====== P-712 ======
**Near-field direction finder P-712**, **Bech NP-4 S**; manufactured by [[Bech|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.

{{:images:p-712-bech-mit-antenne-b.jpg?direct&250 |P-712 (Bech NP4S)}}
===== Technical data =====
  * [[Principle]]: Short distance direction finder [[Bech]] NP-4 S: [[single conversion superhet]], [[IF]] 470 kHz
  * [[Operation modes]]: [[A1]] with BFO, [[A2]], [[A3|AM (A3)]]
  * [[Frequency range]]: 1.5 - 20 MHz
  * [[Sensitivity]]: [[A3|AM (A3)]] / [[Selectivity]]: 

==== Power supply ====
{{ :images:p-712-bech-closed.jpg?direct&200|}}
  * [[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

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{{:images:p-712-bech-front.jpg?direct&600|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 [[1T4]]s ([[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 ====
{{ :images:p-712-bech-kiste-offen.jpg?direct&200|}} From the mid-1940s, the company [[Bech|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.

{{gallery>:images?p-712*.jpg&0&80x80&lightbox&showtitle&titlesort}}

===== Technical documentation =====

==== Further information ====
  * [[http://armyradio.ch/radio-d/p-712.htm|P-712 / NP4S at www.armyradio.ch]]
  * [[https://www.hamfu.ch/de/geraete/geraet.php?id=194|P-712 / NP4S on the Hamfu website]]
  * [[https://www.radiomuseum.org/r/bech_nahfeldpeiler_np_4_snp4.html|P-712 / NP4S on the website of www.radiomuseum.org]]