Antenna Trim?
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?
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 |
Antenna Trim?
"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? |
Antenna Trim?
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 |
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