I have a 2 Meter Beam antenna problem. I purchased a Cushcraft 10 element beam and spent many hours in the process of mounting it approx. 45' above ground, secured by guy wire in 4 places. Just before rasiing the antenna, I measured the VSWR and it was perfect (1.25:1). I connected the antenna with 75' of LM400 RG8. I should also mention the antenna included a rotor. The problem in all of this is that after finishing the job, I found the antenna performance horrible. Both transmit & receive gain were so low as to be almost unmeasurable. Having checked VSWR before raising the antenna, I could not understand what could possibly be causing the problen. At first, I suspected the feedline. So, after pulling the whole assembly down, I measured the feedline. It was fine! At that point, I was completely baffeled. Since I had an older Cushcraft 7 element beam that I knew performed well, I connected it, raised it, and low and behold it worked fine! So, the brand new 10 element now sits in my garage while I try to do a little investigative work to see what could possibly have been causing what appeared to be an "open" antenna connection. I would be curious to know if anyone has ever had a problem similar to this. 73's
Several things come to mind as possible culprits. Some have been mentioned like loose connections.
To analyze the problem, I'd try the following:
1) You say the 7 element was good when installed. Were you using the same feedline (the LMR400)? If so, maybe a bad connector on the new antenna. If not, check out the LMR400 by attaching a 50 ohm dummy load to the far end of the LMR400 coax and check the SWR. It should be low as the dummy should represent a tuned load to the xmitter. If it is high, you've got coax/connectors issues. If everything checks out ok, then it's either an antenna issue or a mounting issue.
2) On HF, having guy lines in the antenna near field can detune the antenna and cause high SWR. While I haven't heard of this being a big issue on VHF, if your guy lines are made of wire (probably are) try breaking up the lengths using 'ceramic egg' insulators. Check the ARRL handbook for length vs resonance to see what would considered the optimum length of the guy line segments.
3) What was under the antenna when it is raised to 45'. Same as under antenna at ground level when tuned ok? I have a 2M 11 element yagi that I temporarily put up at 25' and it had a metal evaporative cooler (mounted appx 8 foot above ground) and was positioned under the feedline end of the boom when rotated south. SWR pointed north was about a 1.3:1 and pointed south was a 2:1. By raising the antenna another 10 foot and the coupling effect reduce to a negligable effect.
4) Since the new antenna indicated correct SWR at ground level, I presume the elements were placed correctly on the boom?? If not, could give some weird results. Was the 10 element tuned at ground level according to the instructions. Antennas will change SWR and tuned freq when raised to higher position. Have you swept the 10 element to see if resonant freq has dropped outside band, etc.
5) Is the bad SWR the same results in all 360 degrees or only in one direction. Could be a coupling problem as mentioned before. Any other 2M antennas close to 10 element position?
6) As a final effort, I'd go through the entire install and look for any changes/differences between the 7 element and the 10 element antenna installs in 7 is good and 10 not like you say. Different coax jumper, antenna height, etc. I would bet there is something there you're overlooking.