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Dr. Ulrich Rohde: Electrically Short Antennas

Discussion in 'Amateur Radio News' started by AB4OJ/SK2024, Dec 24, 2020.

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  1. AB4OJ/SK2024

    AB4OJ/SK2024 Ham Member QRZ Page

    At his request and on his behalf, I am posting an article by my friend Dr. Ulrich Rohde entitled "Electrically Short Antennas". The co-authors for this article are Magdalena Salazar-Palma and Tapan K. Sarkar.

    73, Adam VA7OJ/AB4OJ Antennahouse2.JPG
     

    Attached Files:

    W2QA, KA9P, K3BR and 15 others like this.
  2. WO1B

    WO1B Premium Subscriber QRZ Page

    As the Dr. seems to be a lifelong member of this site, why did he not post this himself? I smell something fishy here.....
     
    N7KO likes this.
  3. N1UL

    N1UL QRZ Lifetime Member #303 Platinum Subscriber Life Member QRZ Page

    The fishy part is that goofed to load it up with a introduction picture so I asked for help rather then make a mistake.
    Is this considered disorderly conduct ?
    I hop that explains it.
    73 de Ulrich, N1UL
     
    WX1J, K4ZH, WA2LXB and 26 others like this.
  4. N1IPU

    N1IPU Ham Member QRZ Page

    Thanks, Good stuff as always. I like QRZ for this and not for "look at me videos" At some point were going to really miss having true Elmers here.
     
    TI3IES, N0TZU, W2CPD and 3 others like this.
  5. WO1B

    WO1B Premium Subscriber QRZ Page

    No apology will be offered considering the amount of effort hackers go to obtaining our information it makes good sense to be suspicious and let's face it, a paper that says its created by a member but posted by another is certainly suspicious. Maybe the better option was to assist the author with posting it under his own identification? All users should be very careful in downloading or opening files (yes including .pdf formated documents] as they may contain malware and other unwanted items.
     
    KT5OT, G1HYD, WZ4K and 1 other person like this.
  6. KQ4EL

    KQ4EL Ham Member QRZ Page

    Thank you Dr. Rohde, and happy holidays.
     
    AJ6KZ likes this.
  7. N1UL

    N1UL QRZ Lifetime Member #303 Platinum Subscriber Life Member QRZ Page

    Thanks and likewise happy holidays, here is a screenshot from a monitoring receiver
    F4FC07ED-8BDE-4ED6-BE1C-E03E4BE723CF.jpeg

    Pretty good up to 21 MHz, RX 2 MHz to 22 MHz display using an active antenna; 73 de Ulrich N1UL
     
    W2ZF, KO4LZ, NR5TX and 4 others like this.
  8. K5SOP

    K5SOP Ham Member QRZ Page

    Excellent paper, very informative. Merry Christmas to all.
     
    KR3DX likes this.
  9. SM0AOM

    SM0AOM Ham Member QRZ Page

    A very interesting paper and presentation.

    It highlights the difficult design considerations which especially designers of co-located systems on small platforms face.

    Former associate Emelie Agnedal wrote a paper "System requirements for co-located equipment on small platforms and cost-effective measures to reduce interference" which was presented at the Nordic HF Conference in 2o13 where many of the questions were addressed.

    Finally, recall these almost 45 year old spectrum plots?

    There certainly is a profound difference in HF spectrum occupancy levels between now and "yesteryear".

    upload_2020-12-25_16-37-20.png

    73/
    Karl-Arne
    SM0AOM
     
    KA9P, N0TZU, KO4LZ and 2 others like this.
  10. N1UL

    N1UL QRZ Lifetime Member #303 Platinum Subscriber Life Member QRZ Page

    Good morning from the USA, I remember this paper very well. This was measured in the USA and it’s equivalent in Europe was about 15 to 20 dB louder, European Broadcast Stations and “Propaganda “ stations including the Voice of America. Most ham radio stations (RX) on 7 MHz in Europe were totally overloaded and I introduced the PIN diode Attenuator and replaced the Si switching diodes.
    To a degree i am afraid that when the sun wakes up again, the A/D converters of the SDRs will be totally overwhelmed without an adaptive PIN diode based attenuator and it’s AGC system. This complex system will not be trivial and expensive.
    My active antenna saw already 60 dBuV around 9 MHz. European broadcast stations.
    There is more to a good receiver then selectivity and sensitivity!
    73 de Ulrich
     
    N0TZU, KR3DX, NR5TX and 8 others like this.
  11. SM0AOM

    SM0AOM Ham Member QRZ Page

    Yes, the unwary will be in for an unpleasant surprise about SDR performance if and when HF spectrum occupancy rises up to former levels, when you could easily measure over one volt RF out of a dipole resonant on 7 MHz.

    In these days it took an EK085 with the FK101 in front or an E1800 to properly handle the massive signal levels from the Continent.

    To quote an R&S application engineer at a seminar about level measuring receivers some years ago:

    "Nothing, I repeat nothing, can replace RF selectivity and level setting..."

    73/
    Karl-Arne
    SM0AOM
     
    KO4LZ and KR3DX like this.
  12. N1UL

    N1UL QRZ Lifetime Member #303 Platinum Subscriber Life Member QRZ Page

    That is both correct and expensive.....
     
    WA2LXB likes this.
  13. PY2RAF

    PY2RAF Ham Member QRZ Page

    Interesting colloquium.

    A question from a non-initiated though curious-

    Isn’t it what the initial band-pass filters and roofing filters (plus selectable attenuators, if needed) are for? To counteract this effect?

    During my curiosity expeditions, I found that the great successful sdr du jour, the ic7300 has an incredible quantity (almost 20) of input BPFs - and that’s it; no further roofing and from there, it is fed to the ADC and digitally treated. But again, lots of filters for specific chunks of spectrum.

    The ft991a, a superhet, on the other hand has 5 or 6 BPFs rather wide - and following suit the signal is greeted by 15 or 3 kHz SAW roofing filters.

    Now comes the question; considering those two designs; is it still of concern by having nearband broadcasts? Wouldn’t the BPF or BPF plus roofing enough to defend from these nearby signals?

    Curious.

    Cheers - and Happy holidays.
     
  14. SM0AOM

    SM0AOM Ham Member QRZ Page

    This is actually the "$64000 question" in receiver system design.

    The direct sampling SDR is somewhat handicapped as they will be subjected to all signals at their input that may pass through the front-end filter. These signals, if strong enough, will consume the available dynamic range, and if just one dominant signal in the BPF passband should be strong enough to use up all the available range, "bad things happen" all at once.

    The superhet design uses more distributed selectivity, where you usually have some RF filtering before the mixer,
    then a first IF filter, all with progressively narrower bandwidths, which reduces the number of signals along the signal path.
    A consequence is that the degradation when unwanted signals become stronger is more gradual.

    However, there will be two crucial components in the signal path before the first IF filter; the mixer and conversion oscillator.
    The signal handling properties of the mixer together with the sideband noise properties of the oscillator will determine the effective out-of-band selectivity properties of the whole receiver. The close-in sideband noise aspects are much less prevalent in a direct-sampling SDR than in a superhet design.

    If it was possible to create a sufficiently narrow RF preselector that only let the necessary bandwidth of the received signals through, and nothing else, the multiple-signal handling properties of the downstream receiver parts would be quite moot.
    Even the most mediocre SDR or analogue receiver would be dramatically improved by such a preselector.

    So, it always pays having RF selectivity, but the improvement is depending of what is downstream from the RF filter.

    If you have lots of dynamic range still present in the downstream parts of the superhet receiver after the first filters, you may get along with fewer BPF filter ranges.

    The above is in essence what I tell clients when I get the question:
    "Should we go the direct-sampling or the hybrid SDR route?".

    This can be expanded a lot by real receiver connoisseurs but I leave this to Dr. Rohde.

    73/
    Karl-Arne
    SM0AOM
     
    Last edited: Dec 25, 2020
    S58DX, KR3DX, WZ4K and 3 others like this.
  15. W2VW

    W2VW Ham Member QRZ Page

    Surplus beer brewing container resonant cavity anyone?
     

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