| Author |
Message |
    
Rollingrock
Junior Member
Username: Rollingrock
Post Number: 10
Registered: 3-2004
| | Posted on Tuesday, March 23, 2004 - 1:23 am: | 
|
O.K. guys, I finally built it. Put together a 1/2 wave dipole and started to get everything ready to hang it in the back yard when my landlord shows up and gives me the big "no go", so I decided to put it INSIDE the house where noone can see it. So here I am with a center fed wire dipole built with PVC "T" connectors for insulators, RG8M coax, 12 gauge copper stranded wire and nylon rope to tie it off with. I also coiled up the coax at the feed point for a make-shift balun (ten turns about 6" in diameter). Anyway, I've got this monstrosity hanging about an inch below ceiling level in my living room, coax hanging off the feed point in a "T" like you're supposed to. The only deviation from standard protocal is that I couldn't get it to run in a straight line(space issue) so it has an ever so slight bend in it (at the feed point) kinda like a vee but so small you can barely tell.
Now on to the question. I started to tune the thing with my cheap but not super cheap SWR meter that has served me well for many years. SWR was high so I checked all the solder points and their all good to go. So I start to trim the hot side of the dipole until I finally get the SWR to about 1.2:1 across the band. Whats the problem? After all the trimming I realize that the + side of the dipole ends up being only about 7' long(!?).
Neg side is still at about 8 1/2'. Checking with the meter and testing my XMit with local stations everything seems O.K. As far as I can see, the inductance is fine. Also the wire seems to be resonating within the band. Can I possibly hurt my XMitter by using this? What could cause this? Am I just an Idiot and am not doing something right? How many licks...Oh sorry. Anyway after that long winded post Any ideas? Over to the experts.
AIR 095 |
Kc0gxz
Advanced Member
Username: Kc0gxz
Post Number: 670
Registered: 3-2003
| | Posted on Thursday, March 25, 2004 - 3:48 pm: |
|
Rollingrock
You never did say what the total length of your dipole is but I'm assuming since it's a 1/2 wave, it's cut to around 18 feet.
Out of curiousty, why are you running a Balun since you have the dipole already cut for 11 meters. I could understand the Balun if you were using ladder line.
In this case, I believe what you have done by coiling the coax is made yourself a choke. Not a Balun. If you have the dipole cut to the frequency spectrum of 11 meters and are using standard HF coax, a Balun will not be needed.
As far as tuning, I was taught to cut a dipole at equal lengths on both ends of the dipole until I reached a relativly good SWR. To me, anything under a 2 to 1 SWR is great - especially when working with a homemade antenna.
The man to ask would be Bruce. He's built more dipoles, groundplanes and beams than Lon (Mr. Tech808) and my ages are put together.
I'm sure you'll be getting other opinions here. Lol. I think I'm about to get one from Lon.
Jeff, kc0gxz. |
Highlander
Intermediate Member
Username: Highlander
Post Number: 470
Registered: 12-2001
| | Posted on Thursday, March 25, 2004 - 6:06 pm: |
|
I think you should use a balun, because you are feeding a balanced antenna with an unbalanced feedline. BALanced-UNbalanced, hence BALUN. It will work without a balun, but you might see a distorted radiation pattern, or feedline radiation, or both without a balun.
David
n9rzf |
Kc0gxz
Advanced Member
Username: Kc0gxz
Post Number: 680
Registered: 3-2003
| | Posted on Saturday, March 27, 2004 - 3:53 am: |
|
Rollingrock
What is the length of your coax from radio to antenna?
-------------------------------
Highlander
I understand what a Balun is and does. I use a 4:1 on my 135 foot Van Gorden All Bander. 4K on the ladder side down to 400 on the coax side. My MFJ-949 absorbs the 400 down to 50-52 without a problem.
Here is my question. Why would he need a Balun when he has the antenna already cut to frequency? If he uses a standard grade of coax cut to a proper length, I'm curious to know where the need for the Balun is.
Out of my shop, I use to sell those cheap little 1/2 wave indoor dipole antennas. I think you may have seen them before. But I have never seen the requirement of a Balun for them.
To be honest with you, I am a far cry from an antenna expert. But I'm having a difficult time trying to understand the need for a Balun on a antenna that is already cut for the operating frequency.
Just so you know, I am not trying to be a wiseguy about this and I hope you aren't taking it that way. But you have my curiousity up now and I'm never too old to learn something new.
You can E-mail if you want.
Jeff, kc0gxz. |
Highlander
Intermediate Member
Username: Highlander
Post Number: 473
Registered: 12-2001
| | Posted on Saturday, March 27, 2004 - 12:22 pm: |
|
Jeff, no offense taken. A 1:1 balun is what you would use for feeding a dipole with Coaxial cable. You don't NEED one to make the antenna "work", but without one you will see a distorted pattern and / or feedline radiation. That may explain why RR is ending up with a shortened "hot" leg on his diplole--the feedline is becoming part of the antenna, due to not having a 1:1 balun correctly in place. You cannot feed a balanced antenna with unbalanced feedline(which ALL coax, regardless of size/type, is unbalanced) and not have some pattern distortion and/or feedline radiation.
Tech833, this would be a good time to check in with a better explanation (or a correction, since I am getting my info from an ARRL publication, and we all know they aren't always technically accurate) ;-)
David
n9rzf |
Tech833
Moderator
Username: Tech833
Post Number: 602
Registered: 8-2002
| | Posted on Saturday, March 27, 2004 - 9:12 pm: |
|
You're doing just fine. |
Tech833
Moderator
Username: Tech833
Post Number: 603
Registered: 8-2002
| | Posted on Saturday, March 27, 2004 - 9:16 pm: |
|
One more thing to consider- The feedpoint of a center fed dipole is about 70 ohms. Shortening the elements will drop the impedance. The zero reactance crossing is at around 0.218 wavelength. |
Dindin
Junior Member
Username: Dindin
Post Number: 35
Registered: 2-2004
| | Posted on Saturday, March 27, 2004 - 11:28 pm: |
|
so about 94.394 inches?and that would make the dipole 15.73 feet,am i correct 833? |
Rollingrock
Junior Member
Username: Rollingrock
Post Number: 15
Registered: 3-2004
| | Posted on Tuesday, March 30, 2004 - 12:36 am: |
|
I'll have to measure the length, I can say that the fluke read 50 ohms at the feedpoint. Because of the TVI I expect that the antenna isnt exactly resonating on freq. I know this prolly isnt accurate but, what do you guy's think?
BTW coax is 6' long, almost exact
Rollingrock
AIR #095 |
Tech833
Moderator
Username: Tech833
Post Number: 604
Registered: 8-2002
| | Posted on Tuesday, March 30, 2004 - 4:32 pm: |
|
Things around the antenna are changing the velocity factor of the elements. Do not be concerned. |
Kc0gxz
Advanced Member
Username: Kc0gxz
Post Number: 690
Registered: 3-2003
| | Posted on Tuesday, March 30, 2004 - 10:07 pm: |
|
Thank you Highlander.
I didn't really think that a 1:1 Balun would really make that much of a difference. That's one of the reasons why I love this forum. I can't help but learn something new.
------------------------------
Tech833
If a Dipole has to be shortened to bring it into frequency resenence, is the correct procedure to shorten both ends at equal lengths?
Jeff. |
Marconi
Intermediate Member
Username: Marconi
Post Number: 293
Registered: 11-2001
| | Posted on Wednesday, March 31, 2004 - 10:09 am: |
|
Jeff, technically Highlander is correct. However, it is very difficult to notice the affect that is mentioned without one. Exception would be RF on the feed line of course. In the shape he has his configured, he probably would have distortion regardless. This affect would be more obvious if the thing was symetrical to start with and was much higher and rotatable. Then maybe you might see this omni affect when moving the antenna while receiving a good signal.
Highlander, I would not worry too much about hurting anything even if the thing is not symetrical or setting funny as long as the meter shows to be good and the radio shows normal output near resonance.
Good work man. You might be surprised at how well these little buggers work DX when DX is good.
Marconi |
Highlander
Intermediate Member
Username: Highlander
Post Number: 480
Registered: 12-2001
| | Posted on Wednesday, March 31, 2004 - 2:47 pm: |
|
I didn't use a balun for my indoor dipole, and it worked fine for me--so Marconi is probably right not to worry about it. Just figured I would post that info in case it turned out to be helpful and/or informative. 
David
n9rzf |
Tech833
Moderator
Username: Tech833
Post Number: 606
Registered: 8-2002
| | Posted on Friday, April 02, 2004 - 2:49 pm: |
|
Jeff,
Off-center fed dipoles work too. It changes the pattern, but on a horizontal dipole at random orientation, so what?
On some shortwave installations, we skew the takeoff angle in small increments by shifting the feedpoint. That is to compensate for a gradual change in skip angle as the sun sets or rises. |
Lowpowerhal
Intermediate Member
Username: Lowpowerhal
Post Number: 177
Registered: 7-2003
| | Posted on Saturday, April 03, 2004 - 11:02 am: |
|
This is how i made my dipole: CB band Channel 20-27.205 divide by 468 = 17'2" . I had some old # 9 copper coated solid wire. Cut off 18'6" and cut it in half.( The reason for extra wire is wraping)I used some plastic egg insulators 3 of them. layed it on the ground next to a tape measure.Now what going to tell you will have scratching your head. I didnt use coax.I had some 300 ohm tv twin lead flat line. wraping and soldering i put into some trees about 15' to 20 ' up in the air. 300 ohm flat line runs into the house and conexted to a Heathkit Balun coil set (Model B-1). I run 3'RG 8 coax from the balun to antenna switch box to a low pass to a Mfj 944 tuner I have 1:1 swr from Channel 1 to 40. Cheap dipole less than $30.00 . The Balun i bought is 30 years old i think i gave $25.00 new.I seen the Heathkit Balun sell at swap meets or internet auctions for what i gave for mine new.Hope this helps someone. |
Bruce
Senior Member
Username: Bruce
Post Number: 1114
Registered: 9-2003
| | Posted on Saturday, April 03, 2004 - 4:18 pm: |
|
cool lowpower good work! |
Lowpowerhal
Intermediate Member
Username: Lowpowerhal
Post Number: 185
Registered: 7-2003
| | Posted on Monday, April 05, 2004 - 8:04 pm: |
|
Thank you Bruce. The fun part of the hobbie is making anttennas and dipoles also putting them up. Also made a j-pole for Gmrs out of 1/2 inch copper pipe works Great. |
Tech833
Moderator
Username: Tech833
Post Number: 611
Registered: 8-2002
| | Posted on Tuesday, April 06, 2004 - 11:49 am: |
|
Hi Hal.
I started making my own antennas when I was a kid. It turned into a lucrative career. Keep it up! |
Lowpowerhal
Intermediate Member
Username: Lowpowerhal
Post Number: 188
Registered: 7-2003
| | Posted on Tuesday, April 06, 2004 - 8:53 pm: |
|
Thanks Tech 833 My Father was in ww11 put up make shift dioples in some of the islands that the bad guys was on. I started with radio at a early age. Tech 808 and i are the same age. And you know i want to learn all i can from you. It helps my hobbie and us all. Thank you Tech 833
|
41magnum
New member
Username: 41magnum
Post Number: 3
Registered: 11-2003
| | Posted on Saturday, April 17, 2004 - 6:12 am: |
|
lopowerhal;
You need to reverse your formula ( divide 468 by frequency in Mgh ). 468 divided by 27.205 = 17.20 feet .
Just my two cents.
41 magnum |
