In article ,
VE2CJW wrote:

From the antenna, I can't see the repeater because of a couple of condos
that are much higher than my house. That's probably the problem because I
can reach a few repeaters about 4 times farther but not in the same
direction. I wonder how high my antenna should be to solve my problem. I
also wonder if more power, say 150 watts would change anything. I am open to
suggestions.

You may be running into a multipath-nulling problem.

I had a similar situation here at my house a few months ago. One of
the local repeaters is located about 5 miles away, with its antenna on
top of a 7-story hospital building. My antenna is a copper-pipe
J-pole, about 20' off of the ground (8' above a 12' roof).

There is an industrial/office building about 50' high, smack-dab
between me and the repeater. It's located around 100' from my antenna.

I found myself unable to open the repeater and get acceptable quieting
even with 50 watts of power. Another local repeater (further away)
gives me an S9+10 reading or better with only 5 watts.

I wandered around on my roof with an HT and a rubbber duck, while
another local ham transmitted through the repeater. I found that
there were places in which my HT received a perfectly good signal from
the repeater, and others where it dropped down pretty far into the
noise.

What I concluded was that my signal path to the repeater was really
messy. There was no direct line of sight, but multiple indirect
paths... knife-edge diffraction from the top of the office building,
with reflections off of numerous nearby trees and houses. By chance,
I had placed my J-pole in a spot where the various indirect-signal
paths summed up to a deep null on the repeater input frequency. When
I transmitted, the power ended up going in many directions, but none
ended up at the repeater :-(

I moved the antenna mast a few feet along the house wall and tried
again... and opened the repeater with acceptable quieting with only 10
watts. Problem solved.

So, I'd say that the brute-force way for you to solve your problem is
to raise your antenna until it clears the roofline of the condos and
has a direct line-of-sight to the repeater. If you do that, you can
probably open that repeater acceptably with only 5 - 10 watts even
using an omnidirectional vertical antenna - less if you aim a gain
antenna towards the repeater.

The subtle way, not requiring a huge mast, is for you to experiment
with moving the antenna around, both from side to side and in the
direction towards which it's pointing. You may find that moving it
places it in a spot where the reflections and diffractions work in
your favor (waveform reinforcement) rather than against you
(cancellation). You may also find that pointing it _away_ from the
repeater, towards some tall building or hill which is located in the
line of site of the repeater, will give you an acceptable (albeit
indirect) path.

You can do some of this testing by simply listening to the repeater
while other people are using it, but the fine-tuning will probably
requiring transmitting - the cancellation and reinforcement path
"sweet spots" can be different in the two directions, due to the
different wavelengths of the repeater input and output signals.