MOBILE RADIO TUANSMISSION 1073 



Use of gain antennas for the land I'cccivers would result in still further 

 lowering the required mobile transmitter poAver. This is not shown on 

 Fig. 3 because the amount of reduction cannot l)e accurately stated on 

 the basis of present knowledge. It appears certain that tiie reihution 

 Avill be at least equal to the antenna gain, and may be appreciably' more 

 than this, as indicated later. 



The system modulation and ])ass-band weic assumed in I lie al)oye 

 discussion to be the same at all frequencies. This would not. be ix^alistic 

 if the tolerance allowed for frequency instability were a iixed jx'icentage 

 of operating frequenc3^ It may be justified, how-eyer, because^ the neces- 

 sity for frequency economy and for best transmission })erfoi"mance 

 demands better percentage stability at higher fre(iuen(!ies. 



A spot check of transmission, ob.serying cii'cuit merits by listening, 

 has been made to determine the \'alidity of the al)oye results in a very 

 general way. T^and transmitter powers were adjusted so that the equiva- 

 lent dipole po\\er at 450 mc was 3 db less than at 150, and power at 

 900 mc was 1 db less than at 150 mc. This approximates the powers 

 shown on the "dipole" curve of Fig. 3. The map of Fig. 4 shows the 

 results of this test. While the comparison of circuit merits generally shows 

 a preferred frequency at any given location, the performance appears to 

 be about equal w^hen all locations are considered. 



TEST EQUIPMENT ARRANGEMENTS 



Tests of transmission out\vard from the land transmitting station were 

 made on signals radiated from antennas on the roof of the Long Lines 

 Building, 32 Avenue of the Americas, New York City. These antennas 

 wxre 450 feet above ground. One of the existing Mobile Service trans- 

 mitters served for the 150-mc tests. Special experimental transmitters 

 were set up for the 450, 900, and 3700-mc tests. All were capable of 

 frequency modulation. 



The mobile unit was a station wagon equipped to receive and measur- 

 signals at the various frequencies. The receiving equipment was ar- 

 ranged for rapid conversion from 150 to 450 to 900 mc. The bandwidth 

 (about 50 kc) and system modulation (±10 kc) were identi(;al at all 

 three freciuencies (equal to the existing standards at 150 mc). The 3700- 

 mc tests were handled separately. It was not possible to employ the 

 same bandwidth and deviation, but this does not invalidate the com- 

 parison of signal propagation at the A'arious frequencies. 



A most useful tool in making these measurements was a device known 

 as a "Level Distribution Recorder", or simply "LDR". This was built 



