Tuning electrical line length using phase of measured s21 – nanoVNA

The nanoVNA has put a quite capable tool in the hands of many hams who do not (yet) understand transmission lines.

A recent online posting asked why phase of s21 of a desired 40° section of 75Ω matching / phasing line did not reconcile with other estimates of its electrical length.

Discussion

Let’s firstly review the meaning of s21.

Considering the two port network above, \(s_{21}=\frac{b_2}{a_1}\) where a and b are the voltages associated with incident and reflected travelling wave components. Implicit in the meaning of s parameters are the port reference impedances which in the case of the nanoVNA are nominally 50+j0Ω. Continue reading Tuning electrical line length using phase of measured s21 – nanoVNA

Last update: 1st January, 2021, 8:33 AM

PERL vs Python pre / post processing NEC

I have used PERL to script NEC runs, and then to read the huge volume of output to produce simpler summary tables. This has provided facility to run a very large number of models with some variation in one or more model parameters. One of the early published web articles was Feeding a G5RV published in 2005, but I had been using PERL for that purpose for quite some time before that, and in my ‘day job’ since early 1990s. Ham projects led to development of some application specific libraries to model transmission lines and ATUs.

Like PERL, Python had its origins in the late 1980s, but it has really only come of age in recent years with the release of v3. Python appears running on all sorts of things from microcontrollers up, and is probably the most popular scripting language today. Continue reading PERL vs Python pre / post processing NEC

Last update: 29th December, 2020, 4:45 PM

The j value

I have noticed growing use of new lingo for reactance on hammy online forums… the term j value is replacing Reactance in the ham vernacular (though of course j does not distinguish Reactance from Susceptance).

A well-designed ferrite transformer operated in the HF spectrum is close to ideal, so if it steps R by 2, it will step j up by two as well.

Now whether general use like that is better educated people reaching hammy Sammy by using his lingo or simply ignorance of the concepts and correct term, it is simply reinforcing the wrong use of terms.

I make the observation that most users of the term j value don’t look like they understand complex numbers at all, they are just wanting to talk like the pros online. The underlying concept of Reactance as a component of Impedance is even further from their grasp.

The concept of complex numbers begins 400 years ago, Reactance more like 135 years old, and it seems modern online hams are happy to ditch to term of the unrecognised Oliver Heaviside… to whom we owe so much.

Last update: 21st December, 2020, 12:37 PM

Applying the RG6/U to a 40m Inverted V Dipole antenna

This article describes an antenna system for 40m based on:

  • an inverted V dipole;
  • Guanella 1:1 balun; and
  • a ‘tuned’ length of RG6/U CCS coax.

The antenna system will be centred on 7.080MHz to suit my own operating preferences.

The coax is that featured at nanoVNA – RG6/U with CCS centre conductor MLL measurement and the matched line loss is taken from measurement as 4.1dB/100m @ 7.1MHz (all conductor loss). The feed line cost $50 for 100m incl delivery, so this project uses $12 worth of cable.

The broad concept is that the dipole is tuned a little shorter than a half wavelength to excite a standing wave on the coax. The VSWR desired is a little over 1.5, and the length of the coax is tuned so that the impedance looking into the coax is close to 50+j0Ω. “A little over” is so that the VSWR at the source end is very close to 1.5.

Above, the topology of the Inverted V Dipole with modelled current distribution in green. The apex of the dipole is at 11m and it is over ‘average ground’ (σ=0.005 εr=13). Continue reading Applying the RG6/U to a 40m Inverted V Dipole antenna

Last update: 19th December, 2020, 6:13 PM

On measuring antennas through integral halfwaves of transmission line

Hams often would like to know the impedance of an antenna at its feed point, sometimes for very sound reasons, and very often in pursuit of a specious goal.

One of the oft given suggestions is that it is convenient to measure through an integral number of electrical halfwaves of transmission line, since as everyone knows, impedance at the end of the line is repeated exactly every half wave towards the source.

Some even tell us that they cut their feed line lengths to exactly nλ/2 to facilitate this at implementation an into the future. So, lets take that idea and cut the feedline to the shortest nλ/2 that will reach the feed point 100m distant. The electrical length of a VF=0.83 feedline will need to be nλ/2 or 1080° at our nominal frequency of interest, 7.2MHz.

To explore the method, let’s use the modelled feed point impedance of a 40m Inverted V Dipole used in some recent articles.

The real feed point

Above is a Simsmith model of the feed point impedance, The blue line overlays the magenta line which is the locus of s11 from the NEC model. Continue reading On measuring antennas through integral halfwaves of transmission line

Last update: 18th December, 2020, 8:50 AM

On the measured phase of s11 in a matched system

I have seen several online posts of hams citing measurement of phase of s11 as a figure of merit of a matched antenna system, indeed evidence of the resonance nirvana.

Let’s review the meaning of s11.

s11 is the complex reflection coefficient at the reference plane, usually wrt Zo=50+j0Ω.

If you were to measure the s11 looking into an ATU, you might adjust the ATU to minimise the magnitude of s11 (|s11|) which is also minimises VSWR. If you do a really really good job of adjustment, you might achieve around the noise floor of the instrument.

You can simulate this near perfect match by simply sweeping your 50Ω calibration load. Let’s do that and look at some relevant views.

Above, |s11| expressed in dB is very low, it is at the noise floor of the calibrated instrument, and it is very jittery… due to the relatively high contribution of noise. Continue reading On the measured phase of s11 in a matched system

Last update: 17th December, 2020, 6:27 PM

Applying the RG11A/U to a 40m Inverted V Dipole antenna

This article describes an antenna system for 40m based on:

  • an inverted V dipole;
  • Guanella 1:1 balun; and
  • a ‘tuned’ length of RG11A/U CCS coax.

The antenna system will be centred on 7.080MHz to suit my own operating preferences.

The coax is that featured at Checkout of a roll of Commscope 4510404 CCS RG11A/U – Zoc, Zsc based MLL calculation and the matched line loss is taken from measurement as 1.2dB/100m @ 7MHz (all conductor loss). The feed line cost $99 for 305m incl delivery, so this project uses $6.50 worth of cable. The feed line is not good because it is cheap, it is good because it suits the application very well, and as a bonus, it is inexpensive.

The broad concept is that the dipole is tuned a little shorter than a half wavelength to excite a standing wave on the coax. The VSWR desired is a little over 1.5, and the length of the coax is tuned so that the impedance looking into the coax is close to 50+j0Ω. “A little over” is so that the VSWR at the source end is very close to 1.5.

Above, the topology of the Inverted V Dipole with modelled current distribution in green. The apex of the dipole is at 11m and it is over ‘average ground’ (σ=0.005 εr=13). Continue reading Applying the RG11A/U to a 40m Inverted V Dipole antenna

Last update: 19th December, 2020, 5:51 PM

144MHz beacon observation from Wellington NSW – 3 aircraft reflections

VK2TP at Wellington has been making observations of aircraft enhancement of the path between my 144MHz beacon and Wellington, an obstructed direct path of 259km. The beacon modulation pattern includes a long steady carrier which permits better observation of the nature of these aircraft enhanced paths. Continue reading 144MHz beacon observation from Wellington NSW – 3 aircraft reflections

Last update: 13th December, 2020, 1:53 PM

Checkout of a roll of Commscope 4510404 CCS RG11A/U – Zoc, Zsc based MLL calculation – nanoVNA

The article Checkout of a roll of Commscope 4510404 CCS RG11A/U – Zoc, Zsc based MLL calculation ended with a comment on making the measurements with a nanoVNA.

This article reports measurements with a nanoVNA-H v3.3 (modified) calibrated and swept from 1-31MHz using nanovna_mod. Continue reading Checkout of a roll of Commscope 4510404 CCS RG11A/U – Zoc, Zsc based MLL calculation – nanoVNA

Last update: 13th December, 2020, 1:15 PM

Steel as an antenna radiator

One sees discussions online regarding the suitability of various conductor materials for antennas and feedlines.

A recent thread asked about steel as an antenna radiator and one response tried to answer the question using NEC models of a half wave 40m dipole.

Above is the graphic offered to support the analysis that steel is only 0.5dB behind copper, the poster stating: Continue reading Steel as an antenna radiator

Last update: 13th December, 2020, 9:30 AM