor front end noise figure.
- Yes... If you have a long run, lossy coax or too many connectors (0.4db/pair “UHF” connectors @144MHz).
- No... In just about every other case.