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Trees as MW Antennas

A

audioguy

I'm not sure if this is the right place for this topic, but the discussion on box loops got me thinking about the use of trees as antennas. I know this sounds like science fiction, but they really do work!

Next time you go for a walk, take along a small portable radio. Tune in a weak station near the top of the dial and try holding it up to various trees to see what happens. (I know, people will think you're weird, but this for the cause of science, right?) Hold the radio so that the internal ferrite bar antenna is parallel to, and in contact with the trunk. In some cases you will observe in increase of 10 dB or more. Signals that are at the noise will become audible. I generally find that tall trees with large diameter trunks work the best. Generally, the bigger, the better-- especially concerning the trunk diameter.

If you find a tree that increases the signal significantly, note how you can orient the radio in a completely different direction and still get the signal by pressing it up against the trunk. In fact, in some cases you can null out the signal completely and then when you hold the radio to the tree, it will come in. This illustrates that the tree is serving as the main antenna when you couple into it.

Be sure to use a weak signal or the radio's AVC will mask the effect.
 
Could be that the water in the tree, with ionic salts, is acting as the conductor. I would think it would be vertically polarized though. Rich, would like to hear your input on this.
 
I've never noticed an effect there, but maybe I'll check soon! (got to get out of Bellevue first to avoid the 50kw gorilla they call KIXI)

-crainbebo
 
I am not sure what would happen if you used the cell phone fake trees :)

Try the same trick on a chain link fence.
 
RE: Hold the radio so that the internal ferrite bar antenna is parallel to, and in contact with the trunk. In some cases you will observe in increase of 10 dB or more. Signals that are at the noise will become audible.

Schroedingers Cat said:
Rich, would like to hear your input on this.
Click to expand...

So far I haven't attempted such with respect to a tree. But over 40 years ago I did note that a cheap, portable AM BCB receiver with an internal ferrite rod antenna had a large improvement in the received nighttime SNR when held next to the "ground" wire stapled to the side of an a-c utility pole.

This was observed when I was in the U.S. Air Force and stationed on the Gulf coast of the US, when the receiver was tuned to WJR (760 kHz, Detroit).

However the best performance always required the ferrite rod to be perpendicular to the vertical conductor(s) on the pole, because such ferrite antennas respond to the magnetic field of an EM wave -- which always is at right angles to its electric field.

Rich
 
The orientation that Rich refers to is the correct one, and naturally you can get quite a bit of reception enhancement if you hold a radio next to a utility pole that has ground wires running down it. But you will find that in most cases, you will also pick up a lot of electrical noise this way. By contrast, trees are much quieter although there are situations I have noticed where they will pick up noise from power lines nearby and you will hear the noise come up when you couple into the tree, though not nearly as much as a down conductor on a power pole.

It's kind of difficult to explain in words, but when I said the ferrite bar should be parallel to the tree trunk I did not mean to rotate the radio to be in a vertical orientation, I meant that it has to be flat against the tree trunk to get the maximum coupling. For most small AM radios the ferrite bar runs parallel to the front of the radio case but there are a few oddball (and generally larger) designs where it is in a different orientation.

I think the simplest way to state it is that the radio should be in the same general orientation it would be for normal reception, but you want to place it right up against the bark of the tree for best results.
 
Listening for my part 15 AM in its extreme end of range, I have also noted "hot spots" with no overhead wiring to
influence fields. I'd be driving along and suddenly hear 10 db of gain within a 10 foot length of road.
At this point, I would then be hearing one of 2 other 1620s, a local township 12 miles away, or a traffic service 20 miles away.
And then suddenly both back into the noise level as normally.

This is repeatable, every time I drive there daily. I will now have a good look at exactly where the trees are.

And I am noticing this with a stright vertical whip on a car, I'm wondering if this happens as I am just
"nearby" a particular tree, or perhaps at the moment I'm under an overhanging branch..?

Now I'm wondering if a pine tree can be loaded up to work as a shunt-fed.....
 
OK, I just did this experiment at 11 AM local time with a Tecsun PL-606. I tuned to a station on 840 kHz 75 miles southwest of me. I placed the radio next to the southwest side of a trunk of a pine tree that is 40 feet tall and saw the dBu reading increase from 17 to within 19 to 21 range. The station is SW of me. Did the same thing with a maple tree and slightly less increase of signal. My guess is that trees may act as having a cardoid pattern when the trunk of the tree is coupled to the radio's ferrite bar. Very interesting. Anybody wants to know what station this is, it's WKTR in central VA; a 8.2 kW daytimer that sends its signal toward south.
 
Schroedingers Cat said:
Could be that the water in the tree, with ionic salts, is acting as the conductor. I would think it would be vertically polarized though. Rich, would like to hear your input on this.
Click to expand...

I would think it would have to be the water inside with some elements dissolved in it as dry wood is an insulater.
 
I would think that all the horizontally polarized elements (branches) would cancel due to approximate symmetry, leaving the vertical component.

Here's another experiment. Fill a very tall graduated cylinder or buret with saline or other electrically conducting liquid and try putting a radio next to that.

I would think that running a wire up the tree would work better though.

I wish John Kraus and O.G. Villard, Jr. were still around to ask these questions. They probably thought of or experimented with all of it.
 
Here are several more data points I just took on this subject. Recording the relative readings on the dBµ and S/N displays of a Tecsun PL-310 tuned to a directional station about 52 miles east of me on 790 kHz, radiating about 1 kW in my direction:

Location dBµ S/N (dB)
(Tecsun 4-1/2 ft AGL)
Area clear for 50 ft around 32 15
Pin oak tree, 3' diameter trunk, 42 24
east side against trunk
Ditto, but west side of trunk 34 14
Gnd Wire of 50' utility pole 48 24
(Tecsun 9 ft AGL)
Gnd Wire of 50' utility pole 50 24

These locations were all within 100' of each other, in my back yard.
 
R. Fry said:
Here are several more data points I just took on this subject. Recording the relative readings on the dBµ and S/N displays of a Tecsun PL-310 tuned to a directional station about 52 miles east of me on 790 kHz, radiating about 1 kW in my direction:

Location dBµ S/N (dB)
(Tecsun 4-1/2 ft AGL)
Area clear for 50 ft around 32 15
Pin oak tree, 3' diameter trunk, 42 24
east side against trunk
Ditto, but west side of trunk 34 14
Gnd Wire of 50' utility pole 48 24
(Tecsun 9 ft AGL)
Gnd Wire of 50' utility pole 50 24

These locations were all within 100' of each other, in my back yard.
Click to expand...
.

If a ground wire is better than the tree, (assume it's grounded) then I guess trees aren't very good antennas. ;D
 
Trees may not work as well as a wire, but a 10 dB increase in signal strength is not insignificant and it demonstrates that they do work.
 
Tried 530 last night (KNCL518 Tukwila, Sea-Tac Airport TIS). I just BARELY had some gain with a former fruit tree. Not much however.

-crainbebo
 
crainbebo said:
Tried 530 last night (KNCL518 Tukwila, Sea-Tac Airport TIS). I just BARELY had some gain with a former fruit tree. Not much however.

-crainbebo
Click to expand...

bebo-- the top end of the band seems to work better for this (probably a wavelength related issue, I would think) AND you need to try different trees. Especially, try a BIG tree! Fruit trees are generally not that tall (or wide). :)
 
I am surrounded by trees (60+ acres of them) and I am situated pretty much in the middle of that 60 acres. My MW DXing has always been pretty good, but I always attributed it to the fact that there aren't any close in neighbors and not a lot of electrical noise on my doorstep.

That said, I will probably go out and play around in the woods a bit to see what I find. I can say that it may make a difference as to what type of tree you are dealing with as well. Most varieties of oak and pine have pretty shallow root systems. The maples and others have very deep roots, and I'd suspect that may have an effect, as well as the type of soil they are rooted in. I'm in Central VA and ground conductivity is pretty poor here.
 
Trees are on average about 80% water. Estimates run from 50% to 90% depending on the tree and condtions. I would think they would have less during the winter. Can't find much on the ions though.

If you live in the woods, you might try to pick a couple of well placed trees and make a directional antenna for DXing out of them. I think you'll have to run wires up the trees though.

You might find a couple that are 1/4 wave apart over the middle of the top end of the band, about 1400 kHz. Then make a phasing network that can be varied over a wide range. Or if you have room, 1/4 wave at near the low end of the band.

It might work well to null out coastal stations to get more inland stations. Say maybe pianoplayer88 could null out KOGO and get WMT, or KECR and get KVIS. Or maybe null out KFI and get WOI.

Maybe some other posters could recommend a phasing circuit that could accomplish this. Remember you don't need anything that handles high current, so that it wouldn't be that expensive.
 
I'd love an antenna that can null out KIRO for KXMR, KGNC or KCMO! All three are needed.

-crainbebo
 
I noticed KSTP got a little bit better tonight using a huge maple tree.

-crainbebo
 
Other things to try.

Run a wire up one side of the tree and close to it. See if the tree still blocks the signal from the opposite side.

Mount a loop on one side of the tree and see if it exhibits the directionality, essentially having a major and minor lobe with nulls perpendicular to the plane of the loop.

Mount two loops on the trees separated by 1/4 wavelength and phase them. You'd have the nulls perpendicular to the plane of the loop, the front to back enhancement from the tree, and a phased set of nulls to adjust. We need a way to feed and phase the loops. Tuning around resonance on one or the other would provide some phasing, but we still need to feed the loops and combine them.

This is keeping me awake and I really need to sleep.
 
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