Loop Padder ?

[60 oldjohn]

How common were they with UK transistor sets? I know they were used in Canadian sets.

John.

[60 oldjohn]

Sorry I should have said as used on the end of the ferrite rod of an AM radio.

John.

[Jon_G4MDC]

Thanks for clarification but still unsure what it is. A moveable shorted turn to modify inductance is my best guess. Please do tell.

[60 oldjohn]

Originally Posted by Jon_G4MDC

Thanks for clarification but still unsure what it is. A moveable shorted turn to modify inductance is my best guess. Please do tell.

It consists of one or maybe two turns of solid insulated wire fixed in place with wax, the tails each about 1 inch long are tightly twisted together to "Tune" the rod or circuits. Apparently one turn is the difference between no reception and excellent reception. A similar "capacitor" is often used on the PCB of transistor radios, this consists of two twisted wires.

John.

[Jon_G4MDC]

Sounds like quite a peculiar thing. The wires tightly twisted together would make a gimmick capacitor but these only manage a few pF at best.

I don't think a couple of turns around the rod tuned by end wire capacitance is going to do very much at all. It would probably make a high frequency trap (a notch) but why would it be needed?

[Radio Wrangler]

Most sets have a padder capacitor in the local oscillator tank to pull the LO of of the frequency of the RF tuned circuit(s). But it's equally possible to leave the LO alone and have a padding arrangement to offset the RF tank from the LO frequency to create the required IF. Of course, the LO tuning range has to be different doing things this way round. The off-tuning of the RF tank can be done by dangling something appropriate on the ferrite rod to pull its tuning.

David

[m0cemdave]

I have seen a ferrite rod antenna that had something similar, that I assumed was a shorted turn to trim the tuned circuit.
A rubber grommet with an aluminium washer fitted into the groove, that could be slid along the rod.

[Graham G3ZVT]

Interesting.
A disproportionate number of Google references point to the Canadian manufacturer "Sparton". Also Philco seemed to like the term.
I'm sure the technique was used more widely, without it being given that (or any) name.

[G6Tanuki]

I have not come across this thing in broadcast radios but do remember a related "shorted turn" loop on the ferret rod antenna was used to desensitize the receiver on a few of the top-band foxhunting setups in the 70s.

[60 oldjohn]

Originally Posted by Graham G3ZVT

Interesting.
A disproportionate number of Google references point to the Canadian manufacturer "Sparton". Also Philco seemed to like the term.
I'm sure the technique was used more widely, without it being given that (or any) name.

There is a closeup photo in this Sparton radio thread, please scroll down nearly half way to a a photo of a Loop Padder and clearly marked on the circuit diagram below. https://antiqueradios.com/forums/viewtopic.php?t=449001

John.

[Radio Wrangler]

The odd name comes from America where ferrite rod antennae were often called "Loop sticks"

David

[turretslug]

Originally Posted by Jon_G4MDC

It would probably make a high frequency trap (a notch) but why would it be needed?

That's reminded me of the days of listening to cheap MW/LW radios late at night when it wasn't unusual to hear the ghostly twang of "Moscow Nights" or Radio Tirana's mournful, downbeat interval signal in the background of the station being listened to- an HF trap might have been useful there!

[Graham G3ZVT]

More likely it was not image reception at all, as Moscow and many other European stations had their external service on a MW frequency.
From memory:
"This is Radio Moscow broadcasting to Great Britain and Ireland in the 16, 19, 25, 31 and 49 meter bands, and on 227 meters medium-wave.“
Moscow was unusual in that the interval signal "Wide is my Motherland" was played after the time signal rather than before it.

"Midnight in Moscow" was used as the interval signal of "Radio Station Piece & Progress" which was just Radio Moscow in disguise.

[Graham G3ZVT]

Peace not Piece.

[Andrew2]

I came across several models of portable radio with a single shorted turn perched somewhere near the end of the ferrite rod. Removing it seemed to have no effect until you tuned down to the bottom (LF) end of MW when the set would oscillate. Putting the short back on stopped it. I may be mistaken, but I think there was a reference to this either in the service sheet or maybe in PW.

[turretslug]

Could that have been in sets that used that favourite of many transistor portables, the self-oscillating BJT mixer? With oscillator feedback taking place between collector and emitter, I imagine that they depend on the base connection (i.e. the tuned ferrite rod) having relatively low impedance to OV at IF for avoidance of instability at IF- tuning towards the low end of MW, the impedance at 455-470kHz would start to rise, and maybe the short is a damping kludge.

[Andrew2]

That would make sense. Yes, AFAIR all these sets used the self-ocillating mixer with the 'not quite earthed' base.