Measuring ambient noise level using a spectrum analyser

The external noise figure Fa is defined (from ITU P.372-13) as:

I have taken a sweep of the 40m band when this is a little activity, but little enough to see the ambient noise floor at the time. It is raining and it is relatively noisy.

Above, the noise floor in 9kHz bandwidth with a CISPR quasi peak detector is about -78dBm. This is 12dB above the instrument noise floor, sufficient to not bother making a correction and we can take the external noise to be -78dBm (see below for correction calculation if needed). Lets allow 1dB loss in the antenna system, and call it -77dBm at the air interface.
Continue reading Measuring ambient noise level using a spectrum analyser

Surecom SW-102 VSWR meter review – v2.6

At Surecom SW-102 VSWR meter review I wrote a review of a meter which I had purchased a little over a year ago, it was at v4.5.

One of the many problems identified was inconsistency of displayed values.

v2.6

Surecom’s versions are confusing, the highest number is not necessarily the latest version. It appears a partial version history from their current page advertising the SW-102 is:

OLD VERSION : V3.3 ,V3.8 ,V4.5,V4.9 ,V5.0,V5.1
2017-8 NEW VERSION : V2.02 ,V2.03

The following image is from Surecom’s current page advertising the SW-102, and I assume that the version shown here (v2.6) is the latest at time of writing.

The image captures the outputs of two tests with poor and good dummy loads.

Let’s check the displayed values for internal consistency. Continue reading Surecom SW-102 VSWR meter review – v2.6

Is |s21| measurement of a common mode choke meaningful to antenna systems?

A common theme among online experts is to measure, or ask for measurement of a common mode choke connected between the centre conductor of a VNA’s tx and rx ports. That raises the question of whether |s21| is meaningful, whether it in any useful way characterises the choke as a component of an antenna system.

Direct measurement of common mode current is not difficult, and it is almost always the best way to determine the effect of a choke on common mode current.

That said, analytical and simulation techniques can be of great value in the antenna design process, well before a prototype antenna is built.

An example choke at 7MHz

Lets perform an experiment using NEC to model the effect of a choke in a 7MHz antenna.

The choke used uses 11t on a FT-240-43 ferrite core. The values are from a calibrated model, values confirmed by measurement.

We will use NEC-4.2 with one of the scenarios detailed in the article Baluns in antenna systems, Model 4, but using the choke described above which has an impedance of 3175+j2502Ω at 7MHz.

Above is a simulation of the connection. Zcm of the choke in this case at 7MHz, 3175+j2502Ω, has been converted to an equivalent inductance and resistance to suit the simulator. (Note that the equivalent circuit it valid only for a narrow band, there is no simple wideband circuit equivalent for this ferrite cored choke (more later).) Continue reading Is |s21| measurement of a common mode choke meaningful to antenna systems?