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I have a metal roof and am interested in mounting my part 15 transmitter up there. How do you wire it all together? I currently have my system mast mounted. But thought the metal roof might work better for my application/
Total posts : 45366It would be best if you read some of these post’s from the past.
http://part15.us/node/765
http://part15.us/node/1450Use the links above as a reference, there is much controversy over mounting a part 15 above a metal structure.
If there is older post i missed then maybe someone can chime in, also our search function works great and will point you to the information your looking for.From my experience with cb radio (11 meters) i had a Fiberglas groundplane without the ground plane kit.
It was mounted on a chimney mount 5 ‘ above a tin roof.
My swr was a bit too high, so I added another 5 ‘ mast pipe to the ground plane and reinforced the whole thing with insulated guy wires.I worked pretty well, but 11 meters behaves a bit differently than the AM Broadcast Band so I had area’s in town my signal was weak and others I was loud and clear to mobile radio’s.
The reason my SWR was so high with the 5’ mast is because of reflection from the tin roof. Basically it was like a mirror.
The signal was bouncing off the roof back to the antenna which is hard on the radio and creates nulls or dead spots in both my recveive and transmitted signals.In your case it might work but there is the matter of the 3 meter rule.
If the part 15 tx is bound to the roof as a ground with wire it would add many feet to your grounding system thus breaking the 3 meter rule for antenna length, feedline and ground.
Total posts : 45366If you mount your transmitter a few inches above the metal roof, there will be capacitive coupling, and the roof should act as ground (with no radiating wire to confuse the inspectors).
All you have to worry about then is grounding the entire installation for safety reasons – but it doesn’t have to be an RF ground.
Total posts : 45366So you wouldn’t ground the transmitter to the roof then? If the spacing is right?
Thanks.
Mike
Total posts : 45366How about a ground clamp on the antenna (if metal to metal), a few inches of heavy wire, inline gas discharge arrestor, then heavy wire straight to ground via shortest path?
Total posts : 45366One of the misconceptions that keeps cropping up on here is that a lightning ground is somehow different than an rf ground. They are so similiar as to be practically the same for all practicle purposes. Lightning is a high energy-short duration pulse the you could say mimics one or two cycles of a very high (megawatt) power transmitter. Using a choke or any other method to stop RF from flowing in the ground lead will also stop lightning from flowing to the actual ground or ground plane thereby causing an energy buildup and possible flashover. The foregoing makes me pretty sure that if you wish to follow both FCC Part 15.219 and the NEC then the transmitter has to be mounted at ground level. Kind of a catch 22.
Total posts : 45366Seems as though the FCC’s retraction on the KENC NOUO for tower mounting a Hamilton Rangemaster should have resolved the question about the need to ground mount.
Or, is that still a case by case action? Maybe this should move to the KENC thread.
Total posts : 45366Still no final ruling on KENC. He is still broadcasting the tower-mounted Rangemaster with the ground disconnected. Odd … the other synced Rangemaster installations haven’t been cited at all.
Total posts : 45366According to KENC’s website as of December 14th, everything was put back as it was before the inspector visited. He maintains that it was and still is Part 15.219 compliant.
He states on his website that an affidavit was requested and sent to the FCC stating that his installation complies with Part 15.219. Doing so, he states that the FCC would dismiss any action against KENC.
I have emailed Ken for clarification as there still seems to be some confusion over the final result.
You can read KENC’s post on his home page.
Total posts : 45366Well thanks to Rich Fry I did a little more checking.
An email to me from KENC states that the transmitter ground has NOT been reconnected and they are waiting for confirmation from the FCC regarding reversal of the NOUO. So, the jury is still out on this one.
Ken tells me that Keith Hamilton has engineered a filter to place in the ground lead at the transmitter. Popular opinion is that this will prevent the “long ground” from radiating which in turn will severely reduce the transmitter range.
Total posts : 45366That was before Christmas and not complete. OTOH, this was posted to the LPAM group list on Saturday (emphasis is mine):
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Gentlemen, Thought I’d let you know that even though I got a phone call just before Christmas, from the Inspector that cited me, telling all is well, I’m still waiting for something in writing from him or the DC office. As of yet, I have received nothing. We still are transmitting, but the one Hamilton that was his concern still has no ground connected to it. Keith Hamilton said that his new ground filter would be ready soon and we hope to install it on that transmitter and reconnect our ground.I’d like to say thanks to all who wrote and to those few who donated to the cause, for your support… As we say at our station. “It’s not how big your watts are, it’s how you use them”.
Kenc
http://www.kencradio.com
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Total posts : 45366“… the final communication I had with the inspector was by phone when he called me to say,” Based on advise from their legal consul, he would suggest that I reconnect my ground, follow the three meter rule in doing so, retune the transmitter to 100 Milliwatts, send him a statement both by email and ground mail verifying that I had done that and they would dismiss the citation.” There was no “Thank you, I’m sorry, or we apologize” and that’s OK with me too. Just that I am free to transmit [h]as I had been doing before the first visit.. He also added that the field strength meter ruling would not apply to part 15.219 transmitters…
That’s it, in a nut shell… Keith tells me he has a filter system in the works that should take care of the grounding. As soon as I have it, I’ll go back up the tower, install it and the ground, retune and go forward. I am currently operating the station with three transmitters. The one transmitter that was the issue has been running without the ground, and it did affect the range without the ground but still get good range…”
Simply put, Ken is not waiting for the sun to rise in the west anytime soon. The FCC inspector is not required to communicate with Ken any further. Ken will very likely not receive any notice of the dismissal of the citation. There will be no public epiphany concerning a generalized rule or ruling concerning the general usage of Part 15 certified equipment used for community micro-broadcasting. Continued contact by the inspector after his notification to Ken could have been construed as harassment and is frowned upon by federal law.
As a suggestion, Ken could have presented the inspector with the certification paperwork supplied by Hamilton to show to the inspector. If he decided to site Ken after that, you can bet he had other motives. Having a 3 ringed binder with all of your paperwork is helpful and instructive to the commission and its staff.
Total posts : 45366I would suggest a study of differences between antenna counterpoises and earth safety grounds. They serve different purposes and frequently in antenna systems are separate for very important reasons.
For instance, an AM BCB vertical radiator works against a ground radial system that operates in the systems as a counterpoise to raise the radiation efficiency of the antenna.
The safety or DC ground is accomplished 2 ways. One, by the use of a static drain choke at the ATU feed to the tower connected to earth ground; the other a pair of static or lightning discharge “johnny balls” or contact points at the base of the tower. The choke also works as an RF block for the AC service to the tower lighting system.
Because Part 15 antennas are so short, static discharge devices are rarely needed. Ground mounting an antenna system does require, under other sections of the FCC rules, a safety fence to keep people and pets away from the hot antenna and ground leads.
A good reference for the differences in ground systems is covered in the ARRL Antenna Book. I have taught ground system theory for years and have had some spirited discussions with electricians about the exclusive use of the NEC Code. The NEC Code, oddly enough, does not cover radio frequency circuits or rules.
Total posts : 45366So I guess if you did a roof mount you wood ground the antenna (example grounding lug on hamilton rangemaster)straight to earth. Does the roof need to be grounded to earth.
Respectfully
Mike
Total posts : 45366Marshall I took a two week class years ago on EMP “hardening” for 911 centers, commercial radio and tower systems in general. Now that being said the things I think I know are from a technicians standpoint as opposed to an engineers knowledge (and yes over the years I fixed many problems caused by engineers but the real knowledge flows from the engineers). In these federal government classes it was stated that lightning is the same as rf for all practicle purposes and if you filter one you filter the other. Now the broadband element of lightning is where I think this”filter” discussed on this forum could work possibly but there would still be a lot of energy backed up behind it to possibly cause damage and in my opinion wipe out any practicle “home” filter. I guess I should say I cringe everytime I hear “ground filter”. To me any filter that keeps rf out of the ground lead also keeps lightning out (not a good thing). You can either block or pass any given frequency, series or parallel resonate circuit, but either way you either are blocking a frequency (then you purposelly dump it to ground from the filter, making the ground radiate) or you pass it, again making the ground radiate on purpose. The energy always has to go somewhere, complete circuit and all. I also wish to comment on counterpoise purposes in lightning protection. If your ground system or soil is poor you can install a ground screen or wires or large underground plate to “hold” the large microsecond charge for a few milliseconds while it bleeds off slowly into the “poor” ground. Is any of this this wrong and if so how? If I am correct then this idea of a cheap”filter” needs to be laid to rest. The only way I can see it working is in an engineered system that has many layers of protection including spark gap, gas discharge and small protectors at the margins etc. Probaby beyound most home installations. Home intsallations= big wire going in a straight line to good ground 🙂 Dave
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