It's the weirdest thing I think I've ever encountered with respect to antennas, definitely. As for testing it "properly" I don't even own a multimeter so there goes that idea, someday I'll actually remember to buy one - even a cheap $10-20 model - just to have around.
I have no idea how the base is assembled on this thing, I've attempted to disassemble it by twisting (assuming it's all screwed together in some fashion) and has zero success, nor is there a way to screw off the BNC end either. Most BNC antennas will have an area (when you pull the BNC outer ring down as far as it will go) where you can insert a very thin crescent wrench, like credit card thickness, and then unscrew the connector assembly but this one doesn't so there goes that idea.
Thing is this: if you have any type of receiver, any at all of any kind, and you touch the antenna connector with any
kind of metallic object, you'll typically get some kind of electrical activity - on an SDR setup that would translate into activity on the spectrum, be it a change in the noise floor (the most common thing because you've just added something to the circuit) or a momentary "spike" of signal. The sheer act of making an electrical connection of any kind translates into something you can visually see on the spectrum.
Now, with my SDR setup (typical RTL stick using an MCX to BNC pigtail), if I touch the inner connector on the BNC end with anything
that's metallic at all, even something as innocuous as a thumbtack, I'll instantly see some activity on the spectrum, regardless of what I use. I've got several antennas around here (RH-C77A, RS 800, a couple of plain old telescopic whips, even random lengths of coat hangers, razor blades, screwdrivers, paperclips, etc) and whenever I make physical contact with that center conductor on the BNC end it always shows something on the spectrum without fail.
However, when I attach the AL-500H I get nothing, and I mean absolutely nothing - it's like it's not even being connected. I figured that perhaps, just maybe perhaps the BNC pin on the AL-500H is thinner than normal for some reason and it was being inserted into the BNC end of the pigtail so precisely that it wasn't even making contact with the BNC inner conductor at all (we're talking a few microns of space but no physical connection) so I took a tiny piece of aluminum foil and did a single wrap around the center pin on the AL-500H, still no apparent electrical connection at all.
And here's where it truly gets strange: if I put a paperclip in the BNC end of the pigtail I instantly get a signal of some kind (even readable on some frequencies, go figure) and then hold the AL-500H in such a manner where I can use the paperclip to touch the center conductor of the antenna - lo and behold I get signal, weak, but I get signal that is of higher strength than just using the paperclip itself (like going from -88 dBi to -82 dBi or something similar).
But whenever that AL-500H is directly attached to the pigtail, I would swear that it's like a null and void of some kind, it's not even acting like a dummy load showing any electrical contact whatsoever, it's truly bizarre. Tried 3 different variations of attaching it to the pigtail using coax and BNC/F connectors, no go. Tried hooking it to an old Radio Shack 100 channel purely conventional scanner that barely works in most respects and get basically nothing - remove the AL-500H and put on the RH-C77A or the RS 800, wham, instant signal.
Wish I could tear this thing apart now but I suppose it's a lost cause... just too weird, I swear.