I have a really nice home brewed 2 element 20M yagi that is only up barely 30′ and which hangs over my garage roof. When I put it up, it confirmed something that Tom, N6BT mentions in his excellent book, “Array of Light”, and that is that an antenna over a roof will be affected by anything in the house that is metal, and even the house itself – that the antenna see’s the house almost like it sees ground.
I know this is true for two reasons:
Now that I moved the horizontal wire of my 160M Inverted L, I can move the 20M yagi back to where I had the N6BT DXU-32, and get it up almost 50′. It will not be over the roof and will be over ground. I will get rid of that annoying RFI with the garage door opener.
Here is a secret that you won’t hear from an antenna manufacturer, but if you design and build your own antennas, you will experience. When you multi band a yagi, you typically drop the gain of each band by 1 dB. This means a 2 element yagi that is 4 dB drops to 3 dB (maybe a little less) when you add a second band, say 15M. Of course, you get 3 dB on two bands, but staggering elements is fine – but does cause interaction – mostly beneficial, but you pay a price. I also think Moxon’s are great antenna’s , but they are much harder to build where they retain the shape they need to have to be effective – at least at 20M. On the higher bands is gets easier to fabricate so they maintain their shape. My point is that a 2 element yagi is a really great antenna, and its where you pick up the most gain over a dipole – and is a fairly compact antenna that does not require a big tower.
Its predicted to be a sunny weekend, so I’ll have moved the 160 and 20M antennas right between two rain storms.
Not bad for a late Fall weekend!