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P-712

Near-field direction finder P-712, Bech NP-4 S; manufactured by R.F. Bech, Zurich.

R.F. Bech, a small RF electronics company located at Badenerstrasse 68 in Zurich, developed several compact near-field direction finders in the 1950s.
After the NP-1 (P-702) and the NP-3 (P-713) with a frequency range of 10.2 - 37.8 MHz, the NP-4 S was introduced with the troops in 1954 as a field use short distance direction finder.

P-712 (Bech NP4S)

Technical data

Power supply

  • Batteries: two heaters batteries 1.5 V; one plate battery 103 V

Dimensions

  • 350 x 250 x 210 mm, approx. 15 kg

Accessories

  • Direction finder antenna**: Frame direction finding antenna with a rod antenna as auxiliary antenna

P-712: Bech NP4S

Operation

The direction finding receiver consists of the actual receiver with a single loop antenna mounted directly on top of it, a rod antenna which is also used for side determination, mounted directly on a support with the compass dial. A small magnetic compass allows the direction finder to be aligned to true north.

The direction finding receiver comes in a simple black sheet steel housing and has a straightforward front panel: the volume control combined with the on/off switch, the manual RF Gain (sensitivity) and the Tf (telephony) / Tg (telegraphy) switch, the latter activates the BFO, are located at the top right.

On the left are the controls for side determination and the frequency range switch, which selects the band ranges 1.45-2.4 / 2.2-3.7 / 3.4-5.7 / 5.3-8.8 / 8-13.5 / 12.5-21 MHz.

Technically, the single conversion superhet with an IF of 470 kHz is equipped with a 1T4 (DF91) in the RF preamplifier stage and the DK92 heptode in the mixer stage, two 1T4s (DF91) operate in the two IF amplifier stages, a 1S5 (DAF91) as demodulator and another 1S5 (DAF91) as AF output stage for headphone operation. A further 1S5 (DAF91) is used in the BFO.

The direction finder receiver is powered by two 1.5 V heater batteries connected in parallel and a 103 V anode battery, all batteries are housed in a drawer.

Technical principle

The receiver has a tuned RF preamplifier stage (1T4), the RF circuits are housed in a coil turret with six chambers. The signal is then fed to the mixer stage; the DK92 tube also serves as oscillator tube. After two IF amplifier stages (1T4), the signal is demodulated, the same 1S5 tube also serves as the AF preamplifier and another one as AF output stage. A BFO (1S5) can be connected for CW and SSB reception.

Tube configuration

V1 (1T4 (DF91), RF preamplifier); V2 (DK92, mixer stage); V3, V4 (two 1T4 (DF91), IF amplifier stages 1 and 2); V5 (1S5 (DAF91), demodulation, AF preamplifier); V6 (1S5 (DAF91), AF output stage; V7 (1S5 (DAF91), BFO).

Development

From the mid-1940s, the company R. F. Bech, Zurich developed several compact near-field direction finders. After only a few units of the NP-1 (P-702) and NP-3 (P-713) sets had been purchased for evaluation, twenty-seven DF sets NP-4 S were procured in 1954/5.

Deployment

The near-field direction finder NP-4 S / P-712 was used for locating spy and illegal transmitters and remained in service until 1970.

Technical documentation

Further information

en/p-712.txt · Zuletzt geändert: 2024/01/05 11:46 von mb