Range check the plane with the tx antenna collapsed into the tx case. Engine off walk away from the plane and determine how far it is before the surfaces start jumping around from loss of signal. Of course this is with a radio that does not have a fail safe set to keep the surfaces at neutral where you can't tell what's happening. The have someone hold the plane and do the same test with the engine running. Check the difference in distance. 10-15% is the max loss that's acceptable. A ground range chek that consistently is good at 200' is a good range check, especially if it's the same engine off and engine running

If you're trying to do the range checks with the tx antenna extended and/or with the plane flying you need to better understand what is meant by the term range check. It is never, ever done in the air, and always with an antenna in it's least radiating state.

Any auto parts store has resistor plugs.