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1-10 of 14 messages
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2 m (also 70 cm) Dipole
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by KB0VYJ on June 24, 2008
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Recently I ventured into my first homebrew project -- a 2 Meter/70 CM Dipole:
http://www.hamuniverse.com/w7lpnvertdipole.html
The plans were easy enough to follow. The problem is I've built this antenna twice now and can't seem to get it to tune correctly.
The setup:
50' RG-58U Coax to feed up to the antenna.
4' RG-58U Coax forms the feedline/balun/connection to elements.
Elements are 1/2" Copper pipe connected by a PVC "T". The elements are about 1/2" apart inside the T.
The coax feed is soldered internally in the T. I stripped back about 4" to give me some play to do the soldering inside the "T".
The Balun is 5.5 Winds around a 1/2" PCV pipe, about 1" from the feedpoint.
The antenna is currently mounted in my attic. I don't have many restrictions on antenna, just didn't want to go through the pain of drilling into the house at this point for a simple project.
The issue with the first attempt was no matter how much I took off the elements while tuning the SWR stayed about the same: 2.5:1 @ 146 MHz. It did favor the lower frequency just a bit (144 MHz). I eventually trimmed the elements down from about 21" (a little more than the instructions asked) to 19" (about an inch shorter on each side than the instructions requested) but the SWR changed very little.
So, I rebuilt and made minor modifications (solder the feeds to the elements rather than screw them down, rewind the balun, etc.). Nothing is making a difference.
I'm at a loss, and I'm still learning about matching and tuning. I know the 1/2-wave Dipole (Vertical mount) is about 73 ohms. This obviously doesn't match the 50 ohm coax very well, which is a small problem but most people say the mismatch accounts for very little SWR.
I also know that the Dipole is "balanced" and coax is unbalanced -- which is the reason for the balun in the design. Not sure if the simple balun is doing the trick or of it would cause the issues I'm seeing. The balun I used is made of 5.5 winds of RG58u around a 1/2" PVC pipe - tightly wound - but not so tight that the insulation is pinching (that I can tell).
I was also thinking if the 4" of coax that I stripped in order to be able to solder internally in the "T" artificially detuned the antenna by accidentally increasing the "element" length (copper + unshielded center conductor) by 4".
If anyone can give me pointers or direct me to more reading on the subject, I would appreciate it. I've been back and forth on reading about all this and I'm not getting anywhere. I would like to move on to a more "difficult" antenna to build that provides better benefits than a Dipole. I just want to make sure I understand better what I'm doing before throwing money into it.
Thanks for any input.
73's
-Joe
KB0VYJ
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RE: 2 m (also 70 cm) Dipole
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by K3ANG on June 25, 2008
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Hi, Joe.
Welcome to the world of homebrewed radio.
RE:The antenna is currently mounted in my attic. I don't have many restrictions on antenna, just didn't want to go through the pain of drilling into the house at this point for a simple project.
Yer NOT gonna like this, but do you have the $$ to build another one per the specs and then mount the new one OUTSIDE (temporarily) for tuning and testing? You would not need to drill any holes for coax, just open a window or door nearest the operating position (hope it's a nice day tho). Perhaps mount it on a pole (or wooden rake handle) bungeed to a deck rail (if you have one) or to a deck rail itself.
If you can't build another one, take the one you have built OUTSIDE and test it. See what happens.
I'm not saying your did (or doing) anything wrong, per say. But what is implied in the specs is that this thing is going to be mounted outside, NOT inside. Also, you might have corrupted your current build by doing the tuning and testing inside. That's why I asked if you have the resources to build another and test that one outside.
Personally, I have built a number of 2m wire dipoles inside with no problems. But the one you're attempted is just a bit more complicated (2-band vs. 1-band).
73 & GL
Greg
K3ANG
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RE: 2 m (also 70 cm) Dipole
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by KB0VYJ on June 25, 2008
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I did attempt to tune outdoors. I knew that much. :)
Had the ground element about 12" off the ground mounted in a vertical configuration. The antenna was about 10' from buildings. The boom is 1/2-wavelength...all as specified in the design.
Does structure (wood, sheetrock, etc.) really mess with a dipole that much? For a 2m dipole, how far is considered the "near field". It's funny they would say it's a great attic antenna but have structures in the attic cause issues with the tuning.
Again...I'm new to this.
I moved it indoors when I could not figure out why I was having the problems that I was having. At least I could monitor repeaters. It's a heck of a receive antenna. At least I got that going for me.
Thanks for the reply. Any additional insights are appreciated.
73's
-Joe
KB0VYJ
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RE: 2 m (also 70 cm) Dipole
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by WW5AA on June 25, 2008
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Joe,
For what it's worth, here is what I see wrong with this design. The gap at the elements should be 1/4 inch if possible and fed at about 1/2 inch out on the elements. The balun location for 2 meters and above is not good due to interaction caused by the "fat" elements. A balun placed several feet from the load would be better. If your coax leads (center/shield) to the elements is long and unbalanced, this could also be a problem. Keep experimenting and you will get it!
73 de Lindy
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RE: 2 m (also 70 cm) Dipole
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by WB6BYU on June 25, 2008
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I'd suspect that the dimensions are a bit long for that
diameter of pipe, though I'd have to check with EZNEC.
Try shortening up the leads on your coax as much as
possible and see if that makes a difference. It may work
better to use a screw through the CPVC T to connect to the
pipe rather than soldering to the pipe above the T, but
these are small changes and varying the antenna length
should correct for them.
How long did you say your feedline was? 50' of RG-58?
That would have about 3dB of loss on 2m, and more on 440.
If so, then the actual SWR at the antenna will be higher
and you may have more of a problem than it appears.
Here is a good check: disconnect the feedline from the
antenna and check the SWR. What does it measure?
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RE: 2 m (also 70 cm) Dipole
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by KB0VYJ on June 26, 2008
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Well...I tried several other things on the antenna as well. Changed to RG-8 coax (not easy to work with). I also fed the elements at 1/2" and mounted them 1/4" apart inside the T. Still almost 3:1 SWR at best.
I guess I'm going to have to abandon this design.
What type of 2 meter (and possibly 70 cm) antenna design would anybody suggest for their "first" homebrew? Are there plans on the Internet for it?
This would be my 2nd attempt at an antenna, but until I get one working correctly, I'll have to still call it my first Homebrew.
With the cost of copper, I'm getting to the point where I would have been better off buying one. :)
73's
-Joe
KB0VYJ
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RE: 2 m (also 70 cm) Dipole
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by KA1MDA on June 26, 2008
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Joe-
I tried building some 2 meter dipoles and a coaxial dipole about 20 years ago, and never had much luck in getting them to tune properly. A much easier antenna to build is a 1/4 wave ground plane. All you need to build one is 5 pieces of stiff wire- each about 20-21" long (old wire coat hangers work great) and an SO-239 socket. Much simpler, doesn't need a balun.
Take 4 of the wire pieces and use needle nose pliers to bend 1 end on each wire into a small loop. Use a small machine screw through the loop to attach each one of the 4 wires to the chassis mount holes in the SO-239. Bend the wires so they point down about 30 degrees. Take the 5th wire, solder it to the center pin of the SO-239. Connect PL-259 connector on coax the the SO-239. That's it! Trim the vertical element for minimum SWR, final length should be close to 19". Should also work on 440 as a 3/4 wave antenna, although the radiation angle will be kind of high.
You can stand it up on the floor in the attic on the ground plane, using the 4 radials as "legs". Or use some electrical tape and fishing line and and hang it from the rafters by the vertical element. When I lived in a duplex, I has one sitting in the attic for years and never had a problem with it.
73 and good luck,
de Tom, KA1MDA
www.ka1mda.org
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RE: 2 m (also 70 cm) Dipole
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by K3ANG on June 27, 2008
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The quarter wave GP is a good first antenna project. Another is the center-fed dipole.
For a center-fed, wire dipole, this formula is 468 / frequency in megahertz.
I cut mine for 146.52 mhz. Just a little over 3 feet long.
If the SWR is high for you, retrim the ends @ 1/4 inch at a time, retest.
Don't feel any remorse about this.
It's all part of the learning process.
EVERYBODY goes through this when they build their first project, whatever it is.
73, GL and keep trying.
Greg
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RE: 2 m (also 70 cm) Dipole
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by WB6BYU on June 27, 2008
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You're problem is not the antenna.
If you use VK1OD's handy transmission line calculator:
http://www.vk1od.net/tl/tllc.php
for 50' of RG-58 coax at 2m, you'll find that the SWR
will never be much worse than about 2.7 : 1, even with
an open or short circuit at the far end! (This is due to
the coax of more than 3dB.)
So I'd suggest that you have a bad or incorrect connection
at the antenna. Have you run a DC check to make sure that
the shield isn't shorted to the center conductor, or that
each lead is really connected through to the corresponding
antenna element? (With an ohmmeter, you should see an
open circuit between the two elements with the coax
connected to the antenna but not to the rig. One side
of the antenna should show a short to the center conductor
at the rig end, and the other side should show a short to
the shield at the rig end.
Look for a major problem like this, not something minor.
If the SWR was really only 2.7 : 1 at the antenna
it would show as 1.5 : 1 at the rig end of the coax.
That explains why adjusting the antenna didn't change
the SWR, though.
Check your coax before using it on your next antenna or
you are likely to get the same results.
(For that length, RG-213 would be a better choice than RG-58
on 2 meters.)
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RE: 2 m (also 70 cm) Dipole
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by KB0VYJ on June 27, 2008
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Yup. The fist thing I checked was for a short in the coax. Everything there was fine. RG-58 isn't very good cable. I knew this. It's cheap...and being my fist attempt I didn't want to sink a whole lot of money into it. I knew I wasn't going to get 1.1:1 out of it, but I was hoping to see at 2:1 at worst. I would then feel better about sinking money into better cable.
But, I haven't thrown it away. It't sitting over there across the room. And when I learn more about what to look for I'll probably go back to it and tweak on it some more.
But your right...regardless of what antenna I build or (gulp) buy, I should probably have better feed line going up to the attic.
I'm thinking about building the 5/8-GP -- just to rebuild my confidence. :) Unless someone else has a "sure thing".
Thanks all and 73's. Maybe hear ya on field day.
-Joe
KB0VYJ
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