SNR and actual rx performance

G4WIMG4WIM
in KiwiSDR Discussion

Hi guys,

new member and Kiwi 2 owner and I must say what a great bit of kit - does a lot more than I was expecting !

I've been fine tuning my setup and haven't quite finished yet but am getting good SNR results in the high 30's, this is using a delta loop 30 ft per side antenna fed at the bottom for now and using a push pull Norton amp with 2N5109 devices.

When I switch over to my 212 ft doublet the Kiwi SNR consistently drops by 10dB or more implying the delta loop is better (quieter) - as indeed it should be given its quieter location away from the house even though it's not as high.

But here's the issue - listening to the same weak station on the doublet using my Icom IC-7800 usually results in a much improved audible S/N ration and better copy than on the Kiwi with the delta loop.

I was hoping / expecting that the Kiwi on the loop would have been just as good if not better than the 7800 on the doublet.

Switching in a 1.5MHz high pass filter doesn't help and actually reduced the Kiwi SNR by a few dB.

I also have an ANAN 100D - I think I'll repeat the test comparing the Kiwi to the ANAN on the same delta loop. I find that the 100D outperforms the IC7800 under noisy conditions due to its superior noise reduction systems.

Maybe the 7800 is just a significantly better rx than the Kiwi - ok I'm not comparing apples to apples here but the difference is significant.

Any thoughts or comments ?

73 Tim G4WIM

Comments

  • G8JNJG8JNJ

    Hi Tim,

    I'm a bit surprised that you need to use an amplifier with the Delta loop, but is the signal level low, or are you using it to make up for splitting or other system losses ?

    How are you evaluating the difference in SNR between the KiWi and your IC-7800 ?

    Are you using similar receive bandwidth and AGC settings ?

    Altering the ACG threshold on the KiWi can make a big difference to the perceived SNR when listening to signals.

    The KiWi and http://rx.linkfanel.net SNR measurement is a very crude tool and is easily fooled.

    Using FT8 signals and their reported SNR may be a better method, but beware of differences in antenna gain and directivity.

    Regards,

    Martin

  • G4WIMG4WIM

    Hi Martin

    the run of coax is quite long at about 70mtrs and even though it is FSJ-450 hardline the loss combined with power injectors etc is 2dB or so at 30MHz. The gain of the amp also drops from 11dB to 9dB at 30MHz. I have a provision to split the RF output to a second rx and will use that in due course for rx comparisons.

    The loop itself is currently on the small side and I notice that switching the preamp in / out does actually help when the bands are quiet but not so much at night as expected.

    I've been listening by ear and there 7800 is clearly better even without making acccurate measurements, but I will experiment with the AGC and FT8 etc. as you suggest.

    I'll also run various rx's side by side on the same attenna's to see what happens.

    Thanks for your help and suggestions, todays project is to improve the antenna, not easy given various trees etc.

    Currently the 'loop' is 100ft of wire in a triangle shape / inverted V with the feed in the middle at only about 4 ft AGL. No termination resistor at the top - but seems to work quite well


    73 Tim

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