Extend signal by running extension off just one antenna?

I'm looking to extend the range of my Billion router from the house to a building at the end of the garden.
The Billion router has 3 antenna. Would it work to remove one antenna, attach an extension cable of approx 20metres to the modem, run the cable through the garden and then attach the antenna to the end of the cable in the external building?

Obviously one of the antenna will be a long way from the modem but the modem will still have 2 antenna so should still work?

Any help will be appreciated.

Why not run an ethernet cable?

So would an ethernet cable run to another router?

Yes. Or you could use homeplugs if mains electricity runs from your house to the out-building.

Terrible idea for many reasons, do not do it. Many devices operate in a way that multiple antennas are used at once to transmit your signal over wifi. Having the antennas so far apart will cause a headache.

Follow advice of others here, run Ethernet and an AP at the end.

In addition to breaking the maths behind the physics for the way the antenna works in conjunction with others, the 20m of cable with such a low level signal is likely to attenuate things badly.

Adding a wireless access point on the end of a piece of ethernet cable would be the preferred option

This explains some basics:

https://www.youtube.com/watch?v=aNq-il-fYNU

& beamforming if your device supports it
https://www.youtube.com/watch?v=8rMtqRObvvU

Imagine moving the antennas to the other side of the home, how would any of this work?

Another aspect is that it would have to be Co-Ax cable of the same, matching, Impedance as the aerial.

Given also the unusually small size of the connectors associated with WiFi Aerials, it seems likely that suitable Co-Ax cable will be very small in diameter and thus physically weak and needing protection, such as conduit.

Given the restricted Low Power of WiFi systems and that generally such cables are normally only found within the Router so very short, I suspect that the losses over that 20 Metre distance may be excessive.

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Trying to use the WiFi directly as a radio transmission, would be subject to both permanent losses such as through walls; and annual ones such as leaf growth; and temporary ones such as persons and animals passing through and absorbing the beam - also rain and humidity.

Additionally if any of the walls are recent, they are likely to have "vapour-barrier" metal screening incorporated.

Again particularly if the garden building is recent, it is likely to have RCB type Mains protection, which tends to block attempts to use the mains wiring.

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So as others have recommended, I think that you should use an Ethernet cable.

As others have suggested, run an ethernet cable to the out-house and install a router/access point.

In reply to a post by tuthai:

So would an ethernet cable run to another router?

Depends what you want to have at the end of the garden. Using an old router with DHCP disabled and an IP address which does not conflict with your main router can be a cheap way of allowing several devices to be connected. It may be convenient to use the same Wifi SSID and password as your main router.