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We have a gate opener with keyfob-type RF remote transmitters that operate at 318MHZ. The range from inside the car is about 75 feet -- less than we'd like due to the time taken by the gate operators to function.

The antenna is approximately 9" long, and mounted vertically on top of a metal fence post; the fence is about 1" metal mesh (similar to poultry netting) with a vinyl coating; I don't believe the fence is electrically connected to the post (at this time) due to the vinyl coating and paint on the post.

The antenna is connected via coax to the receiver; I believe the coax is grounded to the fence post via its mounting bracket.

From an online antenna length calculator, a quarter wave antenna for this frequency should be 8.83", so I believe the antenna itself is OK; my question is whether there is anything that can be improved about the way the antenna is mounted that would improve the range? The gate is at the mid-point of the driveway, so a unidirectional solution doesn't seem appropriate.

Edit: here's an rough diagram of the area; it's about 120' line of sight to the house and maybe 100' to the turn in the driveway toward the street. The area is wooded, and there are some low-hanging branches, but generally a clear line of sight from the house to the antenna. driveway diagram

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  • \$\begingroup\$ Have you spoken to the supplier to see what they can offer? \$\endgroup\$ – Andy aka Jul 5 '14 at 0:03
  • \$\begingroup\$ There is an available amplifier that goes inline in the coax (about $115) - I figured I'd check for basic installation issues before going there. \$\endgroup\$ – TomG Jul 5 '14 at 0:22
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If the antenna is a bare 1/4 wave whip now, you could provide a ground plane for it, and if the run of coax to the receiver is substantial, you could replace it with less lossy cable.

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  • \$\begingroup\$ The coax run is only about 20', but it could be shortened a bit with better routing. \$\endgroup\$ – TomG Jul 5 '14 at 0:32
  • \$\begingroup\$ Any tip on making a ground plane? Reading wikipedia can I use 4 wires radiating from the base? Should the radius or diameter be 1/4 wave length? Should I ground to the coax shield? \$\endgroup\$ – TomG Jul 5 '14 at 1:07
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    \$\begingroup\$ You could use the wires - "radials" - and they should each be 1/4 wave long, grounded to the shield, and perpendicular to the antenna at its base. Better yet would be a solid sheet-metal plate, like an 18 or 19" diameter pizza tray if you could mount the antenna to it like to a car rooftop. \$\endgroup\$ – EM Fields Jul 5 '14 at 17:44
  • \$\begingroup\$ I'll give radials a try first, trying to keep it somewhat unobtrusive. \$\endgroup\$ – TomG Jul 7 '14 at 3:23

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