Performance difference between two MXT275’s

[Guest Nermol]

My son and I bought each other a Midland MXT275 this Christmas and just finished installing them in our trucks yesterday. We were doing some range tests and found at the edge of our reception (4.5 miles) he could not hear me, but I could hear him. 
We do have the same +3db antenna but his is mounted on a hood mount, mine is on a roof mount. 
Any ideas why the performance difference? It seems my transmitter power is not the same.

[PACNWComms]

Quality Control and Manufacturing Tolerance. I know of a corporate site that bought many different Midland products to support work and office operations in two buildings that take up about one square city block. They noticed the same thing, and boxed up the radios that did not work as well as others. Many of these were Midland T7x series handhelds, and we had to break the fixed antenna off to test them with Aeroflex and Hewlett Packard/Agilent Technologies test equipment. Their performance was all over the place. Receive sensitivity and selectivity was anywhere from about 0.27uV to 0.50 ish uV (most being toward the 0.50 uV end of the scale).....for UHF 0.35uV is considered "good" by many. 

In your case, there could be other issues as well, being mobile radios, as the antenna connectors could be better/worse, bends in the coax and the center conductor to sheathing/ground, quality of coax, ground plane around antenna. Lots of factors. Even power from the vehicles could impact range, power output, and radio function. You can take piles of Midland MXT275 radios, place them into  the same make, model, and year of vehicle, and still get some variance based on many factors. The more of those factors that are the same, the easier it is to narrow down.

A co-worker of mine, installs Motorola XTL1500 mobiles into Chevy Suburbans, same make, model year, and places the antennas int he same location. He still has some variation in function, but much less deviation due to manufacturing tolerances and quality control. A thousand dollar radio is usually built better than one that costs only a few hundred. Couple that with better coax, a quality antenna, and install it with attention to detail, and it will outperform a cheaper model.

Likewise, two different installs of the same equipment can have variations based upon that installation. Are the coax connectors soldered or crimped, is the ground braid even around the connectors, are there any tight bends or twists, is there a coil of extra antenna cable at either or both ends (radio and antenna if applicable), what is the output power to the radio (12v -13.8 VDC), was the radio made at the end of a shift, where was it made, etc.

[Guest Re:]

Thanks for the reply. I will order a SWR meter and see if I can rule out some of the items Inhave control over on the list.

[AdmiralCochrane]

Grounding makes a difference too.  Possiblity of better grounding AND possiblity of antenna location on the vehicle promoting propagation better in some directions and worse in others, depending on the placement of the antenna and orientation of the ground plane.  In general, you will have gain across the longest path over the ground plane; possible signal degrading along the shorter paths over the ground plane. 

An antenna connected to a SWR meter functions as a field strength meter to survey and determine this. 

[axorlov]

What PACNWComms said: sample to sample variation, and also what Admiral said: installation difference. It's pointed out in the original post: roof mount vs hood mount. Car with hood mount can hear but can't reach out. That would hint to better antenna efficiency on the car with roof mount (duh, no surprise).

I would also add that location of the test matters. The car with roof mount, at the time of testing could've been under heavy interference. Before you invest in test equipment (crappy SW-33 or such) and time to learn how to use it, and then buy it again now the proper test equipment (major $$$), I'd suggest to switch locations and run the test one (five) more times. And to decide if performance difference is really that big and worth investigating.

[mbrun]

Actual output power of one radio vs the other can be a factor, which can be caused by a combination of factors include the operating voltage during transmit and manufacturing tolerances.

I have a tendency to discount the antenna location factors. The reason be this being that the polar radiation pattern of an antenna is the same as the polar receive pattern for any given frequency. If the antenna emits a strong signal in some given direction, it will pick up weak signals well in that same direction. So performance is always complementary. This being the case, I would tend to look at other factors, including actual difference in radio performance as has already been said.


Michael
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