The H-trap needs to be installed at close vicinity to horses. Horse flies see the infrared signature coming from horses from a big distance. If a choice has to be made by them, they will prefer to travel to a large group of potential victims (the horses), rather than going to one isolated victim (the H-trap)
Placement of the H-trap nearby the drinking area of your horses is often a good location. If this location is not an option – or simply not applicable  – then use the general rules of siting, as described here below.