Aerial Analysers

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Keith Rawlings has more details on aerial analysers, looking at the development and uses of these devices

 

 

 

Keith Rawlings has more details on aerial analysers, looking at the development and uses of these devices before he reports on some professional receiving equipment recently on display at a Duxford air show.  

 

 

Hello and welcome to this month’s Aerials Now column. A Happy New Year to you all.

I ended last month’s column mentioning aerial analysers, and I thought that I would follow up this month by starting to describe what an analyser actually is and what can be done with it.

In reality, anything that can be used to take or read measurements relating to an aerial could be called an analyser. A Standing Wave Ratio (SWR) meter, as used with transmitters, is an analyser. It enables a user to analyse the SWR on a system. At the other end of the price scale, an expensive vector network analyser (VNA) could also be used to do the same (plus a lot more).

My example of an SWR meter may not be of much use to the majority of RadioUser readers, as we tend to concentrate on receiving, and a VNA may not be practical!

In the middle, there are a vast number of analysers available to suit the pockets of most enthusiasts who wish to get the best out of their systems.

 

 

Cambridge Kits

My first 'analyser' (after an SWR meter) was bought back around 1980 and it was a simple noise bridge, which was then sold in kit form by Cambridge Kits (Fig. 1). This basic unit enabled me to measure resonance and radiation resistance, from the short wave bands well into the VHF region.

An aerial was connected to one port and a receiver, used as the 'detector', to the second port.

The unit was switched on, and a wide range of noise, generated by the analyser, could be heard on a receiver. The resistance control was adjusted until a null in the noise could be perceived.

At this point, resonance was indicated, and the value read off of the scale.

The drawback was that, with an amateur-band-only transceiver, if the point of resonance was outside of the band coverage of the receiver, the user was none the wiser as to whether the aerial was too long or too short.

Furthermore, the scale was not supremely accurate. However, it was possible to 'calibrate' the analyser against a known resistance across the ‘unknown’ terminal.

The analyser was cheap, and – after I purchased a decent HF receiver – I was able to make a lot more meaningful measurements. I note that a kit is still available:

http://cambridge.eu5.org/2013/kit/kitanb.htm

 

Palomar and Further

An improvement on the Cambridge Kits noise bridge was one by Palomar Engineers from the USA. It was called the R-X Noise Bridge. This was a lot more expensive than my previous model, especially when I experienced my first encounter with Her Majesty’s Customs and Excise, for their pound of flesh (otherwise called VAT and Import Duty).

However, despite the cost, I found this unit a lot more useful. Not only did it produce a dip in the noise, in the same way as the Cambridge Kits model did with the resistance reading, it also had a second control, which read reactance. Now the coverage of the receiver being used no longer mattered (within reason) because the reactance control could tell me if the aerial was too long, too short or just right.

This is because it is capable of measuring the exact impedance of an antenna and both the resistive and reactive components of the system (Fig. 2).

Both of these analysers work by means of a wideband noise generator and what is called an RF impedance bridge. The 'known' leg of the bridge has a calibrated variable resistor and a calibrated variable capacitor which are on the front panel. (fig2) The 'Unknown' part of the bridge connects to the aerial being measured. The output is fed to a receiver that acts as the 'detector' and is tuned to the measurement frequency where a high level of white noise will be heard

The circuit is tuned until a null in noise is detected in the receiver, the impedances are now matched because the circuit is in balance.

The user then reads off the values from the scale to obtain resistance and reactance.

The devices more recognised now as true analysers are the likes of the MFJ259/269, those made by RigExpert, Sark and a number of others.

These are far more sophisticated than the very basic noise bridges and require no receiver as a monitor.

They are standalone devices and have either inbuilt LCD displays, which graphically demonstrate their readings or, in the case of the MFJ259/269, readings are taken from two analogue meters and a digital LCD display. Many have the option to output data to a computer.

A reasonably new model on the market is the DG5MK FA-VA5. It is supplied in semi-kit form and measures from 10kHz to 600MHz with readings displayed on a graphical display. Its output can be fed into a PC, and, using free software, the VA-5 can be turned into a sophisticated single port network analyser (Fig. 3).

 

In Use

What can be done with an analyser depends on the model and the facilities built in.

Even with the simple ‘noise bridges’ described earlier, aerials such as dipoles, trap dipoles, verticals, beams, and even end-feds, can be measured.

Transmission lines (for example, coaxial cable) BALUNS and tuned circuits can also be analysed, and you can even use one to tune your ATU!

Analysers such as the MFJ269, cannot measure the sign of reactance, whereas the simple Palomar R-X Noise Bridge can. The reading is taken from either the ‘Xc’ or ‘XL’ part of the scale.

 

 

Noise Bridge.

If an R-X noise bridge is used for adjusting a dipole it can be connected to a receiver that was tuned to the desired operating frequency. The aerial would then be connected to the ‘unknown’ port.

If we take a dipole and assume it has been cut longer than required, both the resistance and reactance controls would be adjusted in turn, for the ‘deepest’ null in noise on the receiver. The reactance control should be reading on the XL side, as it will be too long.

Next, carefully trim a few millimetres off of the dipole and take a further measurement.

Repeat this until the null is equal to X=0. Remember from the June AN if an antenna reads -Xc (capacitive reactance) it is too short, +Xl (Inductive Reactance) it is too long.

The feed point resistance can be read off of the R scale (50Ω).

The disadvantage here is that the readings need to be taken at the actual feed point of the aerial. On a dipole, this may well not be practical.

The reason for this is that the bridge will read what it can ‘see’, and the readings at the end of a run of the feeder may well be different from those at the feed point.

This is due to resistance and reactance on the feed-line.

However, if an electrical half-wavelength of the feed line is used (or a multiple of a half-wave), the reading seen at the end of the line will be the same as those at the feed-point.

Of course, this is valid for only one frequency, and measurements must be taken at this frequency.

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Needless to say, an electrical half-wave on a coaxial line can be found using the bridge!

For those interested, a half-wave line can be found by the following formula:

Length L in feet =492/f V

F = Frequency

V = Velocity Factor of the line.

V is usually about .66 for most coaxial cables, although foam dielectric cables are around .80.

Measuring a ground-mounted vertical, on the other hand, may be easier, as the feed-point will be accessible. Sophisticated analysers such as the VA5 can get around this problem with feeders as they allow you to calibrate out the cable. You still have to get to the feed point to make a calibration at that point, but readings can then be done at the receiver end of the cable.

 

Traps.

Parallel-tuned circuits, such as traps for trap dipoles (a capacitor in parallel with an inductor) can be measured. This is done by inserting a link of one or two turns of wire through the circuit and connecting it to the unknown terminal on the bridge. Set both the ‘X’ and ‘R’ controls to zero, tune the receiver until you get a null and read off the frequency on the receiver. This is then the resonant frequency of the trap.

https://www.sotabeams.co.uk/content/antenna-traps.pdf

 

Microprocessor-Controlled Units

Later commercial analysers are usually microprocessor controlled. There is a great advantage here because the chips are able to undertake a lot of number-crunching and also drive some form of display to give a graphical output in a completely stand-alone unit.

Taking the older MFJ269 (mainly because I have one!) it can measure SWR, impedance, reactance, resistance, resonant frequency and bandwidth and it can further be used as a rough frequency counter and as a signal source.

The downside of the MFJ269 is that it only covers 1.8-170 (actually 184) MHz and 415-470MHz.

This was part of the reason I upgraded to the VA5, for its continuous coverage to 600MHz.

The MFJ displays its measurements, in digital form, on an LCD display. In addition to this, it shows SWR and impedance on two analogue meters.

This analyser has given me great service and still does.

It has been up on the roof of a hangar and even been about 75ft up in the air with me, while I adjusted a VHF telemetry aerial. This is not something I made a habit of as, fortunately, most of the other aerials we used to do were no more than ladder-height, on the side of small buildings or on the sides of plinths.

 

MR100

As mentioned earlier, last month I suggested that I use a 'standard' analyser for the column that would enable readers to become familiar with analysers, would be suitable for HF use, and would not break the bank.

I had in mind the MR100, which can be found on eBay for around £40-50 (although, at this price, they may attract VAT). The specifications are 1-60Mhz frequency coverage, SWR measurement 1-9.999; impedance 5-2000 Ω; RF Output of 2V pp.

It has many measurement features that are comparable to the MFJ269, but it can also be connected to a PC, which obviously increases its versatility. Users may also find its ability to measure the value of capacitors and inductors useful for aerial projects.

Next month and thereafter I will describe in more detail some of the techniques described here; in the meantime, here are some links to be getting on with.

https://tinyurl.com/y8htlvrm

https://tinyurl.com/y97d5b2c

 

Royal Air Force @ 100

At a recent Duxford air show, I noticed that 3 (FC) Sqn of 90 Signals Unit, RAF Leeming had a display, demonstrating some of their equipment. 90 SU's mission is, and I quote ‘To support airpower through the delivery of assured information and communication services.’ 3 FC’s task is to 'Provide Force Elements at Readiness to allow 90 Signals Unit to meet its Mission'.

There were a number of items on display such as a Reacher transportable communications hub. Reacher is capable of supplying tactical satellite communications to all of HM forces when required, and it has been regularly deployed on all recent battlegrounds (Fig. 4).

A vehicle-mounted Falcon data network and telecoms hub could also be seen (Fig. 5).

It is used to send and receive voice and data information from sources such as landlines, satellite terminals and commercial internet connections.

A number of other devices, such as Mantis portable data terminal system (Fig. 6) and a Tac-Sat portable UHF SATCOM Yagi aerial, covering the range from 240 to 318MHz (Fig. 7) were also on display.

Using Right Hand Circular Polarisation (RHCP), Tac-Sat can be deployed quickly, operate in winds up to 80mph, handle 200W on CW and offer between 10 and 15dB of gain.

This is all very different from the equipment used in 1918!

My thanks to the staff of 3 Sqn 90 SU, for the time taken to explain the equipment to me on what was a very wet morning!

Well, this is all for this month. As always, I will reply to readers’ questions through this column.

Table 1 is a glossary of the main electronics terms used throughout this article.

Until then: Good Listening!

 

Table 1: Glossary of Electronics Terms

 

Capacitor: a device used to store an electric charge, consisting of one or more pairs of conductors separated by an insulator.

Impedance: The effective resistance of an electric circuit or component to alternating current, arising from the combined effects of ohmic resistance and reactance.

Inductor: A passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it.

Reactance: The non-resistive component of impedance in an AC circuit, arising from the effect of inductance or capacitance or both and causing the current to be out of phase with the electromotive force causing it.

Resistance: The electrical resistance of an object is a measure of its opposition to the flow of electric current.

Resonant Frequency An aerial is a form of tuned circuit which consists of inductance and capacitance, and as a result, it has a resonant frequency. This is the frequency where the capacitive and inductive reactance's cancel each other out. At this point the antenna appears purely resistive; the resistance is a combination of the loss resistance and the radiation resistance.

 

 

 

This article was featured in the December 2018 issue of Radio User