RF Coaxial Cable

[sohaib_a]

I need to decide on suitable RF cables for an RFID system.It is UHF 915Mhz band,My main concern is the distance....the cables should run a maximum distance of 60 meters with minimum loss.
I have some LMR 400 series coaxial cables from Times Microwave which I used for 25 meter runs and worked well.But i think i might face problems if use them for much longer distances.I have seen some other LMR series cables online with higher specs but they are quite big in size (diameter) and very rigid outer jackets which aren't suitable for sharp bends I think, will they be suitable for this application or is there some other type of cable available?

 

[Sceadwian]

If this is for receiving you should put your receiver as close to the receiving antenna as possible then you don't have to worry about transmission line loss at all and you can use a simple low data rate cable for your long run without worrying about high frequency impedance matching.

If it's for transmitting your power levels can't be that high no more than a few hundred milliwatts? If you're getting loss on the line just boost the power output a little to get the desired output power. At lower power ratings you're not going to be generating any significant heat on the line and with a 60 meter feed line.

 

[RadioRon]

Losses in coaxial cable are closely related to cable diameter for competitive cables. If you must have a certain minimum loss, then you will be stuck with a certain minimum diameter. For example, Heliax, which is a good quality low loss coax from Andrews has losses vs diameter as follows:
dia loss at 60 m (est)
0.,25 inches 7.5 dB
0.375 inches 6.5 dB
0.50 inches 3.5 dB

This type of cable is not very flexible since the outer conductor is corrugated solid copper. Typically the more flexibility, the greater the losses, so it probably doesn't get too much better than this.

 

[kchriste]

You'll lose about 8db with 60 meters of LMR400 at 915Mhz.
You'll lose about 3db with 25 meters of LMR400 at 915Mhz.
You'll lose about 3db with 60 meters of LMR900 at 915Mhz.
Try one of the many online cable loss calculators to try and find a suitable cable:
https://www.ocarc.ca/coax.htm
Sceadwian's idea is much more practical. Simply move the RF portion of the ID system closer to the antenna.

 

[Nigel Goodwin]

Yes, it seems crazy to have the electronics so far from the aerial, run the long cables at data levels, not RF.

 

[Sceadwian]

3db loss is 50% of the power, that doesn't do much for the range of an RFID system. You definitely want to put the basic RF front end decoder AT the antenna location then you can use current loop or some other transmission method to send the raw data to the tailend of the system with no loss. The extra cost of the 'sub circuit' at the antenna will likley be cheaper than having to deal with a low loss expensive transmission line and interconnects.

 

[kchriste]

You'll lose about 3db with 60 meters of LMR900 at 915Mhz.

Here I am quoting myself.
Actually, it is worse than that since the loss is in both the transmit and receive directions, making a system loss of 6db by adding 60 meters of LMR900.

 

[Sceadwian]

Transmit loss is irrelevant in an RFID environment, the power output is so low that all you have to do to compensate for line loss is to increase voltage/power at the transmitter. This is not so when receiving where every fraction of a decibel counts for increased range from a static source. The power you're dealing with transmit in RFID is so low the heat loss can generally be ignored. For receiving the closer to the antenna feedpoint you get that analog signal the higher the gain from any given antenna. I know of no antenna design that benefits from a long feed line length.
The system length is not twice the 60 feet. It's 60 each way. 3db loss one way, 3db loss the other, it's only 6db if you're trying to receive the transmitted signal directly, since transmit loss can be ignored if it's adjusted for, and if the receiver is placed at the antenna feed point the loss is 0 or as close as you can get it as the parasitics in the connectors allow.

 

[whiz115]

I have seen some other LMR series cables online with higher specs but they are quite big in size (diameter) and very rigid outer jackets which aren't suitable for sharp bends I think, will they be suitable for this application or is there some other type of cable available?

LMR400/Αircom+/ecoflex-10 are suitable cables with simular losses, and you can bend them a little bit... other types of cables might have less losses but they are too thick to bend them! best thing to do is what Sceadwian proposed. bring your device closer to the arial.