Antenna question base GMRS - MURS

[CaptDan]

I have a antenna question for a long term - temporary base station. My “office” is located inside a one story concrete building – with a concrete ceiling/roof that is then covered with a fake A frame wood truss roof, which has a metal roof on the trusses to make it look like a normal framed roof.

The point being I cannot install an antenna on the new metal roof. While the walls are all 8 inch thick solid concrete walls, there was some pipe installed underground to be used as conduit. So, I can get an antenna wire to the outside without too much trouble.

I don’t need 50 miles of range, I would be happy as can be with 10 miles. I was wondering if I could just get a couple pieces of galvanized pipe – they seem to be some what strong, dig a hole and cement one into the ground 3 or 4 feet deep, then connect the second piece of pipe to the first, I was thinking maybe the second could be a bit smaller – if I used 3 inch pipe, 4 feet in the ground with 16 feet out, then maybe a second piece of pipe say 2 inch pipe adding 20 more feet with an antenna attached at the top ?

Ok thise seems like it would be too easy and I can’t be the first one to even think about it – will it work ? Not work ? Major issues ? Minor issues ? Suggestions ?

I am not that easily offended - if this is really the dumbest idea you ever heard of please let me know and maybe why so I can try and rethink something. I know a tower would be best, but not in th ebudget this year or next.

I am a fan of only using as much power as needed – we do have to share these frequencies. I would be using both – GMRS and MURS frequencies depending upon who was calling in and how much range I needed.

Thanks for reading this and any input you may have.

 

[mmckenna]

Since UHF tends to be line of sight, you'll need to have your antenna up about 65 feet to see the horizon 10 miles out. Of course if the far end is higher up, your antenna can be lower.

What the pipe will support really depends on the antenna wind load (how much pressure the wind pushes on the antenna), how big your foundation is, and if the mast is guyed or not. That involves some calculations that would take a long time to explain here. But 65 feet high will require guying if you are going to use pipe.
You can get self supporting towers that high, but you are looking at something in the tens of thousands of dollars as well as a substantial base foundation to properly support it.
At 65 feet, you'll also want some pretty high grade coaxial cable to get enough signal to/from the antenna. At minimum you'd want to be using something like Andrews LDF4-50A 1/2" heliax. Grounding is important to, especially in your area, that means grounding the coaxial cable near the antenna, at the bottom of the tower, and before it enters your building. You'd also want a lightning suppression device.
A proper grounding system can be important, too, and that will require one or more ground rods, as well as having that ground system bonded to your building ground.

It's not impossible to do all this, it just gets to be a substantial amount of work.
If you can do with less than 10 miles of range, you can certainly go with a shorter supporting structure. Galvanized pipe can work well, but you should probably look at push up poles. These will still require guy wires, but are properly designed for what you want to do.

 

[CaptDan]

Thank you for your response - I was hoping that you would respond, as I have read many of your posts and you seem to be very knowledgeable. I was hoping to avoid guy wires, but even if I use them I can only pull the pole in 3 directions as I was planning on keeping it close to the building to avoid running extra lengths of cable.

When the foundation of the building was poured - 7,000 sf of 6 inch concrete, the concrete contractor ran a piece of the re bar out of the footing about 6 inches to serve as a ground I was told. Would this suffice for a ground or would I need additional ground rods ?

An actual tower is out of the question at the moment, I am exploring my options. The property is the high ground here, 10 miles inland from the gulf of mexico, but 127 feet above sea level.

I appreciate your response and will use google for some education and I am sure I will be back with some more questions, if you don't mind

 

[mmckenna]

I wouldn't go much above about 20 feet if you want it to be self supporting. You can gain some advantage by placing it against the structure and supporting it part way up by bracing/attaching to the building.

The rebar can be used as part of your grounding system, as it is likely used as part of your electrical service ground. You'd need to confirm that, though. Considering the weather you guys get, I'd still want a deeply driven copper ground rod directly beneath the antenna support, though. Usually a professional site would have someone look at ground conductivity in the specific location, soil conditions, etc. and use that to tell you how much of a ground system you need to properly handle a lightning strike. At commercial sites it's not uncommon to have several - many ground rods to make up the system.
Only way to know for sure is to have someone test it with a ground resistance meter.
But, if the rebar was all installed correctly, it's probably a pretty good system, but don't quote me on that. It would, no doubt, be better than what many amateur radio operators get away with.

If your land is above the average ground level, then that will work to your advantage.


As for grounding, do a search on Motorola R56. That's an oft referenced guide used in the industry for site designs. It'll cover most of what you need, and provide some guidance as to the National Electric Code requirements. A lot of it is overkill for hobby use, but it'll give you a good idea of what you'd find at a commercial site.

You can get pretty creative with antenna supports. Building a "tilt-over" design can work well. Basically install two uprights anchored well into your foundation. Near the top, have a pivot point that will support a 20 foot or taller mast to support the antenna. That way the whole thing can be tilted over for maintenance, adding antennas, etc. The railroads around here use a commercial version. You can get pretty creative with the right stuff, and it can save some money.

Since it sounds like this is your first foray into GMRS and MURS on a bigger scale, I'd suggest keeping your options open as sometimes GMRS/MURS can be a gateway into amateur radio. While you may only want those two services now, you might get "bitten by the bug" and decide to expand into amateur radio later on. Having an antenna structure that can support a small amateur radio antenna can really be helpful down the road.

Most important part though, be safe. Don't install any of this where it can contact power lines, or fall on something expensive.