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There are several cell towers on the hills around me. Usually I just walk around their base in a circle with one 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

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!)

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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.

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.

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  • $\begingroup$ "What you are describing is a directional antenna for your phone." Thanks but I am trying to reduce other signals, not increase the target signal. I notice mountains very effectively block cell signals. Hmm, maybe an "igloo" made out of soil, with its entrance pointed at the target tower? Or a cave, with its opening pointed at the target tower? How long would I have to be from the entrance down the "shaft" for best results? More practical would be a narrow valley from which one can only see the target tower. Found from a topo map + viewshed programs to confirm... $\endgroup$ – Dan Jacobson 積丹尼 Apr 3 '19 at 3:45
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    $\begingroup$ It's almost the same thing - increasing strength in one direction and reducing it in others. Your use case, of isolating one cell tower, is best served by a well designed directional antenna. (Subtle difference - antennas can be optimised only for gain, or for low sidelobes, you want the low sidelobes version). An igloo might be better than a pipe, but signals bounce around and so will come out of the door in all directions, not just line of sight. A directional antenna is designed to focus energy in one direction and exclude others. A valley would work of course. $\endgroup$ – tomnexus Apr 3 '19 at 7:16
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You could try a parabolic reflector, such as an old satellite dish, with your phone at the focus.

We think of radio waves as travelling in straight lines, but that's not quite right; they also need some open space around the straight line, known as the Fresnel zone. A long metal pipe will interfere with the Fresnel zone, while a parabolic reflector stays out of the way.

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  • $\begingroup$ I see. I should concentrate on increasing the signal from the target. Any merit in me still persuing methods of reducing the signal getting to my phone from other towers? $\endgroup$ – Dan Jacobson 積丹尼 Mar 28 '19 at 12:47
  • $\begingroup$ This answer would be improved with an explanation of why you think a parabolic would be an improvement over a "shotgun" director. $\endgroup$ – Zeiss Ikon Mar 28 '19 at 13:29
  • $\begingroup$ @ZeissIkon This has a lot better chance of working than a pipe. $\endgroup$ – Mike Waters♦ Mar 29 '19 at 13:55
  • $\begingroup$ @MikeWaters I don't disagree, I was just saying that it's a one-liner answer. Even if correct, these are usually frowned on (at least in other SE sites). $\endgroup$ – Zeiss Ikon Mar 29 '19 at 13:58
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    $\begingroup$ I added something about the Fresnel zone, which is why I /think/ it will work more effectively - but really, the proof of the pudding is in the eating. If it's observed to work, then it's an improvement over the pipe, which has been observed /not/ to work. $\endgroup$ – Mike Playle Mar 29 '19 at 14:05
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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 has a considerably better chance of working than sticking a phone in a pipe.

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If the pipe is tied to the antenna you're essentially using the whole pipe as part of the antenna, so it's no wonder it doesn't isolate the signal -- and there's no effective way to prevent this when the pipe is smaller than the signal wavelength.

So, make sure your pipe is big enough (maybe use a metal garbage can, or a plastic one lined with foil, instead of a pipe?), and make sure it's grounded.

The pipe you have might work for range extension on wifi -- I've seen many reference to using Pringles cans as shotgun antennae for those (2.4 and 5 GHz, so around 6 and 12 cm wavelength).

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    $\begingroup$ There's no way to ground a pipe at cell phone frequencies because any ground lead is going to be large compared to wavelength. The ground lead will act like a monopole antenna, and the opening at the end(s) of the pipe will act like a slot antenna. $\endgroup$ – Phil Frost - W8II Mar 29 '19 at 14:30
  • $\begingroup$ @PhilFrost-W8II Someone who is obviously not a radio geek is much more likely to have heard of a Faraday cage than a slot antenna. I'm pretty fuzzy on the latter, myself. $\endgroup$ – Zeiss Ikon Mar 29 '19 at 14:35
  • $\begingroup$ Yes, I was dreaming that there would be some way to reduce the signals coming from other directions, not increase the signal coming from the target direction. $\endgroup$ – Dan Jacobson 積丹尼 Mar 29 '19 at 22:30

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