I have a double Yagi antenna setup that I used successfully in the past, with both antenna connected directly to a 4G mobile broadband WiFi router, achieving 50-80 Mbps. The antenna have different polarities, one vertical and one horizontal. This was the recommended setup for max performance and stability.

Now I have moved this setup to a remote cabin in the mountains, but with a new addition: a GSM remote starter for my kerosene furnace. This requires a splitter, ref picture. My question is how do I minimize loss of signal?

There are three parts to this: the splitter itself/the splitting of power between two devices and the unused terminals.

Will splitting the signal half the power hitting each device, as in "input signal power"/"number of devices"? I suspect the math/physics to be slightly more complex than this ...

I understand that every additional thing will decrease the power of the signal passing through, but I seem to recollect someone saying that it is wise to terminate the unused terminals, for some reason. How and how much (relatively or absolutely) will an unused and unterminated output affect the signal?



  • \$\begingroup\$ On a 2 way splitter, the 3.5dB loss is reduced to <0.5dB when one port is left open. I'm not sure if that applies to converting a 3 port to a 2 port but I suspect it is. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jul 10 '20 at 17:28
  • \$\begingroup\$ The devil's in the detail of your splitter - there is no generic answer. \$\endgroup\$ – Andy aka Jul 10 '20 at 18:13
  • 2
    \$\begingroup\$ You don't do this at all, period! What you have neglected to consider is that both devices are transceivers and one transmitting into the receive port of another with just a few dB of loss is going to exceed safe specifications, far more than something like a phone in a stack of others for the who-pays-the-check challenge. If you can only have one antenna, consider an IP-network based furnace control with autonomous freeze safety that needs repeated confirmation to maintain a higher comfort temperature. \$\endgroup\$ – Chris Stratton Jul 11 '20 at 0:16

Yes, it is "slightly more complex". In general, if you don't terminate all of the outputs of the power splitter with the correct impedance, then the impedance at its "common" port will not match the impedance of the transmission line and antenna, and some power will be reflected back toward the antenna, reducing the amount of power going into the splitter in the first place. The specific amount depends on the design of your particular device, so I can't give you a number.

So yes, with a 3-way splitter, each output gets 1/3 of the input power (or actually slightly less, depending on how efficient it is), regardless of whether you use it or not. Since unused ports are terminated, the power there is simply wasted. If you really only need 2 ports, get a 2-port splitter.


The unterminated port will REFLECT energy back into the splitter.

I'd expect about an INCH from the reference_plane of that SMA connector into the splitter's mechanical structure.

Thus the wavelength is 4 inches, for some serious misbehavior.

Although quarter_wave opens are transformed into SHORTS at the other end.

I suggest a termination be used, or you should expect horrid holes in that passband.


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