## Exploiting your antenna analyser #22

On a transmission line with standing waves, the voltage varies cyclically along the line, and is dependent also on power.

This article explains a method to use an analyser to predict the peak voltage level at a point for a given frequency and power based on measurement or estimation of complex Z or Y at that point using a suitable antenna analyser.

## The problem

Lets say you have some critical  voltage breakdown limit and want to use your analyser to find any non-compliance at the proposed power level.

Let us assume that the not-to-exceed voltage at that point is 1000Vpk. Let’s allow a little margin for variation due to factors not fixed, let’s actually use 800Vpk as the limit. We will use the maximum permitted power in Australia, 400W.

## Silver vs alkaline button cells

I purchased a new digital caliper recently (no, they are NOT vernier calipers, though modern usage seems to have misused the term vernier to the point of it having no value).

A pic of the back reveals their recommendation for a battery, it is in the upper right corner of the pic “Battery 1.55V”. This is really subtle and a departure from previous practice of marking them more clearly SR44.

The nominal voltage of a silver button cell is 1.55V, an alkaline is 1.5V. Continue reading Silver vs alkaline button cells

## Exploiting your antenna analyser #21

A correspondent wrote about the apparent conflict between Exploiting your antenna analyser #11 and Alan, K0BG’s discussion of The SWR vs. Resonance Myth. Essentially the correspondent was concerned that Alan’s VSWR curve was difficult to understand.

## K0BG’s pitch

For convenience, here is the relevant explanation.

By definition, an antenna’s resonant point will be when the reactive component (j) is equal to zero (X=Ø, or +jØ). At that point in our example shown at left, the R value reads 23 ohms, and the SWR readout will be 2.1:1 (actually 2.17:1). If we raise the analyzer’s frequency slightly, the reactive component will increase (inductively) along with an increase in the resistive component, hence the VSWR will decrease, perhaps to 1.4:1. In this case, the MFJ-259B is connected to an unmatched, screwdriver antenna mounted on the left quarter panel, and measured through a 12 inch long piece of coax. This fact is shown graphically in the image at right (below).

Note that the graph is unscaled, and that frustrates interpretation. The text is also not very clear, a further frustration. It is easy to draw a graph… but is the graph inspired by a proposition or is it supporting evidence. Continue reading Exploiting your antenna analyser #21

## Desk study of opportunity to improve linearity.

At Chinese AD8307 power measurement module #2 I showed measurement of the linearity of an AD8307 based RF power meter.

The specification linearity is +/-1dB, which is poorer than one might like in a power measuring instrument.

The diagram above from the AD8307 datasheet shows the internal architecture, including 9 stages of cascaded log detector cells that attempt to give a log response over around 100dB range. The issue is that in the transition region between detector cells, error is worse than well inside an individual detector cell’s range.

Above is a sweep from -65 to -6dBm at 10MHz after calibration of slope and offset. The linear fit to the blue curve shows slope is 20mV/dB and intercept 1.8015 for 0dBm means the offset is -1.8015/0.02=-90.08dBm. Log conformance is 0.2dB (well within spec at this frequency, temperature etc).

## Finding resistance and reactance with some low end analysers #2

Exploiting your antenna analyser #8 was about finding resistance and reactance with some low end analysers that don’t directly display those values of interest. The article showed how to calculate the values starting with |Z| from the analyser and included links to a calculator to perform the calcs.

This article describes an extension to that calculator Find |Z|,R,|X| from VSWR,|Z|,R,Ro to use R, VSWR, and Ro as the starting point. Note that the sign of X and the sign of the phase of Z cannot be determined from this starting point, there just isn’t enough information.

You will probably not find the equation for |X|(R,VSWR,Ro) in text books or handbooks, and the derivation is not shown here but if there is interest, I may publish a separate paper.

Lets say you knew VSWR=2, R=75Ω, Ro=50Ω, what is |X|?

Above, entering the values in the calculator we find that |X|=35.4Ω. Continue reading Exploiting your antenna analyser #20

## Exploiting your antenna analyser #19

A recent post on an online forum provides a relevant example to discussion of this subject.

I have personally seen ratios similar to 3:1 or higher at the feed point become 1:1 at the rig over 100 or so feet of coax cable.

First point is that in good transmission line, it takes an infinite length to deliver the observations made above. Less might deliver almost VSWR=1 at the input end of the line.

Let us consider a practical scenario, 100′ of RG58A/U with a load of 150+j0Ω at 14MHz, the load end VSWR(50) is 3, the input impedance is 32.50-j22.86Ω and input VSWR(50) is 2.01. In this scenario, the line loss is 2.5dB which might be unacceptable for some applications. Continue reading Exploiting your antenna analyser #19

## Chinese AD8307 power measurement module #3

At Chinese AD8307 power measurement module #2 I concluded that the modified AD8307 was useful on HF, and through to 54MHz depending on accuracy requirements.

This article looks at combining the AD8307 module with a display option based on an Arduino Nano.

Above is a demonstration of the display prototype. The module in the foreground is an Arduino Nano (~\$6), and behind it a 16×2 LCD with I2C module (combined, ~\$4). The Android tablet is connected to the Nano using a OTG cable (~\$1) and is logging the measurements (optional) and powering the equipment. The red and black clips are connected to a power supply to simulate the voltage from the AD8307. The same configuration should work with any Arduino phone or tablet if it supports OTG. Continue reading Chinese AD8307 power measurement module #3

## Chinese AD8307 power measurement module #2

At Chinese AD8307 power measurement module #1 I documented the first phase of checkout of a low cost AD8307 module.

There are two requirements for accurate power measurement:

1. input impedance of the power meter must be very close to 50+j0Ω (say input VSWR<1.2); and then
2. gain from the SMA terminals to the AD8307 input terminals must be independent of frequency.

Though the module was clearly junk in terms of criteria 1 as supplied, it was possible to modify it to present a low VSWR 50Ω input impedance, and that was documented in the last chapter.

## Application requirements

The application requires an adjustment of the AD8307 calibration to 20mV/dB with -90dBm intercept, meaning it will produce 1800mV at 0dBm input and have a slope of 20mV/dB.

Though the original circuit shows the necessary components R3 & R4, and R5 & R6, they are not fitted and must be fitted to the board. I have used 50kΩ 20t trimpots for R3 and R6, and 33kΩ for R5 and 47kΩ for R4.

The technique used to calibrate slope and offset is that described at (Duffy 2014).

## Slope and offset calibration, and log conformance / scale linearity at 10MHz

Above is a sweep from -65 to -6dBm after calibration of slope and offset. The linear fit to the blue curve shows slope is 20mV/dB and intercept 1.8015 for 0dBm means the offset is -1.8015/0.02=-90.08dBm. Log conformance is 0.2dB.

Above is the fuller plot from -65 to 15dBm, and it can be seen the linearity degrades above -5dBm, but the error is small for this class of measurement chip.

## Frequency response

The module was swept from 1 to 500MHz and response at 0dBm captured.

Above is the response from 1 to 50MHz. Response is down by 1.5dB at 55MHz, but within 0.5dB to 30MHz so quite suited to the intended application, a HF common mode current meter.

## Conclusions

• The module as supplied was cheap Chinese junk, it had 35dB slope from 10MHz to 1MHz, input VSWR  from 1.6 to extreme over the range 1-500MHz.
• Reworking the input circuit delivered very good input VSWR to 240MHz.
• Amplitude response with the reworked input circuit is within 0.5dB from well below 1MHz to 30MHz.
• Flat response at VHF – UHF would require an equalised input circuit and appropriate PCB layout.

## Chinese AD8307 power measurement module #1

I purchased a ‘ready to use’ AD8307 RF power measurement module on eBay for a project to develop a HF common mode current meter sensor for use with the RFPM1 (Duffy 2014). Price was A\$22 incl post.

Above, the AD8307 module on a small PCB with shield enclosure. Note the prominent labeling DC-500MHz, but the abundant Chinese language must sound a warning. Continue reading Chinese AD8307 power measurement module #1

## Measure velocity factor of open wire line

One of the measurement tasks that one often encounters is to measure the velocity factor of a transmission line.

Often this is an indirect task of tuning a tuned line section, my method is to often measure some line off the role, find the velocity factor (vf), and use that to cut line for the tuned section making appropriate allowance for connectors etc.

Measuring vf for an open wire line includes all that is done for measuring vf of coax, but requires measures to ensure that common mode current does not affect measurement significantly.

To minimise common mode current effects, I will use two measures:

• a high common mode impedance Guanella balun; and
• form the line section being measured into a loose helix supported on some fishing line to spoil any common mode resonance.

Above is the balun used, it is described at Low power Guanella 1:1 balun with low Insertion VSWR using a pair of Jaycar LF1260 suppression sleeves. Continue reading Exploiting your antenna analyser #18