## 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.

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

## 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

## 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.

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.

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

## Calculate Loss from s11 and s21 – convenient online calculator

I often need to calculate loss from marker values on a VNA screen, or extracted from a saved .s2p file.

Firstly, loss means PowerIn/PowerOut, and can be expressed in dB as 10log(PowerIn/PowerOut). For a passive network, loss is always greater than unity or +ve in dB.

$$loss=\frac{PowerIn}{PowerOut}\\$$

Some might also refer to this as Transmission Loss to avoid doubt, but it is the fundamental meaning of loss which might be further qualified.

So, lets find the two quantities in the right hand side using ‘powerwaves’ as used in S parameter measurement.

s11 and s21 are complex quantities, both relative to port 1 forward power, so we can use them to calculate relative PowerIn and relative PowerOut, and from that PowerIn/PowerOut.

### PowerIn

PowerIn is port 1 forward power less the reflected power at port 1, $$PowerIn=P_{fwd} \cdot (1-|s11|^2)$$.

### PowerOut

PowerOut is port 2 forward power times less the reflected power at the load (which we take to be zero as under this test it is a good 50Ω termination), $$PowerOut=P_{fwd} \cdot |s21|^2$$.

### Loss

So, we can calculate $$loss=\frac{PowerIn}{PowerOut}=\frac{\frac{PowerIn}{P_{fwd}}}{ \frac{PowerOut}{P_{fwd}}}=\frac{1-|s11|^2}{|s21|^2}$$

Noelec makes a small transformer, the Balun One Nine, pictured above and they offer a set of |s11| and |s12| curves in a back to back test. (Note: back to back tests are not a very reliable test.) Continue reading Calculate Loss from s11 and s21 – convenient online calculator