Noise figures for VHF/UHF rigs... what is typical?

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by K3RW, Oct 23, 2019.

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  1. K3RW

    K3RW Ham Member QRZ Page

    Today I was looking at online benchmark tests of some HF rigs running transverters for VHF.

    Depended on the rig and the NF of the transverter, but sometimes the high-end transverters on otherwise average base station HF rigs had NFs close to 1, 1.2, etc. Excellent, compared to the IC-910H (around 8 I think) or even the IC-9700 (around 3 something).

    What I could not find, is what is the typical noise factor of a VHF or UHF rig. Say the IC-7100 in FM or USB on 2m or 70cm, or a random VHF mobile. Maybe thats in brochures and called something else. But a ballpark figure--its like '20'--would be nice if you know it.

    Trying to compare that on HF is difficult too. On some bands the SDRplay 1A as a NF of 8. Then above 100MHz or so its around 3. At 3 thats already better than a IC-910H is supposed to be on the chart I saw.

    In a perfect world I run all SDRs to high quality transverters even on 2m, on up.

    I've done some FM-only contesting and I'm curious where the average 65w FM mobile rig is, the average HF + VHF rig running SSB, etc. Maybe running a simple SDRplay is enough to get a NF down to 3 really cheaply. And a LNA to get it even lower. And get abilities to see a whole band to catch infrequent activity, without having to co stantly spin a dial.

    Its almost enough for me to think to run a SDRplay for RX on these bands (NF around 3) and just use the rig for TX. Economically speaking I mean.
    Its also in my head that a decent LNA sort of makes up for a lousy rig NF. Is that basically correct?

    When I compared HF figures, SDR rigs to non-SDR ones, initially I was surprised that SDR rigs (even high end ones) were frequently less sensitive. But then again, their dynamic range seems to more than make up for it.

  2. SM0AOM

    SM0AOM Ham Member QRZ Page

    I suggest that you read up on what noise figure or noise factor actually means.

    It is a measure of the effective sensitivity of a receiver system, and has a formal definition of

    (S/Nin)/(S/Nout), which is a power ratio always 1 or larger or 0 dB,

    or how much the signal/noise ratio at the input is degraded by the amplifier or receiver.

    What is an "acceptable" noise figure is open to discussion, ideally the noise contribution of
    any receiver should be negligible compared to the amount of external noise that the environment

    Also, there is a trade-off between large-signal handling and low-noise performance in a receiver,
    as low noise figures most often require a lot of signal amplification.

    Looking at the "state-of-the-art", realistic noise figures (NF) for current HF equipment
    are in the 9 - 15 dB range, which make their contribution to the overall noise small in the HF range.

    Production VHF equipment may have NF:s in the 3 to 7 dB range, depending on design objectives,
    and UHF even lower.

    However, very low NF:s are moot today in terrestrial communications, as the ambient noise has increased considerably, so the addition of an LNA, which may reduce the system NF well below 1 dB,
    seldom makes any difference today.

    What NF an SDR has is very implementation-dependent. Most of the better HF
    SDR:s use preamplifiers before the A/D that permit a system NF in the 13-15 dB range which is entirely sufficient for most locations. Some also are able to switch in another preamplifier that reduces the NF further, but at the expense of degraded large-signal handling.

    K3RW likes this.
  3. K3RW

    K3RW Ham Member QRZ Page

    Excellent info. Thanks so much.

    What I was looking for today was whether a transverted HF rig is more advantageous than a dedicated VHF+ one. The average HF rig has better filters and other useful features. I suppose if a VHF rig was equivalent to that, it would be a close option too.

    My FT-817 is a transverter driver if I want it to be one, mainly for a 144 and 432 IF. And because I already have it, and it does not have a history of RF spikes. But its also infuriating to have limited filter options, no IF DSP, and so on. I'm pondering a 'better' IF rig, but not sure what that would be. Perhaps that would be a GPS locked 2m transverter off my IC-7600... running a higher band transverter. A transverter to a transverter! I tend not to like that solution, but I'd also like results.

    Its definitely a different task to dig out faint, weak signals among just the usual noise. And another to try to dig out a faint DX station among strong signals.

    Thats sort of where I'm at for 2m. 2m SSB gets really crowded during contests. People here don't space out much at all. But 1296 isn't like that... well, at least locally here.

    True, the goal really isn't to get a supposed sub-1 NF only to never realize that due to terrestrial noise!
  4. SM0AOM

    SM0AOM Ham Member QRZ Page

    It is much easier to optimise a transverter than a "multi-mode multi-band" transceiver.

    My own VHF/UHF station consists of a chain of series-connected transverters and converters;
    starting with professional-grade HF exciters and receivers going upwards through mostly
    homebrew and Microwave Modules gear.

    A key aspect is to use the available receive and transmit dynamic ranges as efficiently as possible.
    Here, nothing can "beat" homebrew optimised equipment where you can choose an optimal noise figure and gain distribution by yourself.

    K3RW likes this.
  5. AI3V

    AI3V Ham Member QRZ Page

    Typical modern vhf/uhf radio using a mosfet, gasfet, or hemt device will have a noise figure of a db or 3.

    For terrestrial work, with a modern rig, there is little to be gained with a preamp, a 3 db nf radio can "hear" a signal 3 db above the theoretical minimum.

    For extraterrestrial work, (satellite) and directional antennas the improvement can be dramatic, and gets better the bigger the antenna.

    Now for some light reading:)

    When you read thru the above there will be a short quiz... :)

    N0IOP likes this.
  6. DF2JP

    DF2JP Ham Member QRZ Page

    K3RW and AI3V like this.
  7. KN4ZKT

    KN4ZKT Ham Member QRZ Page

    I hate to bring this thread back from the dead, but seemed appropriate given my question.

    I've been reading and studying noise figure in radios, along with pre-amps and wondering if I could benefit from one. The conclusion in this thread is that it typically won't help given the noise level typical hams experience on 2m now, but I live in a very RF quiet place, specifically on 2m and 70cm. On both bands I can unplug my antenna and literally hear no difference in noise level. I assume then, a pre-amp could make a good difference? I'm currently running a Kenwood TS-2000, but getting more serious about weak signal and thinking about replacing that.
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I can hear a difference between a shielded dummy load and a real outdoor antenna on 2m for sure (using an FT-736R). Aiming a good beam at the sun on the horizon makes the noise level increase slightly.

    Don't use FM for this, obviously: Use CW and "listen" to the atmospheric noise as you rotate the beam(s). If you hear no difference at all, I'd say you could use a preamp!:)
  9. KN4ZKT

    KN4ZKT Ham Member QRZ Page

    Yeah, sorry I didn't explain my test further. I was doing this in SSB, and I've done it with a SDR that has a dBm scale to compare as well. I'll try it in CW as well for interest.
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    SSB or CW should be fine; just not "FM."

    I can't use an SDR here to test this, because their front ends are wide-open and have no rejection to out-of-band signals, which are a-plenty around here -- I'm line of sight to dozens (maybe hundreds) of powerful VHF transmitters which create issues trying to receive weak signals on VHF.

    An SDR with front-end filtering, like my little IC-9700 which I've only owned for a few weeks, is a bit better; however it's still not as good as my older IC-736R which has dual helical resonators in the front end (preamp and mixer) and strongly rejects signals outside the 2m or 70cm bands.

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