Advice for roof-mounting GNSS antenna - Page 1
Forum section: electronics / projects

author: matthuszagh
author_rank: Frequent Contributor
timestamp: June 08, 2023, 04:57:49 pm
content: I'll be using a Novatel GPS-703-GGG pinwheel GNSS antenna (https://novatel.com/support/previous-generation-products-drop-down/previous-generation-products/gps-703-ggg-antenna) for precision timing applications. I'm planning to mount it to my roof and, as I've never done this before, would appreciate suggestions for achieving best results.I've attached pictures of the roof I'll be working with. I suppose the first consideration is roof location. The easiest location for me to access is the lowest section of the roof (in image 3). This would also leave the cable to the GNSS receiver the shortest. But, maybe I should be concerned with reflections off the higher roof segments at this location. Another possible location would be the lower left corner in image 2. That's a bit further from the window (i.e., longer cable) but should have a slightly clearer sky view. Finally, I could try to mount it near the top for the best sky view, but that would require clambering up the roof, which I'm not eager to do. It's worth noting that the pinwheel antenna should provide pretty good multipath rejection.Now for mounting. The cinder block method used by sparkfun (https://learn.sparkfun.com/tutorials/how-to-build-a-diy-gnss-reference-station/all#introductionunder "Affix Your Antenna") seems like a good, cheap option. My roof does have a bit of a grade to it - about 13deg at the lower sections (I didn't measure the steeper grade of the upper section). If I need to be worried about the cinder block sliding off, I could drive in some nails downhill of it (other suggestions welcome too!). Another, more expensive option, would be to use something like the Pasternack non-penetrating antenna mount (https://www.pasternack.com/non-penetrating-antenna-peak-roof-mount-1-pole-version-60-inch-mast-galvanized-steel-with-powder-coating-peprm1-p.aspx). This has a 5ft mast. This would use cinder blocks to weigh down the mount, or drill holes with screws for fastening. How much benefit, if any, would there be in having the antenna mounted further off the roof? Another option, which would require drilling screws into the roof, would be something like this:https://www.ebay.com/itm/124845460270. That still mounts the antenna closer to the roof but is cheap and I wouldn't need to worry about a cinder block sliding off the roof and crushing someone below or breaking my antenna and pulling my GNSS receiver out of my window... Any special considerations for drilling into a roof?I'd probably go for the last mounting option (short, screw mount) with the easiest location (3rd picture), unless people recommend I do something else. Thoughts?In case it's relevant/useful, I'm in Oakland CA. I'd been planning to mount the antenna in the direction normal to the roof, since that's easiest to mount. With the slant, that would have the antenna tilted a bit north (actually about 15deg east of north). But, if it's better to have the antenna facing straight up or potentially even tilted in a different direction, I'd be happy to consider that too.The novatel antenna has a 5/8"x11 thread.

author: bingo600
author_rank: Super Contributor
timestamp: June 09, 2023, 12:45:07 pm
content: The "Birds" are far out in space, so a few meters of extra height doesn't matter.What matters is view ... Best is 360 degrees , and no roof blocking.Reasonable cable length (10..30m) usually don't matter if you use a decent cable.I'm using a good quad shielded 75ohm cable (Sattelite cable) on mine , and even use F-connectors. As they are so easy to fit.The impedance diff between 50ohm & 75 ohm receiving, on that length of cable doesn't seem to matter.I have an N-F adapter on my timing "Ice-Cone" , and another N-F on my GPS Splitter.Re mounting:I'd just use some of the Antenna mounting brackets , or even a Sattelite dish mount.Just had a look on the antenna now, seems like it's using the "survey pin mount"Something like this would prob. fithttps://www.aliexpress.com/item/1005002341113865.htmlMaybe a aluminium would be better (doesn't sway)https://www.aliexpress.com/item/1005005090757843.html/Bingo

author: jmelson
author_rank: Super Contributor
timestamp: June 09, 2023, 02:43:04 pm
content: If you have lightning in your area, be SURE to provide a stout ground to the coax.Jon

author: matthuszagh
author_rank: Frequent Contributor
timestamp: June 09, 2023, 03:53:24 pm
content: Quote from: bingo600 on June 09, 2023, 12:45:07 pmThere's a flat section on the top of the roof. That should provide a pretty good, 360deg horizon to horizon view, and should be large enough to install multiple antennas with adequate spacing. My house is one of the tallest in the area, which should help. It's around the same height as the tallest surrounding trees too. I'll have to get up there though. The roof is asphalt shingle and provides a good grip. Plus, the grade isn't too steep. I can install a roof harness as extra protection and I'll probably just leave it up there so maintenance is easy.Quote from: jmelson on June 09, 2023, 02:43:04 pmYeah, lightning protection is definitely something I plan to incorporate. I'll use coax with good shielding/ground anyway. From a quick look, it seems that the common solution is to use an inline lightning arrestor outside the house with a good ground connection to earth ground. Is it enough for me to connect this ground with suitable AWG wire (eg 6 AWG or less) to an ground prong outlet on the outside of the house, or should I try to track down the earth ground rod and attach it to that somehow? Is this enough protection? Anything else I should do? Lightning is pretty uncommon where I live, but I really don't want an incident frying all my lab equipment.

author: EE4all
author_rank: Regular Contributor
timestamp: June 09, 2023, 06:29:30 pm
content: The best protection will be coax to ground level outside of the structure, to the common structure ground, where the protection device should also be located. If this is at a separate ground rod from the structure ground system, then the two should be bonded together. From that protection device, ingress to the structure and to your equipment location. That is the right way to do it. Bonus points if you then have a common signal and AC ground point at the room before it goes off to individual gear AC or signal connections.Or you can just do what many people do and throw out those best practices, running a ground wire from your antenna to ground and believe you are protected, but actually be worse off than if you didn't have any ground. I would avoid this. There will be pushback to my statement here, as yes many satellite installers do it this way, and it is maybe acceptable to some, but I don't like it because you can end up with different ground potentials in your system, and those surges easily take out equipment as the potentials equalize through the equipment to AC ground.FYI, some insurance companies will not cover if you do not follow the best practices. YMMV

author: EE4all
author_rank: Regular Contributor
timestamp: June 09, 2023, 06:38:34 pm
content: I also agree on the 75 ohm common TV coax usage. I do that too and it's fine.

author: matthuszagh
author_rank: Frequent Contributor
timestamp: June 09, 2023, 09:01:06 pm
content: Quote from: EE4all on June 09, 2023, 06:29:30 pmI'm keen to do it the right way - don't want to take any chances with lightning entering my room and killing me and all my equipment...I'm not sure I totally understood your description though. It sounds like the coax should go down to near the earth ground pole, where the lightning arrestor should be placed. Then, the earth ground connection on the lightning arrestor should be attached to the earth ground pole, or something leading into it. I then connect the other side of the lightning arrestor to my equipment (further around the house and then into my window). Is that what you're suggesting? Please correct me where I've misunderstood.I found this (https://www.waveform.com/pages/signal-booster-antenna-grounding), which seems like good advice. Based on that, I was going to attach a copper wire to the metal mounting pole. This would then attach to the metal earth ground pole, or the attaching thick metal wire. I would also use an inline lightning arrestor, with the ground connection attached to the same point on the metal earth ground pole with a similarly thick copper metal wire. It sounds like you're saying the arrestor should be placed near the earth metal pole and not just before it enters my room near the window. Is that right? That would mean the coax would need to be quite a bit longer, but I suppose that's ok.By the way, am I right in thinking that the following pictures show my earth grounding pole? Do I expose some metal on the wire and weld it to that? Or, maybe, should I bolt it to those tabs on the electrical panel? I can check for continuity between that and the wire.

author: matthuszagh
author_rank: Frequent Contributor
timestamp: June 10, 2023, 12:27:04 am
content: It appears there are ground clamps I can use to attach the wire to the grounding rod wire. Do I need to scratch off some of the grounding wire insulation/paint to expose some metal surface and improve the conductivity, or is this sort of insulation gap totally negligible for a lightning strike?I also found this link helpful:https://www.groundedreason.com/how-to-ground-an-outdoor-antenna/. And, I downloaded and am reading a copy of the national electrical code.

author: EE4all
author_rank: Regular Contributor
timestamp: June 10, 2023, 12:36:24 am
content: Yes, run the coax down there and mount the lightning/surge protector nearby, bonding its ground to your house ground. Do not try to solder it. Only clamps or an exothermic weld should be used. A clamp is fine. Use a wire brush or similar to clean away the oxidation there. Coat the clamp and wires where they meet with Jetlube SS30 (can be found from Amazon) or similar. Connect the coax that will go inside to the protection device and have it exit the area away from (at least not right alongside) the antenna down coax. Most all GNSS antennas are amplified and need a DC bias on the coax to power them, so make sure you don't get a DC grounded protector.

author: helius
author_rank: Super Contributor
timestamp: June 10, 2023, 01:08:37 am
content: Quote from: matthuszagh on June 08, 2023, 04:57:49 pmBecause roof materials (like asphalt shingles) are full of small stones, it is more common to use nails rather than drill and screw. There are specific types of nails, copper plated and ridged, used for roofs. You also need to know the joist spacing, usually 2 ft but can differ, so as to go into a solid roof beam and not just make a hole in shingle. If you did drill holes using a hardened bit, I would consider using lag bolts instead of screws along with nuts on the inside of the roof beams. To keep the roof watertight, a "scypolymer" roof sealant such as Geocel should be used on the holes before tightening the bolts. You could also use bitumen sealant.

author: jmelson
author_rank: Super Contributor
timestamp: June 11, 2023, 01:09:58 am
content: Quote from: matthuszagh on June 09, 2023, 03:53:24 pmAgghh! DON'T tie the lightning ground to the house wiring. If you do, a lightning strike will likely blow out every electronic device in the house. You want to send lightning current into the ground away from the house wiring.Jon

author: matthuszagh
author_rank: Frequent Contributor
timestamp: June 11, 2023, 01:31:08 am
content: Quote from: jmelson on June 11, 2023, 01:09:58 amThat's no longer the plan. I'm planning to connect it to the thick wire connecting the electrical panel to the grounding rod with a ground clamp. This will all be on the outside of the house.