While browsing some miscellaneous RF test equipment supplies I stumbled across a two-port component sold as a "feed-thru load".

Take for example the Pasternak PE6026 (datasheet), which looks physically like an attenuator (a slightly extended barrel with an "SMA male to female coaxial interface") and is rated like an attenuator (e.g "handles up to 2 Watts") but does not specify any actual attenuation (e.g. in dB) and is described instead as a "terminator" or "load":

Pasternack’s PE6026 is an RF feed-thru termination (also called RF feed-thru load) that operates from DC to 1,000 MHz and handles up to 2 Watts (CW). This feed-thru SMA load is designed to be used with test equipment, like oscilloscopes, to match the equipment to the 50 Ohm device under test (DUT).

How can something be both a "terminator" and also "feed-thru"?


I think the correct way to understand this is basically as "a dummy load which makes it easy to measure the voltage across itself".

Likely as simple as:

a 50ohm resistor in parallel with both a DUT signal source and a scope/analyzer

simulate this circuit – Schematic created using CircuitLab

That is, for a feed-through terminator, the input port provides a characteristic impedance load assuming the output port is connected to a load of (ideally) infinite impedance. So the terminator provides the whole load and only a ± open circuit (i.e. a non-load) is meant to be "fed" by it — power in, voltage [only] out.

As the quote in my question hints at, the idea is that this lets you connect a device-u'wanna-test (DUT) which is expecting a 50Ω load to something like a high-impedance oscilloscope input without a mismatch.

Whereas with an attenuator there's a more complicated resistor network which provides a load assuming its output is also a load of characteristic impedance. So the load is shared between the attenuator and what it feeds — power in, power [reduced] out.

  • 1
    $\begingroup$ This is correct. In particular, they're frequently used with oscilloscopes, which conventionally have 1 MΩ input impedance (though usually on a BNC connector, not SMA). [feel free to / please do add this to your answer] $\endgroup$
    – Kevin Reid AG6YO
    Dec 3 '21 at 23:06
  • $\begingroup$ @KevinReidAG6YO Thanks for the confirmation. The note about oscilloscope usage is kind of already in the question — though admittedly that's my fault… the downside of figuring out an answer while typing up my own question :-) $\endgroup$ Dec 3 '21 at 23:09
  • $\begingroup$ Sounds like you're saying, that a feed through test load, is designed to be monitored in parallel by a high impedance device like a scope ?? I've never heard of them before, thanks for the post. $\endgroup$
    – wbg
    Jan 13 at 17:31
  • $\begingroup$ @wbg Yep, that's my understanding. $\endgroup$ Jan 17 at 1:55

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