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Author Topic: Three Wire Dipole  (Read 4450 times)
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W9GT
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« on: July 29, 2009, 11:03:19 AM »

Has anyone else tried one of these?  Its an ancient design from the old handbooks.  Anyone have access to modeling software who could model this thing?  I have had very good luck with this antenna on 75M using open wire feeders and a remote tuner which is essentially a KW matchbox.  I'm running coax back to the shack (about 230 ft.)  This thing never ceases to amaze me at how well it seems to work vs a plain dipole.  I haven't tried to use it on other bands yet, but I expect the results would be good.

I believe that, theoretically at least, the only real advantage of this "double folded dipole" might be bandwidth capability, however, empirical results have been outstanding vs a regular dipole or inverted vee.  Lots of variables, however, and there may be other things coming into play.  Anyway, this antenna works better than other 75 M antennas that I have tried.

It might prove to be an ice catcher in the winter....lots of wire and spacers up there...heavy antenna, but the performance sure has proven to be good.

73,  Jack, W9GT

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« Reply #1 on: July 29, 2009, 11:21:56 AM »

Has anyone else tried one of these?  Its an ancient design from the old handbooks.  Anyone have access to modeling software who could model this thing?  I have had very good luck with this antenna on 75M using open wire feeders and a remote tuner which is essentially a KW matchbox.  I'm running coax back to the shack (about 230 ft.)  This thing never ceases to amaze me at how well it seems to work vs a plain dipole.  I haven't tried to use it on other bands yet, but I expect the results would be good.

I believe that, theoretically at least, the only real advantage of this "double folded dipole" might be bandwidth capability, however, empirical results have been outstanding vs a regular dipole or inverted vee.  Lots of variables, however, and there may be other things coming into play.  Anyway, this antenna works better than other 75 M antennas that I have tried.

It might prove to be an ice catcher in the winter....lots of wire and spacers up there...heavy antenna, but the performance sure has proven to be good.

73,  Jack, W9GT

Looks like a cage.  What's the spacing, and I'll see if I have one in the antenna program I can slightly modify to see what your's is actually doin for ya.

--Shane
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W9GT
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« Reply #2 on: July 29, 2009, 11:32:42 AM »

Hi Shane,

Its not really a cage because the only wire that is fed in the center is the middle one.  A cage antenna has all of the dipole elements paralleled and they are tied together at the center as well as the ends.  A fan dipole just ties them together at the center.  This is essentially a double folded dipole.  The spacing between the elements is 6" (same as the open wire feeders).



73,  Jack, W9GT
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KD6VXI
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« Reply #3 on: July 29, 2009, 12:00:15 PM »

Hi Shane,

Its not really a cage because the only wire that is fed in the center is the middle one.  A cage antenna has all of the dipole elements paralleled and they are tied together at the center as well as the ends.  A fan dipole just ties them together at the center.  This is essentially a double folded dipole.  The spacing between the elements is 6" (same as the open wire feeders).



73,  Jack, W9GT

Blah, I went and looked at the pic while I read your description, and I got it now.

I'll see if I can get it to model.

--Shane
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W9GT
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« Reply #4 on: July 29, 2009, 12:15:13 PM »

Thanks Shane.  By the way, the average height of the antenna AGL is about 50-55ft with the ends about 60ft.  Length of the feeders to the tuner is about 60 ft.

73,  Jack, W9GT
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« Reply #5 on: July 29, 2009, 01:39:14 PM »

It should have the same pattern as a regular dipole.  It is basically an impedance transformer.  But it is supposed to be broader tuning.

A regular dipole has about 75Ω impedance at the feed point.  The conventional two-wire folded dipole is nothing but two half wave antennas in parallel, with the ends tied together and the power fed into only one of them.  The current will divide equally between the dipoles so that each will have half the current of a single wire dipole.  Since we are feeding only one of the wires, to get the same amount of power (P) at half the current (I), we need 4 times the impedance (Z).

                                     P = IČ Z

If we cut I in half, IČ is reduced to one fourth, so Z must be quadrupled to maintain the same power.

With the three wire folded dipole, each wire carries one third of the total current.  Since we are cutting I to one third, IČ is reduced to one-ninth.  Therefore, the feedpoint impedance Z is 9 times that of a regular dipole, or 675 ohms. 

This would allow a 600Ω open wire feed line to run almost flat.

One drawback with the folded dipole is that it won't work at even harmonic frequencies like a regular open wire fed dipole will.  The phase is reversed at the central point of the fed wire, but the central point of the other wire(s) is solid, unbroken conductor, so no phase reversal can take place. Therefore  the fed wire will be out of phase with the other wires, and it won't work as a multi-band open-wire fed dipole.
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W9GT
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« Reply #6 on: July 29, 2009, 01:55:19 PM »

Thanks Don, good explanation.  Perhaps if I wanted to use this antenna on other bands, it would be wise to tie the elements together in the center, making it look like a cage dipole.  I'm not sure if that would work as well on 75M, however, as it presently does. 

Like I said, I don't know what other factors might be contributing to the good performance that I have experienced with it on 75M.  Maybe it is best not to over analyze success and if ain't broke, don't fix it.  Anyway, it is a pretty neat old buzzard antenna.

73,  Jack, W9GT
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flintstone mop
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« Reply #7 on: July 30, 2009, 01:29:43 PM »

It looks like one of those B&W NVIS antennas, minus the dummy load at the feedpoint, which was coax fed.

Fred
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« Reply #8 on: July 30, 2009, 01:39:03 PM »

I cant see the attachement on this PC for some reason.. Is this the same as a Cobra?  The Cobra uses 3 wires on each side in parallel.

Clark
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W9GT
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« Reply #9 on: July 30, 2009, 02:03:39 PM »

It looks like one of those B&W NVIS antennas, minus the dummy load at the feedpoint, which was coax fed.

Fred

It works very well as a radiating antenna, not as a dummy load!  I understand that many of those B&W antennas (or those made by others and containing resistive elements) make excellent dummy loads, certainly not good antennas except for the fact that they might look like a good match to the transmitter.  Wow.......this is a tough crowd!  

Anyway, I was just looking for a little data or insight on how/why this antenna works well compared to other configurations I have tried at my location.  I am fully aware of the fact that it is essentially a folded dipole and should work like any other dipole  (i.e. 0 dB gain, more accurately, 0 dBd) and is effectively an omnidirectional radiator.   I wonder if "capture area" as a function of the larger physical size of this or other multi-element dipoles like the cage or fan congurations is any sort of a contributing factor.  Certainly, it has been proven that the larger combined cross section of the elements in these antennas makes them useable over a wider bandwidth.

As Don points out, the feed point impedance is a good match for the 600 ohm open wire feeders, however, the losses there would probably be negligible anyway.

Hey...I like to experiment and I like to try different things that, perhaps might be a little out of the ordinary.  This antenna works well for me.

73,  Jack, W9GT
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K3ZS
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« Reply #10 on: July 30, 2009, 02:18:53 PM »

According to the ARRL 1964 Antenna Handbook, this antenna should be broader band than a regular dipole.    This is because the effective size of the conductor is approximated by the spacing of the individual conductors.   Impedance is 9 times (as Don as shown) over the regular dipole if the conductors are equal in diameter.     Do you notice that you don't have to retune the tuner over a wider frequency range on 75-80 meters?
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W9GT
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« Reply #11 on: July 30, 2009, 02:28:55 PM »

According to the ARRL 1964 Antenna Handbook, this antenna should be broader band than a regular dipole.    This is because the effective size of the conductor is approximated by the spacing of the individual conductors.   Impedance is 9 times (as Don as shown) over the regular dipole if the conductors are equal in diameter.     Do you notice that you don't have to retune the tuner over a wider frequency range on 75-80 meters?

Yes, but I haven't taken any detailed measurements to determine just how wide of a bandwidth it exhibits.  The tuner is remote, so until I come-up with a way to remotely control the tuner, it is a little awkward to make changes.  For initial set-up, I used an antenna analyzer at the tuner.  I could probably play with the analyzer at that point and come up with a plot.


73,  Jack, W9GT
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« Reply #12 on: July 30, 2009, 04:13:14 PM »

Jack, in your first post you give the impression that the three-wire dipole is somehow superior to the single-wire dipole. In what way did you make that determination?

And regarding the bandwidth, we know that the three-wire will be somewhat greater than the single wire.

However, IMO, when using open-wire feeders with a tuner, the difference in off-resonance losses between the two types would be insignificant. Consequently, the greater bandwidth of the three-wire dipole in this case would be unimportant.

Walt
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W9GT
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« Reply #13 on: July 30, 2009, 04:39:41 PM »

Hi Walt,

I appreciate your input and certainly welcome your suggestions.

Here is what I said in my original post:

I believe that, theoretically at least, the only real advantage of this "double folded dipole" might be bandwidth capability, however, empirical results have been outstanding vs a regular dipole or inverted vee.  Lots of variables, however, and there may be other things coming into play.  Anyway, this antenna works better than other 75 M antennas that I have tried.

I realize my observations are based on somewhat subjective criteria and, as I indicated, there are possibly other variables coming into the picture.  I am only trying to figure out what mechanism(s) might be coming into play which have resulted in better performance from this antenna vs others I have played with.  Maybe....sort of a backwards analysis.  However, over some period of time, comparing the 3 wire dipole to an inverted vee and a standard dipole ( both fed with coax), I generally got much better and more consistent reports with the 3 wire dipole.  The location and avg height were nearly the same, although the other antennas were closer to my tower.  Maybe some interaction going on?  Anyway...I have enjoyed the results...whatever the reasons.

73,  Jack, W9GT


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« Reply #14 on: July 30, 2009, 05:15:37 PM »

I ran an unterminated 2 wire rhombic as a kid and found the johnson match box was easier to tune than a single wire rhombic. I suspect the 2 wire antenna had a lower Q.  I wonder if you opened the 2 connections at the center you would have a dual KB3AHE configuration and get it on 160 meters??
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« Reply #15 on: July 30, 2009, 11:28:52 PM »

Jack, in your first post you give the impression that the three-wire dipole is somehow superior to the single-wire dipole. In what way did you make that determination?

And regarding the bandwidth, we know that the three-wire will be somewhat greater than the single wire.

However, IMO, when using open-wire feeders with a tuner, the difference in off-resonance losses between the two types would be insignificant. Consequently, the greater bandwidth of the three-wire dipole in this case would be unimportant.

Walt

Would not the higher feedpoint impedance be better for broadbanding?

Seems I ran across something that said that somewhere, but I can't say for sure (what a scapegoat statement there!).

I modeled it, my model shows pretty much a dipole.  A few more lobes, but....  I modeled over "perfect ground", though.

--Shane
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The Slab Bacon
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« Reply #16 on: July 31, 2009, 07:57:52 AM »

I ran an unterminated 2 wire rhombic as a kid and found the johnson match box was easier to tune than a single wire rhombic. I suspect the 2 wire antenna had a lower Q.  I wonder if you opened the 2 connections at the center you would have a dual KB3AHE configuration and get it on 160 meters??

You know, Frank, I have wondered that myself. I have been pondering trying it, but mine has been so reliable I have been kinda complacent with "If it aint broke, dont fix it".

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« Reply #17 on: July 31, 2009, 08:56:29 AM »

I cant see the attachement on this PC for some reason.. Is this the same as a Cobra?  The Cobra uses 3 wires on each side in parallel.

Clark

Not the same as the "cobra".

The cobra's elements are just one wire on each leg of the dipole folded back on itself twice.

This ant is 3 wires in parallel, with the feed being in the center of the middle wire.

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« Reply #18 on: July 31, 2009, 09:40:09 AM »

I have read somewhere(?) that antennas with more wire produce a higher RF voltage at the receiver, making it seem a more sensitive antenna.    Receiver sensitivities are in microvolts, not microwatts.    Seems that this antenna being a higher impedance and several wires in series would do that, or is this a bunch of BS?
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« Reply #19 on: July 31, 2009, 10:09:12 AM »

A center fed dipole - is a dipole is a dipole, is a dipole - no matter how it's fed or how many wires it has. The horiz and vert radiation patterns are the same whether it's a folded, three wire or one wire. The only reason for these configurations is for better matching (feedline efficiency) to the particular feedline used.

When using coax or 75 ohm twin lead, a single wire is the best match. When using 300 ohm line, a folded dipole is a convenient match. When using openwire line, a three-wire dipole is a reasonable match.  However, openwire, because of its low loss can be used on any style dipole (one,two or three wire) and perform about the same efficiency.

The more wires, the broader the swr curve, but when using an antenna tuner with openwire, the complexity of construction and weight of a three wire dipole is a poor trade-off for slightly less fussing with the dials. 

Again, the antenna pattern is the same for all three, just the match (or method of delivering power to the dipole) is different.

I agree with W2DU's post above.



BTW, I once had up a three-wire dipole fed with openwire for about a year. When constructed with standard spacers it was a bear to keep up and support, especially for 75M. The strain on the supports and complete system was unreal during ice storms. For multiband use, structural integrity and simplicity, I would always recommend a SINGLE wire dipole for openwire or coax use.  Folded dipoles have their place for feeding certain kinds of arrays.

Tom, K1JJ



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« Reply #20 on: July 31, 2009, 10:15:14 AM »

Quote
It works very well as a radiating antenna, not as a dummy load!  I understand that many of those B&W antennas (or those made by others and containing resistive elements) make excellent dummy loads, certainly not good antennas except for the fact that they might look like a good match to the transmitter.  Wow.......this is a tough crowd!  


The B$W is a very effective antenna for emergency use and foreward deployment by the military. Hams OTOH try to milk every last tenth of a dB out of an antenna.

I ran a 2 vertical 80M cage array using 6 wires in a 12" circle at a prior QTH. The VSWR was under 2:1 from 3.5 to 3.9 mHz over an elaborate ground system.

Since that 3 wire FD is a single band antenna Id consider feeding it with a 9:1 broadband balun and coax. Im one who has no use for open wire feeders and antenna tuners.

Carl
KM1H


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wb1aij
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« Reply #21 on: August 05, 2009, 08:08:42 AM »

Hmmmm, a three wire dipole; isn't that an oxymoron? Maybe it should be called a tripole.
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« Reply #22 on: August 05, 2009, 09:07:27 AM »

Hmmmm, a three wire dipole; isn't that an oxymoron? Maybe it should be called a tripole.

Only has 2 'legs'.

Not 3.

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« Reply #23 on: August 05, 2009, 10:07:22 AM »

By the way, the three wire dipole is still working very well, Thank you.  Yes, the three wires are paralleled conductors....not three legs!  That would be a bit unsymetrical, but who knows...maybe a three phase radiator Huh Grin.

(See diagram in original post)

I sure would like to have one of these beasts up for 160 M, but just don't have the room.  Might be an interesting experiment to add a drooping or vertical section on each end to extend the length. 

73,  Jack, W9GT
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« Reply #24 on: August 05, 2009, 10:47:54 AM »

The term dipole has nothing to do with the number of wires but rather that there are electrically two poles or a pair of equal and opposite charges. One wire, 100 wires, there are still only two poles.
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