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ANTENTOP- 03- 2003, # 004

Half-Loop Antennas

 

 

 

 

Jean-Pierre GOUIN & Daniel LAFARGUE

In France

means
ANTENNAS

 

http://perso.wanadoo.fr/starec

 

 

 

STAREC S.A.

7, chemin de Vaubesnard

91410 Dourdan FRANCE

e-mail: starec@wanadoo.fr

 

Credit Line: Starec

 

http://perso.wanadoo.fr/starec

 

 

The present describes a HF loop antenna and its agile coupler which can be adapted to the new designs of ALE and FH (frequency hopping) radiosets. The original specification in 1993 was : "a small mobile antenna and coupler for HF voice and data communications in driving from 0 to 600 km without silent zone, in association with a 125 Watts CW radioset.

Frequency range 2-12 MHz Channel tuning time < 5ms Bandwidth > 3,5 kHz in a military environment" Following on from this product other versions with wider frequency range (3-15 MHz, 3-30 MHz, 2-30 MHz), a higher power and various dimensions and shapes for fixed, land-mobile and naval applications have been developed.

1. GENERALITIES

1.1. on the HF tuned loops

The HF transmission tuned loop antennas which are designed for HF transmission have small dimensions (< 0,1 ) compared to the wavelength, in order to conduct a quasi constant current and to be considered as magnetic dipoles. Their radiation impedance and efficiency mainly depends on their surface which creates a magnetic flux in the near field and an electromagnetic field in the far field. Their diameter, height or width (round or square shape) run from 1 to 3 meters, and their radiating surface generally do not exceed 5 m2 in order to coincide with the small dimensions required.

These types of antennas differ from open antennas (like whips, horizontal dipoles, log-periodic antennas,) by their impedance which is reactive and can be adapted by capacitor only. Their radiating resistance is low (< 1 m ) at the lowest frequencies of the range. As the efficiency is given by the ratio radiating resistance/ total resistances of the tuned circuit, it is necessary to minimise the radiating element resistor, using a good conductive metal (aluminium, copper), and to use low loss capacitors.

 

These conditions beeing fulfilled, the loop antennas deliver a high current and have a high selectivity with a high quality factor (Q-factor), typically 10 times higher than the Q-factor of the best traditional couplers designed for 5 to 10 meters whips or 10 to 40 m dipoles.

1.2 On fast frequency tuning

For future fast ALE procedures the tuning target time is 50ms, while the "low speed" frequency hopping (F.H.) procedures already require a 5ms tuning time, with all calculations and control exchange times beeing included or already done. This can't be done using electromechanical tuning. Digital switching devices are cost effective today at low and medium powers. Their switching time run in milliseconds using low loss vacuum relays, and in the microseconds using electronic relays like PIN-diodes. But the PIN-diode technology cannot be used in loops for transmission, due to their inability to withstand the high currents and due to the losses they bring (0.5 to 1 W) which would drastically decrease the overall efficiency at the lowest frequencies. Vacuum relays, including REED relays, are the only technologies available to switch the capacitors of a transmission tuned loop antenna efficiently.

1.3 On the power requirements

 

Based on the experience of 2 previous generations of tuned loop antennas, and the proprietary propagation simulations, it was calculated that two 100W radiosets and 4m2 loops having a -15 to + 5dBi typical gain figure from 2 to 12 MHz would insure voice and data communications at any distance from 0 to to 600 km at least.

This mission cannot be fulfiled by any 5 to 10 m whip antenna on a medium soil, even in association with a 400W/1 kW radio set: a vertical whip or a bent whip on a vehicle in move do not transmit and receive enough energy to cover the typical 50-250 km silent zone.

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