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outdoor antenna question

motorstereo

motorstereo

AK Subscriber
Subscriber
I recently moved my outdoor omnidirectional antenna from a chimney mount to being mounted on a metal building. Even though it's up higher and in a more open spot the reception isn't as good. The antenna pole is in contact with the metal building. Would this be the source of my poorer reception?
 
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I would think it may be the metal building itself. Lots of outdoor antennas are grounded with a copper rod and ground wire. I'm not an antenna engineer (ME) but I have a buddy who did study antenna design. He called it "magic engineering". I remember looking through his text book. Too many partial differential equations for me!
 
motorstereo said:
I recently moved my outdoor omnidirectional antenna from a chimney mount to being mounted on a metal building. Even though it's up higher and in a more open spot the reception isn't as good. The antenna pole is in contact with the metal building. Would this be the source of my poorer reception?
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What kind of omnidirectional antenna? Mounted on the roof? Offset from a vertical wall? Metal pole? Coax or twinlead feedline? And, is the reception poorer on all stations or just one or two?

If the mast is not in electrical contact with the antenna itself, it's not automatically a big deal if mast touches metal building. But, good practice, if a metal mast is attached to a vertical wall, is to use stand-offs to put some distance between wall and mast.

The problem could be due to the building itself, or not. Other factors such as objects in the signal path (trees, buildings, etc.), bad ground, a longer run of feedline (coax? twinlead?) than before, old feedline, even bad / leaky feedline connections, could be the problem.

EDIT: If you have a horizontally polarized antenna above a metal roof, I'd suggest getting it, at the very least, a few wavelengths above the roof. 1 wavelength at 98 mhz is basically 3 feet.
 
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Bad connections come to mind first, did you add a coupling or anything? Maybe a new cable that could be defective.

I have a buddy that calls antenna design black magic, especially for the higher frequencies.
 
c.coyle said:
What kind of omnidirectional antenna? Mounted on the roof? Offset from a vertical wall? Metal pole? Coax or twinlead feedline? And, is the reception poorer on all stations or just one or two?

If the mast is not in electrical contact with the antenna itself, it's not automatically a big deal if mast touches metal building. But, good practice, if a metal mast is attached to a vertical wall, is to use stand-offs to put some distance between wall and mast.

The problem could be due to the building itself, or not. Other factors such as objects in the signal path (trees, buildings, etc.), bad ground, a longer run of feedline (coax? twinlead?) than before, old feedline, even bad / leaky feedline connections, could be the problem.

EDIT: If you have a horizontally polarized antenna above a metal roof, I'd suggest getting it, at the very least, a few wavelengths above the roof. 1 wavelength at 98 mhz is basically 3 feet.
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Yes it's a metal pole attached to a vertical wall with a 6'' standoffs. Antenna is in contact with the pole and the pole in turn is in contact with the metal building via the metal standoffs. It's a 4 element antenna and it's mounted about 7' above the building. It's in a nice open area. More open than the previous location. There is about 50' more co-ax line for a total of 100' of co-ax. Almost all the stations are coming in poorly compared to the old position. It's also grounded with a 4' metal stake and 4 gauge wire.
 
faber12 said:
Bad connections come to mind first, did you add a coupling or anything? Maybe a new cable that could be defective.

I have a buddy that calls antenna design black magic, especially for the higher frequencies.
Click to expand...

Brand new fm splitter and 50' of brand new co-ax cable were the only things that were added. I wonder if the 100' run of co-ax cable is a bit too much?
 
RF ground's not the same as DC ground -- I doubt the metal building per se is the problem. According to my old man, who used to do this stuff for a living, every coax union has about a dB of insertion loss, a splitter is probably good for about 3 dB of loss. A couple of unions and a splitter and you can give away an antenna's gain pretty quickly (I think).

Can you test the signal strength closer to the antenna (even a portable FM radio and a cliplead)?

FWIW, usually when I've had problems with signal strength in an outdoor installation with coax, I've had trouble with water infiltration into the coaxial cable. Was the reception poor from the get go?

Actually I think antenna design is a science -- it's just one that I don't understand.
 
If the building has a metal roof it can certainly affect the "omni" pattern. The metal roof can cause it to be more directed across the longest path across the roof. Example: If the antenna is at the SW corner, its pattern will be directed more toward the NE and will loose some signal to the SW.
 
That's certainly true. Indeed, I used that 'ground plane' trick on the CB radio antenna on my Bronco way back in the mid-1970s :-) Antenna was on the rear bumper, driver's side, so there was some directivity towards the passenger's side in the front (even with a quarter-wave whip).

bronchusspiffed by mhardy6647, on Flickr

No 'ears' on it yet when that photo was take, ca. '75.
 
mhardy6647 said:
RF ground's not the same as DC ground -- I doubt the metal building per se is the problem. According to my old man, who used to do this stuff for a living, every coax union has about a dB of insertion loss, a splitter is probably good for about 3 dB of loss. A couple of unions and a splitter and you can give away an antenna's gain pretty quickly (I think).

Can you test the signal strength closer to the antenna (even a portable FM radio and a cliplead)?

FWIW, usually when I've had problems with signal strength in an outdoor installation with coax, I've had trouble with water infiltration into the coaxial cable. Was the reception poor from the get go?

Actually I think antenna design is a science -- it's just one that I don't understand.
Click to expand...

Ok; I've tried a tuner closer to the antenna (25' of co-ax with one splitter) and it did just as badly if not worse than my tu9900 which is 2 splitters and 100' of co-ax further away. And yes reception was bad right from the start. Being up higher and in a more open area is what made me think the connection to the metal building is killing the reception. The antenna worked great where it was before mounted on the chimney unfortunately it cannot be returned to that spot.
I'll keep that in mind about the signal loss due the splitters. That's good to know.
 
Update; and the reception problem appears to be solved. I isolated the mast from having continuity with the metal building and now I have decent reception. Like I expected it to be it's a bit better than the older lower setting. I'll have to file this one in the memory banks and keep that mast isolated.
Thanks to all for the helpful suggestions.
 
From what i have read it is ok to put an antenna amplifier at the antenna if you get a good signal there. This will boost the gain so you can feed it along the 100' of coax. If you are getting the stations you want in clean stereo, then nevermind.
 
motorstereo said:
Brand new fm splitter and 50' of brand new co-ax cable were the only things that were added. I wonder if the 100' run of co-ax cable is a bit too much?
Click to expand...
Maybe do a continuity check on the cable, and bypass the FM splitter just in case.

Sorry, just saw the fix. That's great!
 
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