When simple explanations target a simple audience

Modern hams live busy lives and it is difficult to fit everything in to the available time / resources etc. So, there is an appetite for the skinny on some key topics, the inside info that took the wise a long time to learn.

This article discusses one of those articles containing the skinny on VSWR, What is VSWR: Voltage Standing Wave Ratio, it takes only a minute or two to read and there is a six minute video for those who prefer that.

The issues discussed here are common in the ham world explanation of VSWR and analysing them provides a learning opportunity. The video contains the issues mentioned below… and some.

Right up front, eager readers are given a take home message. If something prevents them finishing the article they have learned something they can repeat as pros. So satisfying!

In order to obtain the maximum power transfer from the source to the transmission line, or the transmission line to the load, be it a resistor, an input to another system, or an antenna, the impedance levels must match.

In other words for a 50Ω system the source or signal generator must have a source impedance of 50Ω, the transmission line must be 50Ω and so must the load

Ok, it states clearly and unequivocally that a necessary condition for maximum power transfer it that source must match line and line must match load.

We will test that proposition, but firstly the detailed explanation follows… Continue reading When simple explanations target a simple audience

Last update: 8th October, 2018, 10:33 AM

Australian amateur radio licensing reform (2018)

It has become clear that ACMA intends to progress the WIA’s initial actions to partially integrate the qualifications requirement for issue of an amateur radio licence into the Australian Qualifications Framework (AQF).

The AQF is the national policy for regulated qualifications in  Australian education and training. It incorporates the qualifications from each education and training sector into a single comprehensive national qualifications framework. The AQF was introduced in 1995 to underpin the national system of qualifications in Australia encompassing higher education, vocational education and training and schools.

That push on integration includes the use of Registered Training Organisations (RTO) for assessments, RTOs are an element of the Vocational Education and Training Sector (VETS).

The definition of Vocational Education and Training can be taken from australia.gov.au:

Vocational education and training

Designed to deliver workplace-specific skills and knowledge, vocational education and training (VET) covers a wide range of careers and industries, including trade and office work, retail, hospitality and technology.

So, the WIA and ACMA have over a couple of decades acted to integrate amateur radio qualifications in the AQF to some extent, and current actions are intended to perform assessments within the VETS (ie by VETS qualified assessors under an RTO). Continue reading Australian amateur radio licensing reform (2018)

Last update: 18th October, 2018, 8:07 AM

Measuring ambient noise level using a spectrum analyser #2

The article Measuring ambient noise level using a spectrum analyser was a walk through of measuring ambient noise using a spectrum analyser.

This article details a method that uses an online calculator to conveniently perform the calcs that permit more accurate answers by factoring the internal noise of the spectrum analyser into the calcs.

Step 1: measure instrument noise figure

Measure the noise floor of the instrument with 50Ω input termination using an average power (RMS) detector.

Now calculate the Noise Figure (Field Strength Noise Figure on output report). Continue reading Measuring ambient noise level using a spectrum analyser #2

Last update: 27th September, 2018, 7:04 AM

AE7PD’s transmitting loop measurements

AE7PD documented his measurements of a 3.16m perimeter circular transmitting loop, 1.8m centre height above ground, that he made using 16mm copper tube and a split stator tuning capacitor:

AE7PD gives the radiation efficiency on 20m as 30.5% or -5.2dB.

I present here an alternative analysis of the antenna as measured on 20m.

Assuming the measurements were made with the antenna clear of disturbing conductors etc, and that 5/8″ tube means 16mm OD.

The key measurements were:

  • centre frequency 14.165MHz, VSWRmin=1.0;
  • VSWR=2.62 bandwidth 22kHz.

A NEC-4.2 model of the antenna at 14MHz was built and calibrated to the measured half power bandwidth (22kHz). Model assumptions include:

  • ‘average’ ground (σ=0.005, εr=13);
  • Q of the tuning capacitor = 2000;
  • conductivity of the loop conductor adjusted to calibrate the model half power bandwidth to measurement.

Note that the model may depart from the actual test scenario in other ways.

Above is the VSWR scan of the calibrated model, the load is matched at centre frequency and half power bandwidth is taken as the range between ReturnLoss=6.99dB points. Continue reading AE7PD’s transmitting loop measurements

Last update: 22nd September, 2018, 8:28 AM

Findling & Siwiak 2012 measurements of an Alexloop – discussion

I mentioned in Findling & Siwiak 2012 measurements of an Alexloop issues with their efficiency calculation.

Above is an extract from (Findling & Siwiak 2012).

(Siwiak & Quick 2018) give an equivalent circuit of lossless loop structure in free space.

When tuned to resonance, the response is simply that of a series RLC circuit where R=Rr (the radiation resistance) which is dependent on frequency, but varies very slowly with frequency compared to the net reactance X.

Above is a NEC simulation of such a loop. Continue reading Findling & Siwiak 2012 measurements of an Alexloop – discussion

Last update: 18th September, 2018, 2:42 PM

G3CWI 2018 measurements of an Alexloop Walkham

Richard, G3CWI, measured the impedance and bandwidth of a Alexloop Walkham, a popular small transmitting loop (STL). The antenna was situated in the clear at 1.65m centre height above natural ground.

The key measurements were:

  • centre frequency 7.014MHz, |Z|=51Ω, VSWR=1.1;
  • VSWR=3 bandwidth 16.2kHz.

The step size of the analyser prevented measurement exactly at resonance, but R changes very closely with frequency near resonance so we can estimate it quite well. The above figures can be used to find R close to resonance.

Within the limits of measurement error, we can say that R at resonance should be very close to 51Ω, and VSWRmin close to 1.02. Continue reading G3CWI 2018 measurements of an Alexloop Walkham

Last update: 14th September, 2018, 5:38 PM

Findling & Siwiak 2012 measurements of an Alexloop

(Findling, A & Siwiak 2012) documented measurements they made of a popular small transmitting loop (STL), an Alexloop Walkham.

Now Alexloops seem to have undergone some evolution, and there does not seem to be a clear list of model names or numbers with features or specifications, so to some extent the antenna is a little non descript.

The article did not document the environment of the test antenna, but Findling explained in correspondence that it was relatively clear of conducting structures and about 1.2m above natural ground.

A NEC-4.2 model of the antenna at 7MHz was built and calibrated to their measured half power bandwidth (19kHz). Model assumptions include:

  • ‘average’ ground (σ=0.005, εr=13);
  • Q of the tuning capacitor = 1000;
  • conductivity of the loop conductor adjusted to calibrate the model half power bandwidth to measurement.

Note that the model may depart from the actual test scenario in other ways, it is challenging to glean all the data that one would like from the article.

Above is an extract from (Findling, A & Siwiak 2012).

Above is the VSWR scan of the calibrated model, the load is matched at centre frequency and half power bandwidth is taken as the range between ReturnLoss=6.99dB points. Continue reading Findling & Siwiak 2012 measurements of an Alexloop

Last update: 13th September, 2018, 8:41 PM

Single turn coaxial loop resonator analysis

Recent discussion online of a purported commercial HF small transmitting loop (STL) was challenged in analysing the structure, questioning whether such a connection was ‘correct’.

The STL used a main loop resonator and a separate small auxiliary loop for the 50Ω feed, a very common arrangement.

The main loop is a coaxial cable with, in this case, a tuning capacitor inserted between the inner conductors at each end. Above is a diagram of the main loop. Continue reading Single turn coaxial loop resonator analysis

Last update: 13th September, 2018, 7:20 AM