RADIO EXTENSION LINKS TO TELEPHONE SYSTEM 639 



wave, a single antenna may be used for a number of wave-lengths 

 without readjustment. 



When the rhombic antenna is used for a number of frequencies 

 without change in size or form, the directivity is different for each 

 frequency. The maximum directivity for the higher frequencies will 

 be a lower angle than for the lower frequencies. This works in very 

 well for long distance operation since the angles at which the high and 

 low frequencies come in tend to agree with this characteristic of the 

 antenna. 



The rhombic antenna has come into extensive use for transoceanic 

 short-wave links during the last few years. Due to its multi-wave- 



Fig. 19 — -Rhombic antenna invented by E. Bruce. 



length characteristics it is replacing the other types of antennas de- 

 scribed previously. The initial cost is much less than the type sup- 

 ported on steel towers, the land required is less, and the upkeep is 

 also less. 



In Fig. 20 is shown a photograph of another type of directive an- 

 tenna. Two antennas are indicated, one for transmitting and one for 

 receiving. These antennas are known as "pine tree" ^ antennas 

 because of the connections of the radiators to a transmission line 

 passing up from below. This particular antenna is for ultra-short- 

 wave operation around 60 MC and is one end of the Provincetown- 

 Green Harbor circuit. Each antenna contains 8 radiators in a plane 



