Often a point of discussion in car radio restoration articles, there seems to be some mystery about the coil represented in the above diagram as L1. It consists of only a few turns of wire, and is connected right at the aerial socket. Suggestions have been made that it is some kind of matching network. However, such few turns will have virtually no effect on the MW frequencies the set operates at. The inductance of the coil is very low; typically less than 5uH. In actual fact, as the AWA Radiotron Designer's Handbook book states:
It so happens that ignition interference peaks around 40Mc/s, and in fact this was a major problem with the original 405 line TV broadcasts in London, operating around this frequency. It effectively mandated the fitting of suppressors fitted to cars in the UK.
"Radio Television & Hobbies" for June
1959, describes a car radio project which includes this coil. It is described
as a "hash filter" in the circuit diagram. The text of the article states
that, "It acts in conjunction with the input capacity of the receiver to
form a low pass filter and prevents cross modulation of the first stage
by high frequency interference, e.g. the car's ignition system."
Either way, as a resonant circuit, or
as a low pass filter, the 40Mc/s energy entering the RF input stage is
reduced.
How might 40Mc/s energy cause interference
in a radio which only operates up to 1.6Mc/s, you might ask? If sufficient
40Mc/s energy gets into the RF amplifier (or frequency converter) stage,
grid rectification will result, and this of course will intermodulate with
the lower MW receiving frequency. You might assume that any resulting frequency
products are still too high to get through the RF or IF stages.
But, consider this: That 40Mc/s signal
is being interrupted by the frequency at which the engine rotates, and
the number of spark plugs it has. Consider a four cylinder engine rotating
at 1000 rpm. The 40Mc/s signal is causing grid rectification, and thus
modulation of the broadcast signal at 250 c/s. No wonder it is clearly
audible.
Examples:
Spark filter in this Ferris 106 car radio is the red coil of wire.
It measures only 0.3uH.
Spark filter coil in the Philips NT346V is a moulded choke.
In the Walbar 1255A, the filter coil is wound on an air cored former.
In the AWA 946AZ, the coil is a few turns wound on a high value
resistor for the former.
Note in the above examples, the coil has few turns and is of low inductance. Their effect at 1MHz is minimal.
Loading Coils.
The spark filter coil is not to be confused
with another coil, sometimes fitted to domestic sets, which are of much
higher inductance (typically 100uH or more ). This is used
for matching purposes, to connect an external aerial to a set using a ferrite
loopstick, and the parts list for these sets describes the coils as such.
Input matching coil for this domestic set measures 195uH.
It should be clear from the above examples, that despite schematically appearing as the same thing, the coils are completely different between the car set and domestic set.
