LoRa Coax Losses.

Recently, I installed a test solar repeater at a remote location. This was entirely to assess the reliability of such a system and whether anything might need improving in time.

My test solar repeater, with its short RG174 coax (now changed).

 

As a test, I simply used a 1m length of RG174 coax as part of a pre-made connector to hook up the McGill Microwave Systems 4.5dBi colinear vertical to the Heltek V4 board.

On showing a picture for interest on Facebook, the inevitable 'oh, you shouldn't do that!' comments began to roll in. 

First, I 'shouldn't put an antenna in somebody else's field at random, without permission'.

Er, I've been given a personal easement to use this field and structures 35 years ago.

Then, I shouldn't use 'that very thin wire for a long run; it will defeat the gain of the antenna'.

Er, the cable is 1m long. 

The pole the antenna is fixed to is simply to hold it clear of a roof and little else. Great height isn't feasible and neither is a larger antenna at this very exposed, remote location. The antenna-coax combination was an entirely reasonable system, given what I had available and that it was a short run and that a 1/4 wave antenna using the same RG174 cable assembly had already been shown in earlier tests to give perfectly adequate performance from the same location.

Another factor, significant for me, was that thin coax needs only a narrow hole to be cut through brick walls - in my case, by hand drilling. Wider cable needs a larger hole - very difficult to drill by hand, especially above head height, and otherwise excessive and unsightly for some on a house wall. 

All the same, the question as to whether RG174, which is widely used, almost as standard for LoRa applications, is really so lossy as to require replacement in most cases.

Undoubtedly, RG174 has extremely high losses at or around 868MHz; Farnell's datasheet gives 105dB per 100 metres, or 1.05dB per metre. So any runs much greater than my own 1m length would, indeed, present losses that most people would deem unacceptable, except perhaps where a directional antenna was in use. Even then, a run of more than 2m would really need better coax.

LMR400 coax.

 

The usual alternative is LMR400. In ham circles, we've been pre-conditioned to expect high prices for long runs at HF or VHF, made worse by US-centric discussions online, which usually have very much longer runs to antennas than those in the UK.

In fact, a pre-assembled LMR400 coax of (standard) 3m length for LoRa comes in at just  £19, delivered (mainland UK). This is the kind of length many would choose for an antenna situated on a house wall to be fed from indoors, for example.

RG174, thin coax.

 

The benefit of LMR400 is the low loss: at 868MHz, it's around 12.8dB per 100 metres, or 0.128dB per metre. With a 3m length, the loss is 0.38dB, or already 0.66dB less than just 1m of RG174.

So, is the use of LMR400 and its relatively high price, worth using?  For runs of 2m or less, for most people, no. Beyond that kind of run, LMR400, or even the cheaper and slightly higher loss (~0.193dB/m) X-400, rapidly leaves RG174 in the dust, keeping losses within tolerable limits until one reaches around 15-20m runs or so. Above this length, a better antenna and/or even more expensive, lower loss coax will come into consideration.