CFA project report references.

DJJ's site navigation page
The Poynting vector, power flow, and the CFA
Editorial in the on-line magazine AntenneX, Nov 1999
Sir James Jeans's remarks on the Poynting flux 1933
Electromagnetics and antennas

Reference 1.
Crossed Field Antenna
by Colin M Davis
Final year undergraduate project report 18 June 1992 P021
Department of Electronic and Electrical Engineering
University of Surrey
Guildford GU2 5XH (UK)
Supervisor Dr G G Johnstone
74 pages including constructional details and measurement results

Summary. A 50MHz double-ended version was built and extensively tested. The E-plate cylinder was 125 mm long and 100 mm in diameter; the D-plate was 200 mm in diameter. Care was taken in positioning on the test range. The antennas were suspended on wooden tripods a couple of metres above the top of tall buildings and could be raised and lowered through half a wavelength to check for interference effects from ground reflections and reflections from adjacent buildings. There was a clear line of sight between transmit and receive antennas. Signals received were more than 23dB down with respect to those of the reference antenna at the same range. All three antennas (the CFA, the receive dipole, and the reference dipole) were horizontally polarised. The plate phasing was carefully checked with a vector voltmeter, and adjusted over a range around the suggested 90 degrees lag for the D-plate with respect to the E-plate, both for the terminal voltages, and also for the forward wave voltage complex amplitudes.

Measurements were also made of the power reflection coefficients using dual directional couplers. It was found that although a good match could be made in the single feed to the phasing unit, most of the energy was dissipated there, as the power reflection coefficients to the D and E plates taken separately were close to unity. The power output from the phasing unit reactive components was routed by way of inbuilt attenuators to stabilise the phase shifts; the power was in all probability mostly absorbed in these attenuators, resulting in the possibility of a match at the input to the phasing unit. The test range polarisation was horizontal. There was no indication of any variation in absorbed or transmitted power as the phasing was varied. Careful measurements of cross-polar signal strength indicated that there was no appreciable radiated power from the vertically-hanging cables feeding the CFA, which were transverse to the axis of the antenna.

Reference 2.
The Crossed-Field Antenna
by James N. H. Wong
Final year undergraduate project report 18 May 1999 P012
Department of Electronic and Electrical Engineering
University of Surrey
Guildford GU2 5XH (UK)
Supervisor Dr G G Johnstone
72 pages plus appendices, including constructional details and measurement results

Summary. A single-ended 50MHz version was built and extensively tested. The plate dimensions were the same as the antenna measured in the first study (above). It was placed on a ground plane in a large open expanse of field, and fed all its power from a petrol generator to eliminate the possibility of coupling through the power supply leads to the receiver. Measurements were conducted with all antennas (the CFA, the reference 1/4 wave monopole, and the 1/4 wave receive monopole) vertically polarised, except for a series of measurements that indicated that the received E-field from the CFA was indeed vertical, as it was said to be. The results broadly confirmed the measurements reported in the previous study.This version also showed no evidence of working in the way claimed for the design by others, as far as its acceptance of power is concerned. However, the reported field strength measurements table gives the following relative results:

Reference 1/4 wave monopole over ground plane

Distance (wavelengths of 6 metres): relative power dB
Received power figures +/- 1.5 dB

0.5 -30
1.0 -32
2.0 -36
4.0 -42
8.0 -44
18.0 -52

0.5 -47
1.0 -47
2.0 -47
4.0 -46
8.0 -47
18.0 -52

0.5 -47
1.0 -49
2.0 -47
4.0 -46
8.0 -47
18.0 -50

0.5 -47
1.0 -49
2.0 -47
4.0 -46
8.0 -47
18.0 -50

A quotation from the discussion.

quote (sic)

"According to the proposed theory made by the CFA builders, such impedance mismatch could occur if the phase of the feed to one of the electrodes is reversed....Full-blooded attempts have been emphasised on getting the phasing right....since the phasing requirements are correctly done, then the assumption that the impedance mismatch due to phasing errors is clearly eliminated."

and again...

"On the other hand, interestingly, if the ground-plane CFA could be perfectly matched, then as shown....it could well be an effective small radiator!"

unquote

Of course, if an antenna can be made to accept power, and it is reasonably loss-free, then it must be "an effective small radiator" so this statement is tautological.

There is enough comfort in these figures for those who claim that the close-in fields from the CFA are depressed with respect to a monopole reference 1/4 wave antenna, to encourage them to persist in their efforts to match the antenna.

Note that 18 wavelengths is 108 metres; 8 wavelengths is 48 metres, and the power cable feeding mains to the receiver was 60 metres long, across the end of the range. It is not clear from reading the report that the RF pickup by this cable was adequately excluded from the receiver. Thus the absence of fall-off with distance of the CFA-induced fields could well be due to capacitative coupling to this 60 metre lead; this hypothesis is supported by the observation that the received signal was the same with both the whole CFA, and only the E-plate, and only the D-plate connected. As we have remarked elsewhere on these pages, it is exceedingly difficult to do accurate HF and VHF antenna experiments.

Note also, that there was loss between the phasing unit and the plates. This loss is not present in other peoples' implementations, but was put there to stabilise the driving point impedance to get the phasing accurately 90 degrees. Thus we might conclude that it is not sufficient just to have the phasing right; it may be necessary for stored energy to move back and forth between the E-plates and the D-plates by way of the phasing unit. Could the phase shift be provided by a quarter-wave length of lossless tranmission line?

"The CFA - intellectual itching powder"

An interview with the proponents of the CFA technology, 1999.
John Belrose's report of careful attempts to measure and match a scaled CFA.
DJJ's polemic on the honest reporting of genuine experimental results, and on the value of the reporting of negative or null experimental results.