Antennas in trees

[K6JEK]

AKA, "Possibly the World's Second Best 75M Antenna (*)" 

I have some nice, high trees one half wavelength from the current dipole and in the right place, too.   If I could get an antenna in place and feed it 90 - 110 degrees out of phase, a lot more of my signal would go towards people instead of towards fish.

But part of it would be just clearing the tops of some Monterey Cypress trees, indeed maybe even touching them.   Assuming insulated wire, am I going to set the trees on fire?   On 75, are the trees going to seem like something to the antenna or non-entities? 

Incidentally, this is at the auxilliary QTH, a little place in Monterey County, which, nevertheless, has antenna restrictions.   Towers and other convenient supports would get me thrown in prison.  Life sentence.   Wires in trees they don't see.

Jon

* see "Possibly the World's Best 75M Antenna",  K1JJ

[K1JJ]

Hi again, Jon,

Two identical 75M dipoles fed with two coaxial feedlines, spaced 65' apart.  Make them 55' or higher to get the needed lower angles for USA work.

Yes, I think that's the whirl's best (and simple) antenna because it will not only beam east or west separately, (-90 degrees phased, unidirectional)  but it will beam both directions at once (180 degrees phased ... bi-directional)  or it will beam straight up as a zenith beam for locals. (0 degrees phased)  Axe Gary W2INR who now runs this antenna and can be heard switching quite often to better advantage

Here's that article:  http://amfone.net/ECSound/K1JJ21.htm


Anyway, for arcing problems, you get maximum voltage at the end insulators of each dipole with tapered off voltage towards the center insulator to a low voltage point. If you can keep the ends from touching anything within the last 20' or so, you probably will not have any arcing problems with the rest of the insulated wire. This is assumng 375W of AM carrier. Worst case, just keep the end insulators clear and try it.

Of course, much will depend upon the points of contact, chaffing, moisture in the tree, etc.

The good thing going for you is that the power is split equally between the two dipoles, so it will be like running 1/2 power into a standard dipole.

Getting the coaxial relays set up to do the switching takes a little work, but Gary has built up my switching design and he can probably give some details. I faxed him a copy long ago and I no longer have it, but I'm sure it's in his files if needed.

73,
T

[WA1GFZ]

I ran the same ant and hope to put it back up in the fall. I found a cool motorized transfer switch. 2 single poll double throw relays will do it.
A winkenson divider with a pair of 43 foot 75 ohm sections off the transmitter.  A TEE fitting drives the teo sections of 75 ohm coax. Each coax transfers the 50 ohms of each antenna up to 100 ohms. Then driving both 100 ohms in parallel gets you back to 50 ohms. Then add a 43 foot section of 50 ohm to one of the two outputs 90 degree delay line. The switch puts the front antenna on the extra delay line and the back antenna on the other. A second switch is required to feed both in phase. I used a bird trandsfer switch to go zero phase by removing the delay line.
I found the best feedine length was a multiple of 45 degrees or 21.5 feet. Both antennas need the same feed line length. gfz

[K6JEK]

Thanks, guys.

I think I'll put it up and leave the fancy switching for phase II.  In phase I I'll manage by plugging and unplugging cables.

Thanks,

Jon

[flintstone mop]

I had a "low flying" (30 feet off the ground) dipole running 300 watts on 160M and the antenna current was out of this World!! I discovered burn marks in the trees.
This was an untuned dipole going through an abortion of a tuner burning up baluns and melting everything in sight until I got the Unihat Vertical up and running.
You should be ok if your system is a resonant 50 ohm antenna . The voltages and currents are more reasonable at 50 ohms.

Fred

[Jim, W5JO]

What, back around WW2 or after the military tested loading trees for an antenna.  They found it dried the sap.   It didn't have much range until the sap driied, then the range really dropped.

[k4kyv]

To be politically correct, the proper term for a tree is "natural antenna support."

[Glenn NY4NC]

Jon;

I plan on putting up that same ant system here at my QTH in North Carolina.

Tom, did Gary post your plans for the switching system here, somewhere on the forum?

Thanks!

[K1JJ]

Hi Glenn,

I've not seen it posted. Send Gary an email and maybe he can scan the copy for you. Might as well get it posted here somewhere too.

Remember that there's a few ways to do the phasing and switching. The more complex ways let you maintain good matches under all phasing conditions. The simple methods may produce a 2:1 swr under some configs.

I would look at Frank's suggestions as well as Gary's diagram. And then think of other methods using those vertical four square box torroidal units.  You might call ComTek and axe them about their switchbox for this application.

T

[WA1GFZ]

I have a quad hybrid design in swcad at work if anyone wants to try that design but the wilkenson divider with coax delay line will provide less loss. Quad hybrid is usually 1 dB extra loss per port. 42 feet of RG11 on 75 meters would be a lower even with the delay line port of 42 feet of 50 ohm cable. Cable TV hardline is very low loss.
It would make a great splitter.

[K6JEK]

The Roy Lewallen section on phased verticals in the ARRL antenna book goes on at length about how tricky it is to get this really right.   To prove the point there are are antenna plots showing distorted patterns.   Heh,  I'd be thrilled to have a pattern as good as any of the distorted patterns.

He doesn't think much of a Wiklenson divider for this purpose, incidentally.

Jon

[Todd, KA1KAQ]

I can attest that bare wire installed at a less-than-optimum height will indeed work, even if it's contacting several branches. I can also attest that smal branches will burn through, and larger branches will chafe the wire enough to break strands and give it that 'afro' look (see 'JJs avatar).  So, even though I'm not practicing it, higher/clearer has got to be better. 

Tom, we talked about the double dipole system during dinner last May at Hopkinton. It sounds like this will be a simple yet excellent choice to get me back up this way when I move south. I have some old broadcast phasing switches (scroteful transformers used to pull the large contacts to and fro) that could be utilized, but I'm not sure of the best approach. Would probably help to find out the voltages needed.

Once I get down there and settled in, I plan on pestering you, Frank, and anyone else who will listen. I can read an article all day long, but it's always better to talk with those who have done it. Next time I hear G on, I'm gonna ask him for a demo. In the meantime, I'll be watching Jon's progress and taking notes.

[wa1knx]

An old thread here, but I wanted to chime in that Garys array years past
works well as I worked him a few times out in AZ.  There are a few others thinking of putting up 2el arrays back east!  w6pss put one up in CA a couple years ago
and put himself on the map back east.

Feeding the dipoles with exact multiples of 1/2 wave feedline to
the shack is a great way. you can measure the mutual coupling,
you can "see" the impedance and phase of each dipole, and can
thus set the phasing and amplitude precisely.  I did this 10 yrs
ago with a larger array. I used toroid cores with a few turns to
sample the rf to each ant. worked great!