Whilst a common feature of the communications receiver in the past,
the antenna trim control no longer appears.

Would this be because modern gear expects a 50 ohm pre-matched feed
whilst in former times a long wire might come directly to the antenna
socket?

In message , gareth
writes
Whilst a common feature of the communications receiver in the past,
the antenna trim control no longer appears.

Would this be because modern gear expects a 50 ohm pre-matched feed
whilst in former times a long wire might come directly to the antenna
socket?

"Antenna trim" was usually simply a variable shunt capacitor to
chassis/earth, and only served to attempt to tune out any inherent
antenna inductive reactance (or maybe to provide a bit of independent
fine tuning of the input of the RF stage). It was usually associated
with a receiver high(ish) impedance antenna input. When the input
impedance is low, an antenna trim capacitor will have very little
effect.
--
Ian

"Ian Jackson" wrote in message
...
In message , gareth
writes
Whilst a common feature of the communications receiver in the past,
the antenna trim control no longer appears.

Would this be because modern gear expects a 50 ohm pre-matched feed
whilst in former times a long wire might come directly to the antenna
socket?

"Antenna trim" was usually simply a variable shunt capacitor to
chassis/earth, and only served to attempt to tune out any inherent antenna
inductive reactance (or maybe to provide a bit of independent fine tuning
of the input of the RF stage). It was usually associated with a receiver
high(ish) impedance antenna input. When the input impedance is low, an
antenna trim capacitor will have very little effect.

True, but as a trimmer across the RF stage tuning capacitor?

In message , gareth
writes
"Ian Jackson" wrote in message
...
In message , gareth
writes
Whilst a common feature of the communications receiver in the past,
the antenna trim control no longer appears.

Would this be because modern gear expects a 50 ohm pre-matched feed
whilst in former times a long wire might come directly to the antenna
socket?

"Antenna trim" was usually simply a variable shunt capacitor to
chassis/earth, and only served to attempt to tune out any inherent antenna
inductive reactance (or maybe to provide a bit of independent fine tuning
of the input of the RF stage). It was usually associated with a receiver
high(ish) impedance antenna input. When the input impedance is low, an
antenna trim capacitor will have very little effect.

True, but as a trimmer across the RF stage tuning capacitor?

Maybe not directly, I think. More likely on the high Z antenna input.

In the circuit diagram of the R107, C2A (bottom left, near the dipole
twisted leads) will be the aerial trimmer.
http://70.33.246.110/~radio100/allan1942/r107.html
It's on the tap into the input tuned circuit, so not quite in parallel
with the C4A ganged tuning capacitor.
--
Ian