DAS construction and testing

[Bill_G]

Anyone in the radio biz has watched the deployments of DAS increase greatly over the last decade. What used to be a good idea that was too expensive for most businesses to consider is now required by many municipalities to ensure public safety in building radio coverage. We're seeing more and more DAS going into parking structures, hospitals, factories, malls, public buildings, and anywhere that needs additional coverage. The problem is nobody seems to know what they are doing.

The contractors usually install the cable fine. Occasionally we find low budget installations made from long runs of LMR400 with numerous low profile antennas jammed into awkward spaces above the ceiling tiles. We expect to find problems with those. But, something I'm finding over and over again in otherwise well done professional installations is cut shields on the radiax and heliax mainlines. I'm seeing it often enough to wonder if there is a DAS industry best practices advocating it.

The connector spins on the end of the cable. After you cut back the heat shrink, the only thing holding it on is the foam dielectric. The cable prep is good. The center pins are continuous, but they have deepened the cut of the plastic coating through the shield so there is no continuity between the two ends of the segment on the outside. I understand the logic and possible need for DC blocking in some circumstances, but you can do that with DC blocks.

It seems to me that it would defeat the purpose of installing expensive radiax. I imagine it all becomes a form of waveguide with no way to predict losses or guarantee performance. It definitely looks strange on the Anritsu. In the 700/800 band, lengths of 7/8ths under 100ft have return losses greater than -10db while lengths greater than 300ft actually look reasonable in the -20db range. Blows me away that it almost kinda sorta works ... but I was called in by the customer because the kinda and the sorta played a bigger role than they wanted.

Has anyone else seen this or heard of this?

[Jim202]

I ran into the same problem when I was working for several cellular companies. Seems the crews that are installing the connectors don't seem to care or have much training on the connector installs. Many times it was due to not having the correct tools. I would have a talk with the people doing the work and give them a second chance. They were informed that the next talk would be with their company. Also told them that they had better not put any tape on the connectors before I inspected them.

Low bids generally end up with the same quality type of work. Maybe we should start a list of companies that have been doing poor quality work so others don't get suckered into the same boat. If the word gets around about the quality of work these people are doing, one of two things will happen. They will either pick up the quality of their work, or they will starve from lack of people not hiring them.

Jim

[Bill_G]

We're in the same camp - it's not SOP - it's lack of training and no leadership. The rest of the material handling was perfect. The 7/8ths was mounted conduit straight. No kinks in the cable. Conduit pulls - yes, they pulled radiax through conduits - was difficult, but done without damage to the cable. Service loops on jumpers. Heat shrink on every connector, and well sealed. Wall cores perfect. Fire stop perfect. Kindorf perfect. Cabinet grounding perfect. They did everything as you would expect a true professional to accomplish except for cutting the shield on every single connector which is why I asked if this is something I don't know about.

Mass Electric is all over the place putting DAS into buildings, and I'm following up a couple years later as things fail.

[Jim202]

We're in the same camp - it's not SOP - it's lack of training and no leadership. The rest of the material handling was perfect. The 7/8ths was mounted conduit straight. No kinks in the cable. Conduit pulls - yes, they pulled radiax through conduits - was difficult, but done without damage to the cable. Service loops on jumpers. Heat shrink on every connector, and well sealed. Wall cores perfect. Fire stop perfect. Kindorf perfect. Cabinet grounding perfect. They did everything as you would expect a true professional to accomplish except for cutting the shield on every single connector which is why I asked if this is something I don't know about.

Mass Electric is all over the place putting DAS into buildings, and I'm following up a couple years later as things fail.

They might be great in doing electrical wiring, but obviously have no clue on how to install coax connectors. Sounds like you and the customer are going to spend some serious time and money making over the coax connections. Looks like every connection might have to have the joining coax cable disconnected and then do a simple Ohm meter to ground to see if the shield was carried through. Good luck on this one Bill.

Jim

[Bill_G]

I use the Anritsu to prove (and document) the cable after making repair. DTF (distance to fault) shows a failed connector right in front of me, and the RL (return loss ) tells me how well I reworked it. On a good length of radiax, beyond 300ft the S332D gets kind of mushy and indistinct like you're looking at an infinite length cable which kind of makes sense. You'll see a wedge shape start at about 300ft and increase with distance. Thankfully, it shows line failures quite clearly all the way out to it's theoretical max of 1200ft. What looks like a good line on heliax is actually a failure on radiax beyond 300ft. I should post some sweeps.

[Bill_G]

Here we go - a bad sweep and a good sweep. Both show the wedge shape as the distance increases. On both you can see the 7/8ths connector in front of me after I've repaired it. It passes, and now I can look beyond it to the other end. In the good sweep, the end of line is indistinct disappearing into the wedge. I know where it is, and have it marked, but it does not stand out. However, on the bad sweep, the open connectors and cables through a conduit show up quite well. If you didn't know better, you would say they actually pass. Then you get to the other end, open them up, and they go off the top of the chart.

[FMROB]

So, as they say the larger the connector, the easier it is to install, lol. We do a considerable amount of site work, more so than most radio companies. I have to say personally, I have found the 7/8 connectors the hardest to install. 1/2, 1 1/4, 1 5/8 no issues, 7/8 errr. Bill while I never would think of putting heat shrink on a connector at all, they shouldn't twist or move once installed, at all. Once placed into service and properly supported no reason to look back. If the connector is going outside, the tried and true wrap of Scotch 33+, dum dum, and wide tape done properly and it should last 20 years.

With that being said, installer error being numero uno and just sheer crap equipment comes in at a close second. For example, we use LDF5 alot here, and the L5TN connectors are poorly manufactured. I have gotten use to using drill driven prep tools, a blade, and the flare tools. I always install the connector and take it apart to make certain it flared properly and such and then crank it back together. We had a few crap connections and were pulling our hair out, come to find that the center pins on the connector are not spread out enough from the factory and was making poor contact with the cable center. Needless to say we put a little spread on the pin now, and no problems.

I was scratching my head at a tower job when the new EZ connectors came out for 1 1/4. I still am not use to cranking the back part of the connector body back down on the cable. Tried to read the directions (yes, directions) but they were in Chinese, and I am not lying. When we first put it on, it would twist around after some pressure after we figured out that it needed to be threaded onto the outer jacket of the cable.

The reason I bring up automated prep tools is it takes some of the guess work out of connector install. It certainly speeds up the process, but if used properly it maintains consistency with the cuts. Guys still cutting by hand with razor blades if not trained can make a mess quickly.

Just my 2 cents. Rob

[Bill_G]

We ran into similar problems a couple years ago with the new CommScope/Andrews captivated center pin 7/8ths connectors on NOS Andrews LDF5-50. We had reels of real Andrews cable left over from another job, but had to buy new CommScope AVA5-50 connectors for this job. Then we ran into maddening intermittent high vswr problems. And we found the same thing - the center contact was not spread wide enough for the slightly larger diameter old stuff. If we had done the reverse - old Andrews LDF5 connectors on new AVA50 cable, they would not have fit at all. Again, the Anritsu is what found the problem showing a bad return loss at the connectors.

[wavetar]

When I first read your post, I thought it was probably a case of a contractor who wasn't adjusting blade depth properly in his drill driven prep tools.

[Bill_G]

When I first read your post, I thought it was probably a case of a contractor who wasn't adjusting blade depth properly in his drill driven prep tools.

I tried changing the depth of the cut for the plastic coating on ours, but it has a fixed depth. I think this is a hand tool with a deep blade.