testing leaky cable

[johnny1225]

I was wondering is there a proper way to test LMR 600 Leaky or radiating cable for watts at the end of the line. I have 150 feet of LMR600 leaky cable in a tunnel that i am trying to see what type of watts i am getting at the end of the 150 ft of cable. It look like to me that you can not test it with a wattmeter or can you. Do you need more specialized equiment to measure this cable. The cable is from time microwave and i can't seem to get watts at the end of the cable. Does this look like a connector problem or do you need something else in ways of test equipment to measure this cable.

Thanks

John

[Bill_G]

John - Theoretically you can test it with a wattmeter, but depending the band, and the drive from the BDA, you may not see much. Use a spectrum analyser with a port that will accept at least a +30db or better.

[tvsjr]

A power meter would probably be the right answer... failing that, a service monitor or spectrum analyzer. Start out on the RF input for some power handling in case there's more power on the end than you think... step down to a low-level input if you need additional sensitivity.

[Jim202]

I would suggest using a transmitter and measuring the power out at the radio into the cable. Then go to the far end
of the cable and measure the TX power again. Make sure there is no other devices connected to the cable under test.
Make sure that you have a termination on the far end of the cable while making these measurements. Use the same
watt meter for both measurements.

You will need to know what the spec is for the cable loss and the total length of the cable. There are a number
of web sites that have calculators that will do the math for you. Don't forget that the loss of the cable is dependent
on the frequency being used. The results will give you the loss of the cable. If the loss is greater than the spec,
you should change out the cable. If the loss is close to the spec, the cable is OK.

An example is a transmitter of 50 watts output. You have a cable that is 100 feet long with a loss of 3 db. You
should see 25 watts at the far end.

Jim

[johnny1225]

ok i just did a test here at our office. Using a UHF Repeater. The repeater is putting out 25.7 watts. I have a 150 ft of LMR 600 leaky cable with N Connectors on both sides. At the end of the 150 ft of cable i am getting 2.75 watts. This by time microwave says the lost is 2.2db per 100 ft of cable, i have changed the connectors twice with the same results. Is there something i am doing wrong with the test or do you think i have a bad run of cable.

thanks

john

[Bill_G]

ok i just did a test here at our office. Using a UHF Repeater. The repeater is putting out 25.7 watts. I have a 150 ft of LMR 600 leaky cable with N Connectors on both sides. At the end of the 150 ft of cable i am getting 2.75 watts. This by time microwave says the lost is 2.2db per 100 ft of cable, i have changed the connectors twice with the same results. Is there something i am doing wrong with the test or do you think i have a bad run of cable.

thanks

john

I just confirmed the 2.2db/100 for TRAD-600 which means you should be expecting approx 3db loss in this installation, but are experiencing approx 10db. I would sweep the line with an Anritsu or equivalent from the radio jumper to the line termination. Look at the return loss and the DTF. Could be a bad crimp, a bad polyphaser, or a damaged cable. Is repeater operation noisy / crunchy? That would confirm a physical break as reflected xmit comes back to clobber the rcvr.

[Jim202]

Well now you have something to go looking for. Not to assume anything here, have you very carefully
looked at the cables entire length? Your looking for anything out of the normal. Like a clamp on the cable
that was installed too tight and smashed the cable. Maybe some work was done in the area of your cable
and it was cut or flattened. It may take running your hand along the length of the cable to feel for any
damage.

It has been know for vandals to drive pins into coax to short it out. have you even tried to measure to
see if you have a short on the cable. Disconnect both ends an see if you get infinity on an ohm meter.
Then try shorting the distant end and see if you see a low resistance. It won't be zero ohms, but I
would expect it to be under 25 ohms or less.

Bill G has the right idea if you can get your hands on a Site Master test set or a good TDR if nothing else
and look at the cable.

Jim

ok i just did a test here at our office. Using a UHF Repeater. The repeater is putting out 25.7 watts. I have a 150 ft of LMR 600 leaky cable with N Connectors on both sides. At the end of the 150 ft of cable i am getting 2.75 watts. This by time microwave says the lost is 2.2db per 100 ft of cable, i have changed the connectors twice with the same results. Is there something i am doing wrong with the test or do you think i have a bad run of cable.

thanks

john

I just confirmed the 2.2db/100 for TRAD-600 which means you should be expecting approx 3db loss in this installation, but are experiencing approx 10db. I would sweep the line with an Anritsu or equivalent from the radio jumper to the line termination. Look at the return loss and the DTF. Could be a bad crimp, a bad polyphaser, or a damaged cable. Is repeater operation noisy / crunchy? That would confirm a physical break as reflected xmit comes back to clobber the rcvr.

[Bill_G]

I'm suspecting a 6db attenuator in the line along with normal loss. 25w is pretty hot for a 150ft tunnel unless it is needed at the donor to reach dispatch. 1w would have been sufficient to drive that short length and achieve good rsl the entire tunnel even with a curve. 2w might be over engineering, but I do not know the exact application - transportation tunnel, building service tunnel, or mine shaft. And I don't know if this is a bda or a stand alone rptr.