RADIO EXTENSION LINKS TO TELEPHONE SYSTEM 625 



period during which the shore subscriber is expected to talk. The noise 

 at this time in the receiver on shore must not actuate the Vodas or 

 the transmitting branch will be interrupted. A suitable rectifier with 

 a relay can, under certain conditions, be a satisfactory Codan but 

 not in all cases. There is always a certain amount of static, strays 

 and so forth, reaching an antenna and if the relay is adjusted to operate 

 on a very weak carrier the strays and the static may also operate 

 the relay. Now, of course, it is possible to adjust the relay so it will 

 not operate on the noise occurring at a particular time but will operate 

 on a carrier which is slightly stronger. If that adjustment is made 

 during the day on short waves when the noise is low, when night 

 comes the noise level rises and the noise may then be able to operate 

 the relay. It would be necessary with a simple Codan of this type to 

 have the operator continually adjust and readjust the Codan for 

 different parts of the day and night. 



However, it is impracticable for an operator to be continuously on 

 watch and continuously and satisfactorily adjust the sensitivity of 

 such a relay, with the consequence that a satisfactory Codan necessarily 

 involves automatic adjustment as provided in the circuit of Fig. 9. 

 In this diagram the part above the middle dividing line is the receiver 

 while the part below is the Codan. The Codan here consists of two 

 parts. It consists of a part which selects the carrier by a crystal 

 filter for operating the relay, and instead of a spring to hold back the 

 armature an electrical arrangement is provided whereby the noise 

 coming through the second part of the circuit will produce current in 

 the relay in the opposite direction. The noise is picked out by another 

 crystal filter Y4 which selects all the energy in the two sideband posi- 

 tions minus the carrier position. This noise is amplified and rectified 

 so that whenever the noise is high it will require a large carrier coming 

 through filter Y3 to operate the Codan relay S2 and when the noise is 

 low through the noise branch a smaller carrier coming through the 

 carrier branch can operate the Codan relay. This Codan therefore 

 automatically adjusts itself to the noise level in the ether so that the 

 carrier can connect the receiver to the telephone line whenever the 

 carrier appears. 



Since the development of a successful Codan it has been found 

 practical to dispense with the Vodas at terminals that connect with 

 radio stations which radiate their carrier during transmitting periods 

 only. This is brought about by using the Codan to operate the relay 

 that switches from transmitting to receiving. Since the Codan is 

 operated by the incoming carrier and not by outgoing voice signals, 



