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For the 20m - 10m and 6m bands, will having my dipoles below the tops of trees (I live in redwood forests) drastically affect the RF power out or radiation patterns of my dipoles if I can still get them at least 1/4 wavelength above the ground?

Or will the greenery absorb and/or distort the radiation patterns?

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    $\begingroup$ What frequency band are you talking about? On microwaves trees also emit RF energy themselves. With a parabolic dish antenna you can easily pick up their thermal noise ... and hear them grow! $\endgroup$ – on4aa Nov 2 '13 at 18:51
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Trees do absorb some RF energy. So do buildings, hills, and other structures. There are very few cases where higher isn't better for antennas.

However, I'd guess that most amateur radio dipoles are below the treetops. Will some energy be lost? Yes. Will the antenna still work? Absolutely. Without the specifics of yours trees, and your antenna, we can't say exactly what effect trees will have, but it's a fair guess that the absorption by trees will be somewhere between negligible to significant but probably not drastic.

Part of the reason trees aren't a huge problem is that at HF, they are electrically small. That is, their size is smaller, relative to the wavelength of interest. Longer wavelengths pass through things more easily, such as the thumping bass through those annoying cars.

So, get your antenna as high as you can, but if you don't have a tower sticking above the redwoods, don't worry. Your antenna will still work.

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    $\begingroup$ Electrically small is the significant part here. Dish Network (950 Mhz-2200 MHz) works great through the trees...in the winter. In the summer, not so much. $\endgroup$ – WPrecht Nov 5 '13 at 15:16
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The greenery will always distort, but you're right, the distortion will be less as you increase the height of the dipoles above the greenery. You get some reflection. You get some polarisation effects as well, which may or may not be important depending on the transmission mode and the path.

Get thee to a library and put in a request for Radio Science, Volume 17, Issue 5, pages 1027–1036, September-October 1982, which contains a paper:

Brown, G. S., and W. J. Curry (1982), A theory and model for wave propagation through foliage, Radio Sci., 17(5), 1027–1036, doi:10.1029/RS017i005p01027.

Or measure the RF power many wavelengths away, at various antenna heights.

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