Groundplane and grounding on a flat roof

RichPowers, while reading your question about flat-roof grounding, this idea flashed in the brain:

WHAT IF there is a professionally installed lightning rod up on the roof, and one grounded their transmitter to that ground wire. Would the transmitter than actually become a lightning rod?

Here is one experience, from a micro FM broadcaster.

I had a transmitter, mounted in a weatherproof box (much like the Rangemaster), about 6 feet above a metal roof, no RF ground wire.

Circumstances forced me to change my mount type and installation, so that the box was mounted a few inches above the metal roof, again, no physical connection to that roof. Imagine my surprise when the signal strengthened considerably, to the point where I put it back to where it originally was, as I was afraid of violating Canadian field strength regulations.

I came to the conclusion that I had obtained a capactive coupling between the metal roof and the transmitter (since FM unlicensed broadcasting is governed by field strength only, unlike the FCC AM Part 15 regulation of limiting the power input to the final of 100mw, there was no point in leaving it there). Now, I never attempted this with my Rangemaster; I always wanted to but never got around to it for various reasons. But I see no argument why, if you put some metal sheets underneath the transmitter, and then mount it several inches above those metal sheets, you would not obtain the same effect. And no worries about long ground wires (whether it's the actual wire itself, or grounding to an elevated mast, which the FCC has frowned upon). How an FCC inspector would view this would be an interesting question.

WHAT IF there is a professionally installed lightning rod up on the roof, and one grounded their transmitter to that ground wire. Would the transmitter than actually become a lightning rod?

Actually Carl, I spoke on this before. If you'll take a look at this picture; - See the octagon shaped roof of the pier?.. My original Rangemaster was mounted on the very center point, and grounded to the existing ground system which ran down each section of that octagon. And there were 8 little 2 foot spikes protruding up from each grounding cable about halfway up the roof.. (lighting rods).

So in essence the transmitters ground was also a huge circular elevated ground plane consisting of eight 3/4 inch copper cables..
I suppose that is why I was getting a clear strong signal in excess of 5 miles at only 100mw! - It is also probably why I had to send the transmitter back after a few months to have the lighting protection gas tube replaced.. It seems to me that my transmitter did indeed become a lighting rod.

By the way, this new location I'm considering is exactly 1 block away from my original location on the pier.

And David, are you suggesting to try a metal roof and not ground the transmitter to anything?.. I thought that was dangerous.

"So in essence the transmitters ground was also a huge circular elevated ground plane consisting of eight 3/4 inch copper cables..
I suppose that is why I was getting a clear strong signal in excess of 5 miles at only 100mw! - It is also probably why I had to send the transmitter back after a few months to have the lighting protection gas tube replaced.. It seems to me that my transmitter did indeed become a lighting rod.
"

Yes, that would definitely constitute a great ground system, but I think the elevation, considering that the cables ran down outside, constituted multiple radiating elements above ground, like a dipole with an array on the ground side, both horizontal and vertical. I'd say the cables were radiating as much as the whip ... perhaps even more. The thing would likely be considered illegal for those reasons.

With those very heavy ground cables, perhaps a lightning strike along with the gas tube break (did its job) went to ground before it fried your TX. Anything less would surely have turned it into burnt offering.

Also, it is in very close proximity to the sea, which can carry a signal quite a ways along the coast line, especially if there are buried power lines along there as well. The ocean is one heckkuva counterpoise, even if not directly connected to the TX.

http://www.fcc.gov/mb/audio/m3/index.html

Yes I'm familiar with that map.. mine's just a 4.

But I'm trying to determine the best way to install a certified am transmitter on top of a flat roof while staying within the rules.
What's a legally acceptable ground for a transmitter, when on a roof?

In recent FCC cases (KENC, Liberty Radio) involving elevated ground systems, inspectors and the OET lab have made it clear they don't consider these installations legal under Part 15, if the ground systems add to the radiated signal. The simple test used by inspectors is to temporarily disconnect the ground. If the signal goes down or changes, the length or height of the ground system is considered to be included in the 3 meter length allowed under regulations.

KENC had one transmitter on a 40 foot tower and another on a 100 foot public water tower. Both installations were cited by the same FCC inspector on 2 different occasions. Caution and forethought should be exercised when planning an installation of a Part 15 AM transmitter on top of an elevated metallic surface or structure

These are recent developments in the field interpretation of the rules by FCC inspectors. Your results may vary.

KENC had one transmitter on a 40 foot tower and another on a 100 foot public water tower. Both installations were cited by the same FCC inspector on 2 different occasions. Caution and forethought should be exercised when planning an installation of a Part 15 AM transmitter on top of an elevated metallic surface or structure

True, but KenC also stated:

" We had one of our 3 around town Hamilton's atop a high school football stadium three feet from the metal roof over the stadium grounded to it. It was acceptable to the FCC inspector that cited us for the other two that were on towers.

So, he was only cited for attaching to radiating structures - correct?

The problem with the school installation was that we got very poor range with it. The roof had a 5 degree pitch for water to run off and we found that it acted as a directional deflector. ..."

And later said..

"..Though you don't get as much range without the ground connected at 40' it does work and it does work well enough for the effort. It's my feeling that in the Pacific Northwest, unless you are willing to disconnect your ground and put it up high, you will get cited.

And then his last statement just answered one of my questions!
I've read it before but missed it!...

One other way is set it atop your house roof, attach it to your metal vent stack and ground it there."

So now that I have an idea what to ground the transmitter to.
There is still the question of the ground plane..
Can I legally construct a ground plane under the transmitter, as long as it is not connected to it? And if so, does it matter how I ground that?

By the way, the bookmark I got those quotes from is http://groups.yahoo.com/group/Rangemaster-Transmitters/message/2397

Good research, too. As your research shows, one FCC field test for a radiating ground is to test the field strength of the operation and then to disconnect the ground lead and test again. If there is a big difference, the assumption is that the ground is radiating.

If you have no connection then at least you're not subject to that test 🙂

I'm not sure you'll get the kind of parasitic influence you are hoping for in the large wavelength AM band, but it might be worth a try.

Great photos and animation, btw!

Another option would be to abandon the roof top and go for a location where you can sit the transmitter right on the ground above a network of buried radials and use a small capacitive hat to pull the signal up the vertical radiator a bit.

Good research, too. As your research shows...

Well, thanks, but that wasn't really research.. I just copied and pasted his words! Although previously I did spend ample time searching forums and the web for details of what happened with KenC. That whole story is finally pretty clear to me now.

Another option would be to abandon the roof top and go for a location where you can sit the transmitter right on the ground above a network of buried radials and use a small capacitive hat to pull the signal up the vertical radiator a bit.

Thats not gonna happen! I can't even begin to phantom an idea of where I could sit the transmitter on the ground. As for the capatance hat.. that would void the FCC certification.

"Parasitic influence"... hmmmm

I was speaking about an RF ground. The transmitter is isolated from the mast and the roof. You can still ground the mast itself to the roof ground for lightning protection.

I don't know either if my experiences translate to the AM band. But, as stated, it might be worth the effort to find out - it would be pretty easy to just mount the transmitter close to the roof and see what happens. I seem to recall another post where someone had mounted an AM transmitter close to a metal roof in a barn(?), he disconnected the ground wire and actually got a better signal. And, as also stated, you don't have to worry about the FCC disconnecting your ground to see if it radiates.

It doesn't really matter if your station is 100% legal in every way. I may be redundant here as I've made this point before.

The FCC doesn't snoop around looking for Part 15 Broadcasters. Someone most likely complained for whatever reason. If the FCC has enough incentive to respond, all they have to cite is an interference problem, legal installation or not.

Yes, if your signal sounds really decent 5 miles away you're much more noticable and probably in violation. But let's say your signal is being inductively coupled to power lines or water lines or sewer lines, etc. Your antenna may be ground mounted. Everything 100% legal. If someone complains, the FCC responds and shuts you down.

So, don't worry. Be happy and don't argue with the Inspector.

Say Rich ...

Is there any way you can re-install the TX back where it was on the octagonal metal roof on the beach?

What I mean is that if it is only a block away, it shouldn't be too difficult or expensive to establish a wireless audio connection from your new studio location to that building on the beach. That would leave you with the power supply and legal ground to deal with on site ... meaning you'd have to leave its ground disconnected from the roof and establish another legal ground.

With the roof and the ocean both as counterpoise rather than connected ground, you should still get out OK, but not 5 miles.

"Can I legally construct a ground plane under the transmitter, as long as it is not connected to it? And if so, does it matter how I ground that?"

Short answer: Sure. If you have enough metal laid out on the roof, it might help. Dunno about grounding it, but I'd think that would be OK, too.

The only other caveat is, as already mentioned, the non-interference rule. If the signal is getting into someone's audio system or partially blocking an adjacent station, and they complain to where the FCC shows up .... you're done, at least on that frequency.

I've forgotten what your goals are, if you said, couldn't find same. What do you want to do with your Part 15 station? Feel free to write me off forum if you wish.