Mark,
Good question! There will be ~some~ gain/loss of signal in
various directions. Probably won't be much gain or loss,
just depends on 'other' things (how near the fence, fence made
of wood/metal, and other mysterious things). Wouldn't worry
too much about it, and if it really bothers you, sneak a radial
or two under the fence for a ways. Hide it well and don't get
caught running stuff in your neighbor's yard!
'Doc