Looking at the site for my station's antenna, I see that there is a metal roof very nearby. I know that a metal roof can cause possible reflections in the signal. Is there a way to treat the roof to minimize potential problems or is there a type of antenna that works best for metal roofs? Is there a minimum distance that should be used?
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Antennas and Metal Roofs
- Thread starter ZebraRadio
- Start date
What kind of station? AM, FM, LPFM (I assume LPFM from the section you posted in)? How high will the antenna be over the roof? What kind of mast or tower will it be on, and, if FM, how many bays and of what kind (manufacturer and model)?
Very basically speaking, if you have a multi-bay antenna, the downward radiation is reduced the more bays you have and at some point (depending on size of roof) there is no reflection as there is little to reflect.
For example, if you have a 2-bay antenna 50 feet up over a barn sized building, you likely have no issues. But there are folks on here who know a lot more about RF than I do who can be more precise.
In general, more bays focus the radiation horizontally towards the horizon and not up and down to the sky and the ground (or roof).
The station is an LPFM. The height from ground to antenna is 48' according to the CP. The last 12' has a metal roof on one side of the antenna. The antenna itself is a VP dipole. Only one bay.
At that low level, my experience is that you will not have issues.
Years ago, I installed a dozen or so 50 watt FMs that I used as relays to link an FM network in Ecuador. Most were around 15 meters above ground with corrugated roof small buildings with the transmission gear in it. An antenna on another pole picked up the link station and the other post retransmitted on a different FM channel.
I never had directionality or other signal issues. Most, though, were directional towards the next relay but some had over 100 watts to cover a smaller market. The main station was the first FM in the country. We built all the gear ourselves.
Good to know. Thanks.
My wife and I are frequent visitors to California's central coast. I've listened to your Part 15 AM station while staying in San Simeon and have been very impressed by the professional sound. Coincidentally, we were in Cambria this week and I looked for an LPFM antenna at the coordinates shown in your CP (near Robin's). Best wishes for success. Perhaps, our paths will cross next time we're in the area.
I really appreciate that, Dr. DJ. As you can guess, the Part 15 AM station was a dress rehearsal to see what techniques/technology would work best should the day ever come that the FCC would open a window for LPFM. They did and I applied. Nearly everything is in place. I just need to complete the final steps with the antenna installation and getting the station licensed. Hope to be finished by October.
Is the entire roof below the antenna? If so, I think you will be fine. Don't worry.
I recommend doing your best to build the exact facility specified in the CP.
If your LPFM CP facility is fully-spaced and did not require a waiver, it should be possible to file for a modification of CP for whatever you can build, provided it does not exceed the maximum technical facility permitted for LPFM.
Greg, The metal roof is about 12' below the antenna and behind it. I thought the roof might alter signal propagation but that doesn't seem to be an issue.
I had a friend in Perú tell me of an interesting case of an FM on the roof of a building with a slightly sloped roof (just enough to have rain run off rather than pool) made of zinc coated metal sheeting which was layered over tarpaper seals. The sheets were square, and almost exactly the size of the station's FM wavelength. They ended up soldering copper screen between them all and some odd signal issues in nearby areas (up to about 2 kw away) disappeared.
This is the kind of thing you only learn about from experience and having friends like you, Greg, who tell us those stories. We file them away in the back of our mind and know what to do if we have a similar experience.
For a low gain antenna, the metal roof will usually be strongly illuminated from the antenna. The resultant pattern is a combination of the direct path and reflected path off of the roof. One can easily get a pattern that is much different from the antenna located in free space (lower radiation toward the horizon). I'd recommend modeling the antenna and metal roof using a free method-of-moment program like EZNEC. Not hard to learn. The further away the antenna is from the roof the better, lower illumination. This also depends on the size of the roof.
Looks as if the metal roof question is now moot. The site owner's wife doesn't want my antenna anywhere near their business (even though it's well beyond the 21' requirement from people). The owner offered me another site nearby that is an open field and has offered to build a cement pad. So now I'm looking at getting a Rohn 50' self-supporting tower with a hinged base.
Is she afraid of RF, or is this an esthetics issue? Just curious, since there seem to be so many people that are afraid of RF... yet they go around with their cell phone "attached" to their ear for hours on end!
That is an elegant solution, particularly the hinged base part. Is it winch operated by hand or does it require an external motor source like a vehicle attachment?
Yes, she's afraid of the RF exposure. I couldn't convince her that the antenna is at a safe FCC-mandated distance. Actually, I was looking at the towers that you can just tilt into place, no motors, no winches.
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Yes, that was kind of the idea I had as well. I mentioned the Rohn towers which are galvanized steel. But I'm thinking the Universal Towers which are made of lighter weight aluminum might be easier to manage and a better choice.
I used aluminum towers up to about 300 feet in Ecuador; customs was as much about weight as value (You can cheat on the commercial invoice, but not the weight) so all my FMs and most of my AMs used aluminum. Finally a local company started building towers out of galvanized steel and they were even less expensive and the company did installation, relieving me of liability issues.
And that brings up insurance: make sure you are insured for the installation process or that the installer has insurance that covers you as well.
Have you considered leasing space on an existing structure? I'm thinking of the tower next to the water tanks on Richard Avenue that currently supports K215AF's single-bay antenna. A community partnership with CCSD may be possible to cover the cost of tower space and a transmitter enclosure.
Yes, I did. I spoke with the station manager of KBCX which has that translator you spoke of. He actually gave me the contact information of the engineer who set up the translator. He said they had nothing but interference problems with that location. The antenna sits on top of one the water tanks, surrounded by state and county communication antennas. It took them months of trial and error experimenting with different filtering methods to make that site useable. It turned out to cost them a lot more money than they had planned. I may contact that engineer. Maybe he can help me with my installation.
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