Month: September 2023

Welding modern star pickets

I was chatting with a chap the other day, and he said “pity you cannot weld star pickets anymore” wanting to raise an existing fence 600mm to try and keep kangaroos out (exclusion fencing).

You can buy a section of star picket with a socket to put on top of an existing picket, but they are quite expensive.

So, the problem arises because a lot of star pickets sold in Australia in the last 20 years or so are made in China from “rail steel”, a higher carbon steel (~0.6%) that allows thinner section, less weight, lower transport cost etc.

If you arc weld these with ordinary general purpose electrodes (E6013 in Australia), they are likely to crack and fail under load.

I have used two options:

  • stick weld with E7018 2.5mm electrodes; or
  • MIG weld with AWS A5.20 E71T-GS 0.9mm flux cored wire.

To extend an existing picket in situ, you can lap the extension and do two vertical welds, or but the sections (see jig below) and do horizontal three welds.

Above is a jig using a short piece of water pipe and two locking chain clamps. Of course you would straddle the join to clamp the pieces and remove the jig after two welds are completed. Continue reading Welding modern star pickets

Last update: 21st December, 2023, 8:05 AM

IC-7300 VSWR protection

A ham consulting the experts on QRZ asked:

On 30 meters, my SWR reads 3:1 to my antenna (an EndFed 53 feet long wire up about 25 feet). Reading a chart I have, I see that at 80 watts output, my reflected power should be 20 watts. I verified this by looking at my Diamond SX-200 Meter which also indicates the reflected power is 20 watts. My questions are these: does the 20 watts reduce my 80 watts output to 60 watts at the antenna? I have a choke on my feed line in my shack (near my transceiver) & the SX-200 Meter is between the choke & transceiver….

The OP later explained that the transceiver is a IC-7300 and it appears that the internal tuner is in use above… so let’s proceed on that basis.

Analysing the OP’s report, his SX-200 indicates VSWR=3 Pf=80, therefore Pr=20, and P=60W. Note that \(P=P_f-P_r\) is valid because Zref is real, so the answer to his question about power to the antenna is 60W, he is quite correct.

He went on to ask where the 20W reflected goes to… but I will leave that to Walt Maxwell devotees to discuss energy sloshing around and re-re-re reflections… the stuff of ham lore.

Understanding the IC-7300

As an example of what might be expected of the IC-7300 with a mismatched load, I did a series of measurements at 7MHz with a sample variably mismatched load.

Above is a plot of power output vs VSWR for a sample mismatched load. Also plotted is the measured reflected power and the calculated power output based on the ham lore \(P=P_f (1-\rho^2)\). Continue reading IC-7300 VSWR protection

Last update: 3rd October, 2023, 7:12 AM

Designing a Gamma Match – Simsmith design tool and confirmation of as-built antenna – comparison with Healey

A correspondent reading Designing a Gamma Match – Simsmith design tool and confirmation of as-built antenna referred me to (Healey 1969) and questioned my method.

I have tried several times to reconcile built and tuned antennas and NEC models with Healey and failed, leading me to think of the problem and devise a good approximation that did reconcile (for me).

This article attempts to reconcile the example given at Designing a Gamma Match – Simsmith design tool and confirmation of as-built antenna, an example where the two built antennas reconcile well with the ARRL published design article and NEC model.

Example for reconciliation

The example antennas are 4 element 144MHz Yagis built around 1970. They were originally designed with a 50Ω split dipole feed, or the option of a folded dipole with 4:1 half wave coax balun. Continue reading Designing a Gamma Match – Simsmith design tool and confirmation of as-built antenna – comparison with Healey

Last update: 19th September, 2023, 8:37 AM

Transmission lines – forward and reflected phasors and the reflection coefficient

Let’s consider the following transmission line scenario:

  • Lossless;
  • Characteristic Impedance Zo=1+j0Ω; and
  • load impedance other than 1+j0Ω, and such that Vf=1∠0 and Vr=0.447∠-63.4° at this point.

The ratio Vr/Vf is known as the reflection coefficient, Γ. (It is also synonymous with S parameters S11, S22… Snn at the respective network ports.)

Above is a  phasor diagram of the forward and reflected voltages at the load. Continue reading Transmission lines – forward and reflected phasors and the reflection coefficient

Last update: 17th September, 2023, 7:45 AM

Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #7

Seventh part in the series documenting the design and build of a Guanella 1:1 (current) balun for use on HF with wire antennas and an ATU.

  • This article describes a measurment of common mode impedance Zcm of the packaged balun.

Packaging

The prototype fits in a range of standard electrical boxes. The one featured here has a gasket seal (a PTFE membrane vent was added later).

AtuBalun201

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. Continue reading Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #7

Last update: 9th September, 2023, 2:23 PM

Overheating balun cores – how much power does it take?

Overheating balun cores – an explanation discussed a scenario where an operator reports unstable VSWR after 30s of ATU adjustment.

Where the antenna system incorporates ferrite elements, a possible / likely explanation is that loss in a ferrite core has been extreme and raised core temperature to the Curie temperature at which it quickly loses its magnetic properties.

In that scenario, theoretically, the complete temperature curve would look like this.

The initial rate of temperature increase here is 5°/s, and we can safely assume that almost all of the power absorbed by the core is stored as heat energy, little energy is lost the the air when the temperature difference is very small. Continue reading Overheating balun cores – how much power does it take?

Last update: 9th September, 2023, 2:47 PM

Designing a Gamma Match – Simsmith design tool – how to

Designing a Gamma Match – Simsmith design tool and confirmation of as-built antenna posted a Simsmith design tool to assist in designing a Gamma Match.

Let’s walk through an example.

Above is an example for discussing the Gamma Match. In this case, the assumed feed point impedance of a simple split dipole feed is 17+j2Ω, and the challenge is to design a practical Gamma Match to match it to 50+j0Ω.

The design tool assumes that the connections to the open circuit stub are at the feed point, ie that the gap in the gamma arm outer is at the inboard end. There are other ways to build a gamma match and the model may not suit them without tweaking. Continue reading Designing a Gamma Match – Simsmith design tool – how to

Last update: 9th September, 2023, 6:47 AM

Designing a Gamma Match – Simsmith design tool and confirmation of as-built antenna

Designing a Gamma Match – confirmation of as-built antennas was based on some online calculators to provide key values to a Simsmith model of a Gamma Match.

This article provides an updated Simsmith model that incorporates the necessary calculations (ie without depending on external calculators).

Much is written about the virtue of the Gamma Match, and near as much about how they work, and the difficulty in design and implementation.

Designing a Gamma match using a Smith chart showed a design method for a simple Gamma Match using a Smith chart as the design tool.

This article visits the implementation on a pair of antennas that I built 50 years ago, and are still in use today (albeit with some small preventative maintenance once during that interval). The basic antenna is a four element Yagi for 144MHz copied from an ARRL handbook of the time, probably based on NBS 688. It was designed to deliver a split dipole feed point impedance of 50+j0Ω.

I built them using a Gamma Match, partly to get some familiarity, but mostly to implement a Gamma Match that was reliable, weatherproof and lasting… features that are alien to most implementations I had seen at that point.

Both antennas were constructed and the Gamma Match adjusted for VSWR<1.1 using a Bird 43 directional wattmeter. The dimensions of each (including the key gamma dimensions) are the same, not surprising, but a confirmation of repeatability. See Novel Gamma Match Construction for more discussion.

Above is a dimensioned drawing of the construction. Continue reading Designing a Gamma Match – Simsmith design tool and confirmation of as-built antenna

Last update: 3rd December, 2023, 7:22 AM

Overheating balun cores – an explanation

Correspondents raise instances of damage to baluns with me from time to time, and there is a steady stream of reports online.

One of the very common reports is of something unexpected happening while adjusting an ATU, after perhaps 30s of power applied, VSWR suddenly becomes unstable, changing for some unknown reason, and attempts to find optimal settings of the ATU fails.

A likely cause of this is non-linear behavior of the ferrite core in a balun in the system.

Let’s talk about that.

A theoretical model of temperature rise

A simple model that gives useful insight is to consider the case of a toroid core in still air, being heated by constant applied RF power giving rise to core loss.

Core temperature rises quickly initially, then more slowly as the core heats up and loses more and more heat to the surrounding air.

We can write and expression for core temperature T: \(T=T_{max}\left(1-\mathrm{e}^{-\frac{t}{\tau}}\right)\) where τ is the thermal time constant and Tmax is the final temperature if things continued without disruption.

Above is an example where τ=20s and temperature rise is 100°.

Note that at the beginning (t=0), the slope of the line is pretty constant, but as temperature increase, slope decreases until eventually it is almost zero. Continue reading Overheating balun cores – an explanation

Last update: 8th September, 2023, 10:56 AM