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.
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);
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.
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
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.
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.
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
(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
(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
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).
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.
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.
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.
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.
Calculate initial load line of valve RF amplifier was written as a companion to my RF power amplifier tube performance computer tool to provide a starting point for building a model, but as it turns out, the initial load line (and related values) is a very good estimate and further modelling may not be needed.
Although written for an application to valves, it is quite applicable to any active device, keeping in mind that it assumes a linear transfer characteristic.
The update provides for both single ended and push-pull configurations.
For example, the requirement is for a single ended Class C bipolar amplifier to deliver 25W from a 13.8V DC supply. What is the ratio for a broadband output transformer to 50Ω.
Above is the solution. The required Rl is 3.3Ω, and the required turns ratio is (50/3.3)^0.5=3.9. a 1:4 (turns) transformer would be selected for a prototype. Bear in mind that output power would fall to around 20W at 12V DC supply.
Another example is the common 100W 13.8V Class B push-pull design.
With a requirement for around 3Ω collector to collector (or drain to drain), a transformer with 1:4 turns ratio would be selected.