Foliage Blockage?

[druid]

I'm trying to get a SW antenna set up, probably 20M, maybe 40M and/or 15M, and I'm wondering about if the trees on my property will make a difference.

I live on a West-facing slope and have a BIG cedar tree in the front yard (East) so I think LOS or ground-wave is pretty much out of the question East. To the West I can see for miles... through the trees. We have at least 5-6 giant (>100ft tall) trees in the back yard.

How will these trees affect my propagation to the West? Stop it altogether? Will it make a difference how far from the trees the antenna is (dipole or loaded dipole)? Would frequency make a difference (between 40M and 15M - I can get out pretty well to the West on 2M).

There's also North and South, but I want to orient my dipole for propagation E-W so I can try to contact Eastern Canada on a sky-wave.

(The only advantages of all these trees are:
1. Natural lightning arrestors, and
2. If aviation calls about my antenna, I can just say "Yeah.... that wire will really damage the plane after it's crashed through those trees..." )

druid - VA7LAS

[K7KBN]

You'll have a worse problem with arboreal attenuation (like that term? I just invented it!) at VHF and above. Shouldn't be a problem at HF. At least not unless you're using a benchful of lab-grade instruments to measure it. Remember that much of the signal arriving from a distant station will be coming from ABOVE, having been refracted through the ionosphere and redirected earthward. But why ask when you have the freedom to experiment? Put an antenna up and see just how it works on YOUR particular site. What works well for you might not work at all for me, and vice versa.

[5B4AJB]

You'll see most attenuation when the leaves are wet, on VHF/UHF.

If you are planning to operate 23cm or above, then it might be a problem, but for H.F., you will hardly notice...

[druid]

That's great to hear! I can pick up a repeater 30 miles west with my little 2M HT, so if the arboreal attenuation is less pronounced at HF, I should have no problems. Can I actually run the antenna wire THROUGH the trees? (These are heavy cedars - lots of cover) If I can, I may be able to set up an unloaded 40M dipole.

druid - VA7LAS

[K7KBN]

Lots of hams do run dipoles right through the trees. One thing to try to avoid is actual contact. Some might be unavoidable in winds, but if you can possibly prune or lop off branches here and there, you might improve the antenna's survivability. This would be a place to consider counterweights and/or springs to reduce the pull on the antenna wire itself.

[AB3NK]

Radio waves are expressed either in Megahertz - proper name is Mega Cycles - since the only reason why call them Hertz is due to the man who discovered them.

Or they are expressed in wavelength - which is expressed in Meters.

Hence a 11 meters radio wave is 27.253 meters in length.
27.253 MHz equals about 36.17 feet or 433.273 inches.

When a wavelength is the same as or larger then the object that it needs to diffract around, it will diffract easier then if it was smaller then the object.

Hence a UHF signal lets use 400 MHz as a example - which is .75 meters or 2.46 feet, will not or does not refract well around a building, unless the building is 2 or more Fresnel units away, even then it is iffy if the building is 3 or more stories tall and you are below it.

http://www.csgnetwork.com/freqwavelengthcalc.html

http://en.wikipedia.org/wiki/Radio_propagation

All effective communications is line of sight.
Anything other then line of sight relies on one of these principals.

If you cannot get your antenna's above the trees, then a NVIS antenna would probably work best for you.
It will not propogate thousands of miles away, more like 1500 max - but will be much quieter then most other design antenna's.

It is never a good idea to use trees to string wires for the purpose of constructing long wire antenna's due to the fact that as others has said, the trees are a natural conductor of lightning and there is no way to bond them to ground.
At the same time, trees has a natural tendency to sway in the breeze and when the trees sways the wires has a tendency to stretch - hence the antenna will eventually need to be trimmed back to the proper length.

Using parachute cord does help, but is not a long term solution.