Suggested height above metal building.

[JoshuaHufford]

I know this has been discussed before but I couldn't find anything for the particular setup I'm wanting to do so I'm going to ask as I want to do this correct the first time.

Hopefully sometime this summer I plan on mounting 2 more antennas on this building,

https://live.staticflickr.com/924/43137100211_897daad1cc_k.jpg

I will likely mount a mast to the left of the peak where the current antenna is now. About 8-10 feet to the left of the peak of the higher section is a 2x6 behind the vertical section of metal that I plan on securing a mast to, that vertical section is about 4 feet high, the antenna you see there now is 8Ft. high total. On the mast I want to mount two antennas, an Omni for VHF and a Yagi for UHF,

This is the Omni I would like to buy, I'll be receiving only at 160-162Mhz

https://www.theantennafarm.com/catalog/diamond-f23h-4865

And this is the Yagi I plan to use, and I already have it, I'll be receiving only at 942Mhz

902-17, 900-930 MHz

The VHF omni I plan to mount at the top of the mast, and then the yagi will go below it. The side of the building that the current antenna is mounted to pretty much runs straight E-W, and the yagi antenna will need to point NE, pretty much right over the roof of the higher left side. My questions are,

How high above the roof does the yagi antenna need to be as to not be affected by the roof, and then how much higher will the omni antenna need to be above the yagi?

I plan to use galvanized conduit or pipe, what size do I need to buy at the suggested height for enough strength? Unless someone has a suggestion of a better material to use?

I only want to mount the antennas as high as needed as to not have them affected by the metal of the building. I know the more height the better, but this is my Father-In-Law's shop, and I want to keep the visual impact to a minimum, and I may need to go with a smaller VHF antenna because of that. I have to see if he will even let me do this project, but I need to know the height the mast needs to be to ask permission first. I would just mount them at home but this is a MUCH better receive site than I have at home.

Thanks for the input!

 

[JoshuaHufford]

Anyone?

 

[chief21]

I think that you would be fine with the Yagi at least a full wavelength (at 900) above the roof and with the omni a similar full wavelength above the Yagi. Actually, at 900 MHz, more than a full wavelength of separation might actually be more practical. Generally, if receiving only, these dimensions are not as critical as when transmitting.
For your support structure, I would recommend sectional TV-type mast as opposed to either EMT conduit or pipe. Conduit is relatively thin and pipe is very heavy. I expect that both would bend more easily in a strong wind than purpose-built TV mast.

 

[cmdrwill]

I would also recommend one wave length height and separation.

We use good quality EMT and do not have problems.

 

[JoshuaHufford]

So should the omni be a full wavelength at 160 or 900MHz above the yagi?

 

[cmdrwill]

So should the omni be a full wavelength at 160 or 900MHz above the yagi?

Good question. Usually at the lowest frequency, VHF in this case.
Depending on the VHF antenna and how well the decoupling elements, aka ground plane radials, on that Diamond antenna. I would put the 900 yagi antenna on a separate mast at least 72 inches away from any part of the VHF antenna.

 

[JoshuaHufford]

Good question. Usually at the lowest frequency, VHF in this case.
Depending on the VHF antenna and how well the decoupling elements, aka ground plane radials, on that Diamond antenna. I would put the 900 yagi antenna on a separate mast at least 72 inches away from any part of the VHF antenna.

Well adding a second mast sure complicates things, as does putting the omni 72 inches above the yagi.

 

[cmdrwill]

Do you really need that big of an VHF antenna?

 

[JoshuaHufford]

Do you really need that big of an VHF antenna?

Need? Not a must but it isn't much more expensive than others and I figure if I'm going to go to the trouble of mounting the antenna, running the coax etc, why not get the best performance I can.

 

[chief21]

So should the omni be a full wavelength at 160 or 900MHz above the yagi?

Since you're using these antennas only for receiving, I don't think the spacing would be that critical. I would also think that the radials of the upper antenna would tend to "protect" the active element from the influence of the lower antenna (and vice versa), as long as the radials do not intrude into the beamwidth of the Yagi.

 

[WB9YBM]

I know this has been discussed before but I couldn't find anything for the particular setup I'm wanting to do so I'm going to ask as I want to do this correct the first time.

Hopefully sometime this summer I plan on mounting 2 more antennas on this building,

https://live.staticflickr.com/924/43137100211_897daad1cc_k.jpg

I will likely mount a mast to the left of the peak where the current antenna is now. About 8-10 feet to the left of the peak of the higher section is a 2x6 behind the vertical section of metal that I plan on securing a mast to, that vertical section is about 4 feet high, the antenna you see there now is 8Ft. high total. On the mast I want to mount two antennas, an Omni for VHF and a Yagi for UHF,

This is the Omni I would like to buy, I'll be receiving only at 160-162Mhz

https://www.theantennafarm.com/catalog/diamond-f23h-4865

And this is the Yagi I plan to use, and I already have it, I'll be receiving only at 942Mhz

902-17, 900-930 MHz

The VHF omni I plan to mount at the top of the mast, and then the yagi will go below it. The side of the building that the current antenna is mounted to pretty much runs straight E-W, and the yagi antenna will need to point NE, pretty much right over the roof of the higher left side. My questions are,

How high above the roof does the yagi antenna need to be as to not be affected by the roof, and then how much higher will the omni antenna need to be above the yagi?

I plan to use galvanized conduit or pipe, what size do I need to buy at the suggested height for enough strength? Unless someone has a suggestion of a better material to use?

I only want to mount the antennas as high as needed as to not have them affected by the metal of the building. I know the more height the better, but this is my Father-In-Law's shop, and I want to keep the visual impact to a minimum, and I may need to go with a smaller VHF antenna because of that. I have to see if he will even let me do this project, but I need to know the height the mast needs to be to ask permission first. I would just mount them at home but this is a MUCH better receive site than I have at home.

Thanks for the input!

some multiple of a wavelength of the operating frequency (or center frequency of a band) would be my first reaction. On the other hand, especially in the case of the omni antennas, maybe the metal building could be used as a ground plane which might help antenna performance. On the third hand, if the metal building is not grounded--that means it's floating at whatever RF potential happens to be radiating nearby--it's anybody's guess what that's going to do to antenna performance...

 

[W5lz]

It's always a safe bet to get an antenna as high above other stuff as possible/practical. 'Practical' is always more-better than 'possible', you know?

 

[WB9YBM]

It's always a safe bet to get an antenna as high above other stuff as possible/practical. 'Practical' is always more-better than 'possible', you know?

Yeah--although we're taking it a step further by considering the effect of conductive surfaces in the area...

 

[W5lz]

There's two things I think that you are doing 'wrong'. The first if 'over-thinking' it. And the second is expecting anything dealing with radio as capable of being 'perfect'. You can always figure something could be 'better', but is it worth the effort of making it better??

 

[prcguy]

When Yagi's or most any antenna are very low to the ground or very large conducting surface in wavelengths it will push the pattern upwards some away from the horizon and reduce gain there. The problem is especially bad below one wavelength above the reflecting surface and getting at least 10 wavelengths above the surface will all but eliminate the problem. At 900MHz a wavelength is less than a foot long, so no big deal, 10ft is about 10 wavelengths.

For the VHF antenna sitting on top, I think only the ground radials hanging over the Yagi would be a problem, so arrange the ground radials either side of the Yagi and a good foot above the Yagi and it will be fine. If you use a 10 ft mast with the VHF on top and the Yagi a foot or two below it should be good.

 

[JoshuaHufford]

There's two things I think that you are doing 'wrong'. The first if 'over-thinking' it. And the second is expecting anything dealing with radio as capable of being 'perfect'. You can always figure something could be 'better', but is it worth the effort of making it better??

Well maybe I do tend to "overthink" things, but I would much rather learn as much as I can before I buy anything or get up on a roof, and then do the best installation I can do with the situation I have. But I do realize it will never be "Perfect".

When Yagi's or most any antenna are very low to the ground or very large conducting surface in wavelengths it will push the pattern upwards some away from the horizon and reduce gain there. The problem is especially bad below one wavelength above the reflecting surface and getting at least 10 wavelengths above the surface will all but eliminate the problem. At 900MHz a wavelength is less than a foot long, so no big deal, 10ft is about 10 wavelengths.

For the VHF antenna sitting on top, I think only the ground radials hanging over the Yagi would be a problem, so arrange the ground radials either side of the Yagi and a good foot above the Yagi and it will be fine. If you use a 10 ft mast with the VHF on top and the Yagi a foot or two below it should be good.

Well 10 Ft. above the metal is not an option, hopefully less height will work.

 

[prcguy]

The Yagi 1ft above the roof will work, higher will lessen the pattern tilting up and higher is always better for VHF/UHF.

Well 10 Ft. above the metal is not an option, hopefully less height will work.

 

[JoshuaHufford]

Well I thought I would update this thread. I was FINALLY able to do this install today, life just kept getting in the way and I couldn't find the time, but today it finally happened. I'm happy to say all went well, the antennas are working well and picking up what I wanted, and I didn't fall off the roof!

 

[prcguy]

Looks good but I would have raised the antennas just a little more to get that Yagi away from the metal roof like 2 more ft. A wavelength at 900MHz is about one foot, so raising it two feet is two more wavelengths. I would also arrange the ground radials on the VHF antenna so they fall either side of the Yagi below it instead of one radial hovering right over the Yagi. Otherwise the VHF antenna has no clue there is a Yagi below it.

I also don't like the coax exiting the VHF antenna and being pulled immediately to the side, you should run the coax down the mast a little and gently affix the coax to the mast to take any strain off the antenna connectors. Then run sideways or go down the mast a foot or two and come back up making a drip loop so rain water doesn't run down the coax into the building entry point that's lower than the coax.

 

[W5lz]

I would have raised it too! Something like a 1/4 mile, you know? Hey, it's better than what it was so it's a win! You do what you can and then rejoice in the improvement. Then you get to scheme of some way to make it more better, right? If it's not 'more better' then it was a learning experience (crap!), and then you 'fix' it! I have to think it's better than mine at 10 - 15 feet. What's wrong with that? Keep in mind that it's never going to be "perfect". Improve it till it gets to the point where it just isn't worth the effort anymore. You never know where that 'is it worth it' point is until you go past that point. So... nothing wrong with wanting to improve stuff. Just be realistic enough to know when to quit! I have no idea where that point is... That's what is called 'experimenting'.