so ive been scratching my head on this for a while now. got a diamond x50 up about 25 feet on a mast off the back of the house, running maybe 40 feet of LMR-240 down to the shack. on 2m its fine, hitting the local repeater no problem, but on 70cm i feel like im getting eaten alive. s-meter on the other end is showing me way down compared to what my buddy two miles away is getting with basically the same setup except hes using LMR-400 for his run.

i did the math on the coax loss and yeah 240 isnt great at 440 but shouldnt be catastrophic for 40 feet right? im running 50w out of an IC-2730. wondering if maybe the antenna connector is the issue because i did that PL-259 crimp myself and it wasnt my cleanest work honestly. or maybe the x50 just isnt that great on 70cm, ive seen some people say the pattern gets weird up there. anyone dealt with this?

the connector is almost always the culprit when you start seeing this kind of thing. LMR-240 at 40 feet even at 440 is maybe 2-3 dB of loss tops, which is noticeable but not catastrophic. a bad PL-259 though, especially a crimp on semi-rigid, can add way more than that and it doesnt always show up as a dead short, sometimes its just a slow bleed. if you have an analyzer throw it on the end of the coax without the antenna and look at the return loss or SWR trace across the band. if you see it jumping around or really poor above like 400 MHz thats your connector. i had almost the exact same issue last year, resoldered mine with a proper silver solder job and got like 4 dB back on 70cm. the x50 pattern is a little squirrely up high yeah but its not bad enough to explain what you're describing.

yeah i'd just swap to LMR-400 anyway if you're going back up on the roof. the price difference for 40 feet isnt that much and you'll never think about coax loss again on either band. also double check that the antenna feedpoint connection is clean, those diamond whips sometimes have a little corrosion creep in if theyve been up more than a year or two especially if you're near salt air or anything like that. 70cm is pretty unforgiving compared to 2m with any lossy stuff in the chain.