Poor transmit quality with Midland MXT400 Micromobile

[tkruppa]

Hello everyone,

I currently have a Midland MXT400 set up in my jeep. I'm using the MXTA26 6DB gain whip antenna. Radio is wired directly to an auxiliary battery, does not see alternator voltage but does however see battery charger voltages. Antenna cable is loosely coiled with no sharp bends.

I am having a very frustrating and consistent issue with others on multiple types of radios not being able to understand my transmissions. My radio is used almost exclusively during convoy driving, so very close proximity. When I switch over to my midland walkie talkie I'm told my voice is then much clearer. Repeating myself has gotten old very fast and I'd like to try to figure this out.

For context, I can hear everyone else crystal clear. The 6DB antenna has been great and at high elevations (bridges, elevated highways, etc.) I can almost always pick up signals that nobody else in my group is hearing. My antenna is mounted near the bottom left corner of my windshield, however there is no difference in quality based on whether the recipient is in front of or behind me so I don't think placement is the issue. Any feedback is appreciated.

[BoxCar]

You need to reduce power and check your modulation. Too high of either or both will cause the problems you describe. I would begin by reducing power to a low or mid level if available. The amount of mic gain also plays a part,

[OffRoaderX]

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.

[KAF6045]

1 hour ago, tkruppa said:

I currently have a Midland MXT400 set up in my jeep. I'm using the MXTA26 6DB gain whip antenna. Radio is wired directly to an auxiliary battery, does not see alternator voltage but does however see battery charger voltages. Antenna cable is loosely coiled with no sharp bends.

How much current does that "battery charger" handle. If it is essentially a "float" charger to keep the battery topped off, the radio drain when transmitting -- especially on high power -- could result in you operating at less than battery-full levels, or get charging fluctuations on the power cable as the charger tries to keep up with the drain (analog VOM or oscilloscope would show noise when transmitting; digital DVM won't respond fast enough to show voltage swings). If this is a situation, you may need to add filter capacitors of enough capacity to level out the radio draw.

1 hour ago, tkruppa said:

I am having a very frustrating and consistent issue with others on multiple types of radios not being able to understand my transmissions. My radio is used almost exclusively during convoy driving, so very close proximity. When I switch over to my midland walkie talkie I'm told my voice is then much clearer. Repeating myself has gotten old very fast and I'd like to try to figure this out.

What power is the mobile set at -- obviously a 5W HT is fine; a 50W mobile at those distances may be causing some overload on receiving units (do you sound better if vehicles are a mile apart vs a few car lengths?). Reduce power to the lowest level that reaches the length of the convoy [an overall FCC requirement for most services is to run the lowest power that achieves ones results].

1 hour ago, tkruppa said:

For context, I can hear everyone else crystal clear. The 6DB antenna has been great and at high elevations (bridges, elevated highways, etc.) I can almost always pick up signals that nobody else in my group is hearing. My antenna is mounted near the bottom left corner of my windshield, however there is no difference in quality based on whether the recipient is in front of or behind me so I don't think placement is the issue. Any feedback is appreciated.

As others have stated. Check bandwidth (though NFM heard on FM is usually going to be "quieter", less modulation; I'd expect clipping if FM received on NFM as the signal exceeds the input filters). Try holding the microphone differently -- further away from your mouth, and possibly a bit to the side of your mouth (which should reduce any "plosives" -- the air burst from P, B, V, for example -- it's why studio recording artists often have a mesh screen in front of the microphone). The only microphones I know of that require really close positioning are noise-cancelling types that were a rage for CBs in the 70s-80s. Some of those had rubber "mustache" bars to set position -- since they work by taking environment noise on both sides of the diaphragm [hence cancelling the impulse] and need the voice to come only from the front.

Maybe set up an HT with a recorder -- at a fixed location -- while you make (and announce as such for FCC purposes) test transmissions with both the mobile and another HT from varying locations. Then review the recording (turn off any AGC on the recorder before making the test, set the recording level by hand first -- you don't want it leveling out signal variations). A coupling cable from HT earphone jack to recorder ext-in would be useful; one: no feedback when making test transmissions from the other HT while setting record level.

 

[tkruppa]

Thanks for the in depth responses all.

As far as turning down the power, we usually stay on channel 6 which by default reduces the radio output to 5 watts, so nothing I can do there unfortunately. I haven't noticed any difference based on proximity to convoy but I will look out for that now.

Quote

do you sound better if vehicles are a mile apart vs a few car lengths?

I'm not sure I've ever been able to to talk to anyone near a mile away before, but we do live in NY with lots of hills and obstructions, and I'm the only one in the group with a high power radio.

Re changing from narrow band to wide band, I can give this a try, I do have the programming cable if I can find it. Truthfully my radio knowledge is not in depth enough to know how that effects anything but I'll give it a try.

Regarding the battery charger, its a redarc BCDC 1250D, does both float and high amp charging, and the aux battery is a new optima yellow top, and majority of the time the only thing running on it is this radio, so I'm hoping that will rule out the power draw theory.

Re changing where I hold the mic, a very quick test should yield some results so I'll try this as well!

[KAF6045]

2 hours ago, tkruppa said:

Thanks for the in depth responses all.

As far as turning down the power, we usually stay on channel 6 which by default reduces the radio output to 5 watts, so nothing I can do there unfortunately. I haven't noticed any difference based on proximity to convoy but I will look out for that now.

Well, that does put you in the same range as an HT on high power, so...

2 hours ago, tkruppa said:

I'm not sure I've ever been able to to talk to anyone near a mile away before, but we do live in NY with lots of hills and obstructions, and I'm the only one in the group with a high power radio.

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.

2 hours ago, tkruppa said:

Re changing from narrow band to wide band, I can give this a try, I do have the programming cable if I can find it. Truthfully my radio knowledge is not in depth enough to know how that effects anything but I'll give it a try.

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.

2 hours ago, tkruppa said:

Regarding the battery charger, its a redarc BCDC 1250D, does both float and high amp charging, and the aux battery is a new optima yellow top, and majority of the time the only thing running on it is this radio, so I'm hoping that will rule out the power draw theory.

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.

[H8SPVMT]

My MXT400 mounted in the Jeep (JK) is mounted on the dash with a little cable coiled under the driver seat with the same antenna you have mounted (Magnetic) on the driver side just forward of the cowling.  It is wired directly to the vehicle battery also.  It works well in a caravan and long range (15 miles) on a good day, simplex.

I think getting used to the microphone and holding it away from the mouth about 4-6" was my problem too.  There is a setting on the unit to shut down/off after so many minutes of non use as well.  That way it won't stay on but so long without use.  Might want to try that if you think the issue is current.

My issue is the narrow band.  I have to really crank up the volume to hear the wideband side.  I don't remember for sure but I do think we can swap between the narrow and wide bands with the use of a computer program and a special cable.  I haven't tried it yet myself.