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Author Topic: Fan Dipoles, Has anyone done this?  (Read 15028 times)
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k7pp
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« on: March 04, 2015, 11:10:22 AM »

The weather is getting better and I'm getting ready to start some antenna projects.

I've found over the winter that I need a bit more band width on 75 and 160 meters.

I've played with all sorts of dipoles and feed lines but I've found that in order to be able to
eliminate or reduce electric fence noise from my neighbors that I have to use a coaxial feed.
Antenna tuners and open wire feed are out.
I did make a cage dipole for 160/75 but it was too heavy.  It also didn't give me the bandwidth
I needed.  It did, however have about 50 percent better bandwidth than my plain dipole.

Still,  I wonder if I can configure a fan dipole with two 75 meter and two 160 meter elements on
the same coaxial feed.

The shorter dipoles would be resonant at the top of the band and the longer at the low end of the
band.

75 meters would be the real interest as I like to operate CW on occasion and have a separate
dipole cut for 3550 khz.

Anyone???
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W1ITT
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« Reply #1 on: March 04, 2015, 11:27:46 AM »

Try downloading the antenna modeling program MMANA-GAL.  It's a very capable program, the only distraction being that the dimensions entered are in meters rather than feet.  (I recall, in grade school in the 1950s, we had to learn the Metric System and they told us that in "just a year or so" the USA would use it, and feet and pints would be extinct.)
Just describe the antenna in X and Y coordinates, and use the Z to set the height. (Or you can leave Z at zero and use the "add height" choices on the left hand side of the interface.)  Play around with the lengths and angular spreads of the elements to "tune" it and any interactions among the three dipoles will be accounted for.
The fresh air aspects of antenna cut-and-try work are better for you, but MMANA-GAL will allow you to save work and wire to get to where you want to be.  And I know you can afford the program....

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W2VW
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« Reply #2 on: March 04, 2015, 11:30:16 AM »

I wonder what was the noise ingress point with your balanced feed.

I ditched coax feed here to get rid of noises and it works better than coax.

The antenna has always been partially supported by a telescoping tower and the very first thing to go away when lowering is local noise immunity. The balanced line works better for me when tensioned and away from metal stuff.


Some T matches followed by baluns can be poor performers under certain circumstances.
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WD8BIL
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« Reply #3 on: March 04, 2015, 12:04:42 PM »

I've done it in the past on 80/75 meters and it works fine.
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KD6VXI
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« Reply #4 on: March 04, 2015, 02:59:09 PM »

In mmana,  if you use *F or *I after the number,  it will do the conversion for you.

Ie,  12*I would be 12 inches,  and 12*F would be feet.

All the displays will still be in metric,  though.

It also sucks for modeling close to ground.   This is a known issue with the new engine used.

I love it,  otherwise.   Very intuitive.

--Shane
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WD8BIL
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« Reply #5 on: March 04, 2015, 03:33:32 PM »

I looked up my notes on that dipole. Lower frequency element was 260 feet and the upper frequency dipole was 241 feet. Coax fed at 70 ft.
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Steve - K4HX
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« Reply #6 on: March 04, 2015, 08:31:18 PM »

http://rudys.typepad.com/ant/files/antenna_broadband_dipole.pdf
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VE3LYX
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« Reply #7 on: March 05, 2015, 05:50:34 PM »

This isnt computer modeling but real life. I built a 40M / 80M inverted Vee which is a fan dipole of sorts more or less. I put the 80M on the outside and the 40ms on the inside. I didn't do the angles exact but strove for a bit more then 45 degrees split for the 80M and a bit less then 45 degrees for the 40M.(using existing trees and bushes as it is a hidden antenna))I tried to make sure they were not exactly in the same plane either thinking to minimize interactivity. If you have worked me CW other then BK nights or SSB on these bands then we were on this antenna. I find it very "servicable". It always works whether I am using my Bare essentials 50L6 TX or my Kenwood TS830S. For the first couple of months of my AM career I ran my HB 40M rig and my DX60B  on them as well however built studio B which has no SSB rigs in it and is a long wire antenna downstairs. Would I say it is the most wonderful antenna I ever had etc etc. No probably not but it always works and never gives any trouble. I know I have had it about 5  years and maybe longer. I don't have any complaints about it for what that is worth.
Don
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Don VE3LYX<br />Eng, DE & petite Francais
k7pp
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« Reply #8 on: March 05, 2015, 08:31:38 PM »

After reading all of your kind replies,  I'm guessing I didn't describe what it is I'm aiming for clearly.

Picture a single coax fed dipole with a balun.

Half wave length long.

Resonate at 3900 khz..

Now,  measure the dipole's vswr and I'm guessing you'll find it offering a 2:1 vswr at about 3780 and
1:1 at 3900 khz and 2:1 the top of the band.

Now,   cut another completely different dipole so that it will resonate at 3550 and attach it to the same
balun that feeds the higher frequency dipole.

Space it any way you like.

Has any one done this?Huh  I'm wondering if having two dipoles resonant so close together will effect
the feed point impedance.

I'm wondering if I can effectively make a fan dipole that will cover the entire band without having to
resort to an antenna tuner.   Tuners are not possible for me.
Please believe me I have tried every type of open wire feed possible and I constantly get a 20/9 electric
fence popping from the four farms that boarder my property.

The current dipole I have is in the center of my property and is hung at 160 feet.
It is fed with LMR 400 and exhibits less than an S2 on the fence noise pops.
I've got the noise under control but since I'm the greedy sort,  I'm wanting more bandwidth so I can
operate CW as well on the same feed line.

Anyway,  that's the best I can explain it.

I thank you all for your input.   It is valued.

Pete
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Pete, WA2CWA
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« Reply #9 on: March 05, 2015, 09:06:45 PM »

I think what you need is a fat or wire cage dipole to cover the full range of 80 meters. I would think that cutting two antennas that close in frequency and attached to the same feedline would give you lots of interaction and reactance issues no matter how far apart you separate them.

The ARRL put up a cage dipole to cover the entire 75/80 meter band:
http://www.arrl.org/files/file/Technology/pdf/The%20Cage%20is%20Back%20W1AW.pdf
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« Reply #10 on: March 05, 2015, 09:19:51 PM »

I don't use a balun. I have no trouble working the whole 80M band on my 80M section as is. I run my CW rig on a 3555 rock and work the upper end on SSB with my Kenwood. I think you are worrying about something that isn't really a problem. It is coax fed and I do NOT use a tuner.
If I was concerned about that this is what I would do. I would make an unequal length dipole. One leg tuned for 3850 (1/4wave length) and the other for 3650(1/4 wave length) and I would expect it to work fairly well. However having had several 80M dipole style antennas in the last 30-40 years and having never had a problem I wouldn't worry about it myself. If you want to get rid of noise transmit on a dipole BUT listen on a beverage. The difference is llike night and day!
don  
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Don VE3LYX<br />Eng, DE & petite Francais
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« Reply #11 on: March 05, 2015, 10:54:50 PM »

I have a fan dipole for 80 and 40, no tuner, no baluns, coax fed, no issues. The swr is very low where I operate.

At moderate power (500 watts) you can get away with some swr with tube rigs, not sure how much the loss in the coax from high swr changes the signal at the far end, little would be my guess.
I am not sure I would want a cage dipole in the recent weather, lots of ice, it had better be strong and have very strong supports.
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steve_qix
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« Reply #12 on: March 05, 2015, 11:35:37 PM »

The antenna here consists of a 160 and 75 meter dipole connected to the same coax

The 75 meter antenna is cut for around 3750.  It works very well at 3885 and of course below 3700.

On 160, the antenna is cut for 1830 and it works well on 1885 and 1945 (I operate both of those frequencies often).

The SWR is not too bad.  I use 7/8 inch hardline, so a little SWR isn't going to hurt anything.

Anyway, there's at least one antenna that works.
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Pete, WA2CWA
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« Reply #13 on: March 06, 2015, 03:07:24 AM »

He's not asking about a multiband fan dipole. He wants two dipoles resonant at roughly 350 KHz from each other fed with the same coax feedline.

Here's a thought. Picture a turnstile antenna. Four horizontal legs at 90 degrees from each other. Coax feedline from station is connected to one set of equal lengths (say cut for 3900 KHz) opposite each other. The other two lengths (cut for 3550 KHz) are connected by a quarter-wave piece of 75 ohm coax (dipoles typically have an impedance of roughly 72 ohms). The other end of the 75 ohm coax is connected to the same feed point as the coax going to the station. Since each leg is 90 degrees apart from each other, I would think that interaction might be minimal. However, as you start to reduce the 90 degree angles and by moving the lengths closer to the other lengths,  interaction and reactance issues are probably going to rise. It's probably possible with careful pruning that you could get the SWR down to a tolerable number but I don't really know.
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« Reply #14 on: March 06, 2015, 07:48:05 AM »

I was talking about needing a cage dipole and what that means.
If swr is not critical, you might not need one from a signal at the far end standpoint, if you do need one, it had better be very strongly made if you live in the north.

You would have to test the fan dipole with slightly different lengths, they can work great, cut a few inches off and they can go wack-o.

Ease of install is the reason to hang one dipole below the other, say 1 foot separation, the lower dipole can be made with wimpy wire.
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W1ITT
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« Reply #15 on: March 06, 2015, 08:08:02 AM »

Looking back at the original post, in the middle paragraph, he is indeed asking about a multiband fan, where is posits two 80m elements and two 160m elements on the same feedline.  That's multiband.
It has been stated here that having a single band fan dipole with different lengths will drive things "wacko".  This is not necessarily true.  In fact, one commercial FM antenna manufacturer for which I worked, did just that with a crossed dipole in front of a panel, with three of them spaced around a tower for omni work.  The dipoles were both fed off the ends of a common balun, and were tuned off by a few percent from each other to produce a circular polarization.  I used to spend happy hours on the test rack measuring amplitude and phase at each element to make it work.  So there is no deal-breaker in non-equal fan dipoles.

Again, I suggest an investigation with an antenna modeling program.  It can be done in a soft chair in a cozy place.  And calculation beats speculation any old day.   Also, thanks to Shane for cluing me in to the "feet" feature in MMANA-GAL.  I have been using it so long that I no longer stop to  "read the (**) manual."
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k7pp
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« Reply #16 on: March 06, 2015, 09:47:23 AM »

There is no argument there.  I did indeed ask about a 75/160 antenna which will be the final configuration.

What I was interested in was operating two elements in the same band and you have given me some
actual examples where this was done.

After reading the answers,  many were focused on multi elements on different bands, hence,  rewording
my question to focus on the answer I wanted.

As I said,  I could have worded it better from the "get go".

I'll try harder next time Smiley

Now all I have to do is wait for the standing water around the trees to dissipate, get my waders and
my roll of wire and have at it.

Best to all
Pete
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WD8BIL
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« Reply #17 on: March 06, 2015, 10:53:17 AM »

Quote
After reading all of your kind replies,  I'm guessing I didn't describe what it is I'm aiming for clearly.

Picture a single coax fed dipole with a balun.

Half wave length long.

Resonate at 3900 khz..

Now,  measure the dipole's vswr and I'm guessing you'll find it offering a 2:1 vswr at about 3780 and
1:1 at 3900 khz and 2:1 the top of the band.

Now,   cut another completely different dipole so that it will resonate at 3550 and attach it to the same
balun that feeds the higher frequency dipole.

Quote
I looked up my notes on that dipole. Lower frequency element was 260 feet and the upper frequency dipole was 241 feet. Coax fed at 70 ft.

That's exactly what I described here. These 2 dipoles are spaced 12" along thewires and use the same feed point. No balun! Coax feed.
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k7pp
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« Reply #18 on: March 06, 2015, 09:06:38 PM »

OK,  that was the input I was after.

I've got about 1000 feet of #12 stranded copper and will start cutting as soon as the water level
recedes.   
I'll make a video of it and publish on YouTube when I'm done.
Thank you all for your input.

Pete
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WU2D
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« Reply #19 on: March 08, 2015, 09:54:01 AM »

Going back to the original question: If the two sets of dipoles wires are cut for different parts of the band and are close coupled, like a cage or fan dipole  made of open wire spacers, the elements will act as a broadband element. If the true fan dipole is used where the elements are all the same length, that is also a broadbanding technique.

But if the dipoles are not close coupled and are cut to two frequencies, one shorter (high) and one long (low) for instance, and they are close in frequency percentage, you take the chance of double termination and the impedance going low and the overall SWR actually degrading.

Actually all of these broadbanding techniques will generally lower the impedance of the feed compared to a regular dipole.
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aa5wg
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« Reply #20 on: March 09, 2015, 02:48:18 PM »

Hi Pete,

Attached are two pages from the "antenna Manual" by Woodrow Smith, Editor, Radio Magazine, Editor, Radio Handbook, Published and distributed by Editors and Engineers, 1948, pages 164 and 165.

On page 164, under RECEIVING CONSIDERATIONS, the author talks about atmospheric and man made noise. On page 165, Figure 5-3, there is an example of a "Anti-Static" receiving antenna for short and medium distance sky-wave reception at frequencies between 3 and 7 MC.

To help reduce noise, I once used a center fed zepp fed with twin-coax balanced feed line and a link antenna tuner.  

I enjoy your work on the Hammarlund receivers!  I have the HQ-170A-VHF which I am going repair.  Your YouTube work is going to be very helpful.  Thank you.

73,
Chuck


* Scan3.jpg (913.76 KB, 2097x2697 - viewed 485 times.)

* Scan4.jpg (984.86 KB, 2096x2696 - viewed 473 times.)
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KA0HCP
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« Reply #21 on: March 09, 2015, 05:31:08 PM »

I'm not sure that portion of the Antenna Book holds very well, either in construction  nor in 'anti-static / anti-thunderstorm' orientation or sources.

My interest is piqued, but I don't recall seeing any modern references to our summer thunderstorm static originating primarily from Tropical regions.  It's worth some research.

Construction wise, one can certainly do better with tuned parallel wires or trapped arrangements.  Random length fanned wires having nothing to recommend them other than laziness.

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« Reply #22 on: March 09, 2015, 06:01:34 PM »

http://news.nationalgeographic.com/news/2003/05/photogalleries/lightning/photo3.html

Here is a NASA composite of lightning strikes in the Western Hemisphere.  It appears strikes in the Southern CONUS would out number distant strikes in tropical regions.

My personal experience is that I commonly hear static from storms around 1000-1200 miles away, as correlated to the real time strike finder link here on the board.


* westlightningmap_n.jpg (39.63 KB, 385x461 - viewed 450 times.)
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Steve - K4HX
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« Reply #23 on: March 09, 2015, 09:39:41 PM »

In my experience, at my VA QTH, most of the static in the wintertime comes from a southwesterly direction. I was able to determine this while using a K9AY antenna and a pair of phased beverages. Looking at T-storm maps, the majority of the activity to my southwest was in or near the Gulf of Mexico (coastal Texas, Louisiana) and central and northern Mexico. Those locations cover a range of about 700-1500 miles.

Some nights I would get a huge reduction in the amount of static when the antenna was pointed northeast, away from the static. At times, it was almost like a switch turning the static on and off when switching between a dipole and the Beverages. I'm guessing this was the result of the lightning being located at a bearing directly off the backside of my antenna and at a distance where the preferred elevation angle coincided with a null in my antenna pattern.

Another thing I learned is that even in the winter, on certain nights there will be significant static in the Atlantic, so even with my Beverage pointed Northeast, the static level was high. Looking at the lightning maps verified this. I would have never suspected this previously and just assumed that all static came from southerly (warmer) directions. This occurred maybe 10 percent of the nights in December and January, 20 percent in Oct/Nov and Feb/March. Rarely was the static level from the Atlantic as high as that coming from the southwest. But once in a while, the level would be close.

In any event, I don't see how a fan dipole would reduce static any more than a single wire dipole.
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« Reply #24 on: March 20, 2015, 11:59:48 PM »

Another option is to skip the fan approach and go with a capacitor loaded Extended Double Zepp. You get some gain and a SWR under 2:1 from 3.5 to 4.0 MHz. And with some switching/tuning, you could use it on 160 meters. Bill, KD0HG used this antenna for quite a few years.
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