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unexpected behaviour Moxon antenna




 
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PA0NVD
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Nico and Chappie (Chappie is the dog...)


« on: October 08, 2019, 11:50:15 PM »

I did build a moxon from 4 fishing rods and No 18 wire. I tuned it at approx 8 feet altitude and the antenna behaved fine, Good front to back and 1:1. When I raised it to 6 meters altitude, the frequency changed as expected, but.. The resonance became LOWER, and quite a lot. . It did resonate at 14.1 at 8 feet, and at 18 feet it resonated at 13.6 MHz. That was quite unexpected, I thought that the frequency should go up due to the ground capacitance.
So down again and tuned it to 14.5 MHz at 8 Feet. It did resonate at 14. 15 at 18 feet. Raised it further to 50 feet and the resonance dropped to 14.0 MHz
So down again and corrected in accordance with the resonance and the desired frequency. Now it resonates at 14.25 at 50 feet with approx 25 - 30 dB FB ratio.
That antenne has also a moxon for 18 MHz mounted. That did behave exactly the same, the resonance went down when I raised the antenna
Anyone can explain that? I do read about anywhere that the resonance goes up when the antenne is brought to a higher altitude. Very good conducting soil?  The soil indeed has a good conductivity and is always wet, it rains a lot here.
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KK4YY
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« Reply #1 on: October 09, 2019, 03:45:30 PM »

Nico,

I have no experience with the Moxon design and cannot explain your results. But, I have some questions.

1. How are you measuring resonance? An SWR bridge and your transmitter, an antenna analyzer, a VNA?

2. Did you change feedline lengths when raising the antenna higher?

3. How was the antenna mounted when initially measured compared to the final height? The tower/mast may be interacting with the antenna.

In short, I suspect one of two things. A measurement error or something other than height was changed.


Don
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PA0NVD
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Nico and Chappie (Chappie is the dog...)


« Reply #2 on: October 09, 2019, 04:47:24 PM »

Thanks for the input Don
I measure when the SWR is very low, so the cable length should not  be involved. I get a nice dip in SWR  I measure with a SWR bridge and run the frequency to find a SWR 1:1 or very close. Than I meausre FB in order to know that the antenne is behaving as should and that it is no false resonance. I rotate the antenne to see if reflection interact and point to a place without obstacles. It is on e push-up mast with Keenan guylines, so the lines do not interact. Always the same coax permanently connected.
The behaviour of the 20 meter antenna and the 17 meter antenna were the same, both went down in frequency when raised.
FB is very good so the reflector works as it should, both for 20 and for 17
The coax is attached with a good choke wound from coax at a ringcore. I measured balance (Rf voltage at the antenna wires near the input)and that's quite good.  Touching the coax firmly does not show any effect.
When raising the antenna I had to cut both the driven element as well as the reflector. I tried once without cutting the reflector, but than the SWR became high.
The only explication I can think off is that the soil is that conductive that is shorts the field lines A coil near a conducting surface gets less inductance and the frequency goes up. All quite unexpected and interesting
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KK4YY
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« Reply #3 on: October 09, 2019, 06:11:16 PM »

You've got a mystery on your hands, Nico. Antennas have been installed over every imaginable surface and yours, apparently, behaves differently than all the others. The good news is that it works!

I did some Internet searches looking for others who've experienced this effect and didn't find anything.

I wonder how you're feeding the antenna. Are you using a gamma match or directly connecting the coax? Is the tower/mast grounded at the antenna?

There's got to be an explanation to this.
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PA0NVD
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« Reply #4 on: October 09, 2019, 08:13:28 PM »

Quite a mystery for me as well Don. The coax ends in a choke of coax wound on a ferrite ring core and than directly connected to the antenna. The SWR is nice, with 1 kW input I have approx 4 Watts return. The mast structure is isolated from the antenna. But the antenna behaves like it should, front to back ok on both bands, better than 25 dB, also when it was longer and at low altitude. I get very nice reports, so there is gain as well.
When I raise the antenna there is a smooth decrease in frequency. At 6 meters altitude when cut to length the SWR was very good. Raised 2 meters more,and noticable worse SWR an a decrease in frequency of approx 30 kHz. (a little hard to measure) Raised to 14 meters again worse and a frequency decrease of approx 100 kHz. that is 0,7% too low So shorted DE and REFLECTOR  0,7 % and it was fine at 14 meters and approx 100 kHz high at 6 meters altitude. No mistakes there.
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KK4YY
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« Reply #5 on: October 10, 2019, 01:00:21 AM »

I found this reference to the Moxon:

https://mirror.thelifeofkenneth.com/lib/electronics_archive/ATrulyPortableMoxonRectangleForNearlyNo-toolFieldAssembly.pdf

Quoting from that document:

"For 20 meters, the approximate minimum recommended height is about 3/8 wavelength or 26' above ground. At this height and above, the SWR curve will be very stable."

This would suggest that at a height of 8 meters and above there will be little change in SWR for a 20M Moxon. But, you saw a significant change going from 8 meters to 14 meters in height.

I still find no reference to the resonance moving lower as the height is raised. Perhaps there is some interaction of the two antennas being mounted together as you have done. If you're using separate feedlines try shorting the unused feedline and note any change in SWR on the active feedline to see if there is much cross coupling. Maybe there's a clue there.
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PA0NVD
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Nico and Chappie (Chappie is the dog...)


« Reply #6 on: October 10, 2019, 09:35:17 AM »

Mmmm interaction, a change in coupling between the two antennas... may be. Unfortunately that takes a lot of work to find out.
The two antennas are coupled with 12 inches of 50 Ohms twisted line in phase   The coax is connected to the 17 meter antenna, the 20 meter is at the end of the twisted line..
 The antenna is at full height now, 14,5 meters, and I am VERY reluctant to lower it again just to investigate, They work really nice.
I am a bit older Don, and I am happy I could get them up at all..  Grin.
When I was younger, I should have investigated and tried, but I think I leave it as is. It was an interesting effect, perhaps some day someone else will find the solution.
When time and energy come together, I will make a 1 GHz model and sweep it with the tracking generator and spectrum analyser using a reflection bridge. That gives the possibility to raise and lower by hand and see instantaneously what happens.
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WA2SQQ
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« Reply #7 on: October 10, 2019, 11:10:06 AM »

I did build a moxon from 4 fishing rods and No 18 wire. I tuned it at approx 8 feet altitude and the antenna behaved fine, Good front to back and 1:1. ...at 50 feet with approx 25 - 30 dB FB ratio.

I built a MOXON for 6 meters and found the F-B ratio very poor, maybe ~ 6db. I've looked at Spider beams which are very similar to the Moxon design. My 3 element tri-bander does not even get that much!
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PA0NVD
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Nico and Chappie (Chappie is the dog...)


« Reply #8 on: October 10, 2019, 03:11:35 PM »

The F-B ratio depends quite sharply at the size of the reflector. If it is a little bit too long, the F-B ratio suffers.  I suppose that it affect the coupling between DE and Reflector, which increases rapidly if the reflector nears resonance.  I tried with several stations, and the F-B ratio is close to 30 dB.
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PA0NVD
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Nico and Chappie (Chappie is the dog...)


« Reply #9 on: October 10, 2019, 03:39:44 PM »

here the diagram of a moxon, simulated. But there are quite a few measured as well


* moxon f-b.PNG (63.93 KB, 800x403 - viewed 13 times.)
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