ARRL EFHW (hfkits.com) antenna kit transformer – revised design #1 – part 2

This article continues on from several articles that discussed the ARRL EFHW kit transformer, apparently made by hfkits.com, and the revised design at ARRL EFHW (hfkits.com) antenna kit transformer – revised design #1 – part 1.

This article presents a saturation calculation.

You will not often see saturation calcs (for reasons that will become apparent), though you will hear uninformed discussion promoting FUD (fear, uncertainty and doubt).

Lets assume that the core is capable of maximum continuous power dissipation of 10W (limited by factors like safe enclosure temperature, human safety, Curie point etc).

Now let’s estimate the magnetising current for 10W of core dissipation with 3t primary

Starting with the expected permeability above… Continue reading ARRL EFHW (hfkits.com) antenna kit transformer – revised design #1 – part 2

Fact check: “For an antenna, if it doesn’t resonate, it really doesn’t radiate!”

An example of the utter nonsense posted on social media.

My very first posting as a trainee was to Bringelly HF receiving station in 1970. It had Rhombic antennas every 30° of the compass, and a few other antennas, but the mainstay of operation was the set of Rhombics.

The nearby transmitting station at Doonside had a similar antenna arrangement of Rhombics fed with two or four wire open transmission lines to transmitters in a central building, for most operations, no coax involved between transmitters up to 30kW and antenna feed points. Continue reading Fact check: “For an antenna, if it doesn’t resonate, it really doesn’t radiate!”

nanoVNA – are you fazed by phase?

The NanoVNA can measure and display “phase”, is it useful for antenna optimisation?

Some authors pitch it as the magic metric, the thing they lacked with an ordinary SWR meter.

In a context where it seems most hams do not really have a sound understanding of complex numbers (and phase is one ‘dimension’ of a complex quantity like voltage, current, S parameters, impedance, admittance etc), lets look at it from the outside without getting into complex values (as much as possible).

The modern NanoVNA can display three phase quantities, only two are applicable to one port measurements as would commonly be done on an antenna system:

  • s11 phase; and
  • s11 Z phase.

Let’s look at a sweep of a real antenna system from the connector that would attach to the transmitter (this is the reference plane), plotting the two phase quantities s11 phase and s11 Z phase, and SWR (VSWR) and a Smith chart presentation of the s11 measurement.

Above is the measurement of the antenna system.

Like most simple antenna systems (this is a dipole, feedline, ATU), the most appropriate optimisation target is SWR, and minimum SWR well above 7.1MHz.

The SWR is 2.568 at the desired frequency, it is poor.

Do either or both of the phase plots give useful information on the problem, and leads to fix it?

s11 phase

s11 phase is -179.62° at the desired frequency (the marker).

Some authors insist optimal s11 phase is zero, some with a little more (and only a little more) knowledgeable insist it should be either 0° or 180°, take your pick. In fact the latter criteria essentially means the load impedance is purely resistive… but let’s deal with that under the more direct measurement s11 phase of Z.

Phase of -179.62° is approximately -180°=180°.

This metric is not very useful in this case.

s11 phase of Z

s11 phase of Z is -0.4°, approximately zero, which means the load impedance is almost purely resistive.

Of itself, s11 phase of Z does not identify the shortcoming.

So, what is the shortcoming?

If SWR is the optimisation target as proposed for this type of antenna, the SWR is poor, and the minimum is at a significantly higher frequency.

The SWR plot is revealing.

For more information, the value of Z is reported for the Smith chart marker as 19.47-j0.140Ω.

The reason that SWR is not 1.0 is that the feed point impedance is not exactly 50+j0Ω, and the main reason is that the real component is quite low at 19.47 and less importantly there is some very small reactance.

So, this provides information that to improve the match, the real component needs to increase significantly, and some minor trimming of the imaginary component.

Let’s make some matching adjustments

The sweep above is after some adjustment seeking to optimise the match.

Overall, the SWR plot shows that SWR is now fairly good at 7.1MHz, the Smith chart shows the marker just left of the prime centre so R is a little low and X is close to zero, the marker detail shows that Z is 45.57-j0.426Ω, so a little more information than the SWR curve, and with more resolution than reading the Smith chart graphically, R is a little low, X is close to zero. This is good information to guide the next matching steps if one wanted to refine the match.

The phase plots are of almost no value.

Conclusions

  • Neither of the available s11 derived phase plots are of much use for this matching task.
  • The SWR plot gives the best high level indication of the match.
  • Knowledge of R and X components of Z can be helpful in understanding more detail of the match and guiding matching adjustments.
  • This article has not explained the Smith chart in detail, it requires an understanding of complex quantities, so outside the scope and prerequisite knowledge set out for this article. In fact the Smith chart provides insight well beyond any and all of the other plots.

A low Insertion VSWR high Zcm Guanella 1:1 balun for HF – coax bend radius

I see online discussion of specification bending radius for coax cables, and their application to ferrite cored common mode chokes.

A low Insertion VSWR high Zcm Guanella 1:1 balun for HF and follow on articles described a balun with focus on InsertionLoss.

Let’s remind ourselves of the internal layout of the uncompensated balun.

The coax is quality RG58A/U with solid polythene dielectric. The coax is wound with a bending radius of about 10mm, way less than Belden’s specified minimum bending radius of 50mm.

So, the question is does this cause significant centre conductor migration that will ruin the characteristic impedance: Continue reading A low Insertion VSWR high Zcm Guanella 1:1 balun for HF – coax bend radius

Return Loss Bridge – Dunsmore’s bridge

Jeff, K6JCA, kindly sent me a paper, (Dunsmore 1991) which gives design details for a variation of the common resistive Return Loss Bridge design.

This article expands on the discussion at Return Loss Bridge – some important details, exploring Dunsmore’s design.

Dunsmore’s design

Above is Figure 3a from (Dunsmore 1991). Continue reading Return Loss Bridge – Dunsmore’s bridge

ARRL EFHW (hfkits.com) antenna kit transformer – revised design #1 – part 1

This article continues on from several articles that discussed the ARRL EFHW kit transformer, apparently made by hfkits.com.

This article presents a redesign of the transformer to address many of the issues that give rise to poor performance, and bench measurement of the prototype. Keep in mind that the end objective is an antenna SYSTEM and this is but a component of the system, a first step in understanding the system, particularly losses.

This is simply an experimental prototype, it is not presented as an optimal design, but rather an indication of what might be achieved if one approaches the problem with an open mind instead of simply copying a popular design. Continue reading ARRL EFHW (hfkits.com) antenna kit transformer – revised design #1 – part 1

Jaycar L15 ferrite (LO1238)

Jaycar’s LO1238 ferrite toroid is readily available in Australia at low cost and quite suits some HF RF projects.

The published data is near to useless, so a long time ago I measured some samples and created a table of complex permeability of the L15 material which I have used in many models over that time. It did concern me that measured µi was about 25% higher than spec, which is the limit of stated tolerance. Keep in mind that this is Chinese product with scant data published.

I have measured some samples purchased recently, and µi is closer to the specified 1000, so I intend using this new data in future projects.

Above is complex permeability calculated from s11 measurement of a single turn on the LO1238 (\( \sum \frac{a}{l}=0.000976\)).

Downloads

L15.7z

Probing the popular s21 series through impedance measurement using NanoVNA-D v1.2.29 cf NanoVNA-D v1.2.40

Derivation of the expression for the unknown impedance in an s21 series through measurement arrives at the following expression:

\(Zu=(Zs+Zl)(\frac{1}{s_{21}}-1)\).

The diagram above is from (Agilent 2009) and illustrates the configuration of a series-through impedance measurement. Continue reading Probing the popular s21 series through impedance measurement using NanoVNA-D v1.2.29 cf NanoVNA-D v1.2.40

Fox flasher MkII update 9/2024

Fox Flasher MkII and several follow on articles described an animal deterrent based on a Chinese 8051 architecture microcontroller, the STC15F104E.

Fox flasher MkII update 7/2019 documented a rebuild of the enclosure etc.

This is an update after five more years operation outside.

Above is a pic of the device. The polycarbonate case has yellowed a little. Importantly the cheap PVA has not crazed, it is kept dry by the outer enclosure, and a hydrophobic vent helps keeps the interior dry.

The battery is a pouch LiPo single cell, it is in good condition. A previous trial with 18650 LiIon cells showed they were unsuitable for the environmentals.