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Author Topic: "New" fan dipole calculations  (Read 3293 times)
W4MLO
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Posts: 30




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« on: March 26, 2009, 01:19:59 PM »

Hi All,
I am considering building a "fan Dipole"for 75/40/20 phone use. Has anyone actually built one using the new formulas/feedpoint/end seperation that SRI has come up with as posted on hamuniverse?(see link below) It sounds interesting and I am considering the project. I am rather scared off by all the posts that talk about how difficult the old school fans are to tune because it is VERY difficult to raise and lower my wire antennas due to terrain. It usually takes 3 people to do it.

http://www.hamuniverse.com/multidipole.html

Any thoughts, comments or experiences would be appreciated.

Thanks in advance,
73 Milo W4MLO





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W5GA
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« Reply #1 on: March 26, 2009, 01:51:23 PM »

Put it up and let us know how it works!
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WX7G
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Posts: 5973




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« Reply #2 on: March 26, 2009, 01:52:00 PM »

This looks good. Modeling fan dipoles in NEC I have noticed how forgiving they are when the wires are spaced per the article.

NEC shows less than ideal feedpoint impedances. Because of this the lowest SWR on a band might be only a bit below 2:1. This has gotten me more than once (I have built four different fan dipoles). Pruning them back to find minimum SWR I have overshot the mark.


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KF6A
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« Reply #3 on: March 26, 2009, 05:19:40 PM »

Give it a shot.

I've built them in the past but I cannot figure out why there is so much resistance to using ladder line.
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VE3FMC
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« Reply #4 on: March 26, 2009, 07:13:59 PM »

I have a coax fed Fan Dipole up for 80/40 and I also had 20 M on it for awhile.

I started by adding the 40 M dipole legs to the 80 M legs. No issues and I did not have to do any trimming. My 40 Meter legs were resonate at 7.070 and that did not change. My 80 M resonate frequency was 3.660 and that stayed pretty much the same.

Then I added the legs for 20 and I did have to do some adjusting on those legs. But I did get the resonate frequency right on 14.070.

So you might have to do some trimming. Best to make the legs longer than you want them to and trim them back.

The legs of my fan are made from 14 ga stranded insulated wire. I have to use insulated wire as the legs run through trees.

I have tried ladder line feed right into my shack but I have had RF issues doing that. Now this could be due to the fact my ladder length might be too short. I only have to run about 38 feet from the feed point to the tuner.

So I stick with coax and all is well in the shack!
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N3OX
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« Reply #5 on: March 26, 2009, 09:04:45 PM »

"I am rather scared off by all the posts that talk about how difficult the old school fans are to tune because it is VERY difficult to raise and lower my wire antennas due to terrain. It usually takes 3 people to do it. "

Generally speaking, if you have a fairly simple HF antenna design like this, and you construct it to tight tolerances, you can hit a given SWR right on the nose.

Whether or not the hamuniverse one works, I can't say, but I do know that I've built a few antennas that I modeled and didn't have to trim at all, like my 20m Moxon.

You just have to be precise.  If it says an element should be 16 feet 6 inches long, try to hit that to +/- 1/4 inch including the length of the end loops and whatever you need to connect to the feedpoint.  Don't make the mistake of thinking 17 feet or 16 feet is good enough, because that's more than enough to knock you right out of the 20m band if 16.6 feet is the right length.

For the smaller dimensions, like 5 inch spacing, mark it out with a ruler to within 1/16"

If it calls for bare wire, use bare wire.  If it calls for THHN, use THHN.  Normal insulation makes a 2% to 3% difference, again, enough to knock the sweet spot out of a ham band.  If it calls for 12 gauge, use 12 gauge and if it calls for 18ga, use that.

If you have to install your antenna near other metal objects or very near nonconducting but possibly wet objects like trees (especially if you get element tips near them), all bets are off unless you try to correct in a model.

I try to build HF wire antennas to within about a quarter of an inch over the length and I bought a metric tape measure so I could build my VHF antennas to within 1mm without worrying about 1/32 of an inch increments :-)

I just don't like trimming antennas in real life, and in the case of the Moxon which is on a mast I have to tilt up and down, or my 12 element UHF beam which has, well, 12 elements, it would be very tedious, so I model them in EZNEC and try to build them very accurately after that.

What I don't know is if the hamuniverse instructions are any good.  There's some serious nonsense elsewhere on that site.  If you very accurately build a design that wasn't right it won't work for you, but it *IS* possible to build antennas that you don't have to tweak and trim for acceptable SWR if you exactly copy a good working design and put it in the clear.

If it weren't possible, VHF beam manufacturers would have a hell of a time selling anything.

73
Dan




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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
K4SAV
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Posts: 1837




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« Reply #6 on: March 27, 2009, 12:15:06 PM »

The article on hamuniverse is based on something someone heard someone else say about a fan dipole.  Notice that the author of the article is anonymous and there is no indication that it was ever built and tested. The original source is attributed to Stanford Research Institute but no reference is given, nor can any be found.

The article says the only thing you have to do to eliminate the pruning of a fan dipole is to fabricate a center connecting block of a particular dimension.  It doesn't take a genius to know that is not correct.  Later in the article they did mention that the ends of the wires should be separated by 38 inches.  They apparently think the resonant frequency is independent of the wire size, height above ground, soil characteristics, and any other objects at your house.

I decided to see how close the given dimensions are to reality, so I modeled it in EZNEC using the exact info given including the connector block.  I chose the operating frequencies as 3.7 MHz, 7.1 MHz. and 14.1 MHz.  I used #12 bare wire. Here is the data for the antenna at 50 ft and at 30 ft and over two different grounds, average ground (Av Gnd) and very poor ground (VP Gnd).

Results:
Design Freq __ Av Gnd, 50Ft __  Av Gnd, 30 ft __ VP Gnd, 30ft
3.7 MHz  _____   3.92 MHz  ____  3.92 MHz  _____ 3.94 MHz
7.1 MHz  _____   7.58 MHz  ____  7.53 MHz _____  7.54 MHz
14.1 MHz ____  14.69 MHz ____ 14.78 MHz ____ 14.76 MHz

So it appears that the instructions are significantly in error.  You can argue that EZNEC does not calculate exactly what you will have at your QTH, and that is true because I did not include all the other antennas and metal objects at your house, your ground characteristics and the exact way you have the antenna mounted.  But then neither did the hamuniverse instructions.

Notice that there is also a note on the page that says:
"These lengths are not exact.  Some tuning may be required.  Use the standard formula 468/freq MHz for total feet for each band (frequency) of interest.  Adjust the length longer or shorter as needed"

HUH? I thought the point of the article was to eliminate the pruning.  This is typical of most of the antenna articles on hamuniverse.  It's garbage.

Jerry, K4SAV
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N3QE
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« Reply #7 on: March 27, 2009, 12:57:12 PM »

My advice is to not even try a multiband fan dipole. I tried, and I used NEC to model it, and the resonances came out nowhere near where the model says they should be, and the resonances moved a lot as the wires swung in the wind or got wet.

I got sick of hauling the thing up into the sky and back down trying to make it work. I tried for a while, and sometimes there seemed to be a method to the madness, but the end goal (having an antenna that would work on 80M, 40M, 20M, and 15M) was never realized. A two-band 40M/15M combo worked well but I could never get the other bands to co-exist despite way too much modeling and pruning and dealing with fan spacers.

Just put up an all-band doublet and feed with ladder line and a tuner. It worked 70 years ago and it still works today.
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AB9PM
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Posts: 27




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« Reply #8 on: March 28, 2009, 07:20:43 AM »

I use a fan dipole here. I started with 40 meters and then added to it, pruning to low swr, fed with coax. I am quite satisfied. However I understand that trying to have a fan dipole high in the air and dealing with pruning can be more than a pain in the rear, but it does have it rewards. Good Luck.

Dave
AB9PM
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WX7G
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Posts: 5973




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« Reply #9 on: March 28, 2009, 01:51:59 PM »

The multiband fan dipole can take some effort to tune. Cut to formula the VSWR can be low enough so that a tuner can be used at the shack without excessive coaxial line loss. Of course it depends on your definition of excessive line loss.  
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AE5JU
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Posts: 227




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« Reply #10 on: April 01, 2009, 01:43:21 AM »

I assume you are referring to this article:

http://www.hamuniverse.com/multidipole.html

Jerry, K4SAV said: "Notice that the author of the article is anonymous and there is no indication that it was ever built and tested."

N3QE said:  "My advice is to not even try a multiband fan dipole. I tried, and I used NEC to model it,..."

My fan dipole works just as described in the Hamuniverse article.  Here it is, which Don at Hamuniverse posted as an article:

http://www.hamuniverse.com/ae5jumultibanddipole.html

As you can see, my actual results worked out almost exactly as calculated in the original article.

I have had no trouble with it working.  I check in on several nets in the evenings on 75 meters, make many QSO's on 20 meters during the day.

As you can see from my results, 75, 40, and 20 meters all have very acceptable SWR.

17 and 15 meters have given my my longest DX contacts.  These two bands are about 3:1 SWR, but tune easily with the LDG AT-897 tuner mounted on my Yaesu FT-897.  I have recently made a number of QSO's on 15 meters all over the Carribean, Central and South America.

It just works.  I get great signal reports.  I've been told, don't touch a thing, leave it as is.

Paul
AE5JU
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AE5JU
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« Reply #11 on: April 01, 2009, 02:43:05 AM »

And as an aside... it took less than two hours to tune.  I haven't touched it since.
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K4SAV
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Posts: 1837




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« Reply #12 on: April 01, 2009, 08:46:41 AM »

Paul

Congratulations on the extra class ticket and welcome to ham radio.

I have a few comments about how you built your fan dipole.  

1. For the lengths you stated are those the entire length of the wire you stared with, or the length from the center insulator to the end?  The difference being in one case you would count the wiring on the center insulator block and in the other case it wouldn't be counted.  (Hamuniverse didn't specify this.)

2.  I notice your antenna is erected in an inverted vee configuration while the picture shown on hamuniverse is shown in a flat-top configuration (although it is questionable if this figure even applies to the "no-tune" method).  The resonant frequency of the inverted vee will be higher.

2.  How did you maintain the 38 inches of separation at the ends of the antenna?  I can't see any spacers (not that you really need them but the spacing does effect the resonant frequency of the antenna).

3.  I noticed you used insulated wire while the article on hamuniverse didn't really specify the insulation.  They did specify copperweld in the next method described. That usually makes about 3% difference in length for #14 TFFN, and in this case would help to bring the originally described antenna back closer to being in band.

If the all the details of an antenna are specified and an exact copy is made then you should be able to get a resonant frequency that is very close when building the second one when it is placed at the same height, unless there are other metal objects near the antenna.  The problem with the hamuniverse article is that there are too many details left unspecified.  As a matter of fact they imply that these unspecified things are not significant.  For example, if I vary the insulation and the unspecified parameters, I can get the 40 meter resonant frequency to change anywhere between 6.98 MHz and 7.6 MHz.  That's not close enough to say the antenna will not need tuning.

Nice job on the antenna, by the way.

Jerry, K4SAV
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AE5JU
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Posts: 227




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« Reply #13 on: April 01, 2009, 12:39:03 PM »

The following is my reply to Jerry's (K4SAV) post above.

"Congratulations on the extra class ticket and welcome to ham radio."

Thank you!

Comments interspersed below...

"1. For the lengths you stated are those the entire length of the wire you stared with, or the length from the center insulator to the end? The difference being in one case you would count the wiring on the center insulator block and in the other case it wouldn't be counted. (Hamuniverse didn't specify this.)"

My measurements are from the center connector to the ends.  I did count the few inches of wire in the center insulator.

"2. I notice your antenna is erected in an inverted vee configuration while the picture shown on hamuniverse is shown in a flat-top configuration (although it is questionable if this figure even applies to the "no-tune" method). The resonant frequency of the inverted vee will be higher."

Yes, actually I had built this shortly antenna before reading the original Hamuniverse article. Don asked, "If anyone has built this antenna..." and I responded that mine was similar, only I fanned the ends out, rather than just spacing as in the article.  

I had made the ends longer than 468/freq, on the advice of many, to allow for fine tuning.  

"2. How did you maintain the 38 inches of separation at the ends of the antenna? I can't see any spacers (not that you really need them but the spacing does effect the resonant frequency of the antenna)."

At the center insulator the separate legs are separated by about 2".  The ends of each pair of legs are in farther than the longer legs, so they form three separate V's, each a little sharper than the previous.  So the ends are several feet from the previous longer leg.

"3. I noticed you used insulated wire while the article on hamuniverse didn't really specify the insulation. They did specify copperweld in the next method described. That usually makes about 3% difference in length for #14 TFFN, and in this case would help to bring the originally described antenna back closer to being in band."

Yes, I had already purchased insulated 14 ga wire and built the antenna originally for SWL.  It was later modified by adding the 40 and 20 meter segments.  When I got a rig and license, I simply unscrewed the coax from the Icom R75 receiver and plugged it into the transceiver.  I added the 2 meter J-pole.

"If the all the details of an antenna are specified and an exact copy is made then you should be able to get a resonant frequency that is very close when building the second one when it is placed at the same height, unless there are other metal objects near the antenna."

As it turned out, after tuning, which just involved moving the ladder around, loosening the cable clamps, taking up some wire, tightening the clamps, tuning went quickly.  Also, to get close, I found the frequency of the lowest SWR point, made a ratio of the actual freq lowest SWR and desired freq, times the current length, and that showed me approximately how much to trim or lengthen.  As it was, all were as planned slightly too long.

Then when I got it how I wanted, I saw that it was very, very close to the updated Hamuniverse article.  That was updated with the Stanford fudge factors after I had built the antenna.  If I had used the factors in the updated article, I would not have needed to adjust the antenna at all, or if I did, by very little.

"The problem with the hamuniverse article is that there are too many details left unspecified. As a matter of fact they imply that these unspecified things are not significant. For example, if I vary the insulation and the unspecified parameters, I can get the 40 meter resonant frequency to change anywhere between 6.98 MHz and 7.6 MHz. That's not close enough to say the antenna will not need tuning."

I don't think the article was meant to be a recipe as much as a general "how to", and I accepted it as such.  I had planned on some tuning, and that did not bother me.  As I said, took less than two hours.

And from the copy of my log I sent you, you can see I am getting a signal out.  I really can't complain.

One thing that was really fun, I got to talk to a few hams I had previously listened to as a SWL'er.

And my very first QSO was south Louisiana to Wisconsin, on 20 meters, with my good friend and Elmer, K9ZW up in Wisconsin, just over 1000 miles.  Now is that cool or what?

I think the main thing about all this, it has gotten me on the air quickly, cheaply, and I built it myself.  It gives me confidence, and I hope others, to try some other antennas.

I've also built a (as others decribe it) "Buddipole on steroids" that works on 40, 20, 17, 15 meters.  From the city park, on battery, I've made QSO's all over the country on 20 meters.  Lots of 59's and 57's with that one, too.

Next I want to try a 160 meter coil loaded dipole.

"Nice job on the antenna, by the way.

Jerry, K4SAV"

Thanks, Jerry!

73,

Paul
AE5JU
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K4SAV
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« Reply #14 on: April 01, 2009, 04:40:33 PM »

The fan dipole is a good antenna.  It is a proven design, and the way you built it is like everyone else does it.  Hamuniverse's claim that if you build a center insulator block as described you won't need to tune the antenna is the claim that is not true.

It's nice to see a new ham building his own antennas.  You learn a lot doing that.  The 160 loaded dipole will be very educational too.

Jerry, K4SAV
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