Category: Transmission lines

Receive only antenna for 160m – K9AY matching and performance discussion

This article explores the design / analysis of a passive receive only antenna, a K9AY loop, for the 160m band (1.8MHz).

The example and calculations assume linear systems, if there is significant nonlinearity that gives rise to significant IMD, IMD noise is not captured by the analysis.

Results are for the scenarios calculated and may not be extensible to different scenarios.

Another caveat: I have reservations about transmission line modelling in SimNEC, especially for composite conductors, but for the purposes of the discussion, assume that it is reasonably correct.

Above is the K9AY loop from a model by W7EL.

Design objective

The objective here is to design a receive only antenna system that can be relatively remote from local noise sources (like house wiring), and captures enough external signal and noise that the receiver internal noise does not degrade S/N too much. Continue reading Receive only antenna for 160m – K9AY matching and performance discussion

Last update: 9th January, 2024, 5:37 PM

Receive only antenna for 160m – matching and performance discussion – improved SimNEC model

This article is a follow on from Receive only antenna for 160m – matching and performance discussion. It develops a SimNEC model that imports the loop impedance, and transforms it with an ideal transformer to quickly find an optimal transformer ratio and predict system losses.

Results are for the scenarios calculated and may not be extensible to different scenarios.

Another caveat: I have reservations about transmission line modelling in SimNEC, especially for composite conductors, but for the purposes of the discussion, assume that it is reasonably correct.

Above is the SimNEC model, it assumes reciprocity of the antenna system. Continue reading Receive only antenna for 160m – matching and performance discussion – improved SimNEC model

Last update: 9th January, 2024, 4:25 AM

Receive only antenna for 160m – matching and performance discussion – 8000pF?

A reader of Receive only antenna for 160m – matching and performance discussion referred me to an online discussion with a simpler solution.

A couple of quotes from posters…

It almost doesn’t matter what the actual receiver Zin is. What matters is the 100ft of 50 or 75 Ohm coax that is used to get the Rx-only antenna signal from where the antenna is placed to the receiver.

and…

That’s what I thought, also.

A few hundred feet of coax can be ~8000 pF of parallel capacitance which should short out signals by having only about 11 Ohms of reactance.

So the gist of this is that:

  • receiver Zin does not matter;
  • the 100′ of coax or 300′ of coax matters; and
  • 300′ of coax can be adequately represented by a shunt capacitance of 8000pF.

Receiver input impedance is not necessarily a tightly controlled parameter, but it is measurable, so lets take a real example.

IC7300 example

Let’s measure Zin of an example IC7300 around the 160m band.

Above is Zin plotted from a saved .s1p file. At 1.85MHz, Zin=56.2-j5.4Ω, a little off nominal, but pretty close (VSWR50=1.35). The dotted line is X and the dashed line is R, ignore the solid line… it is for people who don’t understand impedance. Continue reading Receive only antenna for 160m – matching and performance discussion – 8000pF?

Last update: 8th January, 2024, 3:27 AM

VNA measurement – small is beautiful

I have written online and in many many emails that a very common failure of VNA measurements of components is the test fixture, and the standout problem is most often the length of connecting wires.

This article works a couple of theoretical designs based on a validated model and experience of building and measuring many baluns of similar or identical design. We will then look at extracts from a Youtube video by ferrite manufacturer Fair-rite and appraise the results.

Validated theoretical choke designs

FT240-43

It is possible to calculate a pretty good estimate of the impedance of a common mode choke wound on a #43 material ferrite core over 1-30MHz. Measurement of a real choke suggests an equivalent shunt capacitance to calibrate the model to measurement. Whilst I have given the generic name to this core, it is based on Fair-rite’s 5943003801 and Fair-rite’s published 2020 #43 mix characteristics. There are imposters, and they may be significantly different.

Let us take a practical example design and calculate the expected choke impedance and from that, the expected |s21|dB in a VNA series through measurement setup.

Above is a SimNEC model of a FT240-43 with 11t winding and 2.5pF equivalent shunt capacitance to calibrate the self resonant frequency. The model calculates and plots choke impedance, and |s21|dB in the series through measurement configuration shown. Continue reading VNA measurement – small is beautiful

Last update: 6th January, 2024, 6:20 PM

VNA fixture for measuring Zcm of a common mode choke – twisted pair wound

VNA fixture for measuring Zcm of a common mode choke – coax wound discussed issues with common ham practice for measuring coax wound common mode chokes.

The article left readers with some homework:

  • Does the same thing occur if the core is wound with twisted pair that is well represented as a uniform two wire transmission line?
  • Are the resistors beneficial?
  • Do they degrade fixture behavior?
  • Then, why are the used so often?

This article addresses those questions.

Does the same thing occur if the core is wound with twisted pair that is well represented as a uniform two wire transmission line?

Let’s treat the common mode choke as a black box with two input terminals at left and two output terminals at right with voltages as annotated above. Continue reading VNA fixture for measuring Zcm of a common mode choke – twisted pair wound

Last update: 5th January, 2024, 6:33 PM

Receive only antenna for 160m – matching and performance discussion

This article explores the design / analysis of a passive receive only antenna for the 160m band (1.8MHz).

The example and calculations assume linear systems, if there is significant nonlinearity that gives rise to significant IMD, IMD noise is not captured by the analysis.

Results are for the scenarios calculated and may not be extensible to different scenarios.

Another caveat: I have reservations about transmission line modelling in SimNEC, especially for composite conductors, but for the purposes of the discussion, assume that it is reasonably correct.

The example antenna is a K6SE 14’x29′ Pennant optimised for 160m.

Design objective

The objective here is to design a receive only antenna system that can be relatively remote from local noise sources (like house wiring), and captures enough external signal and noise that the receiver internal noise does not degrade S/N too much. Continue reading Receive only antenna for 160m – matching and performance discussion

Last update: 9th January, 2024, 8:00 AM

VNA fixture for measuring Zcm of a common mode choke – coax wound

A common online question is what sort of fixture is appropriate to measure the common mode impedance of a common mode choke.

Above is a screenshot from a Youtube video by Trx Lab, probably about 2016 vintage. I see many problems with the fixture, lets start with the resistors. Continue reading VNA fixture for measuring Zcm of a common mode choke – coax wound

Last update: 3rd January, 2024, 11:17 AM

NanoVNA setup for shunt matching task – scripting the setup

NanoVNA setup for common antenna system measurement tasks showed a display configuration better suited to those tasks.

It is tedious to set the display up using the device menu, and setups may vary with different NanoVNA hardware and firmware.

The firmware I used was NanoVNA.H.v1.2.20 which allows some setup using serial port commands. This article describes the technique.

The command used is the trace command

trace {0|1|2|3|all} [logmag|phase|delay|smith|polar|linear|swr|real|imag|r|x|z|zp|g|b|y|rp|xp|sc|sl|pc|pl|q|rser|xser|zser|rsh|xsh|zsh|q21] [src]
trace {0|1|2|3} [lin|log|ri|rx|rlc|gb|glc|rpxp|rplc|rxsh|rxser]
trace {0|1|2|3} {scale|refpos} {value}

For this exercise, I used Teraterm5 which allows setting delays after each character and after each line so that the NanoVNA is not overrun.

Above, the Teraterm serial port setup. Continue reading NanoVNA setup for shunt matching task – scripting the setup

Last update: 26th December, 2023, 10:39 AM

NanoVNA setup for shunt match tasks

NanoVNA setup for common antenna system measurement tasks offered an example NanoVNA configuration well suited to the most common antenna system tuning / adjustment tasks.

This article looks at a different case, a configuration to support measurement, design, and implementation / tuning of a shunt match.

A shunt match scheme is one where the antenna with low feed point R at resonance is detuned to add some capacitive or inductive reactance, which is then offset with a shunt reactive element of the opposite sign, for the outcome of a load impedance of 50j0Ω.

VNA Calibration

The VNA is OSL calibrated at its Port 1 jack.

Measurements in this example will be made through a 50Ω coax tail of about 1m, so we need to adjust the reference plane to the feed point. In this example, the native reference plane is the NanoVNA jack, and e-delay is used to approximately offset the reference plane. It is a good approximation in this case.

You could instead calibrate the fixture to include the coax tail, but you will need appropriate cal parts… and if they are poor, the previous method may be more accurate.

Above, a measurement is made of the coax tail with an open circuit (OC) at the far end, and e-delay iteratively adjusted so that the Smith chart plot is a dot at R=infinity+j0, the right hand end of the Z=0 axis above. Continue reading NanoVNA setup for shunt match tasks

Last update: 27th December, 2023, 5:40 PM

s11 and phase wrapping

Fazed by s11 phase magic? mentioned the effect of phase wrapping on s11 phase plots, and the apparent discontinuity that is actually an artifact of the wrapping process.

Phase wrapping is the presentation of phase values to always appear in the range typically -180° to 180° (or sometimes 0° to 360°).

F1AMM published an example .s1p file of an antenna system measurement. Let’s use that as a real world example to demonstrate the effect of phase wrapping.

Above is a Smith chart presentation of the data. You might interpret the curve near the marker to show a sudden flip from -ve phase to +ve phase… but is phase discontinuous? Continue reading s11 and phase wrapping

Last update: 23rd December, 2023, 7:27 AM