APRS duplicate removal – trial #3

The undetected long-delayed duplicate posits that are a feature of APRS VHF are a significant corruption of mapping.

In an attempt to limit the propagation of posits and hence the probability of corruption / delay etc, I have experimented with a path of WIDE1-1 on a recent trip to Canberra (about 400km for the round trip).

Whilst this should prevent packets getting to the Wagga, Newcastle and Tamworth regions which have been the main cause of corrupted posits and mapping defects, it does so at the risk of some loss of posits as some digi infrastructure was never updated to the “New N paradigm” of more than a decade ago and they ignore WIDE1.

Screenshot - 06_03_16 , 14_15_51

Above is a zoomed in view of the Canberra end of the trip, and I am pleased to say that the zig zag double backs that have been evident in recent trips did not occur. The principal reason is that with a path of WIDE1-1, the packets did not pass through VK2RWG-1/VK2KAW. Continue reading APRS duplicate removal – trial #3

Last update: 6th March, 2016, 7:51 PM

Chinese 18650 Li-ion cells – Ultrafire capacity test

I purchased a torch (flashlight) on eBay recently. It was described as using CREE T6 LED array, and supplied with two 4200mAh 18650 Li-ion rechargeable batteries with charger for A$25 inc post.

Ultrafire18650Above, the cells are clearly marked 3000mAh, way short of the advertised 4200mAh… but what is their actual capacity.

Screenshot - 04_03_16 , 19_45_49

Above are the results of discharge tests, the first digit is the cell number and the second is the test. The first test is charged with the supplied charger, the second test is with my charger. Continue reading Chinese 18650 Li-ion cells – Ultrafire capacity test

Last update: 4th March, 2016, 8:06 PM

End fed matching – analysis of VK3IL’s measurements

David, VK3IL posted EFHW matching unit in which he describes a ferrite cored transformer matching unit that is of a common / popular style.

My EFHW match box. 3:24 turns ration on a FT140-43 toroid with a 150pF capacitor across the input.

Above is David’s pic of his implementation. It is a FT140-43 toroid with 3 and 24t windings and note the 150pF capacitor in shunt with the coax connector.

The popular belief is that these are a broadband impedance transformer with impedance ratio equal to the square of the turns ratio, 64 in this case and therefore a broad band match from 3200Ω to 50Ω.

To his credit, David took some measurements of several different variations and reported them in his article.

Clip 135

Above are David’s measurements of the subject transformer.

Lets explore the matching detail for the case of a 3.3kΩ load at 22MHz, and using the 150pF shunt cap.

Superficially, you might convince yourself that this is explained by the turns squared story, but the 150pF doesn’t reconcile with that story, nor does the variation with frequency, eg the rapid change above 22MHz. Continue reading End fed matching – analysis of VK3IL’s measurements

Last update: 25th December, 2017, 6:22 AM

ATU efficiency

Much is written about ATU efficiency, about the need for them or not, and often in subjective terms like “lossy ATU”, and most of it lacking quantitative detail.

The little quantitative detail is almost entirely for purely resistive loads… as if that is typical of real life conditions.

The most common configuration used today is the ‘high pass T match’, but a range of other configurations are seen as being superior… though usually without quantitative evidence.

MFJ claims

More Hams use MFJ-949s than any other antenna tuner in the world! Why? Because the worlds leading antenna tuner has earned a worldwide reputation for being able to match just about anything.

… so let’s make some measurements with a reactive load on a MFJ-949E. Capacitive loads tend to be very common for antenna systems at lower HF, so let’s choose a load of 50Ω with a 100pF silver mica cap in series at 3.6MHz. The reactance of the cap is -442Ω, so the load is 50-j442Ω, and the 50Ω part is a RF power meter (RFPM1).

The test setup then is:

  • a standard signal generator (SSG) on 3.6MHz with 20dB precision attenuator so that we are confident that Zs=50Ω (important to the adjustment of the ATU for maximum power as indication of 50Ω match);
  • MFJ-949E;
  • 100pF silver mica capacitor (low loss);
  • RFPM1

The SSG was adjusted for -10dBm out directly into the RFPM1, then the ATU+cap inserted and ATU adjusted for maximum power indication. Power indicated was 1.4dB lower, so InsertionLoss and TransmissionLoss are both 1.4dB.

Screenshot - 26_02_16 , 19_13_52

Above is a simulation of the T network in RFSim99, component values are adjusted for a match and inductor Q is calibrated to the measured loss of 1.4dB. Continue reading ATU efficiency

Last update: 27th February, 2016, 8:34 PM

RG-6/U for lower HF

RG-6 has become a popular 75Ω transmission line for ham stations, and I have used it to good effect in many applications.

(Duffy 2007) extolled the virtues and gave implementation information, but cautioned:

Some types of RG−6/U use a CCS centre conductor and will have higher loss at low frequencies that shown in Fig 1, depending on the thickness of the copper cladding which may vary from cable to cable.

I have used RG-6/U with solid copper centre conductor widely on HF, and measured performance has always been consistent with expectation.

However, RG-6/U with solid copper centre conductor has become very hard to obtain, and products that remain available such as Belden 1694A are quite expensive.

This article documents measurements at low HF on a 100m roll of Quad shield RG-6/U purchased for UHF TV cabling.

The method used was to measure input impedance of the open circuit terminated 100m line section at a range of resonant and antiresonant frequencies, and from those to calculate Matched Line Loss (MLL) in dB/m.

Screenshot - 25_02_16 , 10_17_41

Above is an example measurement around 3.74MHz. Zin is 213.4Ω at 3.74MHz. In this case I have used an AIMuhf one port analyser, but any instrument that can measure impedance in the range 10-500Ω would suit this particular scenario. Measurement of short low loss cables will yield more extreme impedances and may not be within range of some instruments. Continue reading RG-6/U for lower HF

Last update: 18th September, 2016, 5:32 PM

KL7AJ quick quiz 21/02/2016

Eric posed a quick quiz for the masses to test their knowledge under his heading “Do you really understand impedance matching?”

For your convenience, I will quote his challenge here.

Screenshot - 21_02_16 , 08_04_51

All connections are made with low-loss coaxial cable. The antenna tuner is high quality with negligible losses.
According to standard conversion charts, we find that 4:1 SWR will give us 36% reflected power. Keep that number handy.
Now, we set up the experiment. First, set the slugs on BOTH wattmeters to read REFLECTED power.
Turn on the transmitter, and adjust the antenna tuner for zero reflected power on Bird #1. Switch to forward power, and set transmitter output to exactly 100 watts. Readjust antenna tuner if necessary to achieve zero reflected power, while maintaining 100 watts forward.
Go to Bird #2 and confirm that reflected power is 36 watts.
Question: What is the FORWARD power on Bird Wattmeter #2? How you answer this question determines if you understand the conjugate match theorem or not.

Let us assume that the transmission lines are 50Ω, and that the Bird wattmeters are calibrated for 50Ω.

So, to extract the key information, we have a lossless system (KL7AJ is a lossless kind of guy) and the load is stated to be VSWR=4. Continue reading KL7AJ quick quiz 21/02/2016

Last update: 22nd February, 2016, 10:04 AM

Measuring common mode choke Zcm using a two port VNA

There are some who insist that it is not possible to make practical measurements of a common mode choke using a one port analyser, and recommend the ‘S21 method’

S21 method

The ‘S21 method’ means different things to different people.

Screenshot - 20_02_16 , 14_31_07
Figure 1.

Above, (Agilent 2009) describe the common methods of impedance measurement using a VNA. The first method is often supported with direct display of R,X, and possibly a Smith chart presentation of Γ. Continue reading Measuring common mode choke Zcm using a two port VNA

Last update: 26th January, 2019, 7:13 PM

Rigexpert’s Antscope takes a step backwards

AT Measuring balun common mode impedance – #1 I gave an example of the use of a Rigexpert AA-600 to measure the common mode impedance of a current balun.

Screenshot - 31_01_2015 , 06_08_57

Above is a plot from that article. I cannot be sure what version of Antscope was used to create the graph, but it was no later than v4.2.57, as one of the ‘improvements’ of v4.2.62 and v4.2.63 was to reduce zooming of the Z scales to a maximum of 600Ω. Continue reading Rigexpert’s Antscope takes a step backwards

Last update: 18th February, 2016, 4:40 PM

Review of N6PAA’s 40m STL

(N6PAA nd) describes several small transmitting loops (STL) and gives some meaningful performance measurements. It is rare to see such measurements and he is to be congratulated.

This review focusses on his 40m STL.

The loop is a circle of perimeter 3.83m which at 7.1MHz is 0.091λ which is at the top end of the strictest criteria for an STL, the common formula for radiation resistance Rr of a STL fail for perimeter above about 0.1λ (see Accuracy of estimation of radiation resistance of small transmitting loops). It appears from his pics that the bottom of the loop is about 1.5m above real ground, so we expect a significant ground loss resistance component in Rtotal.

N6PAA gives a measured VSWR curve for the matched antenna, and the VSWR=3 bandwidth as scaled from the graph as 20kHz, from which we can calculate the half power bandwidth and eventually, efficiency. There is some suggestion that some measurements were taken indoors, this analysis assumes that the relevant measurements were taken outdoors as pictured. Continue reading Review of N6PAA’s 40m STL

Last update: 17th February, 2016, 9:10 PM