I've got three 433MHz receivers (all the same type) and I would like to increase the range on them (by which I mean the physical distance that the receiver can detect the signal from the transmitter). Changing the antenna on the transmitter (the little key fob/dongle thing) isn't an option for practicality reasons. One of these receivers is unfortunately housed in a sheet metal box (which can't be readily changed), so I would like to get the antenna outside the box, but I'm unsure as to how running it out of the enclosure interacts with the total length of wire used in the antenna itself, (i.e. the length exposed outside the enclosure vs total length to get back to where it is soldered to the receiver).
The receivers currently have short coiled wire antenna (they look like a spring, but are actually coiled wire). Since the receivers are stationary, I can take some liberty with a larger antenna. Googling suggests that a balanced dipole antenna with a balun is the way to go, with 178mm straight wire on each 'leg' (not sure of the right term) on the antenna. The problem is, I don't know the first thing about RF, so I haven't been able to discern what is a good guide for how to construct such an antenna.
Could anyone point me in the direction of a practical guide in this area that is technically sound?
EDIT - Hardware Details: The transmitter and receivers are Elsema PentaCode series devices: Transmitter: PCK43302 Receivers: PCR43301RE & PCR43302240R
Basic details are on page 3 of this document:
They're frequency hopping spread spectrum devices, with a freq range of 433.100 to 434.700MHz.
Target range is ~200m, not quite line of site (elevation diff of about 10m over 150m due to small hill that rolls off).
EDIT - Directions to Transmitters: So, the receiver's are stationary, but the transmitters are mobile. In the case of the gates, they're on access roads, meaning the transmitters can be sending a signal from either side of the gate (so, 180 degrees from each other). This is part of the problem, as a high gain antenna in one direction will hurt the radiation pattern 'out the back' (i.e. from the other direction), which isn't great given that the goal is to open it from both sides. Energy radiated 'to the sides' is wasted though. Is there an antenna design that can help achieve this?