I "may" have fixed it by changing the join rule to number of message parts to 1. When I had it set to 2 (part.0 + part.1) I was getting the ch char added.
Code and test injects are here...
[{"id":"ba6c3622.ce6758","type":"tab","label":"Join Test","disabled":false,"info":""},{"id":"bbef0ccb.b97248","type":"link in","z":"ba6c3622.ce6758","name":"DogPark Rotor","links":["bd0cd66e.8ce69","5344990e.62b1f","2428135b.8aab5c"],"x":315,"y":1020,"wires":[["51e9912b.3c1b98"]]},{"id":"51e9912b.3c1b98","type":"string","z":"ba6c3622.ce6758","name":"delRight","methods":[{"name":"delRightMost","params":[{"type":"str","value":"]"}]}],"prop":"payload","propout":"payload","object":"msg","objectout":"msg","x":440,"y":1020,"wires":[["bab22fb0.49e14","6dd5ce82.b55f6"]]},{"id":"bab22fb0.49e14","type":"split","z":"ba6c3622.ce6758","name":"","splt":"[","spltType":"str","arraySplt":"2","arraySpltType":"len","stream":false,"addname":"","x":610,"y":1020,"wires":[["db46f5a4.c3db78"]]},{"id":"db46f5a4.c3db78","type":"switch","z":"ba6c3622.ce6758","name":"route","property":"parts.index","propertyType":"msg","rules":[{"t":"eq","v":"0","vt":"num"},{"t":"eq","v":"1","vt":"str"}],"checkall":"true","repair":false,"outputs":2,"x":390,"y":1180,"wires":[["7a2fd0e6.efde98"],["273db89.d38a5c8"]]},{"id":"7a2fd0e6.efde98","type":"string","z":"ba6c3622.ce6758","name":"getMsgId","methods":[{"name":"getLeftMost","params":[{"type":"str","value":" "}]},{"name":"toInteger","params":[]}],"prop":"payload","propout":"reportMsgId","object":"msg","objectout":"msg","x":560,"y":1140,"wires":[["5572afcb.22b858"]]},{"id":"6dd5ce82.b55f6","type":"debug","z":"ba6c3622.ce6758","name":"","active":false,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","statusVal":"","statusType":"auto","x":1190,"y":1020,"wires":[]},{"id":"273db89.d38a5c8","type":"string","z":"ba6c3622.ce6758","name":"getPayload","methods":[{"name":"getLeftMost","params":[{"type":"str","value":":"}]},{"name":"trimRight","params":[]}],"prop":"payload","propout":"type","object":"msg","objectout":"msg","x":570,"y":1220,"wires":[["1704ee2c.fac10a"]]},{"id":"5572afcb.22b858","type":"change","z":"ba6c3622.ce6758","name":"","rules":[{"t":"delete","p":"payload","pt":"msg"}],"action":"","property":"","from":"","to":"","reg":false,"x":770,"y":1140,"wires":[["1010133b.4ffd05"]]},{"id":"1010133b.4ffd05","type":"join","z":"ba6c3622.ce6758","name":"","mode":"custom","build":"string","property":"payload","propertyType":"msg","key":"topic","joiner":",","joinerType":"str","accumulate":false,"timeout":"","count":"1","reduceRight":false,"reduceExp":"","reduceInit":"","reduceInitType":"","reduceFixup":"","x":1030,"y":1120,"wires":[["e4a9af0f.899208"]]},{"id":"1704ee2c.fac10a","type":"string","z":"ba6c3622.ce6758","name":"delLeft","methods":[{"name":"delLeftMost","params":[{"type":"str","value":":"}]}],"prop":"payload","propout":"payload","object":"msg","objectout":"msg","x":810,"y":1220,"wires":[["1010133b.4ffd05"]]},{"id":"e4a9af0f.899208","type":"debug","z":"ba6c3622.ce6758","name":"Manual Join","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","statusVal":"","statusType":"auto","x":1210,"y":1120,"wires":[]},{"id":"a671d340.83dfa","type":"inject","z":"ba6c3622.ce6758","name":"Inject Sat","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"12345678 Test [Sat Radio Report:Down Mhz:435.18000, Down Mode:FM, Up MHz:145.98000, Up Mode:FM]","payloadType":"str","x":360,"y":900,"wires":[["2428135b.8aab5c"]]},{"id":"495fa787.65b808","type":"inject","z":"ba6c3622.ce6758","name":"Inject Radio","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"00000669 Test [Radio Report:RxMHz:7.07400, TxMHz:7.07400, Band:40M, Mode:CW, Power:8]","payloadType":"str","x":370,"y":940,"wires":[["2428135b.8aab5c"]]},{"id":"2428135b.8aab5c","type":"link out","z":"ba6c3622.ce6758","name":"Dogpark UDP Broadcast","links":["283150a.473e93","4a8cc533.a6b02c","68170ff2.47c85","7cd9761d.6c6268","f3e51bbd.479fc","dd7d71e1.c9fca8","abd4ded5.24942","2653cb90.d47ae4","dba74049.648eb8","962cefa1.d43678","bd8f1d12.18b75","3f59b914.1e1bfe","faf74149.41dbd8","88df9653.d8ae68","1df250b8.903d4f","f6bca23b.16c048","83e10ce5.e8e13","f6c74fde.f2b4e","bbef0ccb.b97248"],"x":515,"y":920,"wires":[]},{"id":"ced30019.9d5ea8","type":"comment","z":"ba6c3622.ce6758","name":"Manual Join Ends Up with a \"]\" on the end... but ok since I can remove it.","info":"","x":1250,"y":1080,"wires":[]}]