wire beam antenna?

[loosecannon]

hello gurus,

my house has a pitched roof that faces east/west.
the angle of the pitch is greater than 90*, and could be as much as 120*. (dont know)

therefore, im thinking that with a wire boom running along the top of the roof, and the elements running down the pitch of the roof, that it would act like a flatside yagi beam.

i know very little about the way beams are fed, just what i have gleaned from the ARRL handbook in passing.

yes, i am aware that i would only be beaming to the east, and my communication would be pretty much all in the same direction, but that doesnt bother me much.
i live in the south west, and my main interest is being able to shoot skip to the east.
to tell the truth, attenuating the south would be just fine by me.LOL

i would like to feed it with coax.

so, can anyone tell me where the inherent problems lie?
wire too small of a diameter to be useful over a 50-60 channel spread?

downward angle of the elements causes problems?

can i just use formulas for regular horiz. yagis?

how do i feed it?

thanks for any and all help. if this works,it will be totally stealth, and still bring in the DX.
loosecannon

 

[C2]

try it

you dont even need a boom

just feed an inverted v wire dipole with coax. you will need to keep the angle at least 90*, so stand off the ends a bit.

then add a reflector and as many directors as you can...try a std. design.

 

[Highlander_821]

We did something similar in the attic of the house I used to rent, back in '93. Obviously, it was useless for local contacts, but, if the band was open, it worked OK for DX--but not great. My A99 was usually MUCH better. Still, a worthwhile experiment.

 

[W5LZ]

loosecannon,
As "C2" said, try it, see what happens. Is it going to be the 'best' thing since buttered bread? I very much doubt it, but I don't doubt that you will hear things you didn't before.
Some things to think about, not in any particular order.
The diameter of the radiating element has very little to do with the usable bandwidth of an antenna. It does have a lot to do with the length necessary for resonance. Smaller diameter means the length will have to be longer by a small amount. If you leave the insulation on it, the length tends to be shorter by a small amount. Has to do with the 'apparent' diameter, not just the size of the metal part. Also, since it would be near the roof of the house, expect the length to not be close to what you would normally expect, sort of.
How to feed it? Almost anyway you want to, sort of. Directly feeding it will work, using an impedance matching device of some kind will also work. Simplest way would be to directly feed it and put a choke near the feed point. Remember that when you add elements to the antenna, director, reflector, etc, they tend to lower the input impedance.
The 'angle of the dangle' by following the roof line is going to lower input impedance too. No idea how much, but it will. The greater that 'angle of dangle' is, the lower the impedance.
Using the 'standard' formulas for a yagi will get you in the ball park, but certainly won't get you one of the 'good seats'. Mainly cuz the thing is going to be close to the roof of the house. Cutting things a little long to start with helps cuz it'll give you some adjusting length. Element spacing ought to be sort of right, kind of. But since element spacing is one of the big 'playing with it' thingys, I wouldn't worry a lot about it. Get the rest of it sort of 'close' and then play with spacing.
Some of the "Don't"s.
Don't expect a 50 - 60 channel spread in usability. Ain't gonna happen. (Hell, it doesn't happen with 'real-live' yagi's either.)
Don't expect to have things finished in only one 'climb to the roof'. That would be nice, and I hope you see it, but don't expect it.
Don't expect 'zillions' of dB of gain. Nicest part of most directional antennas is that they tend to get rid of signals not in front of them. If that 'farting' fly (signal) is sitting directly on the end of the antenna, you'll be able to hear it, but not at thousands of miles. (Now that's a disgusting thought, ain't it?)
The whole thing may turn out to be a total waste of time, but playing with antennas is fun, which is the whole idea, isn't it?
Good luck.
- 'Doc

 

[loosecannon]

oh, im definitely going to do this. LOL

even if all the answers came back that it would not work, and would cause my radio to catch fire; i would probably still try it. LOL


yeah, the trips to the roof need to be kept to a minimum, as it is a spanish tile roof, and it is very hot here. those tiles are very breakable too.
also i dont want the neighbors hip to what im doing.
thats my whole agenda. be able to shoot a little skip to the east, while running a couple hundred watts SSB, and not have to worry if im messing with someone's TV and phone.
this antenna needs to be basically invisible.

most likely, i will try to get it as close as possible before going up on the roof, and may just have to live with however it performs.

a couple of questions:
from what i have read, if the angle of your inverted V antenna gets to be much over 90*; that it tends to act like a horizontal dipole.
i thought i might be able to use this principle to my advantage, by being able to use standard horizontal dipole formulas.
am i on the right track?
i will look for some software directed towards wire beam antennas.

second, could i lay the wires out in my yard, and do the tuning with it on the ground, and simply drag the whole assembly up onto the roof and secure it in place?
i guess what im asking is: is having it on the ground similar to having it right against the roof?
my house is a one story, and the peak of the roof sits at about 15-18 feet.
im pretty hip with geometry from my metal fab days, and i really think the angle is about 120-130*.

thanks for any help.
whatever happens, i will post the results.
may take a month to do, but...
loosecannon

 

[W5LZ]

loosecannon,
The easy simple one first. No, you can't do it on the ground. There are some "but's" with that, but unfortunately not with a wire beam, it's too flimsy to stand on end. One of those "but's" is that with a rigid type beam, you can stand it on it's reflector and get it sort of close when tuning it.
About the 'angle of the dangle' of an "inverted - V" (actually a 'drooping' dipole cuz a real live inverted-V has an angle of 90 degrees).
The more 'droop' the legs have means a lower input impedance till you get to something less than about 100 - 120 degrees. After that it gets sort of 'squirrelly', not worth doing. The polarization, even at 100-120 degrees is still horizontal. Not much vertical component at all till you get to 90 degrees, sort of. So for all practical purposes it'll be a horizontal beam.
- 'Doc

And just for 'grins', how about putting up a birdhouse on the ridge of your house with two sets of guys to hold it up? Since they're there anyway, make the guys into crossed dipoles. Feed line down through the 'mast'. Bound to be more bird lovers around than antenna lovers, right?

 

[loosecannon]

ha ha,
i forgot to mention that i dont want to have to go through the rigamarole of asking the landlord for permission.
what he dont see wont hurt me.
this is the main reason it has to be simple and invisible.

thanks for all the help doc, i always appreciate your input.

i guess everyone else is watching LOST. LOL (im actually a big fan, but at work)

anyway, i found this site that does the math for an inverted V beam.
could i use these figures?
or do i need to use figures for a horizontal beam?

it looks to me like i could do 6 elements, with about a 25 foot "boom".
http://www.kimware.com/antenna/vbeam.html
so, the boom just needs to be a sort of centered reference point, and does not have to be conductive?
i guess i could just use a piece of rope pulled taut.

now, how do i feed this thing?
if i were to use a hairpin match, how would i figure its length?

again, thanks for all the help,
loosecannon

 

[lords]

get some fiberglass garden stake..I used the for a quad once!

 

[W5LZ]

I tried that web site. Either I'm reading it wrong or the calculator is wrong. The lengths for the reflector and director are reasonable, but the length of the driven element is only for one 'side' of it, or half length. Something isn't right there.
The easiest way of going about the whole thing is by starting with the driven element. It's a 1/2 wave length dipole. Common way to find that length is to divide 468 by the frequency in Mhz. That makes it slightly long, but that's certainly better than slightly short. Find/make it resonant on the desired frequency by adjusting the length (fold the ends back onto them selves). That's really about all there is to it. Just directly feed the thing. The reflector is something like 5 to 10% longer than the driven element. The director about the same amount shorter than the driven element. Spacing is variable by quite a bit, almost anything from a quarter the length of the driven element to something just short of it's full length.
I'd put a choke at the feed point. Coil up some feed line, maybe 5 to 10 feet of it. The size of the coils isn't critical at all. Each loop should be large enough so that the coax doesn't get 'cramped'. Depending on the size of coax used, coils of about 6 to 10 inches in diameter ought'a work just fine.
That'll get you sort of 'close'. Almost all the dimensions are variable to some extent, whatever works 'best' for your situation.
If you really want to go to the trouble of using a gamma or 'hair pin' match, feel free. (I can't give you those dimensions.) The direct feed ought to do just as good with the choke. Certainly won't be the absolute most bestest thing around, but it'll work. As for supporting the wire beam, as long as it isn't conductive, almost anything stronger than 'thread' will work. I use the white nylon cord from 'Wal-Mart' for stringing up dipoles. Use the white stuff! I've found that the green or other colored stuff is at least partially conductive. ('Trot-line' cord?)
- 'Doc

 

[Marconi]

'Doc the reason is that the driven element is split and insulated at the feed point in the middle, and the other elements are one piece. I think that is a little "thingy" that was noted in the caluclator or in the preface to the calculator.

 

[W5LZ]

Marconi,
That's what I figured it had to be, but I still think it's an odd way of doing it. Oh well...
- 'Doc

 

[loosecannon]

ok,

was up on the roof today clearing out a dryer vent, (bigtime cause of household fires) and im excited!

whether or not it performs halfway decent, who knows, but the logistics of building it seem to be coming together nicely.

most likely will be uninsulated wire, 14ga.
any benefit to using 7 elements as opposed to 6? (noticable difference?)

this is going to be fun!

if i didnt care too much about front to back ratio, and cared more about forward gain; how would this relate to the sizing and spacing of the elements?

thanks to all,
loosecannon

 

[W5LZ]

In general, the greater the number of elements and the greater the spacing between them, the greater the gain. Sort of. More to it than that but that's a sort of general rule of thumb. Making it seven elements instead of six isn't worth the effort. But, if you feel like it, why not?
- 'Doc

 

[Master Chief]

I'd rather have a long boom 6 than the same size boom with 7.

One nice thing about this is you can "space" the elements back and forth and see how that affects performance.

Have fun!

 

[loosecannon]

ok, here is where i am at so far.

i have used a few different yagi calculators online, and one antenna "modeler" program to get the lengths of the elements.
nice thing about the calc. i used is that you can input the element diameter, making it hopefully more accurate for the wire i am using for elements.

i decided to make the beam resonant for 27.385mhz. that way, if it barely works on one freq. i might still have a chance at making a few contacts. LOL

the wavelength at this freq. is 35.932 feet.
i chose to space the elements equidistant from eachother along the boom.
i spaced them at .20 of the wavelength, which is 86.25 inches.

with six elements, this gives a "boom" length of 35.937 feet.
(is this just a coincidence that it is so close to 1 wavelength at design freq.?)

element lengths are as follows:
reflector= 219.5 inches
driven= 207.25"
director 1= 197.25"
director 2= 197.25"
director 3= 197.25"
director 4= 197.25"

i calculated the design using both 14ga. and 12ga. wire diameters for the elements and the results varied very little, so i think i am safe using either.

here are the results of my antenna "modeling":

gain:10.22 dbi
f/b: 11.07 db

the pattern looks "acceptable".

here's the part i need help with:
Zin=49.71 +j 13.07
what does this mean and how does it relate to feeding the antenna?

i interpret the Zin as the impedance of the antenna at the feedpoint. if that is right, then this antenna should be close to a good match without using a beta match.

what is the "j"? is this reactance?

what does it mean for me?

does anyone have a really good yagi modeler program on hand that can plug my numbers in and see what happens?

use .064 inches for the element diameter.

thanks for any advice and help anyone can lend.

i have been having fun learning all the math involved in these things!
this antenna WILL get built!
loosecannon