What I noticed when listening to Washington PD all last evening between 18:27pm, when I heard my first transmission from Washington PD, after about an hour of total silence, and about 22:30pm when I turned the scanner off, the only radio traffic I heard was between the main base TGID 4803 and only one singular car also on TGID 4803, a female officer. No other car numbers were heard all evening through the Weldon Spring tower.
The very odd thing is, there are many more cars on duty in Washington than a singular officer, and I should be hearing transmissions from more than one car over a period of 4 hours. I don't know, maybe it's just me, but I also noticed that the digital audio quality of those transmissions was not up to a par with respect to what I generally hear from MSHP cars on the same and other MOSWIN towers, even though the control channel signal reception at my base station is excellent with respect to Weldon Spring Tower.
When I was up in Washington with my BCD436HP on Monday, I was up there for about the same period of time as last night, but from about 11am until 2pm on Monday, monitoring through the Washington 700 tower, and I didn't really notice any of these anomalies via the Washington 700 tower that I'm noticing when monitoring through the Weldon Spring Tower.
I seemed to be hearing all the cars on duty on Monday through the Washington 700 tower, except I sometimes perceived a slight garbled kind of audio quality to some transmissions, but sometimes that can be typical of some digital transmissions. It wasn't too bad, but I do sometimes notice it, not that analog is particularly any better with respect to its ocassional static.
I also have TGID 4801 and TGID 4802 programmed into the scanners, which are identified within RR as Multi-Dispatch channels for Washington PD, just so in the event they broadcast via a different TGID number, I wouldn't miss anything. I understand that it is not probably necessary to program all the frequencies listed for different agencies, and that the primary and secondary frequencies is all that's probably necessary.
But, due to my confusion with respect to how the MOSWIN system is sometimes working, whether it's always setup as its intended to work, and whether RR has the accurate information, not to mention the changes that radio techs might be making to the system that never make it into RR, I just add all the data in on RR in order to be on the safe side. The card has only about 1% of space used up for all the data I have programmed into the scanners for the small number agencies I want to listen to. So, I feel that I have the luxury of not having to be sparing with respect to adding in all the frequencies for the towers that are listed in RR that I want to monitor. I guess it can slow down the scanning speed, having excess data that it has to scan through, but it's a compromise I'm willing to endure, and a negligible delay, just so I'm sure not to miss anything.
I speculate that we are all missing a considerable number transmissions through the Weldon Spring Tower when monitoring Washington PD and not through the Washington 700 tower, and that it's most likely due to our not having the frequency ranges necessary to copy all the cars, which may be transmitting on different frequencies than what is published within RR. I believe the main TGID is correct, TGID 4803, but who knows, there may be some data missing from RR there too.
But, if some Washington cars are broadcasting on frequencies that are unknown to us, thereby not programmed into our scanners, it stands to reason that communication between the base station and "ALL" the cars whose frequencies we are unaware of aren't going to be heard. At least that's my theory with respect to my understanding of how P25 is supposed to work, and what I'm experiencing via the Weldon Spring tower and Washington PD.
From your testing again, it sounds like your only option to make that happen is probably an antenna outside of your attic and mounted higher in free space. A 7/800 MHz yagi and low loss coax like LMR-400 may work if you can detect a signal at all from the Washington 700 sites control channel. For that to work though, someone needs to determine if there is a signal at all otherwise it would all likely be a waste of time and money.
Yep, I would probably have to have a tower in the back yard to clear the geographical horizon to achieve line of sight with the Washington 700 tower using a Yagi, and even then, it seems to me they're operating on low power, since I can't even pick them up 1 mile outside of Washington city limits with my BCD436HP when I'm right outside Washington city limits just a short distance South on Highway 47, and on a hill at that. That doesn't explain how you're able to sometimes copy them up all the way up in Chesterfield, but it is what it is.
I am so tempted to get a Yagi up outside to see what it would do for me. But, as you said, it would be quite a hassle to go through, getting it outside, elevating via a mast (a tower is out of the question, LOL!), all the antenna system grounding, cable expense, labor, etc...Just too much when I'm not able to elevate a simple mast sufficiently to achieve line of sight. Plus, I don't think the powers that be around here would take too kindly to my adding more outside antennas, I already have a 30 ft. Vertical, and a Rotatable Dipole for HAM radio use out there.
That's why I keep the Discones in the attic, no grounding necessary, cable runs are relatively short, cable itself isn't too expensive, and the results I seem to be getting out of my current setup is quite acceptable with respect to all the other agencies I want to listen to. It would be quite an investment of money, engineering, and labor just on the off chance I could get a Yagi to work.
Of course, the other issue is, half of what I want to monitor is analog, and then there's MSHP MOSWIN and Washington PD via P25. The Discones are working well with monitoring MSHP through the Discones, but I would have to probably erect a Yagi on the remote chance it could pick up Washington 700 tower from my base station location, which means I would not only have to erect a Yagi, but also feed the scanner with a diplexer to combine the Yagi and the Discones.
But, that all having been said, can you advise which Yagi make and model number are you using?
I know, it's too cold, and the cops don't want to get out of the car, other than the brave soul at 18:27pm with a stop on an orange jeep...LOL! It takes a former one to know one...but, 39 degrees, that's not that cold, what gives? 
