K3EUI’s MyAntennas EFHW on 80m

Barry, K3EUI, posted some interesting measurements of his MyAntennas EFHW which he described with some useful detail:

I have been experimenting with a “MYANTENNA” 130 foot long “end-fed” with the 49:1 UNUN*

I replaced their 130 ft antenna wire with a heavier gauge #12 stranded insulated wire (I had a few hundred feet).*

This is classified as a ONE-HALF wavelength antenna on 80m, hence the need for the 49:1 UNUN to transform 50 ohm (coax) to a few thousand ohms*

It has resonances on the other ham bands (harmonically related) but I wanted it mostly for 80m.

One leg runs about 60 ft horizontally to a tree and then the next 70 ft makes a 90 degree bend (to fit into my yard) still horizontal.*

At this time I removed a 15 ft “counterpoise” wire on the GND side of the UNUN.

I will try it later this week (after the snow) as a “sloper” or an Inverted V up to a tall fir tree.

It is only about 20 ft above ground now (with 4 inches snow) for NVIS prop, and fed with 70 ft of RG213 coax (50 ohm) with a RF choke on the coax feed line 10 ft from UNUN (the counterpoise?) and another RF choke just as the coax enters the shack.

His VSWR curve is interesting, a minimum at source end of about 1.32 @ 3.66MHz as built and measured.

Minimum VSWR is about 1.32 @ 3.66MHz. Continue reading K3EUI’s MyAntennas EFHW on 80m

The black art of common mode current and two wire transmission lines

One of the very popular topics on ham social media is common mode current, and it seems whilst opinions are presented as fact, there is little sound science in evidence.

In a two wire transmission line, we can get good insight into the state of current balance by measuring three currents at a point along a transmission line:

  • I1 in one conductor;
  • I2 in the other conductor; and
  • I12 being the sum of the currents.

These can be measured using an RF current probe, essentially a current transformer for RF, and in the case of I12, it is measured by placing BOTH conductors through the centre of the current transformer so the flux is due to I1+I2 (not simply |I1|+|I2|). There are other ways to obtain I12, but in concept they work the same as passing both conductors through one current transformer. Continue reading The black art of common mode current and two wire transmission lines

Youtube – measuring velocity factor of coax cable

I keep being offered Youtube videos showing how to measure velocity factor of coaxial line.

I did indulge one this afternoon. The author explains that measuring s21 phase is the basis of his measurement.

The DUT for the demonstration is 3.76m of coax, no mention of where it was measured from and to.

No mention of the calibration details, so we might assume that a short jumper was used to connect Port 1 to Port 2 for the through test, perhaps the very one shown in the pic below.

Above is the test jig, one end of the coax (UHF plug) attaches to a UHF-F to SMA-M adapter which is attached to the VNA. The other end of the coax appears to connect via a UHF-M to a UHF-F to SMA-F adapter, and the 100mm long jumper cable. Continue reading Youtube – measuring velocity factor of coax cable

Is |Z| a really useful metric for optimising antenna systems?

One often sees some misconceptions about the relationship between VSWR and impedance. The maths of the relation is explained at Telegrapher’s Equation. The relationship is not trivial and will challenge readers who do not understand complex numbers and exponentials.

Even if you do not have the requisite maths, the following examples may dispel some wooly thinking.

By example

|Z|=50

I have created a SimNEC model to simulate a load Z of specified |Z|, and to sweep the phase of Z from -90 to +90°, and to display VSWR50.

Above is the result where |Z|=50 and for phase of Z from -90 to +90°. Continue reading Is |Z| a really useful metric for optimising antenna systems?

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

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

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

Does RBN give a reliable metric for comparing antennas – more detail

Does RBN give a reliable metric for comparing antennas? gave an example of signal strength measurement and the effect of fading over time.

This article goes into a little more depth on the subject using a further data capture of 600 measurements 10s apart.

Above is a plot of signal strength of an 80m A1 Morse (CW) beacon measured in 20Hz bandwidth over 100min (a terrestrial path of length 105km). Continue reading Does RBN give a reliable metric for comparing antennas – more detail

Reconciliation of transmitter power, EIRP, received signal strength, antenna factor, ground wave propagation etc @ 576kHz

This article reconciles measurements with path predictions for a MW AM transmitter on 576kHz. The techniques used could be used to validate / assess the performance of a transmitter.

Source

The source is a MF AM transmitter on 576kHz located about 74km distant.

Above is the station data from the ACMA licence register. Conveniently it gives the EIRP as 132kW, we would expect something a little less than 150kW from the nominal 50kW transmitter, system efficiency calculates to 80%.

The EIRP would have been calculated from a set of field strength measurements at the time of commissioning. Continue reading Reconciliation of transmitter power, EIRP, received signal strength, antenna factor, ground wave propagation etc @ 576kHz

Does RBN give a reliable metric for comparing antennas?

I see that lots of hams depend on HF RBN to compare to antennas, or to compare before and after a change.

Experience says that A/B comparisons on HF are subject to variation in Ionospheric propagation paths, and that variation can be wide in range and rapid.

An example

Above is a plot of signal strength of an 80m A1 Morse (CW) beacon measured in 20Hz bandwidth over 15min snapshot (a terrestrial path of length 105km). Continue reading Does RBN give a reliable metric for comparing antennas?