80m voltage fed Half Square matching workup

A correspondent wrote asking about the design of a matching network for a Half Square antenna for 80m, voltage fed at one end.

Above is the current distribution on the half square voltage fed. It is essentially two in-phase vertical quarter waves separated a half wavelength, a broadside array.

Feed point impedance at resonance is very high 5700Ω, and being a high Q antenna, they are very sensitive to dimensions, nearby clutter etc. Note that this is calculated for an antenna in the clear, it will be different where trees or conductive mast exist nearby. Continue reading 80m voltage fed Half Square matching workup

Checkout of SimSmith v16.3 – spot check of transmission line database – further discussion

The article Checkout of SimSmith v16.3 – spot check of transmission line database raises an issue with SimSmith’s modelling of transmission lines.

The case chosen was Belden 8216, a RG174 type line with silver clad steel stranded inner conductor.

Fully developed skin effect

Most practical transmission lines used for HF and above have fully developed skin effect above some frequency, and are well represented by the loss model MLL=k1*f^0.5+k2*f. For an RLGC model, the R is given by the first term and with fully developed skin effect, it is proportional to square root of f. The loss of good dielectrics is usually simply proportional to f and indicated by the second term.

Under this model, L and C are independent of frequency.

Many calculators use this model, and it works fine where skin effect is fully, or even well developed. The model coefficients are commonly discovered by performing a regression on measured matched loss at a range of frequencies, and the quality of the regression fit is a good indicator of the quality of the model for that particular line. Continue reading Checkout of SimSmith v16.3 – spot check of transmission line database – further discussion

FT82-43 matching transformer for an EFHW

A published design for an EFHW matching device from 80-10m uses the following circuit.

Like almost all such ‘designs’, they are published without supporting measurements or simulations.

The transformer is intended to be used with a load such that the input impedance Zin is approximately 50+j0Ω, Gin=0.02S.

Analysis of a simple model of the transformer with a load such that input impedance is 50+j0Ω gives insight into likely core losses.
Continue reading FT82-43 matching transformer for an EFHW

Ham grade analysers and VNAs often use unconventional meanings for well known terms

Lets use a simple test circuit to review the meaning of some oft misused terms associated with VNA and antenna analyser measurements.

Above, the test circuit is a nominally 220pF COG capacitor connected between tx and rx ports of a two port VNA. An extra 1Ω series resistance is included to model the likely effect of capacitor ESR. Continue reading Ham grade analysers and VNAs often use unconventional meanings for well known terms

Checkout of SimSmith v16.3 – spot check of transmission line database

I am not a SimSmith user, and with upgrade of my desktop computer, I have lost access to the Smith chart application I have used for 20+ years. That has given me reason to evaluate various Smith chart applications for a replacement.

Smith charts are about modelling problems in transmission line terms, and what better test than a simple transmission line problem.

Above, a model of Belden 8216 (an RG-174 type cable) picked from SimSmith’s library of transmission line data (source KN5L). The model is at 1MHz and essentially indicates the Matched Line Loss of 100m by deducting the left hand dBW figure from the next one to the right, -5.88228e-3–2.33357=2.33dB. (Duh, I could not copy and paste these values, they had to be read and typed in by hand which is not only laborious but more importantly gives scope for error.)

Lets check the Manufacturer’s data sheet. Continue reading Checkout of SimSmith v16.3 – spot check of transmission line database

Measuring ambient noise level using a spectrum analyser

The external noise figure Fa is defined (from ITU P.372-13) as:

I have taken a sweep of the 40m band when this is a little activity, but little enough to see the ambient noise floor at the time. It is raining and it is relatively noisy.

Above, the noise floor in 9kHz bandwidth with a CISPR quasi peak detector is about -78dBm. This is 12dB above the instrument noise floor, sufficient to not bother making a correction and we can take the external noise to be -78dBm (see below for correction calculation if needed). Lets allow 1dB loss in the antenna system, and call it -77dBm at the air interface.
Continue reading Measuring ambient noise level using a spectrum analyser

Surecom SW-102 VSWR meter review – v2.6

At Surecom SW-102 VSWR meter review I wrote a review of a meter which I had purchased a little over a year ago, it was at v4.5.

One of the many problems identified was inconsistency of displayed values.

v2.6

Surecom’s versions are confusing, the highest number is not necessarily the latest version. It appears a partial version history from their current page advertising the SW-102 is:

OLD VERSION : V3.3 ,V3.8 ,V4.5,V4.9 ,V5.0,V5.1
2017-8 NEW VERSION : V2.02 ,V2.03

The following image is from Surecom’s current page advertising the SW-102, and I assume that the version shown here (v2.6) is the latest at time of writing.

The image captures the outputs of two tests with poor and good dummy loads.

Let’s check the displayed values for internal consistency. Continue reading Surecom SW-102 VSWR meter review – v2.6

Is |s21| measurement of a common mode choke meaningful to antenna systems?

A common theme among online experts is to measure, or ask for measurement of a common mode choke connected between the centre conductor of a VNA’s tx and rx ports. That raises the question of whether |s21| is meaningful, whether it in any useful way characterises the choke as a component of an antenna system.

Direct measurement of common mode current is not difficult, and it is almost always the best way to determine the effect of a choke on common mode current.

That said, analytical and simulation techniques can be of great value in the antenna design process, well before a prototype antenna is built.

An example choke at 7MHz

Lets perform an experiment using NEC to model the effect of a choke in a 7MHz antenna.

The choke used uses 11t on a FT-240-43 ferrite core. The values are from a calibrated model, values confirmed by measurement.

We will use NEC-4.2 with one of the scenarios detailed in the article Baluns in antenna systems, Model 4, but using the choke described above which has an impedance of 3175+j2502Ω at 7MHz.

Above is a simulation of the connection. Zcm of the choke in this case at 7MHz, 3175+j2502Ω, has been converted to an equivalent inductance and resistance to suit the simulator. (Note that the equivalent circuit it valid only for a narrow band, there is no simple wideband circuit equivalent for this ferrite cored choke (more later).) Continue reading Is |s21| measurement of a common mode choke meaningful to antenna systems?

Effect of shorting turns on a tapped air cored solenoid at RF #2

Effect of shorting turns on a tapped air cored solenoid at RF offers a simple model for estimating the effect of shorting turns on inductance (L) and Q.

A correspondent sent me a set of measurements he made of an air cored solenoid using a Q meter.

The coil was a 22t air solenoid of length 99.5mm and radius 31.56mm. Q of the whole coil (L12) was measured at 6MHz to be 475.

L and Q were estimated and measured with three different tapping points at one end of the coil.

Whilst the method described in the reference article does not attempt to estimate the effect of tapping where the unused turns are left open circuit, we might expect than when the unused section is a small part of the coil, that the effect is similar to that if the unused turns were not there.

A model as described in the reference article was constructed.

The notation is L1 is the used part of the coil, L2 is the unused part, L12 is the whole coil with no taps, Lms is measured L unused shorted, Lmo is measured L unused open, like wise for the Q subscripts.

Above, the model results There is quite good reconciliation with the predicted behaviour. Continue reading Effect of shorting turns on a tapped air cored solenoid at RF #2

Finding velocity factor of coaxial transmission line using the velocity factor solver

This article is a tutorial in use of Velocity factor solver to find the velocity factor of a sample coaxial transmission line using an antenna analyser.

Example 1: Youkits FG-01

we have two lengths of H&S RG223 terminated in identical BNC connectors at both ends. Let’s connect each in turn to a Youkits FG-01 antenna analyser and find the quarter wave resonance of each (ie the lowest frequency at which measured X passes through zero).

Above, the line sections are connected to the Youkits, and the length overall is measured from the case of the analyser to the of the cable.
Continue reading Finding velocity factor of coaxial transmission line using the velocity factor solver