# A Wideband impedance transformer/converter. How does it work?

#### Vilius_Zalenas

Joined Jul 24, 2022
156
Hi,

Lately I got interested in 75 Ohm antennas and wanted to play around with it on my VNA. After a short research on the internet I saw these little impedance transformers/converters.

https://www.ebay.ca/itm/255651004196

From my classes I remember that the most popular impedance transformer type is a quarter wave one. But it only works for a given frequency...
This impedance converter is said to work DC-2GHz and it doesnt seem to pack a whole lot of advanced RF circuitry inside. I am asking this just for the curiosity, does anyone have an idea how this transformer could work in such a wide range of frequencies? Thank you in advance

#### crutschow

Joined Mar 14, 2008
33,355
how this transformer could work in such a wide range of frequencies?
It's not an inductive type transformer, it's just a resistive pad circuit to convert a 50Ω coax (cable) to a 75Ω coax (cable).
That's why it can go down to DC.
Since they are resistive, they do introduce a power loss in the circuit for the transformation, (according to that reference, it can be up to 5.7dB) so you must allow for that.
For lower loss you would need to use an inductive transformer type converter which, of course, don't go down to DC.

#### Vilius_Zalenas

Joined Jul 24, 2022
156
It's not an inductive type transformer, it's just a resistive pad circuit to convert a 50Ω coax (cable) to a 75Ω coax (cable).
That's why it can go down to DC.
Since they are resistive, they do introduce a power loss in the circuit for the transformation, (according to that reference, it can be up to 5.7dB) so you must allow for that.
For lower loss you would need to use an inductive transformer type converter which, of course, don't go down to DC.
Thats really interesting but you left me with more questions this time... If this transformer is purely resistive, what is the point of it? I was thinking that things like these are used to calibrate VNA to a 75 ohm systems: open and short calibrations are made the same the way, but for load calibration you take a 75 ohm terminator, connect it to this 75/50 ohm transformer and plug it to the VNA... (I used to think like that) But if this impedance converter has nothing to do with the coax impedance itself, what is the point of it?

#### crutschow

Joined Mar 14, 2008
33,355
But if this impedance converter has nothing to do with the coax impedance itself, what is the point of it?
But it does.
The reason for the converter is to match the coax cable impedance to prevent mismatch reflections in the cable.

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#### MisterBill2

Joined Jan 23, 2018
16,569
A non-resonant transformer should be able to adjust impedance matches over some frequency range without excessive losses. The range will be determined by the loss, really.At one extreme the magnetic saturation will be the limit and at another extreme the coil inductance will be the limiting factor.