Dipole Antenna using PVC 1/2 inch pipe ?

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cjrjr507

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Hi All

I plan on making an milair dipole antenna using 1/2 inch PVC pipe as the housing. Would the PVC pipe decrease the signal ? I going to use TV twin lead 300 ohm antenna wire and put it in the pvc pipe for weather protection. If it works good I am going to make 3 more dipoles for 118-137 mhz airband, 406-420 mhz band and 450-512 mhz band. Any info would be appreciated. Thanks in advance.
P.S. I'm going to mount the dipoles vertically for omnidirectional use.
 

krokus

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None of those frequency ranges are MilAir.

PVC will have zero theoretical affect on the signals. There would be more affect from weather, on the 300 Ohm lead. (You would need to make sure you have correct velocity factor, for the specific cable you are using.)
 

LtDoc

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There are different types of PVC, some contain more 'metal' than others. I know the 'white' PVC doesn't have any appreciable affect on RF, and the 'gray' does. Can't tell you the 'names' of each type, never heard them. Since that PVC is only for structural strength, holds the wire up, as long as whatever you use isn't 'conductive' (electrically neutral) it should work fine. Use what you have or what you can handle best.
- 'Doc
 

nanZor

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Hi All

I plan on making an milair dipole antenna using 1/2 inch PVC pipe as the housing. Would the PVC pipe decrease the signal ? I going to use TV twin lead 300 ohm antenna wire and put it in the pvc pipe for weather protection.
Are you planning on making a folded-dipole, or are you just going to tie the ends of the twinlead together on each end to simulate a "fat" wire for a standard dipole? I'm assuming you are going to use coax for the feedline back to the radio.

Also note that after stuffing an antenna inside a protective cover with small spacing between it and the antenna elements, you may need to shorten the antenna a little bit, as the covering detunes it lower in frequency somewhat. By all means don't use the black/grey pvc.
 
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cjrjr507

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Well, I got the PVC pipe dipole antenna built useing white pvc pipe. Had to lengthen the antenna a little so I could attach it to the 4 way terminal strip. It should be around 216 mhz, give or take 5 mhz. it should pick up milair no problem. now I just need to get some low loss coax and a preamp for it. I did tie the wires together on one end of the twin lead but just left the other end cut off with a hole drilled in the plastic to attach it to the small screw hook in the pvc caps on the end of the antenna. Have another antenna like that and it does seem to pick up ok. Also used an 300 to 75 ohm balun. I had the antenna cut to 225 mhz but the wire would not reach the terminal screw, rats! So had to make it a little longer, oh well.
 

willgrah

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I use a Dr. Fong PVC Dual Band J-Pole with great success. He is very specific that 200 PSI PVC 3/4" in his instructions. He does state that other types react differently. His write up is all over the internet, just google his name, Dr. Ed Fong, and put in J-Pole. He might have a good explanation of why. Hope this helps, Bill
 

cjrjr507

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Well I put up the antenna in a temporary spot to try it out and it seems to be working great on fm but on AM mode I am getting electrical interference on some but not all frequencies. It might be just the spot it's in as there is a power line in the alley and the one going to my house. I will have to try it somewhere where there is no power lines to see if the interference goes away. Hmmmm.
 
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nanZor

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.. I did tie the wires together on one end of the twin lead but just left the other end cut off with a hole drilled in the plastic to attach it to the small screw hook in the pvc caps on the end of the antenna. Have another antenna like that and it does seem to pick up ok. Also used an 300 to 75 ohm balun
Cool! If I read your description correctly, you really don't need the balun. Just go direct with coax to the terminal screws, as you have a basic dipole, not a folded one.

For mil-air's large spectrum, I would be tempted to cut ONE LEAD of each twinlead wire-pair down to about 8 inches. You could still maintain that hole you use to keep it taught inside. You have covered the low end of the milair-band well already, but what about the high-end up near 350-380 mhz or so? With only one side of each piece of twinlead at 8 inches, and the other wire kept long as you already have it, it might make for a broader response. Kind of like a fan-dipole, with one short wire, and one a bit longer. (on each side of course)
 
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cjrjr507

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Ok, I found the problem what was causing the electrical interference on AM mode. I took out the twin lead I had in there and replaced it with the other twin lead I had cut that was shorter in length; an 1 1/2 shorter than what 225 mhz calls for to allow for the 1 1/2 space in the terminal. That cleared up the interference for milair band but it's still there when the scanner is tuned to the vhf air band, but that is because the antenna is too short for that band. You have to have the balun because you have two different impedences, dipole 300 ohms & coax 75 ohms, the balun is a matching transformer. A dipole is balanced and coax is unbalanced, duh! I might put in a couple of small pieces of hook up wire bent at 90 degrees at the terminal to keep the twin lead straight. Oh, I found out one thing. You can't connect two different antennas, 30 mhz dipole & 225 mhz dipole to an 2 way splitter/combiner, heh heh. The scanner thinks it's one big antenna. Major interference! Ha hah Need an antenna switcher.
 
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nanZor

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Ok, I'm a little unclear on what you've got but I'll take your word for it.

If you must use a matching transformer, know that some of them are auto-transformers that share a common ground. This could be bringing in noise from your coax braid. You can check with a vom and see the continuity on all the terminals.

An isolated transformer might help. I've used the Radio Shack model 15-1140 and they are truly isolating transformers. They also have a 15-1230 that appears to be just a gold-plated version (ahem), which I haven't tested for isolation, but my guess is that it would be isolating too. Some guys even hook these back to back for 75 ohm systems, just to break the braid galvanically - if you can handle the loss.
 
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cjrjr507

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Oops! That should have been 1 1/2 inch space between the terminal screws, heh heh. Thanks for the info on the isolation transformers. I knew they make them. But spaced them out. What I have is just a plain old dipole antenna. Still fiddling with it. I will get some better coax sometime when I can afford it. I know an double or quad shielded coax would be better for interference problems but all I had was some single shield 75 ohm cable coax. Would'nt the dipole pick up electrical interference on the negative side of the dipole ? Or is it just the braid picking it up ? Hmmm. Maybe I can connect 2 different dipoles to an splitter/combiner if it's the coax I'm using. Might just need better coax. The single braid stuff ain't cutting it. It's ok if your in an area with no power lines. Can you get electrical interference from the sun? I noticed this morning as the sun was coming up the interference was increasing more. The frequencies were fairly quiet until then.
 

nanZor

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So we're on the same page - I threw my attic milair folded dipole on the floor. Is yours electrically like this? It's nothing but the RS isolated 300:75 ohm transformer, a terminal strip and a squashed loop of #12 house wiring about 18 inches long total. Nothing fancy. Hung vertically of course.

I could have used twinlead to do the same. Or for a normal dipole, it would just be an 9-inch element on each side (no squashed loop), and no need for the transformer.
 

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cjrjr507

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Dipole Antenna

So we're on the same page - I threw my attic milair folded dipole on the floor. Is yours electrically like this? It's nothing but the RS isolated 300:75 ohm transformer, a terminal strip and a squashed loop of #12 house wiring about 18 inches long total. Nothing fancy. Hung vertically of course.

I could have used twinlead to do the same. Or for a normal dipole, it would just be an 9-inch element on each side (no squashed loop), and no need for the transformer.
Hi hertzian

I was looking at your picture of the dipole and is that a 1/2 wave ? 225 mhz? It looks like a 3/8 wave dipole.
1/2 wave dipole is 2.08 ft at 225 mhz or 24.96 inches in length.You probably know that. Ah, did some calculations and yours is cut for 260 mhz, about the middle of the satcom band (255 mhz). Picture of my antenna is attached. It just shows the pvc pipe. My dipole is around 27 inches wide, tubes are 1 inch to long on each side. Got to do some chopping, heh heh. Should be around 24.96 inches
 

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cjrjr507

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Picture of dipole wiring

Here's a picture of the inside of the dipole cup that shows the wiring, had to do a little surgury to get the length right, ha ha. It was off some.
 

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prcguy

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If you look at the dipole construction use in the post # 5 pdf I think it would be a much better method for UHF mil air. I'm only pointing out making one dipole, not the whole 4-bay array which is not possible for mil air.

The pdf is for a VHF antenna but using copper pipe for the elements will give you some good band width needed for the mil air band and the element length would be around 8" or 9" for mil air. Ideally the elements should be around 1 1/2" to 2' diameter, which is more common for military antennas covering this band.
http://forums.radioreference.com/build-your-own-antenna/109144-4-bay-vhf-dipole-array-project.html
prcguy
 

nanZor

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Here's a picture of the inside of the dipole cup that shows the wiring, had to do a little surgury to get the length right, ha ha. It was off some.
Awesome! Yep, you are right about my center frequency near 255 mhz.

It sure looks like a normal dipole to me. You've got the leads twisted together inside the cup, and the leads are open at the far ends right? In this case, no need for the transformer.

Check out prcguys antenna - nice construction, and the use of tubing helps assure a wider bandwidth - something we'll never obtain with just wire on the wide milair band....
 

nanZor

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If you look at the dipole construction use in the post # 5 pdf I think it would be a much better method for UHF mil air. I'm only pointing out making one dipole, not the whole 4-bay array which is not possible for mil air.
Fantastic! Thanks for making that pdf. Looks like the ideal "upgrade" from a narrow-banded wire antenna. Even my fat folded dipole has a limited bandwidth that doesn't do justice to the low and high end.

In addition to the bandwidth, I also get concerned about lobes going upwards at the low and high end when cutting for the middle of milair.

What do you think about this: Use TWO antennas for milair to help preserve low reception lobes. Cut the milair band in half. Then make two dipoles cut for the centers of these "two" bands (one low, one high), and then phase them together? The objective here is to continue to use wide-bandwidth techniques (tubing, etc), but concentrate on keeping the lobes as low as possible by using two antennas cut for different center frequencies .... Probably not a practical enough problem for most, but now you have me wanting to cut up some tubes and coax!
 
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prcguy

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Using two antennas at different ends of the band and phasing together might be a challenge, it would have to be modeled to make sure there would be no funny business.

For a single mil air antenna I would rather make a fat coaxial dipole using copper tubing around 1.5 to 2" diameter that can sit on top of a mast instead of one held out from a mast like the OP. I have a few Collins AS-1404 225-400MHz coaxial dipoles that would be fairly easy to reproduce or you can just buy them on Ebay here:
AS-1404/PRC-41 UHF MILITARY RADIO BAND ANTENNA | eBay

I also have a military 225-400Mhz 4-bay collinear array but its sealed in a fiberglass radome and I don't know how its constructed. The company (Chu & Associates) has patents that could probably be looked up, otherwise its very difficult to phase antennas that cover a 2:1 frequency range.
prcguy

Fantastic! Thanks for making that pdf. Looks like the ideal "upgrade" from a narrow-banded wire antenna. Even my fat folded dipole has a limited bandwidth that doesn't do justice to the low and high end.

In addition to the bandwidth, I also get concerned about lobes going upwards at the low and high end when cutting for the middle of milair.

What do you think about this: Use TWO antennas for milair to help preserve low reception lobes. Cut the milair band in half. Then make two dipoles cut for the centers of these "two" bands (one low, one high), and then phase them together? The objective here is to continue to use wide-bandwidth techniques (tubing, etc), but concentrate on keeping the lobes as low as possible by using two antennas cut for different center frequencies .... Probably not a practical enough problem for most, but now you have me wanting to cut up some tubes and coax!
 

nanZor

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Using two antennas at different ends of the band and phasing together might be a challenge, it would have to be modeled to make sure there would be no funny business.
I did just that - and would have wasted a lot of material because there really isn't any issue with the lobes going high. Glad I modeled first!

I did learn something about the bandwidth, and one interesting thing about folded dipoles using tubing.

Here come some EZnec pics of the various bandwidths from:

1) Dipole made with nothing but #12 wire
2) Folded dipole made with #12 wire (what mine does in the attic)
3) Tubing dipole
4) Folded dipole with tubing
 
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