Thursday, June 26, 2014

Wireless Site Survey Rigs (a repost)

Site Survey Rigs

(an archive of my CWNP post)

By Jennifer Huber On 03/01/2011 - 3 Comments
I’ve been traveling to customer locations to perform wireless site survey work for over four years now, and I can assure you traveling with survey gear can be somewhat tricky, especially if you’re like me and want to avoid checking your bags at the airport. I’ve seen many a survey kit arrive in pieces as a result of the thorough TSA checked baggage ‘screening’. As a result, I’ve come up with some fool proof ways of flying with survey gear stored in my carry-on luggage, while doings so in a way that I’m not holding up the TSA security checkpoint line. 
The gear I carry with me to do an active site survey is a Tessco battery pack, a Cisco 1140 access point (or a Cisco 3500 CleanAir access point), a roll of duct tape, colored ½ inch stickers, a Sharpie, a roll of transparent tape, and the mounting bracket for the 1140/3500 access point. The mounting bracket uses the metal mounting bracket that ships with the 1140/3500 series access point and a wooden block threaded for a painters’ extension pole. The wooden block was at one time a drywall texture brush.

I removed the bristles with a pair of pliers and then spray-painted the wood block gloss black to look more professional.

A pair of Mr. Longarm Angle Adapters allow me to suspend the 1140 access point from the wooden block bracket as if it were installed on theceiling.

I have seen someone use a plastic backed drywall texture brush as their mount for an 1140 bracket, but I don’t have a dremel to grind down the plastic mount to allow for the bumps on the back of the 1140 flat metal bracket.

Those are the basic parts I need to perform an active site survey at the customer location.
Before I discovered Ziploc XL bags at Home Depot I had to pull out the access point and power supply and put them in a bin, and now I use one of these gigantic bags to carry all the survey gear. I can zip open my carry on suitcase and lift all the survey gear out and put it in an x-ray bin in one swift movement.

Since a survey cart is too large to count as carry on luggage, and often don’t have enough early warning about a site survey to ship anything to the site, I’ve had to make do with things that wouldn’t normally pass as a survey cart.  The customer sometimes will have an A/V cart or a utility cart that I can use to survey with, but sometimes I have to be a bit more creative.  I’ve made do with items you wouldn’t normally imagine passing as a cart for surveying. I use the following as examples: the survey kit pelican case, an office chair (works if it is a really small survey); a furniture moving cart, a furniture dolly, and sometimes I even had a proper Rubbermaid cart to use.

The things I can’t travel with are the Mr. Longarm 12 foot painter’s pole and a dowel rod. These items can be found at any Home Depot, and there has been a Home Depot within driving distance of every site I’ve had to survey. I use the dowel rod to attach the ¼ inch round labels combined with the transparent tape to the suspended ceiling grid to mark the access point installation locations.  I fold the transparent tape into an “L” shape and then stick the tape to the suspended ceiling grid.  This works much better for me than having to find and drag a ladder around with me so I can reach the ceiling to manually mark the access point installation locations.

Several other wireless engineers I’ve worked with have designed their own outdoor survey rigs by using perforated sheet metal strips and crafting survey tripods out of this held together with bolts and wingnuts.  I have also seen survey rigs for outdoor point to point feasibility testing crafted out of sections of PVC pipe.  PVC piping is very sturdy, disassembles easy, and can be used in a multitude of ways to create a survey pole or survey mounting structure.  My friend Jeff Russell has made an outdoor rig for doing a mesh survey from PVC piping, and an indoor tripod mount out of perforated metal (photo gallery at this link).

There are certainly a million different ways to craft a survey rig out of commonly available parts from your nearest hardware store.  The only limit seems to be your imagination. 


  1. This post was really awesome! I love the ingenuity!

  2. Why do you point the antenna down? Do you prefer panel antennas over omnidirectional for surveys?

  3. Jen -

    I wonder if some sections of PVC pipe would work? The only problem is that as you go higher the stability of the pole becomes an issue. I used to do the Lowes/Home Depot thing too until I just decided to have a machine shop make something.

    @Ben -

    I had a really nice long reply for you until the blog site ate my comments as I attempted to publish. So this is the short short version. You want to survey with the AP aligned and positioned exactly as it will be installed so that your survey data lines up with how the system will operate in production. Though it's an omnidirectional antenna, it does have more signal radiating out of the front of the AP as opposed to the back (front-to-back ratio). Surveying with it 'upside down' would be radiating more signal to the floor above and would give you inconsistent performance data when comparing the survey and the production environment. You also have to consider that site surveys are 3D chess and not just 2D. You have to consider channelization, power, and placement both on the current floor as well as those above and below. You must measure this as well while you are on site and to accurately measure that you have to have the AP in the same orientation as it will be when installed. You'll also notice that Jen places stickers on the ceiling indicating the location to install the AP. There's no fudge factor here either, especially when real time applications like voice and location are involved. We often rely on structures in the environment to block or reduce signal for us to be able to provide sufficient separation between channels and limit co-channel interference. This is becoming less and less of an issue as more and more networks are 5 GHz. But wireless performance isn't about just signal strength. You need a clean channel to use the higher compression algorithms that provide for higher MCS rates. Hope that helps.

  4. @Ben -

    Also, the choice for a panel (i.e. directional antenna) vs. an omnidirectional antenna is a combination of what the site layout looks like and the application being used. For real time apps, high density, high throughput, location, etc. where you need a micro or pico cellular deployment, the only way to cram more APs in is to limit the scope of the spectrum spread. We can do this with highly directional antennas with smaller angles that limit the footprint. Keep in mind that you'll have considerably higher front-to-back ratios and more energy pointed down so you'll need to measure through-floor propagation for sure. Genereally these deployments have lower power ratings such as 1/8 - 1/4 depending on the application. For simple data usage and casual internet browsing, omni-directional antennas are ok. But again, this depends on the site. Regardless of the application, if you're going into an area highly congested such as New York City, you've got to stay away from the building perimeter where other buildings are bleeding signal through the windows. You'd almost certainly have to use directional antennas and 5 GHz if possible.