G8JNJ

About

Username
G8JNJ
Joined
Visits
1,935
Last Active
Roles
Member
Points
400
  • Antenna recommendation for listening to faraway DRM broadcast inside a city

    Hi All,

    Well the MLA-30 'sort of works' and it doesn't cost very much, so it's better than nothing for folks who don't want to build their own.

    You get quite a bit for your money, and it's OK for use with say and RTL dongle in direct sampling mode, but I'll probably cry the first time I see one being used with a high end receiver like a Perseus or Airspy HF :-(

    Here are some measured parameters of various popular loops using the same test setup.

    MLA-30 OIP2 approx +46dBm OIP3 approx +20dBm
    M0AYF OIP2 approx +75dBm OIP3 approx +27dBm
    Wellgood (Copy of early Wellbrook) OIP2 approx +57dBm OIP3 approx +37dBm
    LZ1AQ OIP2 approx +79dBm OIP3 approx +36dBm

    I've now added the MLA-30 information, including circuit diagrams to my Active antennas webpage.

    https://www.g8jnj.net/activeantennas.htm#MLA30

    Regards,

    Martin - G8JNJ
    Powernumptycathalferris
  • Need to attenuate LW 252 Khz

    >
    > found that the notch was 34 dB deep, which was more than necessary. I 'softened' it to 20 dB with a series 2.2 ohm resistor.
    >

    Hi Richard,

    That's good, you must have found an inductor with a good value of Q.

    I've only ever been able to achieve a maximum of about 20dB using surface mount or small molded axial types.

    You can widen or narrow the notch bandwidth by choosing different L/C ratios. The values I used were designed to provide some attenuation over the whole of the LW BC band, with a deep notch at the band center and minimal effect outside of the band (148-284KHz @ -3dB).

    You can decrease the depth of the notch by adding low value series resistors as Richard suggests. When using surface mount inductors, I typically add 6.8 Ohms to this circuit to obtain a notch depth of 10dB, with 0 Ohms I can get about 17 to 20dB.

    Here's a plot showing the measured performance of my BC band filters.




    Regards,

    Martin - G8JNJ
    HB9TMC
  • W/F and SND Bad Params

    Hi Glen,

    I was able to enter these commands directly via the KiWi admin console page.

    I don't know if this is possible, but the responses I got back seem to indicate that it worked OK.

    Regards,

    Martin - G8JNJ
    Brendan_WG0LUJ
  • Seeed Metal case and GPIO connector

    Hi All,

    I've had some issues with radiated noise from a KiWi that I installed on a shared site. The main issues were that it was interfering with of VHF satellite reception and degrading the GPS performance of another system in the same room.

    The KiWi was installed in the Seeed metal case, and I had fitted ferrites to the cables.



    As I'm currently building up a KiWi for installation at another site, I thought that it would be a good idea to see if I could further reduce the level of emissions from a KiWi fitted in the Seeed case.

    In order to do this I used an EMC H-Probe consisting of a 1"diameter screened loop connected to a spectrum analyser, in order to see what the level of emissions looked like and where they were escaping from the case.

    The majority of the noise was leaking out through the holes in the ends of the case and also via the additional interconnecting cables I had fitted to bring out the GPIO ports for antenna switching.

    This shows the general level of radiation from a KiWi without a case, with the Seeed metal case and with the Seeed case and additional modifications. Note that the measurements were made without the external GPIO cable connected.



    The next shot shows the difference between the Seeed metal case and with the Seeed case and additional modifications in more detail.



    As you can see the biggest difference is achieved by fitting the metal case, and I would definitely recommend doing this if you are still using the plastic frame supplied with the KiWi.

    However there is still a fairly significant amount of radiated noise throughout the whole spectrum, especially in the VHF / UHF / Microwave range, and the only way to cure this is to close up the holes in the box.

    As I didn't want to restrict airflow and ventilation I used some very fine woven brass mesh that I previously bought for antenna construction.

    I added a section of mesh at each end of the box and sandwiched it between the end plates and the box sides, having first cutout suitable holes for the connectors I still wished to use.



    Note that I had to solder short brass sheet straps between the Ethernet port metal socket surround and the mesh and also the -ve of the DC input connector where it enters the board (prior to the common mode choke on the board) in order to reduce the level of radiated noise from the external screened CAT5 Ethernet cable and DC power cable. There already was physical / electrical contact between the mesh and Ethernet connector, but I noticed that the screening effectiveness varied as I flexed the cable, and I didn't want to risk problems on-site at a later date if the mesh tarnished or developed an oxide layer.




    Here are a couple of external views, showing the wire mesh with the case closed.




    Once I had done this work I still had some noise from the GPIO port, so I added some 0.1uF decoupling capacitors and some 1/8th watt 1K Ohm resistors in series with the wires from the KiWI GPIO pins (not the 0v ground return) and the decoupled pins of the output connector. This also provided some short circuit protection, as previously suggested as best practice earlier in this thread.




    After doing all of this work, the level of emissions are now minimal, and the ambient level of noise in my workshop is higher than the majority of noise produced by the KiWi, even though the close field probe is right next to the case. There is still a bit of radiation along the seams between the top and bottom case halves, but this is very low level, and if necessary can be further reduced by running self adhesive aluminium (ducting) tape over the seams.

    One other interesting observation is that before fitting the mesh and additional earth bonding strap, I didn't find that using screened Ethernet cable made much difference to the level of noise radiated around 20MHz. However after performing the modification it's really noticeable that using screened cable makes a big difference.

    Overall I think the additional mods are worth doing, especially if you are bringing the GPIO pins out of the box. But if you are already housing your KiWi in a diecast box with suitable cable bonding, then you are probably already experiencing most of the benefits I have observed.

    Regards,

    Martin - G8JNJ
    PowernumptyHB9TMCG0LUJ
  • Seeed Metal case and GPIO connector

    Hi All,

    I've had some issues with radiated noise from a KiWi that I installed on a shared site. The main issues were that it was interfering with of VHF satellite reception and degrading the GPS performance of another system in the same room.

    The KiWi was installed in the Seeed metal case, and I had fitted ferrites to the cables.



    As I'm currently building up a KiWi for installation at another site, I thought that it would be a good idea to see if I could further reduce the level of emissions from a KiWi fitted in the Seeed case.

    In order to do this I used an EMC H-Probe consisting of a 1"diameter screened loop connected to a spectrum analyser, in order to see what the level of emissions looked like and where they were escaping from the case.

    The majority of the noise was leaking out through the holes in the ends of the case and also via the additional interconnecting cables I had fitted to bring out the GPIO ports for antenna switching.

    This shows the general level of radiation from a KiWi without a case, with the Seeed metal case and with the Seeed case and additional modifications. Note that the measurements were made without the external GPIO cable connected.



    The next shot shows the difference between the Seeed metal case and with the Seeed case and additional modifications in more detail.



    As you can see the biggest difference is achieved by fitting the metal case, and I would definitely recommend doing this if you are still using the plastic frame supplied with the KiWi.

    However there is still a fairly significant amount of radiated noise throughout the whole spectrum, especially in the VHF / UHF / Microwave range, and the only way to cure this is to close up the holes in the box.

    As I didn't want to restrict airflow and ventilation I used some very fine woven brass mesh that I previously bought for antenna construction.

    I added a section of mesh at each end of the box and sandwiched it between the end plates and the box sides, having first cutout suitable holes for the connectors I still wished to use.



    Note that I had to solder short brass sheet straps between the Ethernet port metal socket surround and the mesh and also the -ve of the DC input connector where it enters the board (prior to the common mode choke on the board) in order to reduce the level of radiated noise from the external screened CAT5 Ethernet cable and DC power cable. There already was physical / electrical contact between the mesh and Ethernet connector, but I noticed that the screening effectiveness varied as I flexed the cable, and I didn't want to risk problems on-site at a later date if the mesh tarnished or developed an oxide layer.




    Here are a couple of external views, showing the wire mesh with the case closed.




    Once I had done this work I still had some noise from the GPIO port, so I added some 0.1uF decoupling capacitors and some 1/8th watt 1K Ohm resistors in series with the wires from the KiWI GPIO pins (not the 0v ground return) and the decoupled pins of the output connector. This also provided some short circuit protection, as previously suggested as best practice earlier in this thread.




    After doing all of this work, the level of emissions are now minimal, and the ambient level of noise in my workshop is higher than the majority of noise produced by the KiWi, even though the close field probe is right next to the case. There is still a bit of radiation along the seams between the top and bottom case halves, but this is very low level, and if necessary can be further reduced by running self adhesive aluminium (ducting) tape over the seams.

    One other interesting observation is that before fitting the mesh and additional earth bonding strap, I didn't find that using screened Ethernet cable made much difference to the level of noise radiated around 20MHz. However after performing the modification it's really noticeable that using screened cable makes a big difference.

    Overall I think the additional mods are worth doing, especially if you are bringing the GPIO pins out of the box. But if you are already housing your KiWi in a diecast box with suitable cable bonding, then you are probably already experiencing most of the benefits I have observed.

    Regards,

    Martin - G8JNJ
    PowernumptyHB9TMCG0LUJ