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VSWR for testing 900Mhz freq hopper antenna & cable

VSWR for testing 900Mhz freq hopper antenna & cable

Our plant has a small network of 4 frequency hopping, 900Mhz, 1 watt, unlicensed ISM band, serial data radios (tlaking Modbus) whose performance seems to be degrading over the past 2 years, as evidenced by an increasing number of time-outs and failed responses.

The system worked flawlessly for the first 1 year, but over the past 2 years has gradually "missed" more readings.   The data is for monitoring & logging purposes, not for control purposes, but I'd still like to figure out what the problem is to come up with a repair stategy.

The various radios are mounted indoors and an antenna cable exits each building to yagi antennas mounted on stand-offs or masts (the base radio has an omnidirectional antenna).  Each radio has a surge suppressor device between the antenna cable and the radio.

Each radio has its own 24Vdc power supply (2 amp rating for a 200mA radio load) and each p/s is dead on at 24Vdc.

Each radio has an output for an RSSI value.   Having recorded the RSSI values over 4 months shows a slight decrease month to month (hundredths of a volt on a 0-4Vdc scale), with a couple radios at the marginal edge of acceptable.

At 1 watt output, I suspect that the actual transmitter part of the radio probably is not defective, but I haven't yet tested that.  

I suspect that the antenna cables or antennas might have impaired efficiency due to weather, sunlight, water instrusion, whatever.

Going back several decades to "Good Buddy" days of CB radio, I remember using an SWR meter to 'tune' the length of a CB antenna.   If my recollection is correct, SWR is supposed to indicate the relative amount of power radiated out from the antenna 'system' vs that which is lost in reflection.  As such I infer that SWR can indicate the performance of the suppressor, cable and the antenna.

I inquired of the radio supplier about whether a 900Mhz VSWR meter would be suitable for
  a) checking the relative condition of the antenna cable and antenna
  b) checking the power output of the radio's transmitter with a dummy load.

The individual I talked to wasn't sure because that function resides in the "installer's domain, the guys who put up the towers" .  The manufacturer only sells radios.   Sigh.  He did tell me that there is an AT command that will force the radio into continuous output mode, which although technically not legal in the ISM band, a 2 second output needed to get a reading is unlikely to disturb much in a fairly rural area.

I have a spare yagi antenna and a spare 25' antenna cable.  My thought is to connect those to one of the radios to get a base line SWR reading.  Then compare that reading to the reading of each of the installed radios.

My questions are
  1) whether a VSWR meter like the Daiwa CN-801SII
might be suitable for doing antenna system & transmitter power testing?

  2) Are there light duty,commercially available 2 watt dummy loads?  (I"ve found 10 and 15 watt dummy loads)

  3) I asssume I have to make some accounting for any error introduced by a dummy load for a power reading, or maybe a dummy load error is essentially a constant?

  4) Is there a test for the surge suppressor?  Do surge suppressors fail incrementally or catastrophically?

RE: VSWR for testing 900Mhz freq hopper antenna & cable

I'm not an RF guy but I agree with your assessment on the 'decay'.  Oxidation, leakage, water, corrosion would all act that way.  Also a tree growing into your line-of-site would act that way over that time frame.

I'm not sure you will see quantifiable differences with a VSWR meter based on what appears to be only a slight degradation that's occurring.

How about just replacing an antenna with that spare one and studying the result?

Keith Cress
Flamin Systems, Inc.-

RE: VSWR for testing 900Mhz freq hopper antenna & cable

It's almost impossible to guess the problem over the Internet, but we can provide a shopping list of possibilities.

But first, some tidbits.

Although range depends on other parameters, one would expect that 1 watt of RF into a Yagi antenna should provide tons of signal over fairly long (more than 1 mile) line-of-sight paths. The exceptions would be, for example, very wide bandwidth applications, or if the receiver circuit is cheap.

Although VSWR should be checked, as a total system measure it actually isn't all it's cracked up to be. For example, a lossy cable might improve the VSWR while making the signal weaker. A high VSWR might indicate, for example, that the antenna is broken; but a visual inspection might reveal the same thing much quicker.

A slow deterioration is often indicative. But, trust nothing. Put it on the list as a clue - maybe.

Shopping list:

1) Water getting into coaxial cables. All outdoor connections must be wrapped in coax sealant. If they weren't then you've probably found the problem.

You can test the cables for loss at the operating frequency. One method is to measure the return loss for an open-ended cable and divide by two. Compare loss against spec.

If the above is difficult, then perhaps cut off about six inches of a suspicious cable and reterminate. Then cut open and inspect the cut-off section. Look at the shield - it should be shiney and new looking. This is not a conclusive test (cables can look good and be bad); but if water drips out, it is.

2) Antenna connections getting oxidized. Disassemble and clean antenna. All metal to metal joints should be clean and tidy. Insulators should be clean. Reassemble with a dab of conductive anti-seize compound on the metal joints.

3) Inspect the antennas for anything broken or bent. Make sure that they're still aimed properly (not highly critical for moderate gain antennas).

4) The noise level on your frequency might have increased. Perhaps someone else is also trying to use 'your' frequencies.

5) Lightning damage to in-line protection devices. Very unlikely. If those devices had taken a hit, then you'd probably have at least a few fried units.

6) Perhaps the units have deteriorated - maybe the units are not designed properly and even a tiny drift of the master crystal throws off the whole system. Very very very unlikely. Might be worth calling the company and asking the appropriate engineer if he knows any reason why the units seem to deteriorate. Do this last.

RE: VSWR for testing 900Mhz freq hopper antenna & cable

I use a sitemaster 322 and record the results for both cable losses and swr on our installations.
Subsequent testing when required tells us if there has been a change in the antena and cables characteristics.
Without a detailed record of initial characteristics every thing you measure is only as good as a guess.
The sitemaster is an expensive but lovely piece of kit, hiring one or something like it may be worthwhile to help you sort out your problems.

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