they are both coax connectors. small one is SMA female SMA male to plug into it and the larger one is PL-259, sometimes called a "UHF" connector, which mates with SO-239.
The adapter you need is an "SO-239 to SMA Male". A pigtail like this, for example:
https://www.amazon.com/DHT-Electronics-Handheld-Antenna-Connectors/dp/B00COW5E3A/
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My pigtail cable came with my antenna. So, it made it easy. It looked something like this:
https://www.amazon.com/DHT-Electronics-Handheld-Antenna-Connectors/dp/B00COW5E3A/ref=sr_1_6?dchild=1&keywords=pigtail+cable+sma&qid=1629741891&s=electronics&sr=1-6
NOTE: THIS IS NOT WHAT I PURCHASED. The link is just for an example. You would have to figure out what ends you need to attach to the antenna that you will use.
Another option is a flat cable...but again....you'll have to match the ends to your scenario
https://www.amazon.com/Ultra-Ghost-Frequency-Window-Door%EF%BC%8812-6/dp/B07QFWNG2Z/ref=sr\_1\_12\_sspa?dchild=1&keywords=flat+sma+cable+window+pass+through&qid=1629742043&s=electronics&sr=1-12-spons&psc=1&spLa=ZW5jcnlwdGVk...
The RT83 uses an SMA connector for the antenna, I don't know if the connector on the radio is male or female.
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If the conector on the radio is female, then get these adapters:
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If connector on the radio is male, then get these adapters:
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cheap dual band antenna:
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So $30 to squeeze a ton more performance from your radio.
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I have that antenna and according to the nanovna it's decent, IIRC. It's been a minute since I ran an SWR check on it, but it was pretty close to the tram 1185.