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Three Band Dipole System




 
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Steve - K4HX
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« on: April 06, 2016, 10:58:37 PM »

See the article in the Handbook/Antenna section at the link below. Discuss here.

http://amfone.net/Amforum/index.php?topic=41574.msg302569#msg302569
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« Reply #1 on: April 07, 2016, 12:47:02 AM »

It may work !

It seems like a lot of complexity.  If one has the space for an 80 meter dipole, then 40 and 20 are no sweat!!

Up at Rattlesnake Island, where it is a lake lot with very limited antenna space - an 80 meter coax fed dipole fits - barely.  So, to get on 40 I hung a 40 meter dipole off the same feedline and below the 80 meter ant.  Works perfectly, and no tuner needed.

It would easy to add 20 meters - that antenna is even easier than 40.

I am a pretty big "fan" of the "fan" (as it is called) dipole.  Use it at my main location - share  160 and 80 meters on the same feedline.  40 is a separate antenna.

The main advantages are NO ANTENNA TUNER, the feedline does not radiate if you put a bunch of type 43 cores at the feedpoint - just run the coax up through the middle,  and the feedline can be buried, run at an disadvantageous angle, etc. without issue.

The dis-advantage is the antenna will only work on the bands for which it is designed.  You have to add dipoles for each band you want to operate.
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« Reply #2 on: April 07, 2016, 01:00:06 AM »


I've a 80m/40m coax fed combo. I am able to load her up on 15 and 10 meters.


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W8EJO
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« Reply #3 on: April 07, 2016, 02:30:43 AM »

See the article in the Handbook/Antenna section at the link below. Discuss here.

You have backed in to a variant of the ZS6BKW antenna which is itself a vaiant of the G5RV antenna.

I put one up this winter down at the Ocala cabin & it worked FB.

W5DXP has some interesting design thoughts which I would like to try.

http://www.w5dxp.com/ZS6BKW80/ZS6BKW80.HTM

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Terry, W8EJO

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Steve - K4HX
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« Reply #4 on: April 07, 2016, 09:12:09 AM »

A lot of these antenna designs are similar or build on previous work. The idea of juggling feedline length and flat top length to obtain desirable impedances precedes ZS6BKW or G5RV by decades. Their antennas were variants of a much older theme. All of these multiband designs have been around in one form or another since the 1920/30s. I like this approach since it is limited to only three bands. The 4,5 and 6 band claims usually have horrible pattern breakup on the higher bands. The only design I've seen than maintains are clean pattern over 4+ bands is the DJ4VM loop/quad.

I like the fan dipole approach too. I've never built one for more than two bands though. I would think at some point it would become unwieldy and/or too heavy. That is the one advantages of this approach, only one flat top wire. The drawback, is the combined open-wire/coax feed (although I consider this a minor drawback as ladder line is lighter and cheaper than coax, YMMV). Also, some do not or cannot bring open-wire or ladder line into their shack for various reasons. Here, the coax feed addresses that problem. It also avoids a balanced tuner requirement, which also trips up some people.

The other advantage of this approach is the shortened length the lowest band. This may of value to some people. I would tend towards putting up a full-sized dipole for the lowest band, if I had the room and use this approach for a second antenna to cover the next three highest bands. I would imagine that the dimensions could be doubled again to cover 160, 80 and 40 meters. Here the shorter length on 160 may be valuable to more people.

In any event, antenna design is about trade-offs. This design might have acceptable trade-offs to some, not for others. It's good to have options and it keeps the bands interesting. Enjoy.
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w8khk
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« Reply #5 on: April 07, 2016, 12:55:55 PM »

Snip..... . I would imagine that the dimensions could be doubled again to cover 160, 80 and 40 meters. Here the shorter length on 160 may be valuable to more people.


Steve, that sounds very interesting.  I have space for a dipole a bit larger than 75 meters, but not enough for 160.  I wonder how much difficulty it might be for you to model the antenna again with double the dimensions to determine the performance on 160 and 75?  Weight would not be an issue, as I have a yardarm at the top of the tower for center support.   40 meters would also be of interest, but I have a separate dipole with OWL to the link-coupled tuner, so 40 would not be critical, but 160 would be very helpful if this might work well.   Thanks for your efforts!

73, Rick
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Rick / W8KHK  ex WB2HKX, WB4GNR
"Both politicians and diapers need to be changed often and for the same reason.   Ronald Reagan

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« Reply #6 on: April 07, 2016, 12:57:11 PM »

I like this approach since it is limited to only three bands. The 4,5 and 6 band claims usually have horrible pattern breakup on the higher bands. The only design

My version of the ZS6BKW worked very well considering the constraints I had at the cabin.
1) I could only put up one antenna.
2) I had about 100' between  two 40' trees
3) I wanted to work 75, 40, 20, 17 & 15.

75 & 40 were essentially NVIS as any  horizontal wire antenna would be at that heigt.
20 seemed to work very good boadside  to the wire as it should have being essentially an EDZ at 1/2 wl up.
17 & 15 was hit or miss but very usable.

It took about 2 hours to cut it, solder it up & hang it from the trees. So all and all it turned out to be an excellent antenna for my situation.

I can't think of too many other antennas that could do all that with low swr & no tuner.



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Terry, W8EJO

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Steve - K4HX
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« Reply #7 on: April 07, 2016, 05:39:16 PM »

Sounds like a nice set up. The fact that it worked on 17 and 15 (at least to some extent) is good bonus. 15 always seems to be the odd band out or the problematic on for most of the multiband designs.

I may have to try putting a second set of wires on the 3-band design to see what happens for 15 meters. Other have used properly placed vertical wires along the horizontal wire span to get additional bands. This approach would be easier to implement and weight less.
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w8khk
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« Reply #8 on: April 11, 2016, 01:29:50 PM »

Steve, would it be possible for you to model this approach, again doubling the dimensions to work 160M and 75M, or should I just double the dimensions you specified and measure the results?  I have no room for a full size 160M dipole, but this would fit between my trees if it appears to be a practical solution.  Thanks for your help!
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Rick / W8KHK  ex WB2HKX, WB4GNR
"Both politicians and diapers need to be changed often and for the same reason.   Ronald Reagan

My smart?phone voicetext screws up homophones, but they are crystal clear from my 75 meter plate-modulated AM transmitter
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« Reply #9 on: April 11, 2016, 02:13:10 PM »

Even if the 160M element is a bit too long you can let the ends hang down with no real effect on performance.
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Steve - K4HX
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« Reply #10 on: April 11, 2016, 11:13:10 PM »

Doubling the size probably won't work. I ran some numbers and it seems getting things to work well on all 160, 80 and 40 meters is a bit tricky. So, I went for the best numbers on 80 at 3.7 MHz and checked to see what happened on the other two bands. The numbers are probably usable though. You'll have less than 3 dB loss on 160 and you'll need a tuner on 40 meters too (although many boat anchors would tune into that SWR without a problem).

Anyway, you'll have to be the judge of if this will work for you. Since the tuner is needed on two bands, you could just skip the length of coax (assuming the ladder reaches to your shack) and just use the tuner on all three. The impedances should be handled without much loss by a good tuner.


Flat top length = 183 ft at a height of 70 feet over average ground.
Ladder Line Length = 78.5 ft
Coax Length = 30 feet

160 Meters

Frequency 1.9 MHz
Dipole Feedpoint Z: 30.19 - j558.71

Z at the end of 75 feet 359 Ohm line: 14.41 - j8.31
Line Loss: 1.68 dB

Z at the end of 30 feet of RG 8: 24.85 + j38.57
SWR: 3.51
Loss: 0.21 dB

Total Feedline Loss: 1.89
Tuner loss: 0.4 dB
Total system loss: 2.29 dB


80 Meters

Frequency 3.7 MHz
Dipole Feedpoint Z: 258.79 + ij817.48


Z at the end of 78.5 feet 359 Ohm line: 42.55 - j1.24
Line Loss: 0.39 dB

Z at the end of 30 feet of RG 8: 53.24 - j6..89
SWR: 1.17
Loss: 0.12 dB

Total Feedline Loss: 0.51


40 Meters

Frequency 7.295 MHz
Dipole Feedpoint Z: 102.050 - j466.69

Z at the end of 78.5 feet 359 Ohm line: 45.84 - j79.39
Loss: 0.71 dB

Z at the end of 30 feet of RG 8: 37.97 + j65.17
SWR: 4.14
Loss: 0.5 dB

Total Loss: 1.21
Tuner loss: 0.1
Total System Loss: 1.22
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w8khk
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« Reply #11 on: April 13, 2016, 12:06:29 PM »

Steve, thanks for modeling the antenna to include 160.  Since I have a separate dipole for 75 and 40, I will use the information you provided to focus on 160.  The thing I overlooked is the necessary space to add the required length of 450 ohm ladder line. 

So I will try using the flat-top length you specified, and support the center with the yardarm at the top of my tower, and run ladder line to the base of the tower where I have feed-thrus  into the house.  I have a run of #10 pair spaced 6 inches from the entry point to the ceiling of the shack in the basement, with large porcelain feed-thrus, then a short run to a balanced tuner based on a coil of 1/4 inch copper tubing which works well on 75 and 40, so I will make adjustments if needed to feed the 160 antenna with that or a similar balanced link coupled tuner. 

Thanks again for running the numbers.

73, Rick
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Rick / W8KHK  ex WB2HKX, WB4GNR
"Both politicians and diapers need to be changed often and for the same reason.   Ronald Reagan

My smart?phone voicetext screws up homophones, but they are crystal clear from my 75 meter plate-modulated AM transmitter
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