Dipole Antenna using PVC 1/2 inch pipe ?
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Of course modeling doesn't take into account real world affects, like me mounting a nice tubing folded dipole into an attic, as compared to someone mounting a skinny dipole outside up high etc. 
I'd go with the latter for sure.
What I see is that the wire-folded dipole is not all that bad when compared to the standard tubing dipole except at the bottom of the band. Of course the tubing dipole can actually be mounted outside like a real antenna should.
One interesting thing about the last one, the very wide folded-dipole made of tubing, is that I saw about a 2 to 3 db reduction of the main lobe on the side opposite the feedpoint when I did the directional plot. It makes me think that if one is using folded dipoles made with tubing, is to put the feedpoint facing the direction of interest, ie furthest from the tower or mount pointed towards the target just to be sure. (wire dipoles not affected by this shift much) And, this effect only started getting noticeable at center frequency (2db) and got worse up at 380 mhz (3db).
Again, this is all just modeling - the real world and my construction techniques can make a big difference - usually for the poorer.
I'd go with the latter for sure.What I see is that the wire-folded dipole is not all that bad when compared to the standard tubing dipole except at the bottom of the band. Of course the tubing dipole can actually be mounted outside like a real antenna should.
One interesting thing about the last one, the very wide folded-dipole made of tubing, is that I saw about a 2 to 3 db reduction of the main lobe on the side opposite the feedpoint when I did the directional plot. It makes me think that if one is using folded dipoles made with tubing, is to put the feedpoint facing the direction of interest, ie furthest from the tower or mount pointed towards the target just to be sure. (wire dipoles not affected by this shift much) And, this effect only started getting noticeable at center frequency (2db) and got worse up at 380 mhz (3db).
Again, this is all just modeling - the real world and my construction techniques can make a big difference - usually for the poorer.
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nd5y
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Of course modeling doesn't take into account real world affects, like me mounting a nice tubing folded dipole into an attic, as compared to someone mounting a skinny dipole outside up high etc.
What I see is that the wire-folded dipole is not all that bad when compared to the standard tubing dipole except at the bottom of the band. Of course the tubing dipole can actually be mounted outside like a real antenna should.
One interesting thing about the last one, the very wide folded-dipole made of tubing, is that I saw about a 2 to 3 db reduction of the main lobe on the side opposite the feedpoint when I did the directional plot. It makes me think that if one is using folded dipoles made with tubing, is to put the feedpoint facing the direction of interest, ie furthest from the tower or mount pointed towards the target just to be sure. (wire dipoles not affected by this shift much) And, this effect only started getting noticeable at center frequency (2db) and got worse up at 380 mhz (3db).
Again, this is all just modeling - the real world and my construction techniques can make a big difference - usually for the poorer.
Sinclair Technologies Online Product Catalog - SD310-HF1P2SNM
That Sinclair looks like it would withstand anything!
Makes me wonder just how/where the actual feedpoint of that folded dipole is - on the element that is clamped, or on the outside facing away from the mount?
Brings up another point - I'm sure the impedance transformer inside for the sinclair has much less loss than my RS tv-type balun!
Update: my mind is racing on the Sinclair - especially what looks like a red piece of plastic seperator. Could this be some sort of offset-feed and the rest folded back on itself? I don't see "folded" anywhere in the manufacturer's description. Perhaps a "C-pole" aka KF2YN design? Back to EZnec I go!
Makes me wonder just how/where the actual feedpoint of that folded dipole is - on the element that is clamped, or on the outside facing away from the mount?
Brings up another point - I'm sure the impedance transformer inside for the sinclair has much less loss than my RS tv-type balun!
Update: my mind is racing on the Sinclair - especially what looks like a red piece of plastic seperator. Could this be some sort of offset-feed and the rest folded back on itself? I don't see "folded" anywhere in the manufacturer's description. Perhaps a "C-pole" aka KF2YN design? Back to EZnec I go!
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oops, I forgot - my enthusiasm for collinear arrays for airband (civil and milair) took a major nosedive when I got great gain on the main lobe, but it was pencil thin. Man, I could hear aircraft going above and below it real fast and it got really annoying. I'm sure some need it, but quite frankly, that pencil thin lobe made me think of my OCFD's on milair. Point-to-point ok, but for flying things - I'll take larger lobes vs gain.
One thing I haven't tried for a simple kitchen-table indoor antenna is a dedicated milair bow-tie. Not sure it would be any better than a folded dipole....
nd5y
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The feedpoint is the black section opposite of the mount. The coax runs inside the loop. The red band is just paint to mark the polarity when you assemble several into an array. I don't think they have a any kind of balun. If they have a matching section it is probably just a specific length of higher impedance coax.
Thanks Tom!
I was getting a little burned out trying to do an EZnec analysis on it and getting nowhere thinking it was something else.
The great part is that everyone here got me to look at this in more detail, and my final tweak to the wire-version of the folded dipole for milair is still 18 inches in length, but now with a 3-inch separation instead of 2 inch. Brought the low end swr down a little bit. Still, the tubing variants, both simple and folded are the champs.
I was getting a little burned out trying to do an EZnec analysis on it and getting nowhere thinking it was something else.
The great part is that everyone here got me to look at this in more detail, and my final tweak to the wire-version of the folded dipole for milair is still 18 inches in length, but now with a 3-inch separation instead of 2 inch. Brought the low end swr down a little bit. Still, the tubing variants, both simple and folded are the champs.
Ah, did a little more searching on this, and it looks like the canonical way to feed the folded dipole that can be used for transmitting is to use an electrical quarter-wave (velocity factor taken into account) of 125 ohm coax like RG63. This quarter wave transformer will then do the required transformation from 288 ohms or so down to 50 ohms. Run it inside the tubing, and you have no major worries about common-mode current skewing the pattern. Neat - now I know. Maintaining that transformation across the entire milair band would be interesting to see if it could be done.
On a related front, I put a 200 ohm carbon resistor across the leads of the RS transformer/balun, and attached it to a Comet analyzer (50 ohm input) It did a nice clean 4:1 transformation, and started to get a little squirly above 250mhz, but appeared to be within reason. No idea of the actual loss however. But yeah, it seems like it will do a nice 4:1 for a 300 ohm folded dipole down to 75 ohms from about 25 mhz to 450 mhz or so for rx only..
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I just picked up the 15-1230 with it's magical gold-plating (ha ha) for nearly the same price as the non-gold one, and it too is isolated.
There are quite a few different ways an 'array' can be arranged/mounted, they don't have to be all on the same 'pole'. They can also be mounted all at the same level, not one above another. Each element of that array can be of several different 'styles', dipoles, folded dipoles, etc. The hardest part of the whole thing is in the impedance matching required, and that's certainly not all that 'impossible' either. It amounts to what you have to work with and the type/shape of the desired radiation pattern (reception pattern). (And how much trouble/expense you want to put into it.)
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blue5011
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Folks sure go to a lot of work building something. Why not just buy a ready-made product and be done with it? Outdoor VHF-Hi/UHF Scanner Antenna Model: 20-176, costs all of $30.
The reasons for building your own vary.
One example is that the 20-176 is a multi-band quarter wave groundplane on the primary frequencies. I prefer at least a dipole with a little bit of gain, but most importantly, a lower lobe than what the 20-176 provides. And it is not resonant on milair, where I do most of my listening. On the bands where the 20-176 is resonant on harmonics, it may actually be looking upwards into the sky, rather than outwards across town.
Plus, building your own is just plain fun. You'd be amazed at how much better performance you can get over the 20-176 with $30 worth of your own materials. I've made many mistakes making my own, but that is part of the process.
One example is that the 20-176 is a multi-band quarter wave groundplane on the primary frequencies. I prefer at least a dipole with a little bit of gain, but most importantly, a lower lobe than what the 20-176 provides. And it is not resonant on milair, where I do most of my listening. On the bands where the 20-176 is resonant on harmonics, it may actually be looking upwards into the sky, rather than outwards across town.
Plus, building your own is just plain fun. You'd be amazed at how much better performance you can get over the 20-176 with $30 worth of your own materials. I've made many mistakes making my own, but that is part of the process.
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I have finally settled on what I think is the best compromise for the wire-version of the folded dipole. I lowered the resonance a little bit by using a total length of 19 inches. And have gone back to no more than 2-inch spacing.
But, the biggest issue here is the loss of the 300:75 ohm transformer itself. Based on reading about half a decade's worth of threads from the TV dxer's, I'm probably looking at 3db of loss at 300mhz. K6STI did a nice job of measuring the loss of the RS transformer as 0.85db, but this was at FM broadcast. So despite the improved bandwidth, and simple construction, there is a significant hit. At least it maintains the right look angles.
Playing around with wire, or thin elements of amateur antennas will only take you so far in milair. To do this right, a simpler dipole made of at least 1-inch tubing cut for perhaps 275 mhz and directly connected to coax is the way to go and is what I'd call the baseline standard. The wire folded dipole is working well - but that transformer is staring me in the face every time I look at it.
But, the biggest issue here is the loss of the 300:75 ohm transformer itself. Based on reading about half a decade's worth of threads from the TV dxer's, I'm probably looking at 3db of loss at 300mhz. K6STI did a nice job of measuring the loss of the RS transformer as 0.85db, but this was at FM broadcast. So despite the improved bandwidth, and simple construction, there is a significant hit. At least it maintains the right look angles.
Playing around with wire, or thin elements of amateur antennas will only take you so far in milair. To do this right, a simpler dipole made of at least 1-inch tubing cut for perhaps 275 mhz and directly connected to coax is the way to go and is what I'd call the baseline standard. The wire folded dipole is working well - but that transformer is staring me in the face every time I look at it.
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prcguy
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I still think for a single element antenna, a very fat coaxial dipole would be the best choice since it can be mounted with no mast interaction.
Consider something like 2" dia copper pipe for the elements with maybe a simple SO-239 to 3/8 stud adapter at the feed point. I would mount the stud adapter in a 1" copper end cap with the connector threads inside the cap then the hot side of the stud will connect to another 1" pipe cap facing away from the connector.
Then use a 1" to 2" reducer to taper the 2" pipe elements to 1" at the stud adapter. I think if you used two opposing 2" pipe caps spaced 1/8" apart at the feed point it would create too much capacitance and reducing to 1" would help.
prcguy
Consider something like 2" dia copper pipe for the elements with maybe a simple SO-239 to 3/8 stud adapter at the feed point. I would mount the stud adapter in a 1" copper end cap with the connector threads inside the cap then the hot side of the stud will connect to another 1" pipe cap facing away from the connector.
Then use a 1" to 2" reducer to taper the 2" pipe elements to 1" at the stud adapter. I think if you used two opposing 2" pipe caps spaced 1/8" apart at the feed point it would create too much capacitance and reducing to 1" would help.
prcguy
I built one out of discarded backyard umbrella mast, and NOW we are cooking! The elements slip over some pvc and the coax runs down inside. We could have gone with a side-feed, but with the wide spacing of the tubing as compared to the thinner coax run down the middle, it is a BIG improvement over the quickie "fold-the-coax-braid-back-down" type.
Thanks!
prcguy
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Glad to hear some progress, I'll make one soon myself. It's also good to keep the coax centered inside the bottom grounded element and a lot of antennas use doughnuts made of closed cell foam rubber or styrofoam for this.
prcguy
prcguy
Uplink
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Here's a mil-air antenna out of some parts lying around the garage. Old umbrella pipe, (cut to 9" each) some old ground strap from the back of a car stereo (reamed out one of the holes), some PVC pipe lying around, an F-Connector jack, and electrical wire. A couple bucks worth of junk, hung it up in the attic, and now it's a nice wide band UHF antenna. LOL
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Fast1eddie
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Nice!!! Hooah!!!!!!!!!!!!!!
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