For years now I have been using an indoor antenna for AM transmitting. This antenna is mounted horizontally on the ceiling in my basement with the tx. grounded to a cold water pipe near the base of the antenna. The antenna is resonated with a loading coil. Being indoors and horizontal it is suspected that this is a less than ideal situation to achieve range.
So to test this I did an outdoor antenna test. I laid down 11 10 foot radials and connected the ground of my "high efficiency" transmitter to the center point of the radials. The transmitter was positioned on the ground next to the antenna. As seen in this picture http://s6.postimage.org/s1vtl2nhd/DSCN0269.jpg the loading coil and antenna are supported by a high tech insulator (an empty wine bottle). The loading coil is the same one I use indoors and has a trim coil for fine adjustment seen at the bottom.
The three meter high antenna is supported with Kynar rope as seen here: http://s6.postimage.org/6e6vamn35/DSCN0268.jpg My wife ventured out and when she saw this she said "Oh, you're playing with an antenna.". Nothing surprises her anymore.
I used my current transformer arrangement to tune the antenna to resonance. It appears that being outdoors above the radials adds capacitive reactance compared to the indoor position since it required adding 8 turns to the coil to get resonance. It is almost amazing how sensitive this antenna system is to artifacts nearby. I found that by moving from about three feet away to 8 feet away shifts the phase angle by 45 degrees. It is really a sensitive adjustment.
Having achieved resonance it was time to do a comparative range check based on past experience with the indoor antenna. First with my portable I walked down the street and there was a very marked improvement in the signal strength. I could hear the signal at a distance where it was lost previously.
Now for a drive around test. The results are surprising. From my home to the exit of our neighborhood the signal was about the same on my car radio but making the turn onto a state highway the signal disappeared and there was very strong interference from the high voltage power lines on the poles (within our neighborhood the power lines are underground). With the indoor antenna I could travel down this road for 2 miles and still hear my signal. This wasn't the case now. Perhaps having my antenna indoors coupled a good bit of the RF signal to the power lines and the pole mounted lines were radiating this signal ala carrier current.
I continued and turned onto a street with underground lines and the noise abated and I could hear my signal at a distance where it could not be heard previously away from the overhead power lines. This was about 1 mile away. My criteria for this test is if I could identify the audio as being from my station then I was in range, if not I was out of range. Because it was rush hour I didn't drive further away to find this limit but at this distance of about a mile the signal had noise but it was listenable. The signal stayed listenable as I approached my home being heard as I entered my neighborhood by the back door route where previously I couldn't hear the signal.
Then I drove the other direction along the state route and again the line noise blocked the signal. About a mile away in this opposite direction I turned into a neighborhood with underground power lines and my signal was again listenable.
It seems that the range is much improved as I heard a listenable signal at distances and in areas where it was not heard with the indoor antenna but the antenna effect from the overhead power lines experienced with my indoor antenna is gone.
The conclusion from this is that an AM transmitter delivering 86 mW into a base loaded antenna over ground mounted radials can be listenable on a car radio out to at least 1 mile.
I haven't decided whether to go back to the indoor antenna or make the outdoor antenna permanent. This indoor antenna needs not be weather, people, deer, falling limbs, and other mayhem resistant but it is not the optimum for getting a signal out.