First of all, I'm not talking about an active transmitter, I'm talking about a passive one - one that doesn't use power or boost the signal as such.

Right...

I live in a part of town with borderline mobile coverage. It's been this way for years, and regardless of my handset, the coverage here sucks. It hasn't normally been a massive problem until recently, since I have dropped my landline and am fully reliant on my mobile for voice at home. I can step outside and it's usable, or stand at the window facing one of the two closest towers, but since it's 2016, I don't think I should have to.

My mobile is with Vodafone (work phone, so no choice about the provider) and I have my broadband with Spark, hence SureSignal is out of the question.

So I've been doing some experiments with a passive repeater (directional antenna on one end, pointed at the tower, fed by coax to an omni antenna inside, to effectively reflect the strong signal from outside inside. So far, I haven't managed to improve my signal at all. I've tried a few different designs, all optimised for Vodafone's 952MHz, as that's what my phone is preferring to use when inside. 

Has anyone else tried this successfully? My outdoor antennas thus far have not been directional, so I'm thinking that the improved signal I'm getting through being outside is quickly getting eaten up by losses in the coax. If I had a higher gain outdoor antenna, perhaps things might be different. But I don't want to spend the money unless I know it's possible.

These guys have a product that effectively does the same thing:

http://cellutronics.co.nz/Passive_Repeater.php

Only, the phone has to be placed on the cradle, and it's overpriced for what it does, imho. It's very hard finding homebrew or DIY guides online, as most people talk about using transmitters, which we can't here. 

Thoughts?