[DavGoodlin]

That VHF antenna in your picture is quite a directional one with a high front-back ratio, so being off by 15 degrees would make a huge difference. Those beasts like yours (looks like an Antennacraft-made, Archer design) must be dead-on, even with strong signals. I have some polar plots from the JFD designs which are similar.

The low-band VHF can tolerate being off beam (no lo-band V in NYC) but by the time you're up to channel 11 and 13, even 10 degrees will result in sufficient multipath (visible only on a spectrum analyzer) to raise the bit-error rate thus disrupting reception.

I found four things that will totally f-up ASTC reception, in order of difficulty to resolve;
1. twin lead or 300 ohm balanced line from the antenna - switch to RG6 coax and matching transformer at the antenna connection
2. switching noise from hi-wattage, multi-lamp electronic lamp ballasts and other noise sources - try a capacitor across/inductor in series with the source of noise or high-pass filter on the antenna, keep electronic-ballasted fluorescent lamps away from the receiver
3. multipath interference (analog "ghosting") due to signal path obstructions, trees moving with the wind, vehicle traffic, aircraft - strengthen primary signal by relocating antenna and/or changing (not necessarily increasing) the mounting height
4. Weather - I am still trying to figure this one out, but clouds, fog and rain are all bad in differing ways - BUT this affects mostly the weaker, non local channels