RF Signal Sampling for Spectrum Analyzer

Discussion in 'General Technical Questions and Answers' started by KI7LFD, Feb 14, 2020 at 3:34 AM.

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  1. KI7LFD

    KI7LFD Ham Member QRZ Page

    I have access to a Spectrum Analyzer for evaluating the output of my radio system. I'd like to look at the transmitted signal coming out of my 9300 and my KPA500. The outputs of these two is 100 watts and 500 watts respectively. I want to ensure that any digital mode I emit is clean and crisp..

    Having said that, what type/brand/model # , RF sampler can I buy that will allow me to sample these outputs without hurting the spectrum analyzer ?

    Thanks in advance for the help.

    Rich KI7LFD
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I've built my own RF samplers, but I've also purchased some in the past. I took one of these and added a 20 dB 50 ohm resistive pad(built with 2W resistors) to its output. That yields a total of 50 dB of sample port isolation which is very safe from the standpoint of monitoring kilowatt or even legal limit signals with a spectrum analyzer that can handle +30 dBm input signals. You could build in a bigger pad if your spectrum analyzer is limited to lower level signals or if you want additional safety margin.

    N2HUN likes this.
  3. KF5LJW

    KF5LJW Ham Member QRZ Page

    An antenna at the input, a very poor one at that. Otherwise you would need an expensive 40 db sniffer, and all that buys you is some measurement reference. But honestly if the analyzer is at the site, the antenna the analyzer came with is more than adequate. IMO even a better real world signal because the antenna has no influence on the TX antenna circuit and you see what is really going out. At that power level a mile away is close enough.

    You would likely find just a small loop of wire with a 2 inch diameter is all the antenna you need at site.
    Last edited: Feb 14, 2020 at 4:06 AM
    AA4MY and W6KCS like this.
  4. SM0AOM

    SM0AOM Ham Member QRZ Page

    There are many ways of doing this.
    One "plug and play" way if you have a Bird wattmeter is to get the RF sampler element:
    Be aware that such a sampler does not provide a constant coupling factor in the HF range, so measurements of any harmonics and VHF spurious emission suppression will not be accurate.

    Another way is to get a directional wattmeter with a sampling port in the coupler assembly. Aim for a coupling factor in the HF range of 50 to 60 dB.

    What digital modes are you using?

    It takes a very good conventional spectrum analyser to actually measure the signal quality of current digital emissions.

    "Real-time" or SDR based analysers are better, but are still quite uncommon.

  5. N2HUN

    N2HUN Ham Member QRZ Page

    I've seen a YouTube video by a knowledgeable ham who used an antenna tuner's unused antenna port (e.g. "Coax 2") to sample an xmitter's signal. I tried it with my MFJ-969 and it works.

    Just be careful and use the least amount of xmit power and if necessary attenuators so you don't wreck the SA.
  6. N3DT

    N3DT Ham Member QRZ Page

    Unless you're looking for a reference value from the transmitter, all you need is a wire in the air, like LJW says. Why spend $300 on some rf sniffer? You're just looking for a signal and the corresponding spurs if there are any. Longer wire, more signal, shorter wire, less signal.
    WA7PRC likes this.
  7. KF5LJW

    KF5LJW Ham Member QRZ Page

    Exactly and that sniffer unless you know what you are doing can burn out the front end of the SA. All you need is a very small sample and just a 1-inch piece of scrap wire stuck in the Antenna port of the SA is all you need. Even that may over load the front end if you are not careful.
    AA4MY and W1BR like this.
  8. K6BRN

    K6BRN XML Subscriber QRZ Page

    Rich (KI7LFD):

    First, be careful not to blow up the borrowed spectrum analyzer. Unlike oscilloscopes, spectrum analyzers are designed to measure very low signal levels and the input to any spectrum analyzer should usually be kept below 0 dBm (1 mW). 20 dBm (100 mw) is often enough to damage or destroy some analyzers.

    There are three choices for coupling large signals into a spectrum analyzer, in order of effectiveness and (to a degree) cost:

    1. BEST: A calibrated high power RF attenuator chain (usual lab method for measurements)

    2. SECOND BEST: RF feed-though coupler, which is generally the equivalent of a few turns of wire around the insulated center conductor of a coax after the shield has been removed - though they can be significantly more sophisticated.

    3. DISTANT THIRD: An "RF probe" ("sniffer") or stub antenna

    The first method provides the best, closed RF system for accurate and precise measurement as the attenuators have flat frequency response across a broad range and the closed nature of the RF measurement system prevents environmental RF not related to the device under test from distorting measurments.

    The downloadable .pdf "ARRL Procedure Manual" (Google: "ARRL transmitter output power test") provides details on how to do this and lists the equipment required. In general, you will need a high power -30db attenuator, such as a Bird 8325, plus 1 watt -20 db and -30 db attenuators from Narda or other reliable makers. New, these are all quite expensive. But ebay offers a lower cost alternative for used attenuators.

    The second method relies on tapping a small amount of RF directly from the feedline. The feedline should be terminated into a dummy load to prevent environmental RF that an antenna might pick up from corrupting the measurement. This method has two disadvantages. First, the degree of coupling varies over frequency, distorting measurement results to a degree. Second - for any given coupler at any given frequency, the attenuation can vary quite a bit - any listed attenuation is approximate and you need to determine how much added attenuation is needed before the spectrum analyzer to prevent damage. Couplers are usually in the range of -20 dB to - 40 dB. You'll need about -50 to -60 dB total attenuation between your 500 watt KPA500 and spectrum analyzer. In general, to avoid spectrum analyzer damage, a 10 to 30 dB attenuator SHOULD be placed in line with the coupler, starting with the higher value first and moving to lower attenuation values as possible, AFTER coupler attenuation is determined.

    As others have noted above - you can MAKE your own coupler: http://om3lz.com/directional-coupler.html

    ... or buy one, for example: https://www.hamradio.com/detail.cfm?pid=H0-015644

    If you do buy one, make sure its a high power coupler for the right frequency range. The examples above are dual directional HF band couplers - you really only need a single coupler, but those are a bit harder to find - yet they are even simpler to build.

    The third method, using a stub antenna ("sniffer") to sample transmitted RF, is good for curiosity purposes and general experimentation, but because the stub antenna will pick up a lot of environmental noise, and because having it too close to the transmitter CAN damage or destroy the spectrum analyzer front end (RF level will vary a LOT and is basically uncontrolled), it's not a very good or reliable method for determining transmitter cleanliness. It's used when there is no other practical choice and when the user has already surveyed the ambient environment for signals that can distort the readings.

    The good news is that you can use an N connector to BNC or SMA adapter to connect a short non-resonant scanner whip antenna to the spectrum anayzer input and you are ready to go. Still recommend a -10 dB attenuator in line with the whip to start with, just in case the signal is stronger than expected.

    An inexpensive non-resonant "sniffer" antenna can be found here:

    https://www.martinrfsupply.com/bnc-telescoping-antenna-rf-sniffer-in-stock-br-7610-fs-element-7601-fs-meter.aspx feed=Froogle&gclid=EAIaIQobChMI8Zygoq3R5wIVicpkCh2WCAh5EAQYAiABEgKYtvD_BwE

    .... or just use a length of wire, as noted by others above. But... remember to treat the spectrum analyzer gently in all respects. The input IS static sensitive, too. They are NOT anywhere near as robust as ocilloscopes.

    So - once you've got a spectral plot to look at ... do you know how to interpret it? It takes a little knowledge and practice to do this well. Otherwise you may interpret a few unimportant artifacts, or even the wavefore spectrum, as a problem. A local mentor with a background in comms systems would be very helpful in this respect.

    Best Regards,

    Brian - K6BRN
    Last edited: Feb 14, 2020 at 3:46 PM
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  9. N2HUN

    N2HUN Ham Member QRZ Page

    I've also wrapped the coax from the xmitter with several turns of copper wire and attached one end of the wire to an o'scope to get a look at my signal. That's one of the easiest ways.
  10. AI3V

    AI3V Ham Member QRZ Page

    You are in way over your head.

    First: what is the maximum signal level your analizer can handle without adding its own distortion?

    Second: exactly what is "clean and crisp?" That sounds like something you would tell a dry cleaner you want your shirt to be.


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