Well put, John.
I have understood the purpose of the 'matching network' for years, but
I have been puzzled by the source impedance expected on the generator
side.
I have several RF generators of various vintages in my collection and
most of the older tube ones have most peculiar output attenuators that
would have impedances that vary all over the place depending on the
setting of the output attenuator, frequency and, to some degree, output
cable length. On 'modern' generators, once you cut in 10dB or more of
attenuator, the output inpedance it pretty close to the specified, but
even many of them are quite a low impedance at 0 dB.
Perhaps someone with spice can characterize the output impedance of the
IRE network when fed from a generator of 0 ohm, 50 ohm, 75 ohm and,
say, 200 ohm output impedance.

Neil S.

John Byrns wrote:
In article ,
wrote:

Michael A. Terrell wrote:
"H. P. Friedrichs" wrote:

I am interested in the origins and purpose of the "IRE (Institute of
Radio Engineers) Standard Dummy Antenna," as used during the alignment
of radio receivers.

Its to match the receiver to the generator.

Or more specifically, it's to present a uniform high impedance
from the generator to the radio. At the time, with the exception
of a few lab grade instruments, signal generator outputs were all
over the map with regards to output impedance. The "dummy antenna"
presented a fairly uniform, and high, impedance to the radio and
kept from detuning the front end while doing an alignment while
connected to a signal generator.
This doesn't sound right, why would it be desirable "to present a uniform
high impedance from the generator to the radio"? The impedance of the
"IRE Standard Dummy Antenna" is not exactly what I would call "high"
especially on some of the short wave bands, exactly what is a high
impedance in this context?

It was my understanding that the purpose of the "IRE Standard Dummy
Antenna" was to provide a standard antenna to use for making performance
measurements, as well as for alignment at the factory and on the service
bench.

As to the exact design of the of the "IRE Standard Dummy Antenna", I
understood that it was meant to represent the impedance characteristics a
typical antenna that might be used by a radio set owner. This topic has
been discussed here before and I seem to vaguely remember that someone may
have even dug up the exact nature of the antenna the "IRE Standard Dummy
Antenna" was designed to simulate. IIRC if you are only doing MW
broadcast band alignments a single capacitor will provide a very good
approximation of the "IRE Standard Dummy Antenna" across the MW broadcast
band. The coil, resistor and second capacitor are there mainly to provide
the desired impedance on the short wave bands where the prototype antenna
has resonant effects

There are at least two problems that the "IRE Standard Dummy Antenna"
solves, the first being standardized measurements for obtaining
performance specifications, as it is possible for a manufacturer to
"improve" his products specifications by using his own specially designed
"Dummy Antenna". The second problem is how to align sets in the factory,
or on the service bench, when the final users actual antenna system is not
available. The problem is that the alignment of the antenna stage of a
radio is greatly affected by the impedance of the actual antenna that is
used with the radio. The "IRE Standard Dummy Antenna" provides an
approximation of what was assumed to be a typical consumer antenna. Of
course what you really want to do is align the set with the actual antenna
attached that it is used with.


Regards,

John Byrns


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