Thoughts on mounting 2m/70cm antenna upside down?

[JMSchlus]

I was a tower site manager long, long ago. When two way radio was the only way to communicate with your vehicles on the road and tower antenna space was tight. We would double up the antennas on the side arms. Making more tower space rental money in the process. Vertical polarization is vertical polarization no matter which way you hang the antenna. But you do loose the few feet of antenna height on the arm when you hang it from the bottom, so that is probably why you don’t see it so much anymore as most sites are ghost towns now as far as two way radio and any one still shelling out for space on a tower for a repeater is going to want those few extra feet of height to gain a few extra feet of coverage.

 

[paulears]

You can put a dipole upside down, but have you ever looked at the radiation diagrams? Vertically polarisation is NOT even around the horizontal axis! In fact, the lobe of the main takeoff angle changes between models, some designed to hug the horizon, others designed to cover a wider beam width for local topography. In fact, there are also downwards radiating antenna, designed to concentrate output locally like on top of a factory with a central high point. Excellent factory coverage, but minimal spread outside the local area. Look at the radiation pattern for these, they had to change the graphs. You could hang a quarter wave vertical upside down and this would do the same thing. This is simply physics. Balanced antennas like dipoles have lobes above and below horizontal and are symmetrical, end fed dipoles are not. Their horizontal plots are anything but symetrical.

 

[JMSchlus]

Picture a guitar string. A resonant wire. Now pluck it. It will resonate at a frequency based upon its length. The sound it makes will be the same and radiate from it whether you held the guitar straight up or straight down. Now in free space you can measure every direction the (sound) EM wave travels and create a polar graph showing the radiation pattern. But on earth everything is either an absorbent or reflector of the radiation, which will cause all sorts of lobes and nulls. The goal is to get the antenna as high above as many of the obstacles as possible. But you still need to have it attached to a tower or roof which has other materials and antennas and cables and such. The real RF environment is not perfect. I guess the short answer is that it’s doesn’t matter unless it doesn’t work, then you can reposition it until it does work. Heck, it might even work better horizontal rather than vertical depending on what’s around it.

 

[paulears]

Crazy analogy. Manufacturers provide a horizontal and a vertical plot. The dipoles, two and four stacks have symmetry above and below 90 degrees to the vertical and do not have any problem with being upside down, bar feeder cable position. The rest have lobes that are different above and below horizontal. These work very differently upside down. Remember Scotty. Ye canna change the laws of physics Captain. If the vertical graph is symmetrical above horizontal, all works properly either way up, but people pick other designs to make use of the radiation pattern. If your tower is on the top of a mountain, upside down could work really well, if the users are all in the valley. That’s an excellent reason to use them upside down. Why waste power going up, if nobody is there. That’s a design choice. You look at the plots, do the maths, look at topography, and pick a model and orientation that does the job. Upside down could be a brilliant choice. at another location a terrible one.

 

[madrabbitt]

Picture a guitar string. A resonant wire. Now pluck it. It will resonate at a frequency based upon its length. The sound it makes will be the same and radiate from it whether you held the guitar straight up or straight down.

a ground plane dipole type antenna (i.e. a vertical whip) is summed up 100% by that analogy. There is some ****yness introduced by the wavelength fraction and base type, but for the most part, a vertical whip is a vertical whip.

guess the short answer is that it’s doesn’t matter unless it doesn’t work, then you can reposition it until it does work.

Exactly this. Try it and see. If it doesnt work for you, try something else till it does.

That being said, i've thrown magnetic mount antennas on metal ceiling vents for years with varying degrees of success. I have also installed a NMO on a metal perforated ceiling plate like the following picture (with the mount and whip aimed downward towards the floor, and the NMO base grounded to the ceiling grid, which was (in theory) earthed to the building. Although this exact installation was the opposite of what you wanted, this was to recieve signals INSIDE a building and not outside, it was still... sort of functional.



And, back to the original question, aside from "try it and see" If it was me, i'd be leaning towards a base mount NMO with a ground plane as the solution to the issue, either mounted to a wall as high as possible, or run outside and mounted to the structure.

 

[paulears]

The string analogy is orientation independent and also introduces components we need to discount for RF discussions such as harmonics. Wave theory is a complex and only marginally linked talking point. We are talking about antennas. I’m out now, with one piece of advice, visit the web site of any commercial antenna manufacturer and look at their horizontal and vertical radiation plots. Then consider the impact of inverting the vertical orientation. That is all that matters.

 

[Echo4Thirty]

Are they? Never seen any upside down ones here?

We do it all the time. As long as we move the drain plug from the top of the antenna to the bottom and properly seal it, works fine. Many VHF/UHF/700/800/900 antennas are installed inverted.

 

[paulears]

Presumably to alter the radiation pattern to suit your needs?