Thursday, July 9, 2015

The Simplified Method and the Near-Field Catastrophe

The simplified method, and in particular the system base-RF-power determination used in the simplified method equation, only applies to far-field data. Why? Well, as an example, the radiation polar-plot provided by antenna suppliers is far-field data and in no way represents how the field-strength varies with angle close to the antenna.

So where exactly is the near-field / far-field divide? That is, where does the simplified method no longer apply? We could talk about this until the cows come home and get nowhere, because basically one man’s near-field is another man’s far-field, with both parties eager to prove how the dividing line was derived quoting various permutations of wavelength and Pi as the precise answer.

The Near-Field / Far-Field Decree

So for the purposes of this blog-thread we are going to create a dictate. As regards RF waves surrounding a radiating antenna, when a wave is an independent entity the wave is in the far-field. By independent we mean the excitation signal feeding the source can do a backward-summersault for all the wave cares, it has already escaped the influence of the source and will carry on along it’s merry way at the speed of light (in vacuo).

Conversely, if a change in the excitation signal causes a change in a surrounding RF wave, that wave is in the near-field. For example, if the excitation source to a resonating dipole was suddenly removed the voltage and current standing waves present across the resonating structure would die away almost instantaneously.

We complete the dictate by stating that the near-field / far-field dividing line is at 3 wavelengths.

Effect on the Simplified Method

The new decree means the lowest frequency at which the simplified method is valid is 300MHz. The determination is easy enough:-

Our test distance is 3 meters. Obeying the decree means that in order for the calibration plane to just be in the far-field, the 3 meter distance must represent 3 wavelengths. Therefore the limiting wavelength is 1 meter. Using c = f/Lambda, this corresponds to a lowest frequency of use of 300MHz.

The Antenna Suppliers’ Get-Around

Given antenna suppliers sell antennas starting at 80MHz (wavelength 3.75 meters) into this 3m RF immunity test market, how do they get around that the supplied gain and radiation pattern data only applies at far-field distances? (This of course is where the “yes it is, no it isn’t in the near-field” brigade kick in and try to claim that 3m distance is far-field even at 80MHz. And this of course is why we ‘cut them off at the pass’ by announcing the decree).

The supplier get-around is to provide data on how much RF power is required to produce a particular test-field at a particular test-distance. The antenna supplier is not forthcoming with (or doesn’t know) how much of the test-field is created by field type, that is how much of the measured field is contributed by the far-field and how much by the near-field. In fact you will find that the only data supplied is boresight data. That is the data represents the field we would measure with a field-probe mounted at the center of the calibration plane. No guarantee is given regarding the field achieved elsewhere across the calibration plane. Why does this matter? Answer – we are designing the test system under ideal conditions (perfect, very large fully-anechoic chamber). The premise being that if we are unable to generate a compliant field across the calibration plane under these ideal conditions, we do not stand ‘a snowflake’s chance in hell’ of achieving one under real 3 meter semi-anechoic chamber conditions. For instance, what if the measured field across the plane was to vary due to multiple peaks and nulls in the near-field radiation pattern?

To be continued...

Tom Mullineaux
Lionheart Southwest

Friday, June 5, 2015

Incorporating Antenna VSWR Loss into the Simplified Method

The Simplified Method of Establishing the RF Power Required by a RF Immunity System Continued...

Previously we presented the novel idea that the RF power required by a RF immunity system is a base RF power level (determined entirely by constants), minus the gain of the antenna. The basic principle is shown pictorially in Figure 1. The picture says the system RF power requirement in dBm equals the base RF power level in dBm, minus the antenna gain in dBi. A quick sanity check says this makes sense in that the higher the antenna gain, the less power required by the system.

Figure 1

The approach is also self-correcting in that should the linear gain of the antenna drop to less than 1 as can happen in lower frequency test systems (say SANT / SISO = 0.9), the gain in dBi will become minus, resulting in the system power requirement equaling the base power level PLUS the antenna gain.

So far so good.

We then built on this by converting all phenomena requiring more system RF power into loss-blocks and adding the loss blocks to the system diagram as shown in Figure 2.

Figure 2

Later on, we will simply add the overall dB loss of the blocks to the antenna dB gain to obtain the ‘overall system gain’ such that we are back to Figure 1, with the rightmost block amended to represent the system gain.

This approach, combined with the graphical representation described in the AH Systems webinar [Link Here] provides superb understanding / visualization of the system behavior across the band of interest, and uses the power of dB notation to simplify power computation.

In this particular blog entry we will concentrate on the mismatch presented to the system by the antenna.

Generating the VSWR Loss-Block

We need to convert the antenna VSWR phenomenon into the ‘basic loss-block form’ shown in Figure 3.

Figure 3

Please Note: this section also gives the solution to the teaser question posted last time.

The VSWR loss-block must obey the form of the equation in Figure 3, that is:

We need to establish Pout / Pin. We start with the classic VSWR representation as applied to an antenna shown in Figure 4a. Note in all cases below, the antenna symbol represents any type of antenna and the measurement plane is at the antenna connector.

Figure 4a

The figure shows the incident voltage Vinc striking the measurement plane, a portion of Vinc passing through the measurement plane to the antenna (shown as Vnet, the net voltage used by the antenna to create the test field), and a portion of Vinc being reflected back along the transmission line (the reflected voltage Vref).

By inspection we can see that Vinc is the input to our loss block and Vnet is the output. Again by inspection

Here we introduce ρ, the reflection coefficient at the measurement plane. This is equal to the ratio Vref / Vinc, so Vref can be written as ρVinc. This is shown in Figure 4b.

Figure 4b

Figure 5

We still want Pout / Pin so we need to convert Figure 4a to the power based diagram of Figure 5

So the

of Figure 4a becomes the

of Figure 5

To do this we convert all voltages to power with reference to the characteristic impedance of the transmission line (cable). That is: -

For future reference that is


So the

Of Figure 5 becomes

Pnet (that is Vnet2 / Zo ) is the output of our VSWR loss-block in Figure 2 and Pinc (Vinc2 / Zo) is the input

Rearranging to make Vnet2 / Zo the subject of the equation

But looking again at Figure 4b, Vref = ρVinc, where ρ is the reflection coefficient presented at the measurement plane

That is

Dividing throughout by Vinc2 / Zo will give us our Pout / Pin (since Pin is Vinc2 / Zo)

The final step in the conversion to a loss-block is to take 10log10 of this

One last little trick and we are there.


And at last we have our VSWR loss-block (Figure 6)

Figure 6

As a sanity check we should test this to see if the block works for ρ = 1 (100% reflection at plane) and ρ = 0 (No reflection at all)

When ρ = 1, none of Vin gets through the measurement plane and the loss should be infinite
When ρ = 0, all of Vin gets through and the loss should be 0

So testing for ρ = 1


And testing for ρ = 0

But log10[1] = 0, so


As a further step, we can now place this VSWR Loss-Block in front of a perfect match antenna (VSWR = 1:1 over the entire antenna frequency range).

Figure 7

To be continued...

-Tom Mullineaux
Lionheart Southwest

Friday, May 15, 2015

Use of Commercial Items

The use of commercial items (CI) or commercial-off-the-shelf (COTS) equipment presents a dilemma between imposing military E3 standards and the desire to take advantage of existing commercial systems, and accept the risk of unknown or undesirable electromagnetic interference (EMI) characteristics. Regardless of the pros or cons of using COTS, any procured equipment should meet the operational performance requirements, including electromagnetic compatibility (EMC) requirements, for that equipment in the proposed installation.

Integration of COTS electrical/electronic equipment on DOD platforms is an increasingly common practice for a variety of good reasons. COTS typically offer the latest technology and can be cheaper and more quickly fielded than military systems developed from scratch. Unfortunately, commercial equipment is not designed for the harsh electromagnetic environments (EME) found in military platforms and theaters of operation.

One of the biggest difficulties with integrating COTS products into complex military systems is achieving EMC. EMC is the ability of electrical and electronic equipment and systems to share the electromagnetic (EM) spectrum and to perform their desired functions without unacceptable degradation from the EME and without causing EMI to other systems. Blindly using COTS carries the risk of increasing serious EMI problems within the platform or system.

COTS equipment has typically been designed, tested and fielded to much less demanding commercial EMC standards, if tested at all, than MIL-STD-461 or MIL-STD-464. However, the simple fact that it is a commercial item should not be taken as a reason to accept lower EMC performance. Rather than forgoing robust EMC requirements, program managers (PMs), system acquisition personnel and E3 engineering professionals must first assess the EMC-related risk to full operational capability performance from the use of COTS equipment. They must impose a detailed methodology by which to assess the risk of using COTS and achieving EMC.

To mitigate the risk, an assessment should be performed to evaluate the equipment’s immunity characteristics against the planned EME and ability to meet the desired performance. Factors to be considered in evaluating the suitability of COTS for military applications include:
• Impact on mission and safety
• The operational EME
• Platform installation characteristics
• Equipment immunity/susceptibility characteristics

After determination of the intended operational environment, the risk assessment process starts with obtaining and reviewing existing design criteria (commercial specs), analysis/test data and conducting additional EMI testing (if necessary.) If, after evaluation of the EMI data, it is determined that the equipment would not operate satisfactorily in the intended EME, then the equipment needs to be modified, or it might prove to be necessary to select different COTS equipment with adequate characteristics.

On the whole, most COTS equipment has less strict EM requirements (lower immunity levels, higher allowable unintentional emissions, lax or nonexistent susceptibility limits) than military equipment and could therefore be more apt to be upset or damaged when exposed to high level radio frequency (RF) fields or could interfere with legacy systems. Therefore the use of COTS introduces additional risk of incompatibility and can result in problems, plus associated extra costs, in maintaining performance through life and for re-use in other scenarios. When considering COTS or NDI in an acquisition, it is important to include E3 requirements and obtain and review any existing EMI test and/or analytical data.

-Brian Farmer