Making a Variable RF Signal Sampler

One of [Brian]’s hobbies is Amateur Ham radio, in which it is usually required to check that the transmitted signals are within specifications. As it isn’t safe to connect the radio’s output directly to measuring equipment due to the high voltages involved, [Brian] made his own dedicated RF signal sampler. It works by using capacitive coupling between the signal you wish to sample and a high impedance output. The latter can then safely be connected to an oscilloscope or spectrum analyzer for monitoring.

In the picture you see above, the air gap between the core signal conductor and the output plays the role of a capacitor. By adjusting its length you can therefore vary the output signal’s voltage range. The sampler is built using a die-cast aluminium enclosure which is 52x38x27mm. As you may have guessed, due to the case geometry the output attenuation will depend on the signal’s frequency. [Brian] tested the unit using a 30MHz signal generator and printed this frequency attenuation graph while also varying the air gap.

18 thoughts on “Making a Variable RF Signal Sampler

  1. The frequency attenuation chart is rubbish. It becomes constant at very low frequencies, where the capacitive coupling becomes so small that it can be neglected. The attenuation chart should show a response rising with frequency propotional to f, until the coupling becomes so high that the input power is split equally between output port and coupling port (3dB loss hence).

      1. Apparently not, and they probably don’t care about a $15,000 fine or jail time, either.

        Maybe I’m wrong, but according to my math a $15 ham radio license is cheaper.

        1. I believe you’re missing the point here, a “$15 ham radio license” doesn’t allow the broadcasting of music, ever, an “FM pirate” is all about broadcasting music and talk and whatever else they want to, a pirate acquiring a Ham radio license can’t legally act as a commercial, non-commercial, or Low Power FM broadcast station.

          See Subpart B—Station Operation Standards
          §97.113 Prohibited transmissions.

      2. Some of them don’t, others do and actually are much more skilled than some licensed HAM operators who own big €/£/$ gear but never used a solder iron in their entire life. A few of them are HAMs themselves operating without their callsign as the HAM license would not allow them to transmit on non HAM assigned frequencies, nor would allow to transmit music or anything non HAM related to non HAM audiences.

        1. Things have gotten much worse. Hams now buy their dipoles ready made and coax with the connectors installed now. They spend big bucks on transceivers, amplifiers and antennas, hook it all up to computers, monitor the internet to find a signal that they can holler 59 (insert state), just to a click on a database to get an award like a dolphin jumping for a fish at Marineland. They can’t send code so they use programs to do it, and quickly go 73 CUL, when you put a little Lake Erie Swing in your signal their computer can’t read. When it all breaks they ship it off to a repair facility. There are a few died in the wool buzzards out there, keeping the old stuff going, aggravating the computer radio rig operators because we are 50 cycles off their TCXO stabilized Japanese creation, with audio that is not sent through high dollar studio equalizers that they buy to make their high dollar POS Heil mic sound good.

          1. True, but some of us start with store bought, then get interested in more, so we learn to repair and build. As for CW, I cannot seem to grasp it. The tones all run together on my, but someday…..

      3. Depends, In the Netherlands the pirates have big parties and transmit advertising, so they have a massive budget. Often more than legal operations even since most people don’t listen to the radio anymore.

        They just buy a commercial FM transmitter so the signal is within spec. The power levels are also ridiculously high, somstimes as much as 40kW for national coverage. I suppose that the probability of a HAM transmitting an out of band signal by accident is higher than a pirate using a professional transmitter installed and setup by the manufacturer.

        They also gladly pay the fine of maximum 40k€, they make alot more than that with a few days of pirating.

  2. Another trick I recently spotted on youtube to accomplish to the same thing was to hook up your device to an unused port of an antenna switch if you by chance happen to have one extra. Theres often enough leakage to get out a couple of 100mV to run to your scope/spectrum anylzer/counter/etc… Of course, you have to remember to never transmit into that port!

  3. This looks promising. If I can find an Al box laying around I’ll try it out. I’m concerned about what looks like a pretty steep roll-off past 30MHz. If we extrapolate, this wouldn’t be that usable at VHF and above.

  4. If you have, say, a TX antenna on the left side of your house roof, place a sensor cable on the right side of the roof. Strip back the shield from coax [1 metre should do]. You can use power meters, VSWR etc. to plot output power vs detected in your band of interest if you want.
    Since you’re checking your signals are “in spec” and assuming you aren’t trying to blast more watts than your license permits, what you really want is sidelobes or frequency “smear”. We’re talking under 30MHz, right? That means mostly sideband and CW. So it doesn’t matter how accurate the sensor is, it just needs to be REASONABLY linear in the small band you are checking for interference.

    1. I typically use a UHF rubber duck antenna (aka: dummy load) when looking at my signals. It seems to do a good job and I don’t have to worry about overloading my scope. Given the distance between antennas and the poor sensitivity of the rubber duck, there is more than enough attenuation to protect the equipment, but not so much that it becomes unusable.

  5. Don’t they make RF couplers and attenuators that are already tuned and work from DC to daylight pretty much? I’m sure with greater power handling comes greater expense, and ham calls for a lot of DIY, but still.

  6. Ah yes the old waveguide below cutoff means of attenuation.
    This works fine for one frequency however, it is nowhere near flat and requires recalibration every time the frequency is changed. A directional coupler on the other hand is easy to build and will provide that both the measuring instruments and the unknown device remain properly terminated. One issue with the above method of signal sampling is the capacitive coupling does effect the characteristic impedance of the system the sampler is placed in. This may or may not be an issue. I do cringe when I see N connectors with a bare buss wire strung between them. On the bright side this should work fine up to about 30MHZ, above that and the sampler will introduce a mismatch into the system for which it is placed.

    At VHF and UHF directional couplers are simple to build using transmission line sections.
    A little skill with hand tools and patients will yield a very high quality coupler.

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