Reuse of 75 Ohm coax?


#1

We are deploying a Max Transit Duo to a building where there are legacy 75 Ohm coax runs (from satellite systems). Are there (reasonable) ways to reuse that cabling for antenna runs to the Pepwave or do we have to install new 50 Ohm runs (which would be expensive)?


#2

How long are the existing 75 Ohm runs?


#3

They vary, roughly 20-50 feet, less than 2 years old.


#4

Thats not so bad. You’ll be fine. 75ohm loses almost twice the amount of dB gain compared to 50 ohm at every 10feet, so at 50ft thats a potential loss of -2dB gain - so you might need to compensate for that loss with amplification on the longer runs.

If it was me I’d get a pair of 12DBi directional wideband LTE antennas and install them on the longest runs of cable and see what sort of signal squirts out at the business end.

If you’re not in europe (where cellular boosters are generally illegal to use), you could get yourself a cell signal booster and use the 75ohm cable between the external antenna and the internal amplifier. Then just use the paddle antennas on the back of Transit.

Speak to the guys at 3gstore about that (@kevin is here in the forums and knows what he’s talking about).


#5

Funny you should suggest that: We have attempted that with some success (getting a usable signal). However, it seems no to play well with the MIMO aspects of LTE (unless we use four runs and four amplifiers, I guess, for the two antennas for each of the two modems) (I am not an RF person, as is likely abundantly clear).


#6

Ah, now you’re firmly out of my scope of knowledge too I’m afraid - simply haven’t used any amplifiers before.
Other partners here have though - so hopefully one of those will step up and enlighten us both.


#7

I think @mldowling can be of service here!


#8

Might I add a comment? The losses associated with nominal impedances of 50 Ohms (as used by Peplink, cellular and the land mobile industry, for example) and 75 Ohms (typically CATV) are easily found in tables scattered throughout the internet. The real issue, other than loss, is impedance mis-match. If, for example, one used a 75 Ohm cable in an application where 50 was expected, the SWR would rise to about 1.5:1 for that reason alone. And, it’d get worse from there.

Frankly, I’d do exactly what @MartinLangmaid said – I’d try it. If it works – great! If not, use the 75 cables to pull in new 50 Ohm cable! You may find that while the losses are noteworthy, the power budget/system gain is quite sufficient to net you better performance – which is really what it’s all about. It takes quite a bit of line loss to overcome the gain from a good quality directional antenna (e.g., yagi or log periodic). :grin:


#9

Rick-DC is correct. Mixing 50ohm and 75ohm systems results in significantly more signal loss, plus because of the number of adapters you’ll introduce more loss into the system. Direct connect amplifiers are always 50ohm so you immediately take a performance loss by adapting it to RG6 cable.

The alternate option would be to use that 75ohm cabling in a wireless repeater kit. You would have a roof mounted antenna, use the existing cable to come in to an amplifier, and from there another internal antenna. This would rebroadcast the signal to cellular devices within range of the system including the Duo, phones, etc.

The systems we have for 75 ohm are designed for use in the US and only work with US carriers, so if we can help you there feel free to reach out and we can put the right option together for you. This is a basic kit that could boost up to 1k square feet, depending on the signal strength outside the building.


#10

Hello @zegor_mjol ,
You must not use 75 Ohm cable runs with professional RF solutions for 3G/4G/LTE/Wi-Fi/PtP, these systems are designed for 50 Ohm impedance equipment only.

To put it simply:
75 Ohm is best used for one way receive signals only (such as TV & Satellite receivers)
50 Ohm is best used for bidirectional RF signals such as mobile/cellular systems, Wi-Fi & PtP (Point to Point).
Mismatches of impedance of connections, cables and equipment rating is going to cause you massive issues and even damage the sensitive electronics…

A good start for reading on this is the results found in a google search of
"what is the difference between 50 ohm and 75 ohm coax cable"
https://www.google.com.au/search?q=what+is+the+difference+between+50+ohm+and+75+ohm+coax+cable

Here are some additional great reads/references on this is from our friends at the Australian company "Telco Antennas"
Guide to Antenna Cables & Connectors
https://telcoantennas.com.au/site/guide-antenna-cables-connectors
Coaxial Cable Theory
https://telcoantennas.com.au/site/coaxial-cable-theory

Happy to Help,
Marcus :slight_smile: