The A99 is an end fed half wave antenna. The only real problem is they are not very well decoupled from the coax and RF can be present on the coax, mast, etc. The length of coax and mast can change the performance some on these antennas. You can fix RF on the coax with a good ferrite based RF choke near the antenna in the feedline. You can't fix RF on the mast unless you insulate the antenna from the mast and use a good RF choke in the feedline.
Antennas in general will have the lowest angle of radiation at the horizon when the high RF current portion of the antenna is at 1/2 wavelengths above ground with the higher 1/2 wavelength points being better like 1 wavelength or 4 wavelengths, etc. With all that said, the middle of an Antron 99 is the high RF current point so a goal would be to get the middle at 18ft or 36ft or 54ft would work better than in between heights. With an A99 at 20ft high at the base it doesn't put the high RF current point at the most desirable place, but it will work ok and make contacts.