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attic antenna grounding


lazarus1024
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I've lately been getting serious about GMRS and radios in general (this summer when my kids are out of school and I have a bit more time I plan to study for and take my HAM technician license test). I know the path I am headed down which is nickel and diming my way to a good setup, spending a lot more money than if I'd just "done it right" when I started. Been down that path with other hobbies. At the same time, it is often an enjoyable path and I learn a lot more than just "doing it right" to start with. I just mean, getting really nice equipment at first. I like to do things properly as I go (installation, licensure, safety, etc.)

Should I ground the coax feed for the antenna? It would be easy to do in my attic as I have a 10ga ground wire from a jetted tub I removed from my master bath when I renovated the bathroom last year. And I am installing the antenna within a few feet of where the wire runs. I removed the ground wire up to the point of the attic, but I didn't bother pulling the 100ft of ground wire all the way back to my panel. As it is an attic antenna, I am not worried about lightning strike protection. And static build up shouldn't be any real concern. Since the ground point would be all the way back to my electrical panel ground bus bar and then to earth, would that help reduce any possible extraneous RFI? Or is that likely to add RFI? A lot of the stuff I read about RFI is informative, but I can't help wondering how widely applicable it is. RFI at 10 meters is very different than RFI at 462/467Mhz. Is RFI much of a concern at the shorter wavelengths? And if so, would this ground path be likely to help? Hurt? Neutral? Everything I've read has said to keep the grounding as close as possible for a short and wide ground strap for the coax. But at times I see it mentioned, seem to be people who are working with HF installations where the ground path can be meaningfully shorter than the wavelength. There isn't a chance in fluffy bunny hades I can have a shorter than 70cm ground path, or even a shorter than 2m ground path if/when I take up HAM. I've seen conflicting diagrams of grounding the antenna coax at the radio and then directly to a ground rod (which is bonded back to the service ground). Other diagrams I've seen have the antenna coax grounded at the antenna and running down to a ground rod that the lightning arrestor is also grounded to, but are often NEC diagrams for outdoor antenna installs. And lastly I've seen diagrams where the coax is grounded both at the antenna and at the radio.

Advice would be greatly appreciated. Another question would be, should the radio chassis be grounded? It is going to be powered off portable 85wh power bank that has a 12v/10a cigarette outlet and then the pack will be recharged using a 30w PD charger. None of those has a ground and the radio (DB-25g) as near as I can tell doesn't have a chassis ground on it anywhere. Though grounding off the SO259 connector should accomplish the same thing I would think, however if the coax is grounded by the antenna, that would also be a ground path (though much longer).

 

Separately for an outdoor antenna, as I might go down that route at some point if the attic antenna doesn't work out. That seems much simpler. Direct ground on the mast to a ground rod, bond the ground rod back to my service panel, ground the coax also to the ground rod directly as close to the antenna as possible. Can that grounding on the coax be a lightning arrestor? Or does it need the coax grounded to the ground rod AND a lightning arrestor installed and grounded (so three connections to the ground rod, mast, lightning arrestor and coax. Or just two, mast and lightning arrestor)? If I were to install a second antenna, does that antenna need its own set of grounds to a different ground rod? can it run to the same ground rod? Can the grounding points for both antennas/coax and combined and then run down to the ground road as one wire to save having a ton of wires all running down?

 

The 2 antenna question is in part because of where I am located I am mostly trying to hit my brother-in-law's house 4 miles away, over a ridge as well as a local GMRS repeater over that same ridge, but about 20 miles away (and I think it is at higher elevation than my brother-in-law, probably not by much though even accounting for the repeater being on a tower). The ridge is about a 100ft rise over 1.5 miles and then 2.5miles and 100ft down to my brother-in-law's house. I haven't tested yet to see if anything is workable between us (he does not have a GMRS license or radio yet, but is considering both). The repeater I can pickup in some spots indoors as well as from my roof (single story rancher) on a UV-9g with a 1/2 and 5/8 wave length antenna. Not well on Rx, but intelligible. Tx I can only hit from my roof and I suspect it is BARELY reaching, but I get an acknowledgement from the repeater when I broadcast from the roof (I haven't tried a conversation with anyone). The two locations are roughly in the same direction, but perhaps around 10-15 degrees separated. I want an omni and I am hoping it'll be sufficient. I am installing an HYS tunable 6dBi antenna in the attic. If that isn't sufficient, but I can tell it gets me close I would try a quick temporary roof/chimney install to get it a bit higher and no attenuation from the gable end wall (just 11/32" OSB, house wrap and vinyl siding. The signal might hit a 2x4. So I assume minimal attenuation at 462MHz). And/or I'd consider installing a 2nd antenna, something like a 9-11dBi Yagi as I think the points I am trying to hit are close enough together that a Yagi would get me a significant amount more gain to those points than the 6dBi omni I am otherwise installing. I'd then just use an antenna switcher. I am hoping to test from my car using another DB-25g with a UT-72 antenna from my brother-in-law's house and use one of my sons to man the radio back at my house. My other plan if that doesn't work and is setup a ladder (he has a 17' ladder) and try from the top of the ladder with my HT just to see if I can get least get Rx with the extra elevation even though my gain would be a bit lower

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Most of the 2/70cm and gmrs antennas are going to dc blocked. Some of the ones I looked at said I need to run a ground to the base of antenna (memory is really fuzzy been years). Ground plane can be over come with chicken wire, pizza dishes etc.

Is the coax going out the side of the house? If so you could get static build up that way. If so I would suggest a way to discharge the static (poly phasers).

In my setup all the antennas coax lines are grounded to polyphasers in a dxenginnering outside box, which is grounded to the main electrical ground rod. The radios are bussed back to the dxengineering box. I don't like using grounding lines in the house, have seen some wonky electricals in houses we have owned. The point of bounding everything in a specific way is to make sure max ground potential is even. If the main grounding for the house is on the other side of the house. You may want to drive 8ft grounding rods ever 16 feet, and bond everything with 1-2 copper straps. If in doubt call an electrician.

A lot of the grounding work is to help discharge the static and help with common mode noises. Not really going to help with lighting in most cases, it just going to vaporize everthing in its path. Even in a properly grounded set up, lighting is going to do a lot of damage most of the time.

Common mode noise, which you will see more of on 10m. Most can be allieviated with air baluns or chockes.Fortunately, a 10m dipole hides well in most attics, barring metal shielding from vents, electrical and insulation.

Shockingly, most of the rfi you will experience will be comming from your house. The proximity to the antennas to the house electrical won't help. Everything from wall warts, led lights, stoves, washers, and tvs will have the potential to interference on different bands. Grab an sdr from a buddy and do a sweep to see what frequencies you will see interference.





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The short version: If the antenna and cable are inside the house, you should not ground the antenna.

The TLDR version: Using a separate ground path to the antenna creates a "ground loop", and the bigger it is (in sq ft), the more RFI it will catch and put up your radio's front end. Since you are not concerned about atmospheric static accumulation, do not bother with grounding the antenna. Whatever static may accumulate, will be dissolved via the cable shield through the grounding of your radio.

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Thanks! I’ve done a ton of permitted electrical work as a home owner (gut and completely renovate, re-plumb and re-wire one house, build an addition on another and finish a 2600sq-ft basement including re-plumbing the entire house and all of the basement and new basement bathroom electrical). Granted that doesn’t make me a master electrician, but I have a very good passing familiarity with electrical. Antennas and radio are just something I am starting to learn though and I don’t know a ton outside of some colleges level physics classes and deep passion for science (though I work in IT). I am familiar with ground loops. I’d bond any new ground rod back to the existing house ground bus bar using at least 10ga wire to ensure continuity. 
 

just wondering if ground back to that main ground on its own would help. Though it sounds like in this instance it would be the last thing I’d want to do as the long ground path literally running over the entire house would be more likely to pickup RFI and conduct it along the coax. 
 

For the attic installation, the coax would run through the attic down to the storage room in the basement below. A few feet from where I am planning to install the antenna is conduit for the attic air handler high and low pressure A/C lines. I planned to run the coax over and down the conduit (resealing around the coax) and then either tack it up along the ceiling to my storage room (where I have my hunting and gun stuff and a workbench). Or just terminate it directly in that utility/storage/exercise room where the conduit to the attic is. 
 

The later is what I am likely to do. It makes the path about 15-20feet shorter. I could probably get away with 30-35ft of coax rather than 50-60ft running it to my storage room. The room has the space for me to re-arrange some stuff for a small desk.

If I end up with a roof/chimney antenna I may just run it down to my storage room at that point. I’d be running the coax in through the exterior wall instead and my storage room would be almost below the roof peak. 
 

where this antenna (and others?) would end up should be more or less okay. A 10m dipole would be SOL for an attic install. I have air handlers in the attic and a dipole at 10 meters (that’s 16ft long for a dipole, right?) the antenna would have to run near them. Also most of the wiring for my

house is trunked along the attic peak down the spine of the house. But is all branched out by the time it gets down to the end of the attic. 
 

Not impossible I guess, but I suspect a 10m dipole would not end up working well in my attic. A 2m/70cm I think will work well as other than a small amount of residential wiring within maybe 5-8ft of the area I have picked out, there is no duct work, air handlers or serious amount of wiring. 
 

though if I want a higher gain 2/70 antenna or GMRS antenna it’ll probably have to go outside. It’s not super low in my attic, but I’ve only got about 6’ max height and my current 6dBi is running about a meter. So a 9dBi would be around 2 meters and not fit (or might just fit). 
 

In pleasing news I just busted that HYS antenna out of the packaging and after about 2 minutes with my SWR meter I have it adjusted right. 1.01SWR at 467MHz and about 1.04 at 462MHz with the antenna set to the 470 markings. At 465 markings it was 1.01 at 462 and 1.5-1.6 at 467. 
 

I’ll see if it needs to be tweaked it all in its final assembly as I was using my 1 meter “patch” cord of RG86 and not the LMR400 I am planning to get. I am waiting till I can get the antenna in my attic and test where it works best (hence the 1 meter coax) and then measure and get the length of LMR 400 I need. I didn’t want to get 35 feet and find out I really needed 40. Or that I got 50 and found out I only need 27. I’ve got a 5’ diameter tulip poplar outside my house about 50’ from where the antenna is going, close to the LOS path to my brother-in-laws and the repeater I am trying to hit. I think it’ll be off the LOS path (well, NLOS really). I figure that won’t do any favors for signal attenuation. But I also figure there may be a sweet spot for locating the antenna and I don’t want to find out it’s 15’ away from where I was planning to put it in my attic. 

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Also to add, the antenna itself is probably pretty close to as far from any source of residential electrical noise as it can be in terms of where any computers, TVs, appliances, etc. are. Not that it can’t pick any of that up. Just that is likely to be about the most advantageous spot possible. 

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What's important to understand, there are two types of the ground: DC ground and RF ground.

Our normal mains electric supply is at 60 Hz AC, with wavelength of around 3 million miles (my napkin math may be bit wrong, but it still in millions), so, any length of ground wire in your house do not produce any wave effects on your mains electricity. It's all like a direct and immediate short circuit, no matter how long it is. In a normal house. This is why it is safe to call your mains ground a DC ground, even if it is for 60 Hz AC.

RF ground is different. When we start to talk about tens of feet of ground wire, that's be tens of wavelengths on 462 MHz (the wavelength is about 65cm or 26 inches), with all kind of wave effects you may expect, reflection, diffraction, resonance, etc. There is also a skin effect, when the higher the frequency, the closer to the surface of the wire current flows.

So, in my view, there is no difference between 60ft and 30ft of ground wire. Both will pick up RFI at all VHF and UHF wavelength. It's better to avoid ground loops, if possible. If antenna is outside, it's better to run separate ground wire to the grounding rod to protect from the static. But if the antenna is inside, protected by the house from static accumulation or from the lightning, no reason to create elaborate protection. I think you'll be fine without grounding the antenna.

With regards to if to ground to the mains ground: yes of course. Ground your station to the mains ground at the panel or at the dedicated grounding rod, connected to the mains ground. Idea is that you have a single point of ground near your station, that is connected to the ground somehow. And to that point you connect your radio ground and our antenna ground, and your antenna switch ground, and your amplifier ground, etc.

I use a flattened 1/2" copper pipe, about 10" long, bolted to the back of my desk, with ground screws taped into it and connected to the radios and power supply. This grounding bus is connected to the rod in the soil via 4ga stranded copper wire about 10ft long (I'm on the ground floor and can drill walls). The rod is connected to the other rod and the electric panel via 4ga solid copper wire outside of the house.

If you do not have an option to run short (relatively short) grounding wire from your station to the ground, you should still have a connection to the mains ground, to be protected from AC faults, and because it's prescribed by the code and common sense.

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A little off topic but what is the roof itself shingle or metal, also does any of the insulating material have a foil back. Metal roof or foil back is going to be reflective just putting that out there.

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Tar shingles with an asphalt felt paper underlayment on top of 11/32 OSB with 2x6 trusses spaced 24" on center. At least in terms of my brother-in-laws and the repeater I am most worried about hitting, the signal is going to be NLOS out the gable end most likely, not through the roof itself. The gable end is 11/32" plywood, 2x4, standard house wrap (not foil thank goodness! My old townshouse was a foil wrap) and vinyl siding. If it would matter, the attic insulation below the antenna is fiberglass with kraft tar paper vapor barrier.

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