I want to build (not purchase) a wide bandwidth power combiner.
It should combine signals from 2 TFD antennas cut for 15 MHz lower frequency and used for receive only. Upper frequency is 30 MHz so desired bandwidth is 15 MHz.

Each antenna has its own balun and the output coax cables are 50 ohm. Output from the combiner must be 50 ohm as well.

I remember a design based on a toroid transformer and I would like to:
- get the circuit schematic of this design
- get the formula to calculate the inductance and turns to wind on a T 50-6 toroid that I already have


  • 1
    $\begingroup$ Why combine two antennas, what are you trying to achieve? Their patterns will interact and produce a directional pattern, also their near fields will interact. Even if fed from a perfect isolated splitter, the patterns won't be what you expect unless you have considered the coupling. $\endgroup$ – tomnexus Jul 1 '17 at 7:29
  • $\begingroup$ @MarioCannistra, have you modeled this to see what the resulting pattern will be? That's good advice from Tom Nexus (and Phil Frost). $\endgroup$ – Mike Waters Jul 1 '17 at 18:31
  • $\begingroup$ Sorry for the late answer, busy days... I'm working at my second amateur radio astronomy project. First one was based on the Radio Jove single dipole. Currently at phase 1 of 3 of the second project, based on a TFD antenna + SDR receiver. Phase 2 will use 2 TFD antennas crossed, each one feeding a separate SDR receiver. Should be able to record separately LCP and RCP emissions from Jupiter/Io. Phase 3 will use 4 TFD placed along a square seen from the top. Two for rcp on horizontal sides and 2 for lcp on vertical sides. $\endgroup$ – MarioCannistra Jul 1 '17 at 19:15
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    $\begingroup$ I'm very open to any suggestion, feedback since i'm learning all of this. I did not model these antennas just because i'm following an approach that already showed to be working. It has been published by Dave Typinski. Some info here: radiojove.gsfc.nasa.gov/library/newsletters/2013Nov/#4 $\endgroup$ – MarioCannistra Jul 1 '17 at 19:17
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    $\begingroup$ Now I can't find the link but Dave has modeled the dipoles for directionality in order to optimize for the planned usage in radio astronomy context and also to have some more signal coming out of the antennas compared to simple or double dipole used by the original radio jove project $\endgroup$ – MarioCannistra Jul 1 '17 at 19:21

You're probably thinking of a single transformer hybrid.


simulate this circuit – Schematic created using CircuitLab

The resistor should be half the system impedance, so 25Ω for a 50Ω system. I'll assume a 50Ω system for the rest of the description.

P1 sees the loads on P2 and P3 in series. If we assume all loads are 50Ω, P2 and P3 in series are 100Ω. Setting the turns ratio with 10 on the P1 side and 14 with a center tap on the other side (with a center tap) makes makes a 1:2 step-down transformer (because $10^2 / 14^2 \approx 1/2$). Thus, P1 sees a matched 50Ω.

P2 sees the matched load at P1 but through a 1:2 step-down transformer (25Ω) in series with another (25Ω) to see a net 50Ω matched load. Thus, signal from P2 is equally divided between being across P1 and the resistor. P3 sees this voltage across the resistor, but it also sees the voltage across P1 in antiphase. These two cancel such that a signal applied at P2 does not appear at P3.

The cost of this isolation is a loss of half the signal power from P2 or P3. For a receive application this may be negligible provided the receiver noise floor is significantly below the (additionally attenuated) received RF noise floor.

Absent a datasheet, I usually look at torroids.info for data, or find a solution empirically.

  • $\begingroup$ Hello Phil. Thank you very much for your kind answer. I will certainly build and test this design. Kind regards, Mario $\endgroup$ – MarioCannistra Jun 30 '17 at 9:22

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