04-06-2012, 07:11 PM
Low-inductance primaries: I think that the thinking goes something like this . . . .
High impedance primaries were popular when the general aerial in use was the ubiquitous 100 foot 'long-wire'. With the general advancement of valve technology and circuitry (increased gain; the s/het., etc.), not only was the long-wire aerial starting to disappear (a 'wet piece of string' often found to be adequate) but the S/W dipole was becoming more popular. By choosing a low-impedance primary, a match to the nominal 73 ohms of a correctly-fed dipole was more appropriate and at the same time, quite adequate for a 'few feet of wire' for an aerial.
My Eddystone 888A has low-impedance primaries for the aerial coupling. Some time ago I did some circuit calculations in an attempt to discover the nominal figure for the aerial input impedance of the set, using the values of the components between the aerial input terminals and the grid of the 1st. R.F. amplifier. I shan't repeat the details of all the calcs. here, but basically it was a matter of calculating the dynamic impedance of the 1st. L/C tuned cct., and having determined the transformation ratio of the input transformer, converting that impedance into the corresponding impedance presented at the aerial sockets. Over a freq. range of 3.5 to 4.0 MHz, the resultant figure was 75 ohms resistive at about 3.7 MHz, deviating to something like 90 ohms plus some small capacitive reactance in parallel at 4.0 MHz; 80 ohms plus some small inductive reactance at 3.5 MHz: overall, a nominal 75 ohms. This set has an aerial trimmer, which, I must confess, I did not adjust after those measurements to see if those unwanted reactances could be tuned out: my curiosity had been satisfied. In practice, however, that aerial trimmer makes only a slight improvement in the received signal strength, but not by much - and, of course, will not affect the overall S/N ratio, anyway - and the set has plenty of gain in reserve (double-conversion; 85 KHz 2nd. I.F.)
Al.
High impedance primaries were popular when the general aerial in use was the ubiquitous 100 foot 'long-wire'. With the general advancement of valve technology and circuitry (increased gain; the s/het., etc.), not only was the long-wire aerial starting to disappear (a 'wet piece of string' often found to be adequate) but the S/W dipole was becoming more popular. By choosing a low-impedance primary, a match to the nominal 73 ohms of a correctly-fed dipole was more appropriate and at the same time, quite adequate for a 'few feet of wire' for an aerial.
My Eddystone 888A has low-impedance primaries for the aerial coupling. Some time ago I did some circuit calculations in an attempt to discover the nominal figure for the aerial input impedance of the set, using the values of the components between the aerial input terminals and the grid of the 1st. R.F. amplifier. I shan't repeat the details of all the calcs. here, but basically it was a matter of calculating the dynamic impedance of the 1st. L/C tuned cct., and having determined the transformation ratio of the input transformer, converting that impedance into the corresponding impedance presented at the aerial sockets. Over a freq. range of 3.5 to 4.0 MHz, the resultant figure was 75 ohms resistive at about 3.7 MHz, deviating to something like 90 ohms plus some small capacitive reactance in parallel at 4.0 MHz; 80 ohms plus some small inductive reactance at 3.5 MHz: overall, a nominal 75 ohms. This set has an aerial trimmer, which, I must confess, I did not adjust after those measurements to see if those unwanted reactances could be tuned out: my curiosity had been satisfied. In practice, however, that aerial trimmer makes only a slight improvement in the received signal strength, but not by much - and, of course, will not affect the overall S/N ratio, anyway - and the set has plenty of gain in reserve (double-conversion; 85 KHz 2nd. I.F.)
Al.
