A belated response but here are my .02;
I've not had much experience with HF verticals but have used a few slopers and a 5btv trap vertical for a few rx sessions at a friends, these antennae being located in Omaha in the south side not too far from downtown, so industrial level noise was expected. That said, the 5btv and slopers did wonderfully on the HFBC bands and more or less noise free, I was impressed and always wanted a vertical for myself. Slopers are kinda a mix between vertical and horizontal polarisation, a true sloper should be around 45 degrees. The one I eventually got to use at home was much more shallow of a slope, going from a window on the second floor to the mailbox at the end of the drive 70ft away. The mailmen asked about the wire, they thought it was for detecting earthquakes or something lol. It did detect t storms from a few hundred miles away however, dust and snow made a racket in it too.
It worked great on all HF bands, especially with a tuner, wasn't a HAM back then so never tx with it.
As to why one wants polarity diversity, even for rx, is along this wise;
I had that sloper and a cliffdweller antenna to select from feeding the glorious Icom IC R-70. I wondered if there would be any diff in performance betwixt them as to polarity response. The cliffdweller can be placed in any convenient position, it was stashed atop the upper window sill. The cliffdweller was a piece of pvc pipe about 6 ft long wrapped with the 100ft rotator multiconductor control cable you could buy at any radio shack. Wrapped it (all 100 feet of cable meaning 300 ft of wire) around the pipe according to instructions and cut it here, splice it there, and a single wire lead to the feedpoint. I was certain I had fooched it in the construction so expected utter failure.
The stupid thing worked.
So you know, the sloper would be more or less vertically polarised, the dweller was horizontal.
In the heady days of the Icom IC-R70, the fabulous Infotec M600 data mode decoder was coexistent, no one had software decoders in those days save for the most elite of snobs and spooks, the rest of us had Infotecs - if we were lucky. With such a setup I shortly found an interesting fsk news feed in EE that came from Rabat Morocco in the high 19/22mhz range on a daily basis, along with myriad other data sigs, from vft to fsk to fax to tdm to piccolo to, well, you guys have no idea how many data modes have come and gone on HF. I oft compared the rx betwixt the sloper and dweller, during the portion of the propagation between fade in and fade out - there was little to no diff in reception pretty much till you got to the end of propagation, I think there were times when the dweller would rx the sig where the sloper heard no trace of it, and sometimes it was like that for an hour or so till fade out, amazingly.
So now back to more recent history, I have the GAP and a horizontal loop about 140ft in dia fed with coax, up around 25ft to compare with the GAP.
The GAP is some 60ft away from the shack with buried coax, the loop feed is some 40 ft long. Anyway what I find is you can normally hear just about every signal on both. That said, I find that while you can hear the guy a few hundred miles away on 40m he can't hear you on the GAP but he can on the loop. The GAP is squirting most of the rf at a much lower angle of radiation than the loop, the loop squirts the rf straight up at lower HF freqs and when the idunnosphere is cooperating the rf rains down all around the sending position for several hundred miles in a mode we like to call NVIS.
en.wikipedia.org
That said, there are surprising differences to that maxim of using the loop for closer-in NVIS work, everything in HF depends upon that most fickle of females, the idunnosphere. If idunnosphere says you need the GAP to talk to this guy, well, you're going to need the GAP to do it regardless if he's only a state away. That said, most of the time my dx contacts cannot hear me on the loop but can with the GAP, the more local contacts hear me better with the loop. As to the dx and overall rf efficacy of the GAP, I run a Xiegu G90 at the moment that produces a max of 20 watts on the HF bands, and I have talked around the world with this G90/GAP setup. The loop excells at vlf, however, the GAP doesn't hear WWVB, the loop does very well.
tldr; you
are missing out of you don't have polarity diversity and a GAP is a decent HF antenna for DX.
