In message , The Natural Philosopher
writes
On 09/02/18 10:58, Ian Jackson wrote:
However, I also guess that as the variable tuning capacitor for
typical MW and LW radio was a physically large 500pF. As most car
radios had an RF stage, this would have required it to be 3-gang

2 gang.

3-gang - surely? RF stage input, RF stage output / mixer input, and
local oscillator.

MW/LW switched both one RF coils AND local oscillator coils

But ONLY those two.

Uh?

In practice tuning the EF stage in mots conmmnercial grade sets was
really veryt briad - the [promary function is image regjection at
900Khz- 940Khz away from the incoming signla. On LW that really meant
so far away you didnt need to worry so the oinly thing te RF tuning did
a bit of was a slight gain to the incoming signal (mostly noise on LW
anyway)

With superhets neartl ALL te selectivey is in et pretuned fixed
frequeancy IF strip, and thats why they were de regeur.

Well..... quite. That's what superhets do. [When was the last time you
used a TRF set ?]

The only tuning as the local oscillator. The RF srages might not even
be PRESENT let alone tuned

Most house/portable LW/MW radios don't have an RF stage [RF stage?
LUXURY!] - but the aerial/mixer input is invariably tuned (as close as
possible) to the signal frequency, and the local oscillator runs the IF
frequency above the signal frequency.

- direct injection into te micxer was not that uncommon spcially on
ferrite rod tuned portables but was frowned upon a little as it pumped
the oslitaor back up the aerial.

You don't get much local oscillator radiation if the set has a ferrite
rod aerial.

So in essence the RF stage MOSTLY stoppoed te oscillatort being
rtertsnmitted , andf provided sa bit of gsaiun., It made very little
difference to selectivity and was pretty low Q really.

Ther was no need for two RF coils.

It all depends on the application. LW/MW car radio are VERY short, and
I'm pretty sure that many had an RF stage to boost the sensitivity. With
those old types (with the 'peak-it-at-the-HF-end' trimmer) it was
obvious that the input tuning was quite sharp (and therefore critical if
you wanted to optimise sensitivity). However, I have a couple of 'more
modern' sets that don't seem to have any provision for LW/MW peaking -
and both seem to work OK when fed with a short vertical loft aerial in
the loft, via about 15' of 75 ohm coax. With the old sets, this would
have been a killer - so I'm not sure what the input circuit is like.
--
Ian