you don't say where you are. therefore, we don't know which rules apply. the following info is for people under the benevolent jurisdiction of the FCC.
RF modules for Arduinos are mostly Part 15 devices. Part 15 is an unlicensed low power radio service. Part 15 devices are limited to a low EIRP ( Effective Isotropic Radiated Power ) output. EIRP is a combination of transmit power and antenna gain
bottom line: Part 15 output power + antenna gain is limited. By contrast, in amateur radio, transmitter output is limited to a very generous level, and you can effectively increase that with a high gain antenna. you can't legally do this with Part 15 devices.
you may have noticed that in your lifetime, the number of people busted for exceeding Part 15 power limits is exactly equal to the number of people who got stomach cramps and drowned from swimming 59 minutes after eating. I only mention this because for some people in some work environments strict adherence to the letter of the law is necessary.
Part 15 is a legal unlicensed use of a licensed band. 433 mhz and 915 mhz are shared bands. The government gets first priority, licensed amateur radio operators get second priority, and Part 15 gets what is left. you may find yourself clobbered by RADAR or the ham up the street bouncing signals off the moon with 1500 watts and a vast gain array. as a user of an unlicensed band, interference from a licensed operator means tough luck, Chuck. the good thing is there are thousands of designs for DIY antennas designed by hams on the internet.
for continuous transfer of data I would use HC-12 modules. This is my current project, I have no results to report yet. The HC-12 sends serial data. therefore, I must have serial data to send. the sensors do not generate serial data, so the plan is ( far end ) SPI & I2C sensors >NANO > serial port > HC-12 > ( the sky ) HC-12 > serial port > NANO > one pin to trigger the MEGA and an I2C connection for the sensor data. I plan on having far too many sensors for direct serial connections to the MEGA. You may need something like this, send values through the serial port that get converted to PWM at the downstream end.
When the distance gets large, which definitely includes 1.5 km. you have to lower the bit rate to get the distance. you have to use specialized modulation schemes, and you have a limit on packet size.
research HC-12 radios, and LoRa. LoRa claims the range you are looking for, but in bursts of data at sporadic intervals
you are not going to get the range you want, high data rate, and compact antennas. the laws of man and the laws of physics won't support that.
a thing to know about antennas: a higher gain antenna on one end bestows it's blessings on both ends of the link. if you replace a standard issue rubber duck antenna with 2.14 dbi gain ( you are on your own if you want to know more about db & dbi ). if you replace your 2.14 dbi dipole with an 18 dbi yagi, both ends of the link see a 16 dbi improvement in signal strength
higher freguency > shorter antennas > higher gain in a smaller package
higher frequency > shorter range > offset by the benefits of better antennas
high gain antennas get their gain by concentrating power out in one direction, and receiving from a smaller aperture in the sky
if you need a high gain antenna on both ends, and have no practical way to aim and support a directional antenna, the highest gain omnidirectional antenna is a collinear. the practical limit for a collinear is 15 dbi gain. if you try for more, losses in the longer antenna counter the gain of the additional elements. those eBay vendors peddling 18 dbi collinears know new laws of physics they are not sharing.
at 433 mhz, 9 dbi is a practical limit for a collinear. more gain = longer = support structure required.