Your 'Max model 8HH indoor antenna' appears to be a typical 1/4 wave 'groundplane' antenna and as such has no 'gain' by definition. The same antenna outside and as high as you can get it would work 'better' than indoors. Increasing the electrical size of that 1/4 wave antenna to 1/2 wave, or 5/8 wave, or a vertical array of any of those electrical 'sizes' will result in a definite and positive gain figure.
Very simply, just moving your present antenna outside and up high would make quite a difference in signal reception. Nothing 'miraculous', but better.
And then there's the matter of the feed line for that antenna. One good way of determining which feed line would be 'best' is by use of a coax comparison chart, or a 'spec' chart of the various sizes of coax cables. Select a cable that has the least loss per 100 feet at the frequency of interest (800 - 950 Mhz?). Sit down before looking at the prices of those cables, the good ones aren't going to be cheap. You should have a rough idea of how much cable it would take to get to that outside and high antenna. If you are like the rest of us, you may have to revise your estimates a few times to get them into a "reasonable" cost range.
Happy figuring!
- 'Doc
(One reasonable guess about what cable to look for is Times Microwave LMR-400. Listen to your wallet scream!
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