Sunday, June 12, 2011

Verifying LMR-200 Coax Cable Loss

For years I have used RG-58 cable for my antenna systems, it is inexpensive, easy to work with, and for short runs it works just fine. Last year I started heavily receiving NOAA weather APT imagery using the same RG-58 cable. During this time, pass after satellite pass I noticed my images were not as pristine as they should be. The cable length I was using was just over 100' in length and looking at the loss of my RG-58 it's no surprise that I had such poor images. As a test I replaced all of the cable with a used length on Andrew LMR-400 cable which resulted in an instant improvement.

Now this past week I finally mounted a wide-band scanner antenna on my home and needed a 60' run of cable to reach my RF bench in the basement. Instead of going to my typical spool of RG-58, I wanted something better. LMR-400 was my first choice but is significantly more expensive than my free RG-58. Because of this and the fact that a smaller diameter cable would be better hidden on the outside of the house I chose LMR-200. I purchased 60' of Times Microwave LMR-200 (arguably the best cable you can buy).

With an attenuation of 9.9dB @ 900Mhz, it was significantly better than RG-58. This cable combined with quality Amphenol SMA connectors I was curious to see if the cable would measure up to it's spec sheet. Times Microwave rates it's LMR-200 at 9.9dBm of attenuation at 900Mhz over 100'. At my 60 feet I should only be seeing 5.94dBm of loss which is what I am looking to verify.

The test setup:

An HP 8656B signal generator would be the test source while the spectrum analyzer in my HP 8922H would be used to verify the attenuation of the cable. As a baseline test, I connected a small 1' high quality 50 ohm SMA cable with Amphenol connectors in between the signal generator and spectrum analyzer. Using a -50dBm baseline, the measured signal through the cable was -50.52dBm at 100Mhz. At 900Mhz, it was -51.65dBm, a good start. Now to place the 60' of LMR-200 inline. Unfortunately I did not have a male N to female SMA adapter so I could not test the cable directly inline. To make the test work I placed a mini-circuits ZFSC-2-5-S splitter inline of the cable.

According the the mini-circuits datasheet, at 100Mhz this splitter has a loss on each output of 3.25dBm. At ~900Mhz it has a loss of 3.57dBm, both of which will be compensated for in the results. So testing the 60' of LMR-200 through the splitter at 900Mhz yielded -59.95dBm:

Taking that result and subtracting the loss of the mini-circuits splitter you end up with -56.38dBm, and the difference of that from the original baseline of -50.52dBm at 900Mhz is -5.86dBm of loss at 900Mhz, which is under the spec sheet of -5.94dBm by .08dBm, a very good result indeed. I love it when numbers work out. As a final test I ran a sweep test on the 60' cable using an HP 8754A Vector Network Analyzer. As expected, the loss from 10Mhz to 1300Mhz exactly matched the expected attenuation in the datasheet:

I will be running this cable to my antenna tomorrow. :)