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. G0HZU

    G0HZU QRZ Member

    If at some point you want an accurate step attenuator for RF use then maybe consider the R&S DPU. This is a 2GHz step attenuator rated to about 400mW. These attenuators are very old, very big, very clunky to operate and very heavy but the RF performance is very good indeed. I bought a couple of these for £15 each a few year ago. They are probably as old as me but they work extremely well. I had to strip/service both of them to get the performance reliable up to 2GHz but it didn't take very long. These are precision attenuators with very accurate 1dB and 10dB steps to 140dB. I think they were meant to be used for test equipment proving and they must have been very expensive when new. But if you can live with the size and heavy/clunky controls these are hard to beat for accuracy. They are often available very cheaply but they will need an internal service if they are old as the ones I have here. By internal service I mean strip and clean the metal bores the attenuator slugs move along as they get selected by the cam mechanism. This needs to be clean to maintain the performance up at 1GHz+. If the attenuators have been stored poorly this can lead to some oxidation in the bores. This can cause the slugs to become sticky and erratic when selected. But a good clean/service can sort this out.
     
    Last edited: Oct 26, 2017
    KK5JY likes this.
  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    Seems like a Ham near you could help you make measurements with no need to buy anything.

    Watt a sad state of affairs. :rolleyes:
     
  3. VK2TIL

    VK2TIL Ham Member QRZ Page

    VK2TIL , your link does not work - wrong site ?

    Yes; my apologies. I copied the link from a site where it was described as the NA5N article but didn't check it.

    I'm afraid that searching does not find this very good article; it's disappeared from the 'net.

    I'm glad that I kept a copy.
     
  4. G0HZU

    G0HZU QRZ Member

    I dug out my oldest homebrew 20dB attenuator and this is an attenuator made from scrap bits of PCB and some leaded 1/5W resistors and it uses BNC connectors. This thing is old, I think I made it >25 years ago when the company was pre SMD.
    But the aim was to use select on test resistors (measured with a DMM) to try and get close to 20dB. The aim was to get an attenuator that could be classed as a precision attenuator to 144MHz and to still be useful at 500MHz.
    Across 300kHz to 144MHz it typically measures 19.98dB +/- 0.03dB. It's hard to measure it very accurately on an analyser because it uses BNC connectors. But that's not bad for something old/ugly and homebrew

    It's easily possible to make a version using cheap SMD parts (each resistor costs less than a penny) and SMA connectors that will typically produce 20dB +/- 0.1dB up to 1296MHz and with a VSWR better than 1.15:1. But this will only be rated to maybe 200mW. Most of the stuff I do here at home is up at many GHz so I use commercial attenuators but for operation in the HF/VHF bands there's no reason not to use homebrew for low power attenuators even if you want something with decent accuracy and VSWR.

    The alternative to a high power attenuator is to make a simple broadband coupler but this will limit the bandwidth up to VHF rather than UHF. It will also be harder to calibrate in terms of measuring the coupling factor. But it will be within a dB or two worst case. The above homebrew attenuators can be checked with a DMM at DC and this attenuation at DC will typically hold true (with a fraction of a dB) up into VHF as long as decent resistors are used and the layout is good.
     
    Last edited: Oct 27, 2017
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  5. G0HZU

    G0HZU QRZ Member

    [​IMG]

    I recommend you check out the Minicircuits BW-N series of 5W attenuators. These only cost about $55 and they are spec'd to 18GHz.
    https://www.minicircuits.com/WebStore/dashboard.html?model=BW-N20W5+

    I've only ever used the 20dB version but I used this on a project at work that needed a decent 20dB attenuator across 5-10GHz. Some might say that these attenuators are OTT for home use but they only cost $55 and they have very good 'typical' performance across a huge frequency range.

    https://ww2.minicircuits.com/pages/s-params/BW-N20W5+_VIEW.pdf

    I wouldn't recommend using these at 5W even though they are spec'd at 5W but they will be fine at 1 or 2W I think. I was really surprised at how well these attenuators still worked at 10GHz and how cheap they were. I tried to get a price for the Microlab FXR AA- attenuators and they seemed to be over $100 each which seems crazy for such poor performance. But maybe this was for a different version of this attenuator?
     
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  6. KK5JY

    KK5JY Ham Member QRZ Page

    I'm not sure whom you were replying to, but around here, there are no such people. This is Oklahoma, not San Jose. ;)
     
  7. KA9JLM

    KA9JLM Ham Member QRZ Page

    Just get on the air. You will find plenty of operators with fancy radios that can tell you that you are 1 cycle off.

    They hate my Collins. :confused:

    Have Fun, It is all good. :)
     
    Last edited: Oct 27, 2017
  8. KK5JY

    KK5JY Ham Member QRZ Page

    You're probably right. Nobody would notice. And it's QRP, so by the time I wash it through a 50W commercial amp, and even better, my ACOM with the huge PI-L network on the output, there won't be many spurs left standing. :)

    Then again, I have this overwhelming urge to follow the rules... and a slightly less powerful desire to understand how to do things myself. After all, if my only goal was to have a 5W radio, I have an 817 kit in a Storm case in the closet. :cool:
     
  9. N1OOQ

    N1OOQ Ham Member QRZ Page

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  10. VK2TIL

    VK2TIL Ham Member QRZ Page

    Makis SV1AFN is making and selling some interesting things; he has this;

    https://www.sv1afn.com/rfattenuator.html

    The specs, after only a quick glance at the data sheet, look pretty good; about US$30 posted.
     
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