The quarter sized G5RV with hybrid feed

(Varney 1958) described his G5RV antenna in two forms, one with tuned feeders and the more popular form with hybrid feed consisting of a matching section of open wire line and then an arbitrary length of lower Zo coax or twin to the transmitter.

(Duffy 2005) showed that the hybrid feed is susceptible to high losses in the low Zo line as it is often longish, is relatively high loss line and operates with standing waves.

Lets look at measurement of a real antenna, broadly typical of the G5RV. The antenna measured is a G5RV rigged in Inverted V form, 11m height at the apex and around 8m at the ends. The feed line is 2mm diameter copper spaced 50mm with occasional plastic insulators.

To some extent, the measurements are dependent on the environment, and whilst there will be variation from one implementation to another, the measurements provide a basis for exposing the configuration challenge.

Screenshot - 01_05_16 , 19_33_56

Above is a plot of VSWR(50) essentially at the lower end of the matching section and low Zo line. The measurement is made looking into 0.5m of RG142 and a Guanella current balun that uses about 1m of 110Ω pair, it is essentially the load end VSWR of a hybrid feed were it used. Continue reading The quarter sized G5RV with hybrid feed

A thermostatically controlled pot for waxing leg hold traps

We periodically have problems with rabbits from an adjoining reserve, and this summer has been bad enough to do something about the crepuscular critters.

Leghold traps from the factory are usually covered in some type of rust resisting oil combined with the residues of the manufacturing processes, all of which needs to be removed to de-scent the traps.

Practice varies, I spray them with ordinary degreaser and then after 5 min jet them clean with water.

There are a range of practices to colour and wax the traps, for rabbits I simply wax them and with supply shortages, I experimented with ordinary soy wax for container candles. This wax does not try hard, so it is not inclined to crack off, and it reduces rusting whilst lubricating moving parts.

Above is a pair of Victor Softcatch #1 traps that have been cleaned, lightly rusted (for colour), cleaned again and waxed. I also use Bridger #1.65 traps which do not rust as easily, so they are used without colouring at all and they seem every bit as successful when covered in sieved soil.

OTS

Above, the 7.6l stainless stock pot from Big W (~$11), a 1100W coil type electric cooker (~$20). Continue reading A thermostatically controlled pot for waxing leg hold traps

The devil is in the detail…

An image from one of my articles has been posted online in some discussions, with attribution of the underlying image, but it includes some changes / annotations.

I think that this is a better image.

The difference is in the two pin assembly at lower centre, an addition to my original image. My recommendation is that the DUT is attached to the same side of the pin strip as was used for the calibration parts, as shown. Though I did not intend that this jig be used much above 100MHz, small details like this might improve its accuracy. Continue reading The devil is in the detail…

Optimal common mode impedance of a common mode choke

In recent days we see two online experts with diametrically opposite views of the optimal common mode impedance Zcm of a common mode choke…

…the inductance of the CMC is responsible for the CM
attenuation.

and…

A practical choke is RESISTIVE, not INDUCTIVE.

Emphatic statements indeed.

They are very unlikely to both be correct, and it is possible neither applies generally. Continue reading Optimal common mode impedance of a common mode choke

Fox flasher MkII update 2/2021

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

This is an update after several years operation outside, and some in-service modifications to improve performance.

Above is the current version after 18 months in the weather. Continue reading Fox flasher MkII update 2/2021

Improving ‘s21 shunt-through’ measurement of low impedances – more detail

Improving ‘s21 shunt-through’ measurement of low impedances canvassed a possible improvement of the s21 series-through measurement of impedance to compensate for errors in VNA port impedances that are not corrected in simpler calibration / correction schemes.

This article provides more detail on the practical test case.

A small test inductor was measured with a ‘bare’ nanoVNA SOLT calibrated, firstly using s11 reflection.

Above is the R,X,|Z| plot from the s11 reflection measurement of the unknown Zu. It shows small negative resistance, a frustration with these low end VNAs that suffer thermal drift after just a few measurements. It is less than 3min since SOLT calibration. Continue reading Improving ‘s21 shunt-through’ measurement of low impedances – more detail

Improving ‘s21 shunt-through’ measurement of low impedances

This article canvasses a possible improvement of the s21 shunt-through measurement of impedance to compensate for errors in VNA port impedances that are not corrected in simpler calibration / correction schemes.

The diagram above is from (Agilent 2009) and illustrates the configuration of a shunt-through impedance measurement. Continue reading Improving ‘s21 shunt-through’ measurement of low impedances

Antenna system resonance and the nanoVNA

With the popularity of the nanoVNA, the matter of optimisation of antenna systems comes up and the hoary chestnuts of ham radio are trotted out yet again.

Having skimmed a presentation published on the net, an interesting example is presented of an 80m half wave centre dipole with feed line and various plots from the nanoVNA used to illustrate the author’s take on things.

The author is obsessed with resonance and obsessed with phase, guiding the audience to phase as ‘the’ optimisation target. Phase of what you might ask… all the plots the author used to illustrate his point are phase of s11.

A model for discussion

I have constructed an NEC-4.2 model of a somewhat similar antenna to illustrate sound concepts. Since NEC-4.2 does not model lossy transmission lines (TL elements), we will import the feed point data into Simsmith to include transmission line loss in the model.

Above is the Simsmith model. Continue reading Antenna system resonance and the nanoVNA

Improving ‘s21 series-through’ measurement of high impedances – more detail

Improving ‘s21 series-through’ measurement of high impedances canvassed a possible improvement of the s21 series-through measurement of impedance to compensate for errors in VNA port impedances that are not corrected in simpler calibration / correction schemes.

This article provides more detail on the practical test case.

A small ferrite cored test inductor was measured with a ‘bare’ nanoVNA SOLT calibrated, firstly using s11 reflection.

Above is the R,X,|Z| plot from the s11 reflection measurement of the unknown Zu. Continue reading Improving ‘s21 series-through’ measurement of high impedances – more detail

Improving ‘s21 series-through’ measurement of high impedances

This article canvasses a possible improvement of the s21 series-through measurement of impedance to compensate for errors in VNA port impedances that are not corrected in simpler calibration / correction schemes.

The diagram above is from (Agilent 2009) and illustrates the configuration of a series-through impedance measurement. Continue reading Improving ‘s21 series-through’ measurement of high impedances