Question

How do I get the poke radar in heartgold?

Answer 1

The Poké Radar is not normally obtainable in the main game; however, a primitive version of the Poké Radar exists on the bundled Pokéwalker.

Poke Radar (Bulbapedia)

If you're set on getting the Poke Radar, be prepared to shell out a bit of money for a Pokewalker. They're not made anymore and they've generally retained or increased in value.

Comments

can I still shiny hunt with it in game?

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According to the same article, no:

"Pokémon caught in the Pokéwalker are assigned a random Nature, and will ignore Synchronize. Due to how these Pokémon are generated, they are both prevented from being Shiny, and will never generate with a Quirky Nature."

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dang. i'll probably still get the pokewalker anyway

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Amethyst are all Pokemon from the pokewalker shiny locked or just the ones from PokeRadar

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Apparently no Pokemon from the Pokewalker are able to be shiny, unfortunately.
https://pokemondb.net/pokebase/15557/