Question: There are several cell towers on the hills around me.
Usually I just walk around their base in a circle with
[one](https://play.google.com/store/apps/details?id=com.parizene.netmonitor) of the many CellID apps
to get their ID. Then I can impress my friends, "That's tower 4664! That's
tower 5995!". You know, a sort of hobby.

But now I'm getting kind of old so I'm thinking can I just sit back in
my armchair and point some kind of homebrew antenna isolator cannon at them...

If I put my cellphone in a long metal pipe aimed at one of them (I can see the hill with the target tower when looking down the "barrel" of my "cannon"), that
will eliminate the signals from the others, and my cellphone will surely
now just lock on the one I pointed at... or so I thought.

What are the correct dimensions to make it work? Putting my cellphone in
the middle of 1.3 meter length, 15 cm diameter pipe didn't give me any
different results than without the pipe.

I'm talking about 4G LTE towers in the 900 and 1800 MHz bands. I put my
phone in the pipe pointed at tower A, even pushed the reboot button, but
it still ends up locking in to tower B. Hmmm, checking the [wavelengths](http://www.hottconsultants.com/techtips/freq-wavelength.html)

```
900 MHz 33.3 cm
1.8 GHz 16.7 cm

```

Do I simply need a wider pipe? (Certainly even more money down the drain!)

Answer: 
What you are describing is a directional antenna for your phone. You can do this by placing the phone Inside the antenna, or by connecting the antenna to the phone.

The pipe you describe is actually used as an antenna - google for the Cantenna wifi antenna made from a pringles can.

A horn antenna like this might not be the most elegant antenna for a phone though. 

First, it isn't very directive. A simple tube has a beam about 60 degrees wide, meaning it's half power or -3 dB at +- 30 degrees. And it only falls to -10 dB at +-70 degrees. So it's not the cannon you imagine. 10 dB is probably only one bar on your phone's signal display. Making the tube longer doesn't make it more directive. [Here's an example of a radiation pattern](https://www.researchgate.net/figure/Cantenna-radiation-pattern-measurements_fig14_260515111).

An analogy, not too far fetched actually: Imagine using a pipe to blast sound waves in a certain direction - they spread out all over the place, they don't come out in a beam like a ray of light. (Try it by playing some music on your phone while it's in the tube).

Second, a cylindrical waveguide horn will only work properly for one cellphone band. If you size the tube for 900 MHz, it will not work well as a directional antenna at 1800 or 2100 MHz.

My suggestion is that you scratch around online for a cellphone booster antenna. These are commercially used by people who have marginal cell signal outdoors, and nothing indoors. They're a passive wideband antenna, usually an LPDA, connected to a cable that runs indoors, either directly to your phone if it has an antenna socket, or to a coupler gadget that sticks to the back of the phone. 

My experience with these is that you can isolate individual cell towers by turning the antenna around, but it's a difficult, confusing process.
It would be nice if it were so simple to make a very narrow "cylinder-shaped" beam, but **a pipe is simply not going to do that**.

[This answer](https://ham.stackexchange.com/a/13145/8717) has a considerably better chance of working than sticking a phone in a pipe.