Need to Measure Transmitter Spurs, w/o Spectrum Analyzer

Discussion in 'General Technical Questions and Answers' started by KK5JY, Oct 19, 2017.

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  1. N1OOQ

    N1OOQ Ham Member QRZ Page

    Unfortunately, no pix of the guts. Nothing special though. The boxes are made of folded up aluminum flashing material. On the inside is a simple Pi attenuator, grounded to a lug under each BNC connector.
    The inside of the step attenuator looks something like VK2TIL's... lots of foil shields and solder. Somehow, it never occurred to me to put it on the network analyzer. :rolleyes: I'll do that at some point. That thing was a bit of work, even for its lack of beauty...not sure I'd bother with all that again.
    KK5JY likes this.
  2. KK5JY

    KK5JY Ham Member QRZ Page

    That's a really good idea. I had never thought of that as an enclosure material. I have tons of that stuff just laying around, too. Thanks for the tip. ;)
  3. N1OOQ

    N1OOQ Ham Member QRZ Page

    In that small size, it becomes relatively rigid once assembled. :) Larger boxes might be pretty floppy.
    KK5JY likes this.
  4. VK2TIL

    VK2TIL Ham Member QRZ Page

    I agree with 'OOQ; I put a lot of work into my attenuator and it was an interesting project but I'm not sure if I would do it again. Here in VK, the second-hand test gear market is small so I really did have to build something; you have much more used test equipment in the US so buying is an option.

    PCB material is another option for small cases; it's easy to cut, file & solder.

    Its shielding properties are not absolutely ideal but it's adequate for amateur purposes.

    I've built fixed attenuators into equipment but never had to build a "cased" one as I have accumulated some good-quality ones from hamfests & ebay over some years.

    Whilst the asking price on ebay is usually astonishing, some digging produces this kind of thing;

    This seller has 10dB & 20dB models as well. Not "name" brands but almost certainly useful at HF & VHF.

    These appear identical to some sold in VK but now no longer stocked by Jaycar;

    The seller has other values.

    I have some; I swept them and they were good to 500 MHz; they must have an internal blocking capacitor as they were no good below about 5 MHz.

    If you want to build, a small PCB could be easily made by cutting with a knife and peeling-away unwanted copper; I do this quite often.

    A track about 1/8" wide on 1/16" FR-4 gives a 50-ohm line; three SMD resistors (sets are available cheaply on ebay), one in-line and two going to an adjacent ground plane, make a good attenuator.

    On-line calculators, eg;

    take-away much tedium.


    might save some work; for about US$5 you get a PCB and two BNCs. Change the resistors to the values you require and wrap it in some copper or kitchen foil.

    It won't be Space-Shuttle quality but should be fine for HF and into VHF.
    KK5JY likes this.
  5. K6CLS

    K6CLS Ham Member QRZ Page

    WA7PRC and KK5JY like this.
  6. N1OOQ

    N1OOQ Ham Member QRZ Page

    About that... I played around with a commercial -20 dB fixed attenutor a little today. I found that I could get anything from -19.2 to -20.1 depending on 'scope settings, on any one particular frequency. The scope is spec'd +/- 3%, so that's about right. (NIST calibrated one year ago). I checked the specs on the attenuator and it's +/- 1.5 dB, depending on how high the frequency.
    The point being, excessive fuss about accurracy is, well, pointless.
    WA7PRC, KK5JY and AI3V like this.
  7. VK2TIL

    VK2TIL Ham Member QRZ Page

    The point being, excessive fuss about accuracy is, well, pointless.

    Agree 100%.

    Our multimeters display voltage etc to two decimal places; that is probably quite accurate in a good meter.

    When we come to measure RF in decibels, we carry that mindset with us but 0.01dB in those circumstances is an infinitesimal amount and only places like NIST should be talking in those terms.

    Here are the published specs for Microlab/FXR AA-25 series attenuators;


    These are high-quality products from a top-line manufacturer, made for general-purpose lab/experimental use; notwithstanding the shape of the above curves, I would be proud to have some of these in my attenuator box.
    AI3V, WA7PRC and KK5JY like this.
  8. KK5JY

    KK5JY Ham Member QRZ Page

    That's a very nice module. I like it. E-craft did a lot of neat kits back in the day.

    That's a very good point.
  9. KD0CAC

    KD0CAC Ham Member QRZ Page

    Last edited: Oct 26, 2017
  10. G0HZU

    G0HZU QRZ Member

    I agree that for this task the accuracy doesn't have to be great but those Microlab/FXR AA-25 series (fixed) attenuators look a bit grim to me if you wanted attenuators for general use beyond this initial task.

    They are rated to 2W+ which kind of makes up for the poor performance a bit, but the attenuation accuracy vs frequency looks so poor I'm wondering if the spec sheet graph is wrong.

    The classic approach for this type of thing is to buy a fixed 30dB attenuator rated to >5 times the power you intend to use. eg for 5W buy a 25-30W 30dB attenuator. You can buy these used on ebay quite cheaply but there is a real risk the attenuator will have been abused at some point and it may prove to have erratic performance. But with patience these attenuators (eg Weinschel) often pop up on US ebay for about $60 new for a 30-50W 30dB attenuator rated to several GHz. I'm not sure if they are factory rejects or if they are being sold out of the back door of a factory somewhere but I've bought several on US ebay and they have all been brand new and sealed with a cal sheet. They were all in spec when tested.

    But if you buy one that has obviously seen some use (worn/scratched paint and generally scruffy) then there's a good chance it will prove to have some problems. eg erratic attenuation and VSWR when hot or when handled. At work we have to dispose of a lot of attenuators like this because they don't work for very long if they are used heavily. eg for soak tests or environmental tests. So buying used 'power' attenuators on ebay is going to be a big risk.
    Once the signal is down by 30dB you can make your own low power fixed attenuators using chip resistors and these will be good to >1GHz if built with some care. So this will extend the initial 30dB by another 20 or 40dB as needed.
    Last edited: Oct 26, 2017

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