Grounding an antenna/tx to a rooftop

If you don't have a metal roof, you could lay ground radials out on the roof, but they must also be grounded somehow for protection.

If you have a metal vent pipe that goes all the way to ground (mostly old buildings), and you can test it's continuity at the ground, you can bond your radial system to that.

Maybe you can drill a hole in your roof and make your ground wire through a conduit to ground?

Otherwise, I'm stuck for an answer ... anyone else?

Ken is absolutely correct in saying you need a ground connection for safety.

A ground plane at roof level won't help antenna radiating performance. Whether it is a metal roof or a bunch of radial wires on the roof doesn't really matter. The metal or radial wires will add capacitance to the overall antenna affecting tuning, but won't significantly change the effectiveness of the overall antenna.

The overall antenna consists of the 18 ft ground run plus the 3 meter antenna on top. Regardless of any roof metal or radials, the performance of the overall antenna is primarily governed by the elevation and earth ground resistance. When people talk about roof grounds, I think the expectation is that the real ground connection at earth level doesn't need to be very good. In reality, it is very important. Earth-level ground radials are crucial to best performance.

A single 8 ft ground rod for lightning protection is the minimum requirement. A ground rod provides maybe 50 ohms or more ground resistance for the antenna. An earth-level radial system can provide 10 ohms or less.

With regard to the liability for FCC action, a roof mounted antenna will put you at some risk. You can reduce the risk by using just a single ground rod to cripple performance relative to a earth-level radial system. For any given ground resistance, adding a roof-level ground plane won't change your signal range noticeably.

Thanks guys, as usual I have found this forum to be very useful.

Take care.

eddieb

My roof-top antenna (over a wood frame house) using 6, 12 foot radials (ungrounded) made quite a difference in my far field coverage. Near field didn't change much due to re-radiating off the power lines running the length of the road.

Using the Talking House ATU remote antenna, I tried it with and without radials. Radials won hands down.

When I operate the station with my hands down I can't operate the station.

With or without radials.

I keep my hands off, the thing runs itself.

"Using the Talking House ATU remote antenna, I tried it with and without radials. Radials won hands down."

I have to wonder how it would do if you actually grounded the radials? With no ground, I sure hope you don't get a lightning strike or any sort of high power short.

I do have a Blitz Bug protector on the coax but I would guess for a direct strike it wouldn't matter.

Hey, MRAM 1500 kHz. How's the range on your broadcast with your rooftop grounding method?

eddieb

mram1500:

There is a possible explanation for your observed increase of signal with the roof radials. The explanation centers on the details of your coax run from the TH to the ATU.

First, my previous post, saying essentially that roof radials won't help, came from NEC modeling for an antenna elevated 18 ft with radials under the transmitter or ATU at the 18 ft level, and a decent earth ground. I tried 10 ohms and 50 ohms for the earth ground. Results from both simulations showed very little current flowing in the roof radials and the rest (most) of the current flowing in the 18 ft ground run.

If a very high impedance is inserted in series with the ground wire to earth ground, then the roof radials will act as a counterpoise for the antenna. The ATU will see a closed circuit with almost equal current flowing at the bottom of the antenna and the roof radials. Any difference in these currents will flow in the ground lead, but when the ground lead has very high impedance, the ground lead current will be very small. So, maybe something is causing your coax ground impedance to be very high.

Can you provide some details about your coax run? As I understand, the standard TH coax to the ATU is very long and they don't recommend shortening it. Are you using the standard, long coax? Did you coil up the excess coax?

With no other earth ground connection, the TH coax connects to earth ground through the indoor transmitter which has a 3-prong grounded power cord. So the coax is connected through your AC wiring to the the service-entrance ground rod.

Coiling up several dozen feet of excess coax in a 1 or 2 ft diameter coil will create quite a high series inductance in the coax shield circuit. Such a high ground impedance would be a candidate explanation for your observations.

The signal coverage for my Talking House with ATU remote antenna is fairly solid for about .4 miles. Afer which, the AVC of the receiver begins to bring up the background noise.

I have "heard" my signal as far as 2 miles away although it's not what the average listener would want to hear.

There are the occasional hot spots around town where out of the blue, the signal pops in very clear.

My coverage map is found HERE on my MRAM Webpage. Although not 100 percent correct given topographical variations, it's reasonalbe.

The coax is about 35 feet. I did try coiling up some of the coax specifically to see if it would choke out signal flowing on the outside of the coax to ground. The coil was about 5 turns, 10 inches in diameter within 10 feet of the antenna. There was no appreciable difference noted.

The coax does not connect directly to the antenna. Rather, inside the ATU, there is a torroid transformer between the coax and the loading coil. I suppose it is for impedance matching and isolation.

Once again, I appreciate your post to my original question, MRAM. Thanks. Really helped me make a determination about how I can mount my Procaster up high and remain legal.

eddieb

Never drill a hole through you roof.

EddieB,

I'm not saying my installation would stand up to FCC scrutiny.

I feel that my installation is legal as the Talking House transmitter is certified for use with the ATU remote antenna.

A heavily advertised unit known as Info Spot is based upon the Talking House with ATU remote antenna. They specifically state that it is legal under Part 15 and say it will work with up to 300 feet of coax. They packaged the Talking House in an expensive box along with some audio support equipment. They also mount the ATU inside another box with the whip antenna mounted to the box.

The big question is about the elevated radial system I use not referred to by Info Spot. A radial (or counterpoise system) is referenced in many antenna info sources as NON-RADIATING when properly installed (symetrically balanced.) As such although it is part of the antenna "system" as is a ground rod, it is not a radiating element. As such it should not be considered as part of the 3 meter rule. But, that's my opinion and up to discussion with your local FCC Field Agent.