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I found some data on antennas that had been tested for a specific radio. I'm trying to decipher what this data means and how one would go about gathering it. Are there any good resource for learning how to read and understand this data?

Specifically, please explain the following metrics:

  • dBS9
  • The relationship between dBm (the power ratio in decibels (dB) of the measured power referenced to one milliwatt (mW)) and antenna length.

For instance, in the following chart, which group of antennas operates optimally: the one in the blue freehand circle, those in the red freehand circle, or neither?

Antenna Length vs. dBm @ 145MHz

What common methods/software exist for obtaining this kind of data?

Dan
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  • This question might be better off if you asked what a specific term or a few related terms mean. Also, it's usually bad practice to have to go to a link to get a question/answer, although it is good practice to link to a source for more information. – PearsonArtPhoto Oct 23 '13 at 12:47
  • @PearsonArtPhoto how's that? – Dan Oct 23 '13 at 15:48
  • This question does seem a bit overly broad - typically you'd want to aim for answers to specific questions that are a subset of a task, rather than asking someone to walk you through the entire task. (And asking for "Common methods/software" broadens the scope further.) – Amber Oct 23 '13 at 21:14
  • @Amber I'm open to an edit to the question that gets me closer to understanding which antennas are best (blue/red circle, neither). I am so new at this I don't even know what to ask. – Dan Oct 23 '13 at 21:20
  • @Dan Consider splitting it up a little - for instance, you got an answer about what DBS9 means; that would have been a reasonable question all on its own. – Amber Oct 23 '13 at 21:26
  • I was afraid asking what dBS9 means would be closed as a 'reference' question, i.e. 'just Google it.' I had Googled it, and I found nothing related to ham radio. – Dan Oct 23 '13 at 21:28

3 Answers3

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Without a lot of information on the person's testing methodology, I'll assume that dBS9/dBm refer to the signal strength received by the radio the antenna being tested is attached to.

There are several different ways to interpret what's in the graph. You could ask "For a given antenna length, which antenna performs best" (look at the cluster around 15 inches for example), in which case it would be the one that gives the highest dBS9/dBm.

You could also look at the graph as one of those cost/performance type graphs. If you define your criteria as "short antenna and high dBS9/dBm", then the best antennas would be towards the upper left portion of the graph while the worst ones would be in the lower right portion.

"Operating optimally" requires you to define what "optimally" means, and this is going to be different for different circumstances. Do you want the highest dBm? Shortest antenna?

For just received signal strength, the ones you circled in red are great performers. Do you want to be carrying around a 35-45" antenna with you though?

Conversely, the one circled in blue is a great antenna if you want something short, but receive performance kind of sucks.

imabug
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For the first question: DBS9 refers to the strength of a received signal. S Units are generally considered to be fairly arbitrary, but the IARU Region 1 recommends a S9 reading be -93 dBm in VHF frequencies higher than 144MHz, which is equivalent to 50 microvolts at 50 Ohms.

Wikipedia has a pretty good article here: http://en.wikipedia.org/wiki/S_meter

Bryan Leenheer
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Acknowledge that the thread is a bit old, just some information for anyon ending up here with the question regarding dBS9. When testing antennas, on must set up a reference system. This is usually don by having a Tx/Rx system equipped with equal antennas. Then the signal strength is measured, and the result refereed to as dBref. In the mentioned article this becomes DBS9.

The signal measured for another antenna will then be compared to this reference. For instance a antenna with a DBS9 of +3 has 3dBm better sensitivity than the referenceat the given frequency.

Regarding dBm and messuring in decibel. Usefull information can be found in an article by Rohde&Schwarz "Everything you ever wanted to know about decibels but were afraid to ask… " (Google know where to find it)

Martin
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