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Author Topic: Coax cable & signal loss  (Read 646 times)
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Hi all, this might be a question for bar sport but as it's electronics related im putting it here,

so i want to test the range on my XRF modules http://shop.ciseco.co.uk/xrf-wireless-rf-radio-uart-rs232-serial-data-module-xbee-shape-arduino-pic-etc/ in everyday built up areas, mounting an external antenna in my garden, is there any point? if yes what coax should i use? 50 ohm?

The plan would be to use one xrf as a beacon, the other will be mobile in my car, when a valid signal is received from the beacon the 2nd xrf will log the position from gps which can then be plotted on google maps to asses the range.

so the main question is the coax length? impedance? how much loss to expect through the coax?
BTW they operate on 868.3Mhz, 75/868.3 = 86mm antenna length, other than that radio is not my strong point

Thanks
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Rather than jiggering that board, and soldering on a reverse-SMA connector or something,
you might be better off finding a board with the correct connector already soldered on,
and then finding a compatible antenna with coax lead.

http://www.google.com/search?&site=imghp&tbm=isch&source=hp&biw=1012&bih=811&q=868+mhz+dipole

http://www.digi.com/products/wireless-wired-embedded-solutions/zigbee-rf-modules/point-multipoint-rfmodules/xbee-pro-868#overview
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At 800+ Mhz , coax cable loss starts becoming pretty significant with any long length.
You need to provide details of what type of coax cable you intend to use.
RG58, which is a common type of 50 ohm cable, has a loss of approx 1 db per metre at 900 Mhz.

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Coax is commonly available in (nominal) 50 ohm and 70 ohm impedance. The datasheet doesn't list the output impedance but if it has a SMC connector it is most likely 50 ohm. There is significant loss in coax when working at that high a frequency. The loss of a specific coax depends on it's diameter and material used. Below is a link to a coax loss calculator where you enter the type of cable, length of run, and operating frequency and the loss in DB is given. Doesn't matter what value of power you put in, just enter 1 and default swr to 1 also. Recall that a 3 db loss is half the power put in one end of the coax is lost before coming out the other end.
http://www.arrg.us/pages/Loss-Calc.htm
Lefty
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These calculations assume you are using an antenna that matches the coax's impedance at the frequency used, otherwise there will be more loss due to the mismatch.
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I would NOT consider using ANY kind of Co-Axial cable @ 900 MHz. The technology is quite beyond most of the users here. If absolutely required, pre made coax jumpers characterized for 900 MHz would be an absolute requirement. Any gain in signal strength either transmit or receive would be lost very quickly. If remoting a transmitter is an absolute requirement consider remoting the whole transceiver with a control cable. Cat 5 or 6 would work well for that.

Bob
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Thanks for the responses, I gather long cables are not going to work well, so im thinking stuff the arduino into a weatherproof box and mount it high up on my house.

The module has a wire whip antenna fitted but it also has pads for an sma connector, i'll just test the range with the whip then maybe with an external antenna after, they have a 3km open air range, I know that will dramatically reduce in urban areas, I'd like to know how much by logging data with a gps
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As d.edison and I mentioned, all in all, probably best to stay with commercial designs for
such high-frequency stuff. Their antennas are matched and tuned for their transceivers.
900-Mhz will have much longer range than 2.4 Ghz, you'll just have to experiment and see.

If you have a couple hundred $$$ to burn, you might try Aerocomm [now Laird] 900-Mhz
AC4490-1000 transceivers. They go 1-Watt [yes, 1000 mW] licensed in the US.
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