PL-310 external MW/LW antenna connection

Laurence Mann

Hello All,
My apologies if this mod has already appeared in the forum.

I recently purchased the PL-310 and, eager to try out some of Gary DeBock's ferrite loopstick ideas, I popped the back off and I noticed, happily, that the 3.5 mm socket for the ext SW/FM antenna is actually a stereo switched socket and that the SW-FM switched connection for the whip antenna is directed to go to the tip of a 3.5 mm stereo plug. The other pair of switched terminals in the socket are for the ring connection to a stereo plug and are conveniently left unconnected on the PCB. Hence I routed the internal loop stick hot connection through these unused terminals so that with no plug in the socket the internal ferrite rod is connected as normal and, when a 3.5 mm stereo plug is connected, the internal ferrite rod is disconnected and the MW/LW input circuit of the radio is available on the ring connection of the plug. The only thing to watch for is potential shorts between the whip antenna base and these pads on the PCB as they are almost coincident with one another when the back of the radio is in place; I put insulating tape over the base of the whip where it overlaps these pads and there has been no shorting problems thus far.

Via this plug connection made with a short length (few inches) of RG-174 coax, I tried large external loop sticks around 2,000 uH and they work very well on LW being tuned by the radio's internal cap up to about 400 kHz. For an external MW ferrite rod it is necessary to avoid too much stray capacitance for the upper end of the band if you want the radio's internal cap to tune the ferrite coil, or else optimise with a lower coil inductance than you might otherwise use. A switched inductance coil on the external ferrite rod will doubtlessly be the way to go but the stray C in the interconnect to the radio will be an issue and I may end up going with short open leads to the 3.5 mm plug to allow minimum stray capacitance.

The mod also allows for the direct connection of external tuned loops and my external amplified tuned MW/LW ferrite rods work very well through this interface.

Regards from Perth, Western Australia
Laurie Mann

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