Dr. Ulrich Rohde: Electrically Short Antennas

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

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  1. 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..."

    KO4LZ and KR3DX like this.
  2. N1UL

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

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

    PY2RAF XML Subscriber 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?


    Cheers - and Happy holidays.
  4. 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.

    Last edited: Dec 25, 2020
    S58DX, KR3DX, WZ4K and 3 others like this.
  5. W2VW

    W2VW XML Subscriber QRZ Page

    Surplus beer brewing container resonant cavity anyone?
  6. N1UL

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

    Thanks for asking, you opened a Pandora box Most explanation are what I would call lying by omission.

    The conventional receiver with a high first IF of
    45 MHz or so is limited outside the first IF bandwidth by the mixer ( IP3 typically 40 dBm ) and the synthesizer noise. And the 45 MHz Filter IMD, if monolithic pretty bad.
    Th input filters protect against second oder Image products, the third order IMD because of the very close spacing like a few KHz can not be improved, here the performance of mixer, amplifier and oscillator determine all ( In band intermodulation ).
    The high end dual conversion transceiver are pretty much all state of the art and expensive.

    In contrary the fully digital receivers using analog/ digital converters and they integrate power. In 1 Mhz bandwidth one million signals of 1 uV result in 1 V vector added voltage and will totals overload the A/D based receiver while the conventional analog receiver will not be impressed and all will be good.

    This should be a technical discussion by its own
    and many may not like an honest answer.

    The best Version is a hybrid RX with the A/D Filter after first VHF crystal fiter.
    W5ZZ, S58DX, KO4LZ and 4 others like this.
  7. N1UL

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

    You are absolutely correct !

    73 de Ulrich
    KR3DX likes this.
  8. PY2RAF

    PY2RAF XML Subscriber QRZ Page


    Thank Dr. Karl and Dr. Rohde for the enlightenment.

    Kind of a Christmas gift; much appreciated.
    WZ4K and N0TZU like this.
  9. W8AAZ

    W8AAZ Ham Member QRZ Page

    I have experienced "overload" on my IC 7300 at least once, at night, on 40 meters, with all the Chinese broadcasters crashing in, besides hams. Full sized dipole. I don't have any large gain antennas for the ham bands. The issue with 40 meters was alleviated by turning the RF gain just slightly to the left. Turning that, and mic gain down, is a difficult task for many of us!
    S58DX, WD4ELG, KA0HCP and 1 other person like this.
  10. W6RZ

    W6RZ Premium Subscriber QRZ Page

    For many of us, AB4OJ is a pretty well known ham. His rig performance tests are epic.


    W0PV, AJ4GQ and KR3DX like this.

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