Determining antenna line-of-sight?

[DaveReid]

My trusty SBS-1 plus loft antenna has been monitoring LHR arrivals and departures for 15+ years now for my website, but over the last couple of years I've noticed an interesting phenomenon.

While departures are still being picked up well (because they get higher quicker), reception of arrivals can be a bit iffy depending on the runway in use.  I'm just under 20 nm to the west of Heathrow - arrivals on easterlies (09L/R) are fine and, strangely, so are westerly arrivals on 27R, but those on 27L are dodgy (and better on some days than others).

The angular difference between 27L and 27R approach paths 20+ miles away is only about 2° and although the loft antenna is directional (from Chris at MLS many years ago) I don't believe it's so sensitive that 2° should make a big difference.

So I think there is an obstacle in the way somewhere.  Geometry would suggest it's likely to be nearer my end than to LHR. but I'm blowed if I can identify a likely suspect (building, crane?).

I'm looking to construct a 3D polar plot to narrow down (literally!) the size of the hole in my reception, but advice on suitable line-of-sight/intervisibility tools would be welcomed.

I do have an outside unidirectional antenna that is probably about 10 feet higher (and isn't looking through the roof, obviously) which of course has much better reception, so Plan B would be to split the feed from that antenna to go to both boxes, but again I'd welcome advice on how best to do that without losing too much signal.

MTIA
Dave

[Anmer]

Morning Dave.

I found HeyWhatsThat useful but not for buildings or other man-made obstacles.

http://www.heywhatsthat.com/

I assume you've looked at Google maps?

[DaveReid]

Morning Dave.

I found HeyWhatsThat useful but not for buildings or other man-made obstacles.

http://www.heywhatsthat.com/

I assume you've looked at Google maps?

Thanks for the pointer - just getting to grips with that !  Nothing that I can discern on Google maps so far.  We'll see what happens once LHR is back on westerlies.

I might be back looking for advice on splitting the feed from a single antenna.