I’ve got the discontinued Harxon GPS 500 antenna (2) and paid about $275/each. They seem to have better, cleaner data. There’s nothing wrong with the helical s and they have good horizontal accuracy but not vertical in my opinion. If you’re not to worried about vertical, they’re light and good for open areas. Another problem is submitting to OPUS, you’ll have to use a “0” antenna height and then deal with the phase centers and ARP.
As for buying a cheap antenna, you get what you pay for. Regardless of the advertised info. For the M2’s, I’m satisfied with the static results with the Harxons. If anyone is considering replacing/using the helical, I would stick with an antenna cost of no more than the cost of the M2 and also make sure it’s in the calibrated antenna database for possible use in OPUS. Harxon has great products, but in my opinion the helical is for UAV use. My helicals are for backup only.
Using the Harxons for a static baseline for boundary work, I’m confident in the elevations after PP with the rover. I don’t really need vertical info for boundary work, but it’s good to have as we have been 3D surveying for years. It’s just as easy to determine elevations as it is not to. Also it’s one of many indicators of a bad fix either RTK or PP. It’s amazing with the LIDAR data in our state, we’ve seen accuracies of +/- 5cm just checking road intersections and other landmarks that haven’t been disturbed since the LIDAR data was flown.
Antennas are just a part of the process/instruments that help determine accuracy using GNSS signals.