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Given the following specs of a directional antenna:

VHF yagi antenna – 12dBic gain, switchable RHCP - LHCP
VHF Low Noise Amplifier (NF: 0.8dB)
Frequency range: 144-146MHz

(Is this description enough to characterize the antenna?)

What will be the specs of a fixed antenna capable to receive the same signal (~145.8 MHz, cw and BPSK1200)?

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  • $\begingroup$ What do you mean by "fixed antenna"? If you need 12 dB of gain, then a Yagi would be the most likely choice. Yes it needs to be pointed at the transmitter, if that is your question. $\endgroup$ – Martin Ewing AA6E Nov 14 '15 at 16:07
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These specifications mean:

VHF yagi antenna

This tells you the general kind of antenna it is: a Yagi intended to operate at VHF: "very high frequency".

12dBic gain

This means that in the antenna's direction of maximum gain, it will receive 12dB more power than a theoretical circularly polarized antenna which receives power equally from all directions. It means that if the antenna is pointed directly at the satellite, and the satellite also has a circularly polarized antenna, you can add 12dB to your link budget. (If the satellite does not have a circularly polarized antenna, you can add only 9dB: there's a 3dB penalty for not having circular polarization on both ends).

switchable RHCP - LHCP

This means that the polarization of the antenna can be switched between right-handed circular and left-handed circular polarization.

VHF Low Noise Amplifier

This means the antenna contains an integrated amplifier. Amplifiers require power, so you'll want to read about how that power is supplied to make sure it's compatible with the rest of your system. It might have a separate power supply that you just plug into the wall. Or, it may have some arrangement to be powered over the feedline.

NF: 0.8dB

This is the noise figure of the amplifier. It is telling you that the amplifier degrades the signal to noise ratio by 0.8dB. An ideal amplifier that added no noise would have a noise factor of 0dB.

Frequency range: 144-146MHz

This is telling you the frequencies over which the antenna is designed to work well.

What will be the specs of a fixed antenna capable to receive the same signal (~145.8 MHz, cw and BPSK1200)?

Antennas do not "know" what kind of signal they are receiving. That is, they don't care what the modulation is. So there are quite a lot of antennas which could have similar capabilities. Hopefully with the explanation of the specifications above you can better understand how a different antenna might be more or less suitable to your needs.

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Whether you can use an omnidirectional antenna to replace a directional antenna depends entirely on

  • the minimum signal-to-noise ratio required by the receiver, and
  • the amount of noise arriving from directions other than the wanted signal.

Without this information, your question does not have a yes-or-no answer. And if you are going to go to the effort of collecting the second piece of information you might as well just replace the antenna with your proposed alternative and see what the results are.

Presumably you are attempting to receive a signal from a satellite. An appropriate omnidirectional antenna for the purpose — one which does not have a null pointing upward, as a vertical does — would be a “turnstile” or “eggbeater” type antenna.

The presence of an LNA at the antenna feed point is largely independent of the choice of antenna — you can think of it just as well as being part of the receiver. And as I mentioned to start, the input signal-to-noise ratio is the key characteristic. If there is too much noise, the LNA will do you no good. A directional antenna avoids your receiving unnecessary noise.

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