Anritsu FAQs

All cables have loss. If there is too much total loss, then not enough signal is reflected back to the Site Master for a valid measurement. The noise floor of the Site Master is ~40 dB, so if there is more than 40 dB of loss in the run of cable the Site Master will have trouble. Here's an example: A typical cable spec'd as follows: 3.9 db/100 ft @ 2000 MHz Let's assume we have 500 ft of cable. The first question I ask is: "how much loss is there with 500 ft of cable?" 0.039 db/ft * 500 ft * 2 = 39 dB (The * 2 multiplier is due to the round trip nature of a S11 measurement i.e. down the cable and back). If the noise floor of the Site Master is about 40 dB and there is 39 dB of loss in 500 ft of cable, I would expect to have measurement limitations with the Site Master. So, for this particular set of conditions we should disregard anything passed 500 ft. So now we can answer the original question properly. The code uses the c able loss value you enter to adjust the plotted DTF measurement. Let's use another quick example: if you have a 20 dB antenna 100 ft down a perfect cable you would expect to see that on a 20 dB spike in your DTF plot. Now if the cable is not really perfect and you do NOT account for the loss of 0.039 dB/ft the actual antenna will show as 27.8 dB (0.039 * 100 * 2 = 7.8 dB). If you entered 0.039 as the cable loss, the code will subtract (0.039 * 2) for every ft on the screen, giving you the actual antenna RL. That's grand until your loss approaches your noise floor. Once you have subtracted loss equivalent to your noise floor, the actual noise floor starts to creep up on DTF display. Don't forget that while the noise floor is generally a fixed quantity (40-45 dB), the cable loss in a system varies greatly. So, the maximum distance you can see down a cable can vary greatly as well. Incidentally, there are 2 ways to "see further down a cable": Reduce your loss or improve yo ur noise floor. They might try sweeping it at 1000 MHz where the loss is usually a bit lower there. The other thing they can do is use a true bench top VNA. It typically can get 50-60 dB dynamic range so you can handle more net loss. Another option would be to use a S251C in Insertion Loss mode. Insertion loss measures the loss from one end of the cable to the other (instead of doing a reflected measurement where you get hit with the cable loss 2x). While this would not permit DTF plots, but it would allow for an average cable loss value calculated.