I have 2 hf antennas, an OCF that includes the 75 meter portion of 80 meter and a 5BTV vertical with 15 radials for 75 meters, actually 3.989 67 feet ea. I use a FT 710 field model and an Icom 7300. I also have a RigExpert rig match and swr on the OCF shows 1.86:1 and the vertical is almost resonate at 1.06:1. When I use the wire antenna I get infinite swr on the radio (either one) checked with rigmatch and got 1.86:1, radio infinite. Does the OCF need a counter poise? I am testing this using all of the same components for testing. I am at a loss like my radios !
I would do DC continuity checks of your feedline and antenna to make sure nothing is shorted or open first. Also, all your coax connectors are properly seated and torqued down tight.
However, my best guess is you may need a Line Isolator (aka Common Mode choke) at the feed point of your OCF.
You may have common mode RF riding the shield of your coax feed line which can massively distort the overall behavior of your antenna.
You can buy pre-made units, of course, but you might want to conduct a test beforehand.
A simple CMC you can make (google “ugly balun” for details) is to find something cylindrical and nonmetalic about 4 to 6 inches in diameter and wind about 20 to 25 feet of coax on it as a coil form. This forms an inductive choke of the shield that blocks HF RF intrusion onto your feedline. Most hams have spare coax patch cables lying around, so this might be a no or low cost test approach.
In the case of an OCF antenna, you would interpose your CMC between the antenna’s 50 ohm feed point and the regular feedline.
Do your SWR measurements again particularly with the rigs and see if you get an improvement. Success criteria would be to get SWR < 2:1 with your ugly balun instead of near infinite.
If it works out, then you might consider purchasing a purpose built unit for permanent use. Me, I keep my home brew CMC for portable operation using End Fed Half Wave antennas.
73;
Bob KV4PC