Diamond X-50A omni antenna obscura question...

[dbrescia]

I'm not an antenna theory or design expert, so I could really use some guidance! Making some changes of a more permanent nature to my home radio systems, and will be installing several new antennas this spring. The subject 2m/70cm omni will be mounted about 26 feet (at antenna base) above ground level and the antenna itself is 5.5 feet long. Directly below and mounted on the same mast will be a Diamond BC-103 143-176 MHz omni. The peak of the house roof is roughly 30 feet above ground level, and the mast itself is offset roughly 34 feet away and tangential to the roof peak. When all mounted the 2m/70cm omni will not be entirely clear of obstruction by the top of the roof, and the VHF omni will be totally obscured by the roof. There's not any other alternative mounting for me that's practical so obscura to the east of my location is something I'm willing to accept. I'd prefer not to mount the VHF omni below; however, I'm financially constrained for now to one mast. The VHF omni will be used primarily to receive public safety, MURS, rail and marine traffic with the occasional RX/TX on the marine side (we are within one mile of Canandaigua Lake). All that stated, how much performance degradation should I expect on the 2m/70cm omni (primary RX/TX)? My understanding of radiation patterns, take-off angles, etc. is basic at best. Would appreciate suggestions and analysis from the many more wise and experienced operators out there! Thank you very much in advance!

David

 

[popnokick]

You didn't note what kind of coaxial cable you'll use. Use good low-loss 50 ohm cable that is low-loss in the UHF 70cm range, e.g. LMR-400 equivalent or better. Without your exact location that could be used to perform a coverage plot it is difficult to tell exact performance. It's likely to work better than you expect if you pay attention to the coax feed and proper connections. It will work better than anything that is indoors or in an attic / crawlspace. If the adjacent roof is all-metal, or the house has aluminum siding it may affect your coverage pattern / directionality.

 

[prcguy]

How far away from the mast will the Diamond BC-103 be mounted? If its close like less than 2ft from the mast it will become a 2 element beam antenna, directional away from the mast. It would need to be a good 6ft or more from the mast to start getting more omni directional.

 

[dbrescia]

You didn't note what kind of coaxial cable you'll use. Use good low-loss 50 ohm cable that is low-loss in the UHF 70cm range, e.g. LMR-400 equivalent or better. Without your exact location that could be used to perform a coverage plot it is difficult to tell exact performance. It's likely to work better than you expect if you pay attention to the coax feed and proper connections. It will work better than anything that is indoors or in an attic / crawlspace. If the adjacent roof is all-metal, or the house has aluminum siding it may affect your coverage pattern / directionality.

Thank you very much for the reply! I will use LMR-400 for sure with in-line lightning protection and WX-protected connections (very few needed). Runs for both antennas will be around 75 feet from antenna inside to radios--worse case a maximum of 100 feet. Elevation is 860 feet ASL at mast base and mast is tapered fiberglass from 3.0" to 1.5" at an average of 3/16" thick. As per Google Maps the location is 42.85909, -77.24159. House roof is asphalt shingles and siding is vinyl. Antennas and mast are on back side of property with house/obscura to the east. We are on top of a hill (within 15 feet of top) with no obstacles north, south and west. Again, I'm a bit nervous because I can't get the entire antenna body completely above roofline. Thank you again for your time and consideration!

 

[dbrescia]

How far away from the mast will the Diamond BC-103 be mounted? If its close like less than 2ft from the mast it will become a 2 element beam antenna, directional away from the mast. It would need to be a good 6ft or more from the mast to start getting more omni directional.

Thank you very much for your reply as well! I neglected to mention in my original post that the mast is fiberglass. As per my last reply, elevation is 860 feet ASL at mast base and mast is tapered fiberglass from 3.0" to 1.5" at an average of 3/16" thick. My selection of non-conductive fiberglass was an effort to minimize antenna interference. The only conductive element adjacent to either antenna would be copper ground wire for lightning arrestors. I could get some standoff from the mast; however, I was thinking 12" or less. Although this mast is strong, I did not want to introduce large, moment arms without negating counterbalance. Thank you again for your time and consideration!

 

[prcguy]

If the coax from the top antenna runs down the fiberglass mast it will have the same effect as having a metal mast. Just make sure any coax running past the lower antenna is far away from it.

Thank you very much for your reply as well! I neglected to mention in my original post that the mast is fiberglass. As per my last reply, elevation is 860 feet ASL at mast base and mast is tapered fiberglass from 3.0" to 1.5" at an average of 3/16" thick. My selection of non-conductive fiberglass was an effort to minimize antenna interference. The only conductive element adjacent to either antenna would be copper ground wire for lightning arrestors. I could get some standoff from the mast; however, I was thinking 12" or less. Although this mast is strong, I did not want to introduce large, moment arms without negating counterbalance. Thank you again for your time and consideration!

 

[dbrescia]

That makes perfect sense--I should have realized that. I suppose I could mount both antennas at the same height with horizontal separation (rule of thumb is 1/2-wavelength or more, correct?). If I leave them as originally described, anything I shield that adjacent length of coax with could only make things worse. Any suggestions? Thank you!

 

[prcguy]

Mounting 1/2 wavelength apart should result in a four lobe clover leaf type pattern, so you would have four gain lobes and four lossy nulls. My personal rule of thumb is at least 2 wavelengths apart or away from a large metal object, which is about 12ft for a 2m antenna. I made a mistake in my above post about 6ft, that is only 1 wavelength. On the other hand if you need better coverage in just one direction you can optimize the spacing for about 1/4 wavelength and get a little gain in one direction at the cost of loosing it in other directions.

That makes perfect sense--I should have realized that. I suppose I could mount both antennas at the same height with horizontal separation (rule of thumb is 1/2-wavelength or more, correct?). If I leave them as originally described, anything I shield that adjacent length of coax with could only make things worse. Any suggestions? Thank you!

 

[dbrescia]

Well, maybe I need to re-think all this. I have everything but the mast and coax, so will look at options for mounting. If I bite the bullet and buy two masts, I can get 14 feet horizontal antenna separation by mounting them each on their own mast. Another option would be mounting at roof peak on two opposing eaves. They are about 24 feet apart horizontally, and that would also reduce the amount of coax I'd need. Problem is I'm disabled and no longer able to get up on the roof. That would mean hiring someone to mount for me. Will have to compare costs, but I think the best technical solution would be mounting on eaves at roofline peak. Early on had looked at mounting with short mast on short tripods or roof, but not crazy about making roof penetrations. Thank you, and I'm open to any other ideas! I still have about 90 days to get this dialed in!