Well, of course a Bird wattmeter is only good to plus or minus 5 per cent, thus you can live with a 47 ohm resistor rather than 50 ohm, and certainly a 51 ohm 1 per cent resistor instead of 50 ohm and never see the difference in your measurement in practical terms. We are talking about fractions of dB difference to RF. There would be more loss than that in your feedline to antenna set up. -Stuart K5KVH