Please be aware that most FM transmitters listed on eBay come as either 1-watt or 5-watt effective radiation (the one listed has a switch to choose the between 1 watt and 5 watts).
Even 1 watt exceeds Federal Communication Commission requirements for Part 15 unlicensed usage by
one hundred million times (see You are not allowed to view links.
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Login -- 0.01 microwatts divided by 1 watt is 100,000,000).
If your transmitter's signal exceeds 200 feet you are breaking the law. It doesn't make any difference about "being out in the country" or that your driveway is 100 yards from the road; 200 feet.
There are transmitters used by many in the hobby that produce a legal signal; at this time of year, few dealers have product, though it appears that You are not allowed to view links.
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If you can only find a 1-watt transmitter, you should consider using the most minimal antenna and/or add an electronic attenuator; you would want a 50-ohm, 20db attenuator with BNC connectors (if the photo of the device listed is correct).
HTH.
\dmc