Two top candidates are probably the Watson Radar Extender (£90) or the DPD ads-b antenna which you have to order from the States (USD89).
RG58 is useless at 1GHz. Westflex 103 should be the minimum for that length of run. N-Type at the antenna end and SMA at the RB end (you can now get the large entry N-types for 10.3mm diameter cable) or a bnc hen use a bnc to sma adapter. The only problem with using thick and heavy cable though is that you might put some strain on the radarbox antenna connector so careful attention to strain relief is required.