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Even setting up a ham radio for use with GMRS repeaters can be challenging for us simpletons. But a GMRS radio like the KG1000G or XS20G is very easy to setup. You dont even need the software, you can do it directly on the radio (but, it is easier with the software). If simplicity is your goal for GMRS, then do not get a CDM1250, XTL5000 or a ham radio - get a GMRS radio.

Chirp is easy to use and very straightforward. But look at this, it may help: https://chirp.danplanet.com/projects/chirp/wiki/MemoryEditorColumns

Because this it the channel nonsense. Channel is only a programming slot in your radio. Drop this "channel" affection and start thinking frequencies. And if you insist on numbering channels here are two things: 1) there are 22 numbered frequencies in FRS/GMRS, no channel #23 2) here is the channel numbering table: https://en.wikipedia.org/wiki/General_Mobile_Radio_Service

On all Post 2017 GMRS & FRS radios GMRS channels 1-22 and FRS channels 1-22 are the same frequencies. No idea why your software does not know this.

The meter is what some of us refer to as 'close enough' for what we do. Keep in mind, the meter is user adjustable. It likely is calibrated fairly close from the factory. You want to keep your SWR under 3:1 for best performance. I wouldn't touch it.

Results are inconclusive without the proper testing equipment, however signal is being radiated from somewhere, so it either can't reach the PA, or PA is shot. The supply voltages on PA are correct, you confirmed it. I'd try to test AC voltage when keying mic at the Q7, Q202, Q204, Q205. Oscilloscope would be very handy here.

How theoretical did you want. As mentioned, the ARRL Antenna Book is probably a first resource. You just have to rescale things for 465MHz as most UHF example antennas will be sized for 440MHz. The overall concepts of gain, etc. would be covered. Real theory? Antenna Physics: An Introduction gets rather crunchy. For designing/optimizing: Antenna Modeling for Beginners (covers usage of EZNEC), An Introduction to Antenna Modelling (focuses on MMANA-GAL). And, if you end up creating some serious gain antennas RF Exposure and You covers the safety aspects, calculating safe distances from the antenna to "controlled environment" (those who know there is a transmitter in operation and can take actions to minimize exposure) and "uncontrolled environment" (neighbors, people on the street, etc. who don't know about and can not control their exposure to your transmitter). While all the above are available from the ARRL, the MMANA-GAL book is actually from RSGB. For software: the creator of EZNEC has retired, and made EZNEC PRO v7 a free download but offers no support https://www.eznec.com/ . The RSGB book DVD provides a version of MMANA-GAL the current version is http://gal-ana.de/basicmm/en/ (the Pro version is 140Euro for personal use license). 4NEC2 https://www.qsl.net/4nec2/ is a somewhat cruder package -- it explicitly uses external NEC-2 executables built for different memory usages (number of wires/segments). I believe it basically started as an Excel macro package for editing NEC-2 command cards and grew into a stand-alone GUI for NEC-2 (if you are a glutton for punishment, you can download the NEC-2 manual and hand edit the Hollerith card type input deck it uses -- EZNEC and 4NEC2 both use the NEC-2 engine for number crunching [unless you fork over a fortune for a license to NEC-4 from Lawrence Livermore National Labs -- hmm, there is now a NEC-5, wonder if it would be compatible with EZNEC]). Given that EZNEC Pro is now free, I'd probably start with it as the books that include datasets for models probably use EZNEC input files.

This is probably a good idea to start out. The XTS/XTL are excellent radios, but the are end of life, and you can really confuse yourself finding the exact model you need with the correct flashcode (Motorola code for features enabled). The nice thing about the XTS/XTL, you can set an entire zone for the GMRS frequencies, and enable MPL (or user selectable tone) which will allow you to change it on the fly, but you have to program each possible PL for RX & TX, and name it appropriately. It does cap out at 60 pairs.

Probably the best way is to pick up something like the ARRL Antenna book and then watch some videos that teach the use of EZNec or MMANA, which are two antenna modeling programs. Free versions of each are available and they’re pretty easy to use. Look for a channel on YouTube by DX Commander that has some tutorials on MMANA or David Casler on EZNec. I would try going to a ham swap meet and getting a used copy of the book; it’ll cost much less than ordering from ARRL and the difference will be minimal. Antenna theory hasn’t changed much lately, except for the gigahertz regime. There haven’t been any landmark discoveries that have changed people’s minds for awhile. I receive a big book quarterly of the Transactions of the Antennas and Propagation Society of the IEEE and I almost freeze at the titles of the articles. I think I’d have to go back to school just to understand the titles, much less the actual articles, but every article is about gigahertz or terahertz uses. As far as I can tell the UHF stuff is relegated to the same dusty shelf as VHF, and HF. There’s probably something still being done for ELF also, but I haven’t seen anything.

Couldn't you just leave the CW ID blank when configuring the controller?

Well, that does put you in the same range as an HT on high power, so... Any reasonable straight freeways... "Mile" was just a suggestion -- mostly I was thinking of increased range than normal "convoy", IF you were running higher powers. Narrow band for GMRS is 12.5kHz using an 11kHz modulation width, deviation [how much the FM signal swings around the nominal frequency] is even less. Normal GMRS is a 20kHz band (some rigs don't support 20kHz and are actually 25kHz, commonly using a 16kHz modulation. The deviation tends to track "volume" while the rate of deviation indicates the audio frequency (high pitches "vibrate" +/- the stated radio frequency faster than lower pitches). So... 11k modulation doesn't swing as far as 16k modulation -- on a radio configured for GMRS (20kHz), the smaller swing translates to quieter audio. Going the other direction, the wider swing may be clipped, causing distortion in the audio, and will sound louder/compressed [less volume variation relating to actual voice changes]. For true GMRS, only the channels 8-14 (in the 2017 combined numbering scheme) are NFM. ALL true FRS radios are NFM on all channels. You may still want to evaluate line noise at the battery. Do you sound different if you disconnect the charger and rely ONLY on the isolated battery? It will be lower voltage, so your output power might be lower, but if you were running on a 5W channel, probably not much change.

Is yours one of the MXT400 units that can switch between wideband and narrowband? If so, make sure its on wideband.. If not, ask one of your friends to switch their radio (temporarily) to narrowband and ask them if it sounds better. if it sounds ok when you're both on narrowband, then, thats just the way it is.. If its not a narrowband issue, make sure you're not eating the mic.. IIRC, the MXT400 does not have an adjustable Mic gain, so your arm is the only way to adjust it. If neither of those help, call Midland and tell them you send you a new radio that works.