# Bias Tee Range

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by G0VKT, Jun 29, 2020. 1. Hello,

I made an accidental discovery last night which leads me to think that I would benefit from some form of DC block on the input of my RSP1a SDR. This lead me on to reading about bias tees. I am struggling to understand how they work over such a wide frequency range? The one built in to the RSP1a covers 6KHz to 2GHz.

Thanks,

Paul G0VKT

2. To calculate the required C for dc blocking, you have to know the lowest frequency you want to pass, and the input resistance R of the device. Make the reactance of C equal to R at the lowest freq of interest, or a bit below that frequency...

1/(2*pi*f*C) = R

solve for C:

C = 1/(2*pi*f*R)  Last edited: Jun 29, 2020
2E0CIT likes this.
3. Many thanks for that. I will do some maths. A question, R2 is the antenna Z. R1 is the Rx Z. Can I assume the Rx input is 50R?

4. Lacking any information to the contrary, that is what I would assume...

I assumed R1 is the source impedance (antenna) and R2 is the load impedance (Rx)

5. Because modern "surface mount" components are so tiny, they tend to act like theoretical , pure parts.

A old fashioned disc ceramic capacitor like this one: By virtue of the wire (inductors!) Leads acts as a capacitor less and less up to its "self resonant frequency", above that frequency it actually acts like a inductor in the circuit.

When it comes to broad frequency response of something like a bias t, (small) size matters Rege

6. I used to work in the component industry. When I started the most common sizes were 1206/1210 and moving to 0805 and 0603. I remember the introduction of 0402 and all the production issues that had to be solved (tombstoning). Towards the end of my time 0201 was coming into use. I even saw samples of 01005. I don't know if they ever went into production.

Paul