Trying TV signal splitter for antenna measurements

I did some antenna measurement with RTL-DSR with help of directional coupler. Before I realised I had directional coupler, I did some experimenting with antenna signal splitter.

RF splitters are widely used in RF applications. The enable RF power to be split or combined within an environment where the characteristic impedance is maintained.

There are two broad categories of RF proper splitters and one improper:

Resistive power splitters:  These power splitters and combiners use resistors. The 6 dB three-port splitter has (ideally) 6.02 dB loss from any one port to any other port. The isolation of a resistive splitter is equal to its insertion loss.

Hybrid power splitters:   Hybrid splitters use transformers and are able to provide low levels of loss. A typical signal splitter that split signal from one port to two ports has 3-4 dB of signal loss. The amount of isolation will depend upon the impedance termination at the ports (typically from 6 dB to 30 dB).

T-piece: BNC Ts are not splitters. Power does NOT split evenly at a T connector and impedance matching does not work (part of input signal gets reflected back). Actually it COULD be made to work if the coax lengths are cut just right (would work well in this case just at one frequency), but that likely isn’t the case.

Typical TV signal splitters are designed to equally divide the signals on the input port of the splitter to each of the output ports. A two way TV signal splitter will has approximately 3.5 – 4 dB of loss on each port. Good splitters promise up to around 20 dB of isolation between output ports (bad ones can use resistive power splitter or even worse ones are just direct T piece with no impedance matching). TV antenna splitters are designed to work with 75 ohms impedance level.

Here is a good TV antenna signal splitter I used:

Theoretically the signal splitter should provide around 20 dB of signal output port to another when the “input” port is terminated with nominal impedance (75 ohms). When the impedance on the “input” port of splitter is other than right one (more or less than 75 ohms), there will less signal attenuation from one output port to another, meaning higher signal level gets through from port to another. All the signal picked buy the antenna and all the possible signal reflections from antenna will be split to splitter outputs evenly (3-4 dB attenuation).

The antenna splitter I used was Biltema 24-022 (no longer sold there, they have some newer similar looking models though). Here is data from the device:

Frequency range: 0-1000 MHz

Insertion loss: 3.5 dB

Isolation: 22 dB

In my case I connected the antenna tube measured (nominally 50 ohms) to “input” port, signal generator (50 ohms from gray cable) to one splitter output and other splitter output connected to RTL-SDR (approximately 75 ohms). I expect to get maybe somewhat lower than ideal signal attenuation, but still something that could be used for something. Let’s make some measurements to see how things work.

Results 50 ohms
2017-09-10-4

Results qith @450 antenna:
2017-09-10-3

The end results from the test is that the antenna splitter worked so that I could see antenna impedance variations over frequency. But the results I got was worse than what I got with my antenna measurements with directional coupler. So I will stick with using directional coupler. But I know that I might be able to use this kind of antenna splitter in situations where I don’t have directional couplers in hand.

6 Comments

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  2. Tomi Engdahl says:

    RF Isolator: Teardown and Experiments
    https://www.youtube.com/watch?v=QtqJU9UjAzM

    In this video, I took apart a 8 to 10 GHz microwave RF isolator and did some measurements. High resolution teardown pictures at

    http://www.kerrywong.com/2016/03/01/teardown-of-an-8-10-ghz-rf-isolator/

    Reply
  3. Tomi Engdahl says:

    Why You Need Circulators in Repeaters
    https://www.youtube.com/watch?v=5kwb1zCZ0KU

    A discussion of circulators and a teardown of a 915MHz circulator.

    RF Isolator Teardown & Explanation
    https://www.youtube.com/watch?v=Whn3QjHUgIw

    Ever wonder what’s inside an RF Isolator?

    Reply
  4. Tomi Engdahl says:

    HP 778D Dual Directional Coupler – Tear Down
    https://www.youtube.com/watch?v=r-OS3BlRWFs

    In this video I take a look at a 778D Dual Directional Coupler that was part of a lot I won at auction – I really wanted 1 but ended up with 6 – 3 of which didn’t pass manufacturer perf test – So I thought I’d take it apart and have a look inside.

    Reply
  5. Tomi Engdahl says:

    #158: Directional Coupler Basics & how to sweep SWR of an antenna | Return Loss | VSWR
    https://www.youtube.com/watch?v=iBK9ZIx9YaY

    This video describes the basic properties and specifications for directional couplers, and shows their basic operation on an oscilloscope. Typical applications are discussed, along with a practical example showing how to sweep the VSWR, or Return Loss, of an antenna using a directional coupler and a spectrum analyzer with a tracking generator. The resulting return loss measurement is compared to the SWR measured on an MFJ-259B antenna analyzer.

    Reply
  6. Tomi Engdahl says:

    #208: Visualizing RF Standing Waves on Transmission Lines
    https://www.youtube.com/watch?v=M1PgCOTDjvI

    This video illustrates how RF (radio frequency) standing waves are created in transmission lines – through the addition of the forward (transmitted) wave and the reflected wave that results from improperly terminating the line or matching the load or antenna to the transmission line impedance.

    Reply

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