Tom Warfield’s Post

When I first started in the industry, the company I worked for only utilized leaky coax. I recall the Motorola engineers scribbling pencil calculations and using the wheel scale for measurements. The engineers would only use $12,000 Tx Rx BDA's (12 week leadtime) and there passives (I believe they were $150-200 each). Before the rebranding the First Responders loved the free In-Building cover for the Nextel phones. The systems seemed to work but I'm not certain how well by today's standards. Several years ago, I saw a design that used crossing leaky coax lines for a 4G MIMO DAS for a large tunnel in Asia. The engineer claimed it performed well. For some reason the industry virtually forgot about this solution. I believe most people who bash it have never tried the solution and are just going by what they were told.

Hope all is well Tom, I have utilized leaky lines in the past working at a radio shop in a local municipality years ago, Wow how time flies! As others have mentioned, it was more of a trial-and-error type situation without any modeling software, and was shown how to do this by a senior radio guy. This was for low band in the 39-46 MHz range in non-harsh lower levels with the donor site close by (back in those days, donor site wasn't even a term!). We used Andrews Heliax 2–3-inch cable with a transceiver and essentially used a hole saw making penetrations through the rigid copper shield at predetermined spacing after doing some crude RF loss calculations. This configuration worked very well hidden above a drop ceiling until we moved to an 800 MHz trunking system which made the coverage intermittent with improved head-end equipment. We even went as far as drilling 1/4 wave 3 1/2 inch antenna rod into the center conductor which improved the performance but still was less than stellar, high input power to the leaky line and low Rx to the receiver end. I would agree, that if modeled correctly, it's much easier to distribute/install with this type of system with less chance of damage from other trades if done correctly.

I have used leaky coax for many applications where the DAS was an afterthought and particularly prior to 2-hour fire rated coax being a thing... I saved in 1 case over a million dollars in change orders on a project for the building plus extensive lawsuits from the project partners for missing occupancy. Now though it seems like design is more scalpel work requiring preservation of signal power to certain areas rather than spray and pray and hope it works due to critical areas requiring 99% radio coverage. Also, I have seen AHJ's scoff at the use of it despite it being a perfectly viable tool in the toolbox. Installation challenges are definitely a concern as most don't understand how to properly install it. Just a few cents, I appreciate the topic being raised.

Some thoughts. Leaky Coax had/has a bad rap. Unfortunately, unless something changed, it was something not easily modeled using tools such as iBwave (part of the problem in "guaranteeing" the performance of a design). Way back in the day it was widely used in Nextel applications (high power, 800MHz). Insertion loss is the killer when used in the higher frequency ranges. This would provide similar challenges in today's environment with multi-carrier and multi-frequency Neutral Host applications. For areas that require conduit, regardless of cable type (think NYC and Chicago), leaky coax does not play well installed within EMT... Andrews Corp (pre-commscope) had some pretty comprehensive design and use-case info specifically for mine applications, tunnels, etc. Probably still available if you look. Private message me if you'd like. Happy to discuss experiences further.

I have seen leaky feeders used widely outside of the USA for hard to serve areas like tunnels and mines where they can work fairly well. They can be prone to damage and one unknown is if there is any good way to fire rate the cable without significantly degrading its ability to radiate. Phil Eldredge i know you have a wealth of experience when it comes to national infrastructure.

It works very well, only issue is isolation if you place it on top floors. Great use case for apartments that are mostly wood. It actually ends up costing the same for materials but saves in labor.

A lot of good comments here. I have seen it implemented in very large high rises, where the DAS is non-duplexed and DL and UL branches alternate every other floor. This "ladder" approach can be implemented to increase reliability of the DAS. Another reason this approach was used was to mitigate IMD which is created in the DL from reaching the UL path (isolation between cables). Some systems have so many channels in the mix that IMD products are inevitable and will land on UL channels. Splitting the DAS and isolating the DL and UL branches (10ft of separation between radiax cables) is usually enough to mitigate the issue. The system I worked on did have discrete antennas in the mix as well. The system performed perfectly! Same type of system design was used in Boston airport.

I think part of it is familiarity with it and part of it is specific case scenarios. I've only ever used leaky coax for tunnels as that's the ideal scenario for them. However, I can see them being useful for long corridor situations with just one room on each side, like a hotel. Install crews might not be familiar with it and risk damaging it along with improper installation. A crew might not be familiar with which side of the coax needs to face the direction you need the RF to propagate towards for example. It's also less forgiving in the design and install portion of it. If a length of cable ends up being longer than expected, you could have a less than ideal DL and UL at the end, which could lead to issues with failing grid tests for public safety. With conventional coax, you can change couplers to redirect power to where you need it easier if say walls are introducing more loss than expected.

I think it has a place in small spaces where distance to coax is so minimal you can hit it with hardly anything on UL. I don't like it for large spaces or long runs, it's incredibly difficult to model the pattern even if you know the distances and losses. If you make the mistake of running too much you risk having lower DL than you need, worse loss on UL than you want, and in many cases if you are running significant lengths of it you may end up needing more remotes in the design to keep it balanced. I've run into issues where it was used and the bleed over from inside to outside was very hard to control. System hand-off from outside to inside was not smooth and occupancy was delayed due to several tries to balance (overseas). I also feel like survivability is much worse by not having it in a metal raceway, also, I like the small gain I can get from an antenna for UL. I am very much a fan of leaky coax but it's gotta be a small space like a tunnel and a really specific use case else I'm doing with antennas, back boxes and taps. *** Yes, VHF is wonky indoors and doesn't play well with hard surfaces. It really shines outdoors. Worse still if it's split VHF and you're running 2x more antennas with spacing to deal with to boot.***

I think leaky coax does have some good applications, but they are pretty limited. The obvious is tunnels, or other structures that are long and narrow. I had an interesting application I designed several years ago where leaky coax was both simple and effective. The building was a high-rise tower in a hospital. The only areas lacking coverage were two central stairwells and in-between them was a riser closet. Installed the leaky coax in the riser and it covered both stairwells. Otherwise I believe it to be too inefficient. Lots of wasted power.