G8JNJ

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  • LZ1AQ LNA and a 1.2 meter copper loop antenna.

    G8JNJG8JNJ
    Hi Ron,

    Reducing the loop inductance (for a given size) is something worth trying if you want to improve the performance still further on the HF bands (>10MHz).

    This is less critical when you are using a loop amplifier with a fairly high input impedance, but in the LZ1AQ design it is only in the region of a few ohms (which is great for the LF bands), so loop inductance becomes a critical factor as you go up in frequency.

    You can either make the loop a lot 'fatter' to reduce the inductance, connect several same size loops in parallel (but spaced by a few inches), or cross-connect four or more independent loops together.

    Chavdar has good notes on this subject on his website.

    http://active-antenna.eu/tech-docs/3_ActiveAA_Antena_11.pdf

    Regards,

    Martin - G8JNJ
    KA7U

  • Home Brew Broadband Magnetic Loop

    G8JNJG8JNJ
    Hi Ron,

    A couple of weeks ago I received a private email about this circuit, and decided to make a copy, as the person who wrote was having problems with it, and I couldn't figure out how the circuit could perform as described.

    I found that the IMD performance was poor and it had several other issues which couldn't be easily fixed.

    If you want to build or buy an amplifier of this type, my advice would be to take a look at the DXE RPA-1 circuit instead.

    image



    Regards,

    Martin - G8JNJ




    KA7U

  • v1.154 released: New user interface, Navtex extension

    G8JNJG8JNJ
    Hi,

    Three other signals I've decoded (so far)

      6322.5 CW FSK 170Hz shift 100bd, Murmansk, Russia
    12603.5 CW FSK SVO, 170Hz shift 100bd, Athens, Greece
    12622.5 CW FSK XSQ, 170Hz shift 100bd, China

    Regards,

    Martin - G8JNJ
    WA2ZKD

  • New Owner Questions [S-meter OV, audio underruns, tags, GPS sensitivity]

    G8JNJG8JNJ
    Hi Kevin,

    1. On the S-Meter, even with a low signal report, I get the "OV" light up, is this normal, and should I worry about it?

    It indicates ADC overload, if you look on the spectrum display you may find some broadcast signals are above the -30dB line. An attenuator, or better still notch filters will help with this.

    2. Occasionally the web page (with my SDR) stops with "audio underrun", I have to refresh the page, and it's back to normal. Is this happening because I am using something else on my PC? (running through Chrome). Checking the log, does show this error message.

    It happens occasionally some browsers are more prone to it than others. 

    3. I notice the tags at the top of the frequency, is it possible to easily to add my own tags, or are they imported from a source file?

    Shift left click on the banner above the waterfall.

    4. I need to put the GPS antenna outside, as it does not show any acquired signals.

    The supplied antenna is not the best. It does work, but it's much better if it is placed outside.

    Most of this is covered in the on-line guide http://kiwisdr.com/quickstart/

    Regards,

    Martin - G8JNJ


    ZL1KFMWA2ZKD

  • Noisefloor of my KIWISDR

    G8JNJG8JNJ
    Hi Dietmar,

    That looks very good.

    The big test is to see if the antenna you are using is good enough for the KiWi :-)

    The ITU publish a noise / frequency curve (Rec. ITU-R P.372-8) https://www.itu.int/dms_pubrec/itu-r/rec/p/R-REC-P.372-8-200304-S!!PDF-E.pdf (figure 2) which shows the variation in natural noise floor with frequency for various locations. 

    As long as you have a receiver and antenna that is slightly more sensitive than the level shown on the graph (best case B <100KHz / >100KHz C <4MHz  / D >4MHz curves), then you don't need to worry about your receiver or antenna. 

    In most cases because of the shape of the curve, this means that you only really need to concentrate on the higher frequencies, as the lower frequencies tend to have a much higher level of natural noise, even in very electrically quiet locations.

    To try this for yourself, pick a suitable clear frequency, perhaps somewhere around 15 - 20MHz, where the natural noise floor is likely to be moderately low and the KiWi has good sensitivity.

    Put a 50 Ohm termination on the KiWi RF input.

    Zoom to level 12 (Z13 has some problems at low signal levels) and increase the WF min sensitivity slider until you can see the KiWi noise floor on the spectrum display at around -150dB.

    Wait a few minutes for the display average to settle and then connect your antenna in place of the 50 Ohm load.

    Make a note of the new level of noise floor on the spectrum display.

    Ideally if the KiWi has enough gain and the antenna is good enough, your noise floor should rise slightly when the antenna is connected (as long as it's not due to unwanted interference). 

    If you find that the noise floor increases by more than 3dB, then the KiWi and antenna you are using are already sensitive enough, as the lowest level signals you will be able to hear are limited by the natural (galactic) noise level

    If the noise floor doesn't rise by 3dB (or at all) then your antenna needs to be improved.

    Generally speaking, when using a good (passive) antenna the Kiwi is usually sensitive enough up to about 25MHz. On higher frequencies the natural noise floor is lower and ideally you need an antenna that has some additional gain in order to achieve a 3dB increase in the noise floor. So if you repeat the same measurement method at for example 30MHz, you may find that you can't see any change in noise level.

    If you are using an active antenna, it's a bit more complicated to conduct these tests, as the amplifier will add it's own noise contribution. 

    In this case you need to replace the antenna part (whip, probe or loop) with an equivalent terminating impedance. In the case of a whip or probe something like a 12pF capacitor from the amplifier input to the common (ground) rail, or in the case of a loop you need to replace the loop element with a small inductor of equal value to that of the loop (typically 2.5uH). As before look for a 3dB increase in noise when the antenna element is reconnected.

    All of this assumes that your existing noise floor is low, but if it's dominated by local interference (as is the case in most urban environments), then you need to concentrate on reducing the level of interference, rather than worrying about the Kiwi or antenna gain. Although moving an existing antenna to a lower noise location, or using a directional antenna may be at least be a partial solution.

    Regards,

    Martin - G8JNJ
    dl2sba