Homegrown Dipole antenna

[longhaireddwb]

I wanted an antenna for my base radio. A Cobra 135. I made a Dipole antenna using electrical conduit and a 6 foot long 2X2. The conduit is screwed to the 2X2 end to end and separated by about 4". I used a couple pieces of 10 gauge wire, a SO239, a few wood screws and two hose clamps. Hose clamp a wire to each conduit and the other end to the SO239. The conduit started at 10 feet each but now tuned they ended up being a little over 8 feet long each. I hung it on a wood fence with some string horizontally and kept cutting it until the SWR final came into speck. 1.3

With the help of a ladder I stuck this antenna vertically in a tree with the top of the antenna being about 30 feet in the air. Strung some cheep Radio shack (RG58/U) coax and gave it a try with the Cobra 135. First contact was 17 miles away! Although I'm on the side of a mountain (Pikes Peak) and my contact was across the valley on a hill.

I'm happy but I have a lot of noise! The guy I made contact with was talking to other people but I couldn't hear them at all. Can I do something to get better receive on this antenna? Also, the Noise canceling on this radio don't do anything. If the switch is in or out its the same. I checked and the wires are connected it just don't do much if anything. Can this be modded to work better?

 

[Robb]

Radio Shack coax is the finest dummy load available. Doesn't work well for getting RX or TX for a radio - tho. The loss of efficiency per foot on this stuff is really awful.

A halfwave dipole should be the length of two 1/4 wave antennas going in opposite directions. That should make each half nine feet long. BTW - did you check the SWR? What did it show? Also - how high off of the ground was it? Did any of the dipole have contact with tree branches or anything else? Close to any metal objects? Just some thoughts/questions about setup...

 

[HomerBB]

Dipole I Made

I wrote this back in March 2009:

I just finished putting up a horizontal dipole made of 1 1/4" chain link fence top rail (I already owned plenty of the top rail).

The top rail slides comfortably into 1 1/4" pvc which is the center of my dipole. There are two pieces of the pvc that are combined by a pvc tee. I feed it with coax. I attached a pl239 with 16ga stranded wire to the two radials via electrical terminals attached by bolts through the pvc and top rail. I made a coax choke directly beneath the antenna feed point and trimmed the length of each end of the dipole to obtain favorable swr.
I began with each top rail being 8' 8". Eventually trimmed off 5" from each end. I suppose that length was because of the large diameter of the top rail.

It is about three feet above the ridge of my roof at its center, and due to the pitch of the roof about six feet at its tips. The whole thing is 21 feet above the ground.

My swr is 1:2:1 @ channel 40 and 1:0:1 @ channel one.

I made a solid contact with McAllen, Texas this evening from Northwest Arkansas during a brief period of skip. Also, Only the closest locals who are vertical can communicate with me while many other local verticals can neither hear me nor be heard by me on the horizontal dipole.

I am pleased with the results.

Photos are here: Hopeful DX Dipole

 

[Marconi]

Longhair, how is the coax leaving the center of the vertical dipole as it goes to the radio? If the coax is dropping straight down and is less than about 9' feet away from the bottom element, you are probably having common mode current problems and noise is very often associated with CMC's among other things.

The coax to a vertical balanced dipole needs to leave the feed point at a 90 degree angle from the plane of the elements for at least a 1/4 wavelength before dropping down toward the radio or the antennas pattern may be skewed in some undesirable direction and for sure will disrupt the balance if not corrected.

Also if the bottom element is very close to the tree or anything else for that matter, and that sounds like a possibility as you describe, that can and will ill-effect the performance also. Trees are not typically thought of as conductive objects that affect antenna performance in a significant way unless the radiating element is close (9') and parallel to the elements.

I think HomerBB's dipole is still a bit too close to his roof, but he discusses a little bit about such concerns as he describes installation. He also is using a coaxial choke to help with the problems of CMC's, even though IMO he is using way too much coax, the choke is too far from the feed point, and it is touching the metal support mast. All of these factors are noted to be detrimental to the function of the choke.

For 11m a correctly installed choke that is about 4' - 6' is all that is necessary to provide the choking impedance necessary to help stop CMC's in a balanced dipole.

 

[Marconi]

Radio Shack coax is the finest dummy load available. Doesn't work well for getting RX or TX for a radio - tho. The loss of efficiency per foot on this stuff is really awful.

A halfwave dipole should be the length of two 1/4 wave antennas going in opposite directions. That should make each half nine feet long. BTW - did you check the SWR? What did it show? Also - how high off of the ground was it? Did any of the dipole have contact with tree branches or anything else? Close to any metal objects? Just some thoughts/questions about setup...

Rob, you have to consider that both HomerBB and longhair's dipoles are made of much larger diameter tubing than the 1/2 wave ss whip antenna that needs to be close to 9' long to be resonant in 11 meters just like Homer mentions.

 

[longhaireddwb]

The conduit I used is 3/4" I believe so that's why its not as long as far as I can figure. And I do have the coax leaving the antenna at 90 degrees or close to 90 degrees for about 10 feet. If having the bottom element close to the tree can cause noise then that and the cheep coax must be my problem. The antenna is screwed to a vertical tree limb and is only 2 " away from it over half of the length of the antenna.

So, do you think building a balun and putting it right next to the antenna and replacing the cheep ass coax and would quite it down?

Oh BTW, I'm at 1.3 on SWR and with my 100 watt amp I'm at 1.6.

 

[W5LZ]

I'm afraid you will find that the length of a 1/2 wave dipole depends on several things, and only deals with resonance, never SWR. That length will depend to some extent on where the thing is positioned (and how) when it's tuned. Doing that tuning at fence level and horizontally, means that if you mount it vertically higher off the ground, it's tuning will change a bit.
That 9 feet 1/4 wave common length is a good starting point (18 feet for a 1/2 wave), but is almost never the length you'll end up with, it'll be shorter than that. The diameter of that antenna also plays a part in length. The larger the diameter of the conductor the shorter it'll end up. Don't expect huge differences in length until that diameter also get's huge. That diameter thingy is related to fractions of a wave length, and can be calculated if you are really into that sort of thing (and love math). Why a 'common' length that's too long to start with? Cuz it's a lot easier to cut it off than to add to it!
The more of that feed line that the antenna can 'see', the more RF it's going to put onto that feed line that you really don't want on it. So, running the feed line perpendicular to the antenna for as far as possible is a pretty good idea. So is adding a choke once that feed line gets 'below' that antenna. That's not a 'cure all', but it can certainly help.
Once you get the length 'right' for resonance, then you start worrying about impedance matching, SWR. Adjusting impedance/SWR is pretty easy with a horizontal dipole. You just adjust the angle between it's two 'legs', droop them a little. It works. You can do exactly the same thing with a vertical dipole, but it get's a little more difficult. May not 'look' right, you know? But, it still works, and if it works who cares what it looks like? One way of making it 'look' better is by addng a second bottom 'leg', make the thing look like an upside down 'Y'. (Oh ho! See where that's going??) If you mess with the lengths of the legs of that dipole you can get the SWR down to a nice number. The problem with that is that you've just screwed up it's resonance point, which is not exactly the best thing you can do.
All kinds of variations are possible, some 'better' than others, and some more practical than others. Do the one's you can do, then live with it. That's what it amounts to. And the really 'biggy' is to just have fun doing it.
- 'Doc

 

[HomerBB]

I think HomerBB's dipole is still a bit too close to his roof, but he discusses a little bit about such concerns as he describes installation. He also is using a coaxial choke to help with the problems of CMC's, even though IMO he is using way too much coax, the choke is too far from the feed point, and it is touching the metal support mast. All of these factors are noted to be detrimental to the function of the choke.

For 11m a correctly installed choke that is about 4' - 6' is all that is necessary to provide the choking impedance necessary to help stop CMC's in a balanced dipole.

I agree with the assessment regarding my distance from th roof, and the length and placement of the choke. After the photos were taken I re-turned the choke to a proper length for 11 meters, and raised the antenna.
I tried to run it vertical at the greater height and found the mast to be negatively impacting SWR. A diagonal slanted position helped with that and both local and DX contacts were possible. I thought that to place the large sized (1 1/4" dia) dipole distant enough from my mast not to be affected by the mast would have required a boom so why not just make a beam instead. I did, a 2 el quad. Nevertheless, I am still quite proud of the results I got with the dipole. Worked really well. All of this was for the fun of it.

 

[Marconi]

The conduit I used is 3/4" I believe so that's why its not as long as far as I can figure. And I do have the coax leaving the antenna at 90 degrees or close to 90 degrees for about 10 feet. If having the bottom element close to the tree can cause noise then that and the cheep coax must be my problem. The antenna is screwed to a vertical tree limb and is only 2 " away from it over half of the length of the antenna.

So, do you think building a balun and putting it right next to the antenna and replacing the cheep ass coax and would quite it down?

Oh BTW, I'm at 1.3 on SWR and with my 100 watt amp I'm at 1.6.

No longhair, IMO adding a coax choke, in your case with the bottom element so close to the tree, probably won't help much with the problem describe. There are times when my noise blanker doesn't work or work good, but if your NB'r never works---then you just have to deal with whatever Mother Narure has to offer like we all do.

I think the main problem you have---is the antenna is not in the clear where it needs to be. You need to try another radio with a good NB'r. In my area in SE Texas, this time of year always produces changing conditions and very bad noise at times. For me, it is just not a good time for comparing or elvaluating our antennas.

 

[longhaireddwb]

Thanks for the reply Marconi.
I'm limited to where I can put this thing so I'm not too sure I can make it any better. I rent an apartment in the lower level of a two story. The tree I put the antenna in is about 15 feet from the building. The building has vinyl siding but I know underneath the siding is a foam insulation with a metal backing so putting it on the side or close to the building would be worse I figure. And standing an antenna on the roof would be hard to do without guy wires and putting holes in the roof. That's a no-no with a new roof installed. I know the landlord would have a fit screwing Eyelets into the roof.

I don't want a horizontal antenna as I want to talk to the Truckers on channel 19 on I-25 that is three miles away as the bird flies. That's why I went with vertical.

I will try another radio and compare the noise levels to see if there's a difference as you said. I'm open to ideas to make this better but with the limitations I have I'm not sure what else to try. I'll do some more reading on antennas. Maybe learn some more.

 

[RatsoW8]

Hey Longhair,

I'm kind of in a similar situation in that I can only put up stealthy wire antennas these days. I managed an OCF dipole for the ham bands, 40-10m meters, and put up a 1/4 wave vertical wire for 11 meters, which would work well in your case. The benefit is that it's constructed around an SO-239 connector(turned upside down) and the coax hangs straight down from the bottom of the antenna with two legs (which act as radials) soldered to it, same length as the vertical radiator,drooping with about about a 90 degree angle between them. And of course the entire antenna and main vertical radiator hang from some thin dacron antenna rope. It's very light weight and I have it up at about 40 feet in a large tree and it works very well on 11m. I can hear local stations at s9 signal levels out to about 15 miles or so and of course "skip" when band conditions are there. I cut for a center freq. of 27.185 Mhz (ch 19) and the swr is almost flat without any trimming. Best of all the entire antenna was built out of wire and a connector I had in my junk box.

Good luck...