Bias Tee Range

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

1. G0VKTHam MemberQRZ Page

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. WA7ARKHam MemberQRZ Page

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. G0VKTHam MemberQRZ Page

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. WA7ARKHam MemberQRZ Page

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. AI3VHam MemberQRZ Page

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. G0VKTHam MemberQRZ Page

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