.85 nF makes sense.
I've quite a few car radios that had two complete tuners in them, side by side with LEDs on the board. There were two antenna inputs and you could plainly see it switch if you changed to the other input. Having worked on said radio, I think they did a pretty good job at making it switch without a transient.
The theory is obviously that the multipath is not occurring in two places at the same time. However, it did not seem as though the AM section was duplicated in that manner. Looking at these boards, as this was years ago, they seemed to be strictly FM.
Following the reason, for now, along these lines, there would obviously have to be a tapoff for the AM section, and this would operate pretty much like a speaker crossover, just a cap and coil. Both inputs, although seperate for the FM front ends, would be summed for the AM section. Not hard to do, pretty much like a summing subwoofer crossover. Only the frequencies have been changed to protect the innocent (c'mon it's Sunday).
So, if my summary of the architechture of the radios on which I worked is correct we can assume the strange capacitor does something to the phase of the signal, but not to FM frequencies, only to AM frequencies.
This makes sense because the two antennas are of different size. So the capacitor makes the two antennae into a basic, unsophisticated, yet probably effective phased array.
Perhaps someone from Russia will ring in with some ideas on the phase they actually chose. The value of that cap will determine the directional sensitivity no doubt. I wonder what pattern they chose, hmmm, for one it is unreasonable to think that someone would drive right past the transmitting tower every day, I guess a cardioid pattern would be good. Actually that holds water because if you are driving right past the transmitter, what if you want to listen to a different station ? So cartioid it is right ?
I dunno it might be spelled cardiod or something. I don't feel like checking right now, but it is most commonly used to describe the pickup pattern for a microphone. But you had me all screwed up at first anyway, a 0.85uF cap would serve no purpose except to let a static charge build up and blow the front end. I don't think that was their goal.
Oh, the thing about Russia, as far as antennas and things, they are more advanced than just about anybody. We might have better ICBMs, but they have better antennas.
So that's what I think it is, the cap brings the two signals into phase. Then as far as matching the radio to it, they are treated as one unit.
That's my theory and I'm sticking to it. Pick it apart and I'll reconsider.
JURB
PS, I love shit like this, WHY did they do this and WHY did they do that.
JURB