11 Element Yagi - grounding?

[rmariotti]

My first stab at making an antenna, I attempted an 11 element yagi using some software and a target frequency of 858.9Mhz. This is the control freqency for a county about 40 miles from my house. I built to spec, hooked to my scanner and was astonished (and proud) of the fact that it actually worked. I held it in my hand and pointed westward. I could barely pick up that county. I mounted in the attic and it stopped working. What I noticed is that when I touched the driven element(ground), it worked and when I let go, I lost the signal. I am assuming that I was providing a better ground.

How can I improve this antenna? Do I run a ground to that driven element (earth ground)?

 

[RevGary]

Please specify the type of connection and matching network that you have used in this project. THAT might be the issue, not grounding as such...

It sounds as if YOU are changing the antenna impedance when you touch it to a point close to resonance. This should be done by the gamma matcher, balun or whatever was specified in your construction to attach the active element to the cable.

Let us know the construction details and perhaps we can sort it out for you.

 

[rmariotti]

11 El Yagi

Sounds right. The software indicated an impedance of 40 ohms. I connected an RG-6 cable directly to the "driven" element. Still on a learning curve here. Thanks for the feedback.

 

[RevGary]

TRY THIS: If you have some extra RG6 left over, follow this BALUN construction for a better match...

Take a piece of RG6 exactly 9 inches long and trim back the outer insullation so you have 3/4 inch of center conductor and insullation plus 3/4 inch of braid. (Twist the braid together at each end to resemble a stranded wire.) LOOP the piece of RG6 and solder the braid together from each end. Strip off the same amount of outer insullation from your main feed coax and also solder that braid to the two on the short loop. Connect the center conductor of your long coax run to the center conductor of the part of the loop that is closest to it. The center conductor on the other side of the loop should be by itself. Take 2 - 2 1/2 inch pieces of BARE solid 14 gauge electrical wire and solder one of them to the center conductors from one end of the loop and the main feed. Solder the other 14 gauge wire to the center conductor by itself on the other side of the loop.

Place electrical tape over all solder joints to assure that the center conductors will not touch the braids anywhere. Form a closed 'O' at the ends of the two 14 gauge wires to accept a small galvanized sheet metal screw. Drill an appropriate sized hole in the driven element exactly 1 5/8 inches out from where it is mounted on the boom on each side. There should be 3 1/4 inches measured between the two holes. Secure each 14 gauge wire to the driven element with a small screw. Tape the coax loop and the main feed line to the boom and to the mast.

This gives you a 300 ohm to 75 ohm resonant balun for the middle of the 800 mhz spectrum that you are after.

If you need a PICTURE of what it should look like, I can scan a copy of it out of the ARRL Antenna Handbook and email it to you if that would help. Send me a Private Message with an email address and I'll get that image to you ASAP.

This will increase the effectiveness of the antenna rather than having a mismatch the way that you have it now.

 

[rmariotti]

TRY THIS: If you have some extra RG6 left over, follow this BALUN construction for a better match...

Take a piece of RG6 exactly 9 inches long and trim back the outer insullation so you have 3/4 inch of center conductor and insullation plus 3/4 inch of braid. (Twist the braid together at each end to resemble a stranded wire.) LOOP the piece of RG6 and solder the braid together from each end. Strip off the same amount of outer insullation from your main feed coax and also solder that braid to the two on the short loop. Connect the center conductor of your long coax run to the center conductor of the part of the loop that is closest to it. The center conductor on the other side of the loop should be by itself. Take 2 - 2 1/2 inch pieces of BARE solid 14 gauge electrical wire and solder one of them to the center conductors from one end of the loop and the main feed. Solder the other 14 gauge wire to the center conductor by itself on the other side of the loop.

Place electrical tape over all solder joints to assure that the center conductors will not touch the braids anywhere. Form a closed 'O' at the ends of the two 14 gauge wires to accept a small galvanized sheet metal screw. Drill an appropriate sized hole in the driven element exactly 1 5/8 inches out from where it is mounted on the boom on each side. There should be 3 1/4 inches measured between the two holes. Secure each 14 gauge wire to the driven element with a small screw. Tape the coax loop and the main feed line to the boom and to the mast.

This gives you a 300 ohm to 75 ohm resonant balun for the middle of the 800 mhz spectrum that you are after.

If you need a PICTURE of what it should look like, I can scan a copy of it out of the ARRL Antenna Handbook and email it to you if that would help. Send me a Private Message with an email address and I'll get that image to you ASAP.

This will increase the effectiveness of the antenna rather than having a mismatch the way that you have it now.

Thanks! I'll give it a try.
Does that mean that the antenna impedance is not 40 but 300? Can I just hook up one of those 300-75 baluns that they use for tv antennas? or should the balun be designed for the specific frequency as you mentioned.

 

[RevGary]

The TV style 300 to 75 will work to a degree, but they are broadband while the balun is frequency specific. The measurements and attachment point on the active element is also critical for optimum operation. Best of luck and I have that scanned photo ready to go if you need it.

 

[kb2vxa]

Hi R and all,

If you make a better antenna than your antenna you have a serious problem! I see "I connected an RG-6 cable directly to the driven element." in another post. (No need to put driven in quotes.) That tells me it's a dipole insulated from the boom which is about 75 ohms so using RG-6 you have a perfect match. The trouble seems to be an imbalance, OK on HF but disastrous on such a high frequency. Like Gary said a balun is the key or at least should be but there is a major problem, it's ratio. A TV balun WOULD work if the feedpoint impedance were 300 ohms because it's 4:1 which matches down to 75 ohms but it's 75 ohms to begin with! 50 ohm coax would match up to 200 ohms but that's NG too, what you need is a 1:1 balun but they just don't make one for 800MHz, you'll have to wind one yourself and for the hobbyist that's usually a dead end.

Hey, INSPIRATION! I just realized what's inside a two way splitter, a 1:1 BALUN! Think about it, it's 75 ohms input with two 75 ohm outputs but each is unbalanced. The key is each is 180 degrees out of phase with the other which provides a push-pull or balanced output except for the ground which BTW isn't connected. Unless I'm barking up the wrong tree, open the case and remove the guts, set aside and attack the case with a hacksaw. Leave enough to mount it VERY close to the element so the leads remain short and don't become part of the element and you have the F connector for the coax. Then wire it up, being exposed it won't matter in the attic where weather won't get at it. Remember, lead length is super critical at these frequencies, what you add you must subtract from the element so try not to add any.

One final note, if you must you can run the coax down the axis of the boom and end mount the antenna. That way it won't detune the antenna and won't cause pattern distortion, to the signal it just looks like part of the boom.

Good luck, if at first you don't succeed at least we'll find out how far you can throw it. (;->)

This paragraph is a left-over from a previous edit but I'll leave it even though it's out of place, it may come in handy if the home made balun doesn't work.
Now go back to the drawing board and come up with a continuous element mounted to the boom without an insulator and a gamma match. This design is used in single channel UHF TV antennas and works fine with RG-6U coax. By this time you're ready to go out and buy an 800MHz Yagi or plug yourself in since you make such a good antenna. (;->)

 

[Al42]

Hey, INSPIRATION! I just realized what's inside a two way splitter, a 1:1 BALUN! Think about it, it's 75 ohms input with two 75 ohm outputs but each is unbalanced. The key is each is 180 degrees out of phase with the other which provides a push-pull or balanced output except for the ground which BTW isn't connected. Unless I'm barking up the wrong tree

Great idea, Warren.

But ... if I'm reading you correctly, you're saying to connect the output windings in series, ignore the tap, and use it for the balanced side. But wouldn't that present a 2:1 transformation? Better, but not perfect. Maybe 1/4 the turns on each half? (It's been too long - impedance relates to the square of the number of turns, right?) I suppose rmariotti could try it with half first, if none of us can find a copy of Terman. (Haven't seen mine in years, and Ghirardi is much too old for real antenna theory.)

 

[rmariotti]

Thanks guys! Well, it is was a dipole connected directly to an rg-6 cable as kb2vxa surmised. I successfully constructed the balun that Rev. suggested but didn't work well (worse). So I thought that perhaps a folded dipole would get me back in check. That did the trick. I could barely pull in Arlington VA, but not enough to successfully monitor (Arlington is 28 miles, is that too far? Am I gagging on a gnat?). The control frequency was displaying talk groups but could not receive any voice transmissions. Need just a little more umph (technical term, right?). I wonder if there is an easy way to tune the antenna? I didn't realize that the connection points on the fed element would have that much of an affect. In the previous configuration, the RG-6 was connected with a gap of about a half an inch; the latter is 3.5 inches as Rev. suggested.

You are correct, by this time, I decided to purchase the dang thang. A Wilson Yagi is on order! (Although, I'm not giving up on my homemade one). I think it's about a 30 yard pitch to the curb

Cheers

 

[Al42]

Look at a few designs for quads. Element for element a quad has more gain (and a lot more capture area).

 

[rmariotti]

Look at a few designs for quads. Element for element a quad has more gain (and a lot more capture area).

Funny, I was just looking at that. What about a Quagi? Would that have more gain than a Quad or Yagi by themselves??

 

[kb2vxa]

Hi again,

Al, by now you've realized I don't know what the heck I'm talking about, I was thinking out loud. That balun inside the splitter is bifilar wound as usual but dang if I can remember what they did with the common ground tap. You're probably right, configured that way with it ignored because the balanced output needs no ground it would be 2:1, close but no cigar. No big deal, it's a receiving antenna so methinks a balanced feed is more importsnt than an accurate impedance match.

At that frequency you may as well go for a loop Yagi made from a Pringles can. No, I think that's for 23 cM and it's for REAL, check out www.southgatearc.org for lots of interesting projects.

Tuning a folded dipole is tricky at best but the really difficult part is the test equipment. I forgot long ago such things as Lecher wires, that's the sort of approach because there isn't an affordable lab instrument available unless you can grab onto a Q bridge that goes that high. Still it's 300 ohms so a TV balun should do the trick.

In any case it looks like a loop is something like 400 ohms or is it 600? Dang, any way you look at it the impedance is known even if my brain is fuzzy so the modeling software should calculate it. Still there is that gamma match...

Babble babble babble...

Speaking of quads, I can't get that old shampoo jingle out of my head! "Squalo everybody, Squalo. For an antenna that shines..." UGH!

 

[RevGary]

Hey, Warren - wouldn't a commercial 800 mhz CORNER REFLECTOR work for him as a wide angle 'beam'? Seems to me that I have seen those on rotators in a couple locations along Interstate 57 in south central Illinois... Just wondering out loud...