Metal building as a reflector for FM band

[mikeformha]

Hi all, great forum.
I'm new here so I'm gonna start off with this...

Call me hair brained but...

I'm contemplating using a steel building as a reflector for my home made horizontally polarized 1/2 wave FM antenna.

It's orientated correctly for receiving the signals I want and blocking the signals I don't want (near by FM transmitter that over runs the band). So instead of mounting a typical antenna on top the building and exposing it to direct waves from that big bad transmitter I don't want stomping the band, mount it over the side about 20' up leaving at least 10' above it to the roof line. I'm guessing that a spacing of 1', 11 inches would be the standard spacing for a resonant reflector...

I was wondering if anyone here could tell me if the reflector for 98 mhz would have to be of the resonant type, or if, in my case, you have an area that is very large (30' high X 300'wide), would suffice as a "planar" style reflector, and whether or not that reflector spacing still holds true with a planar design.

I'm gonna try this soon but hopefully someone will stop me before I put a hole through a perfectly good building.

Any thoughts on this topic would be greatly appreciated.
Thanks,
Mike

 

[Don_Burke]

I would:

1. Figure out a temporary rig that would not involve punching a hole until after the concept was proven.

2. Start with a quarter wave off the building and be very open minded about what the spacing should end up as.

Hi all, great forum.

I'm new here so I'm gonna start off with this...

Call me hair brained but...

I'm contemplating using a steel building as a reflector for my home made horizontally polarized 1/2 wave FM antenna.

It's orientated correctly for receiving the signals I want and blocking the signals I don't want (near by FM transmitter that over runs the band). So instead of mounting a typical antenna on top the building and exposing it to direct waves from that big bad transmitter I don't want stomping the band, mount it over the side about 20' up leaving at least 10' above it to the roof line. I'm guessing that a spacing of 1', 11 inches would be the standard spacing for a resonant reflector...

I was wondering if anyone here could tell me if the reflector for 98 mhz would have to be of the resonant type, or if, in my case, you have an area that is very large (30' high X 300'wide), would suffice as a "planar" style reflector, and whether or not that reflector spacing still holds true with a planar design.

I'm gonna try this soon but hopefully someone will stop me before I put a hole through a perfectly good building.

Any thoughts on this topic would be greatly appreciated.
Thanks,
Mike

 

[mikeformha]

Good point, could be different than what a resonant reflector would be , seems like with a planar style, the reflector distances could be quite different and this is somewhat corrugated metal. Unfortunately, I had to solder a radio shack tv preamp to the front, (300 foot run to the radio), which will make it more difficult to test rig it, but it can be done.

 

[mikeformha]

I have yet to try this theory out but...

I'm wondering if anyone else has had any success using buildings/lakes or other grounded objects as a reflector for an antenna system.

 

[k8mcn]

I have yet to try this theory out but...

I'm wondering if anyone else has had any success using buildings/lakes or other grounded objects as a reflector for an antenna system.

Its done all the time for HF vertical antennas--either a vertical is mounted over a metal roof and the roof acts as a ground plane, or when the vertical is mounted on the ground, several wires (ie 50-60@ 1/4 wave) are laid on the ground in a pattern around the base and tied together....

 

[Don_Burke]

This is not a ground plane. The active element is a horizontal halfwave dipole and it will be mounted on some sort of standoff on the side of the building. The side of the building will be a reflector (and a shield for that matter).

Its done all the time for HF vertical antennas--either a vertical is mounted over a metal roof and the roof acts as a ground plane, or when the vertical is mounted on the ground, several wires (ie 50-60@ 1/4 wave) are laid on the ground in a pattern around the base and tied together....

 

[mikeformha]

Well put Don.

I would look at the "ground plane" on a ground plane antenna, more as a wave guide than a tuned parasitic element.

The problem I have, has to deal with the near bye, unwanted FM transmitter. I'm guessing that six layers of metal building between this antenna and the unwanted transmitter, will do more for attenuation than a roof mounted yagi. I would have used a more conventional antenna but there is parking lot below so there is no place to put a mast and a stand off mount for an antenna with a reflector would be too great of distance away from the build to support it adequately.

It will be a couple of weeks before I know how well this setup is going to work, but when I do , I'll add to this thread.

Mike