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

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

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

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

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

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

Review of KK5JY’s 40m STL

(Roberts 2010) describes several small transmitting loop (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 4.3m which at 7.1MHz is 0.102λ 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 2m above real ground, so we expect a significant ground loss resistance component in Rtotal.

Roberts gives the VSWR=2 bandwidth as 5.4kHz, which if we assume that it was adjusted for a perfect match mid band, we can calculate the half power bandwidth and eventually, efficiency. Continue reading Review of KK5JY’s 40m STL

Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #4

Fourth part in the series documenting the design and build of a Guanella 1:1 (current) balun for use on HF with wire antennas and an ATU.

Packaging

The prototype fits in a range of standard electrical boxes. The one featured here has a gasket seal (a weep hole would be advisable in a permanent outdoor installation).

AtuBalun201

Above, the exterior of the package with M4 brass screw terminals each side for the open wire feed line, and an N(F) connector for the coax connection. N type is chosen as it is waterproof when mated.

AtuBalun203

The interior shows the layout. The wires use XLPE high temperature, high voltage withstand, moderate RF loss insulation. Two short pieces of 25mm electrical conduit serve to position the balun core against the opposite side of the box, and a piece of resilent packing between lid and core holds the assembly in place.

AtuBalun202

Differently to the example shown in the earlier articles, this prototype uses twisted PTFE insulated wires which have voltage breakdown higher than the XLPE shown earlier.

Clip 124

The self resonant frequency of the built balun was measured as 7.4MHz and the predictive model above calibrated. The balun has high choking impedance on the lower half of HF.

Low power Guanella 1:1 tuner balun using a pair of Jaycar LF1260 suppression sleeves

The article describes a current balun intended for use with an ATU at modest power levels. It is lightweight and well suited to portable operations, and can be made with materials readily available in Australia (LF1260 cores are a little over $1 each in packs of six.) Continue reading Low power Guanella 1:1 tuner balun using a pair of Jaycar LF1260 suppression sleeves

End fed matching – PA3HHO design review #2

 

A correspondent having read End fed matching – design review raised a similar design by PA3HHO which uses a#43 ferrite toroid as part of an end-fed matcher, see Multi band end-fed (English).

Further to End fed matching – PA3HHO design review , this article has a solution to PA3HHO’s “original 40/20/10 version” using FT140-43 with a 2t primary.

Continue reading End fed matching – PA3HHO design review #2