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Author Topic: Loading Coil  (Read 3772 times)
K2OWK
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Posts: 1066




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« on: October 03, 2012, 03:43:53 PM »

Hello, I have a question about selecting or fabricating a loading coil. I have a 40 meter dipole antenna I am going to extend to 80 meters. The problem is I can only extend one leg of the antenna to a full quarter wave (property length and position of the dipole can only be lengthened on one side). The other leg will be the original 40 meter  one quarter length. I was told that I can add a loading coil to the short leg to match the impedance of the other leg. I have an MFJ-249 SWR analyzer to test the antenna with. Someone had a calculate on line for calculating coil resonance for diameter, number of windings and length of coil. If anyone knows this website I would appreciate the link? If anyone knows how to calculate the coil I might need to fabricate, I would appreciate the information? What I basically need to do is make a coil  that allows the short leg to match the impedance of the full size leg. Also I can only put the coil at the center or the end (top loading/center loading). Which would be the most efficient position?

Thank you for any information you might have for this problem.

73s

K2OWK
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AI7RR
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Posts: 164




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« Reply #1 on: October 03, 2012, 04:59:05 PM »

I think you find everything you need right here. http://www.ve3sqb.com/

73, Roger
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W9GB
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Posts: 2626




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« Reply #2 on: October 03, 2012, 05:09:08 PM »

Quote from: k2owk
I have a 40 meter dipole antenna, that I am going to extend to cover 80 meters.
The 80, 40 and 15 meter antenna (trapped or shortened with loading coils) was one of the most popular HF wire antennas for USA Novice radio amateurs from 1950s through the 1970s.

15 meters is covered by using the 40 meter section as a 3/2-wave dipole.
Quote from: k2owk
The problem is I can only extend one leg of the antenna to a full quarter wave (property length and position of the dipole can only be lengthened on one side). The other leg will be the original 40 meter  one quarter length.
You need to determine the TOTAL length available.  It sounds like you have 90 to 100 feet.
Unless this antenna is supported from the center insulator (inverted V) ....
You can install by moving center insulator position, installed as a sloper OR running a non-conductive messenger wire/rope from your 2 end supports.

K7MEM Short Dipole Calculator
http://www.k7mem.com/Electronic_Notebook/antennas/shortant.html

HyPower in Bethlehem, PA makes loading coils, like those calculated by K7MEM.
Typical coil set, such as the LC8073, sells for $20
http://www.angelfire.com/electronic/hypower/webdoc1.html

True L-C traps, by Unadilla (available at your local amateur dealer: HRO, AES, etc.)
http://www.unadilla.com/
« Last Edit: October 03, 2012, 05:20:29 PM by W9GB » Logged
WX7G
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Posts: 6134




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« Reply #3 on: October 03, 2012, 06:29:10 PM »

Your asymmetrical loaded dipole will work. To resonate the dipole at 3.75 MHz requires a 21 uH inductor on the short side.

With the antenna 40' above average ground the input impedance is about 40 ohms. The 2:1 VSWR bandwidth is 90 kHz and the 3:1 VSWR bandwidth is 150 kHz.

Give it a try.
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K2OWK
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Posts: 1066




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« Reply #4 on: October 03, 2012, 09:19:31 PM »

Thanks for the information. The antenna I am going to use was an inverted "V" with a canter insulator at about 35 feet high. The legs are about 32 feet long each. The center conductor leg I will lengthen to the calculated 1/4 wavelength (about 65 feet) the shielded leg will stay the same at about 32 feet. I will set the the antenna as a dipole 35 feet high. The 32 foot leg (shield) will go to a tree in the front of my house. The 65 foot leg (center conductor) will go to a tree in my backyard. The 32 foot length to the front is as long as possable at this time and is the leg I am going to try to coil load. The coil will have to be a center load or end load coil. I will try to set up a 21 uh inductor on the short end. This is just a little more information on the antenna I plan to try on 80 meters. I have an S9 I will be using on 40 thru 6 meters (32 foot with a 4 to 1 UNUN)

I do realize that it makes no difference which side I use as the shield or the center conductor. I am giving this information for reference.

I have an MFJ-914 Antenna Tuner Extender that allows my internal autotuner to tune an antenna up to a 10 to 1 VSWR.
73s

K2OWK
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WX7G
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Posts: 6134




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« Reply #5 on: October 04, 2012, 02:13:40 AM »

The 21 uH is correct for a coil placed right at the feedpoint. Placed in the middle of the 32' wire the coil should be about 37 uH.

I build coils like this by close winding #14 THHN (house wire) on a PVC form. The calculated Q is 300. Close wound you get 10 turns per inch.
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W5DXP
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« Reply #6 on: October 04, 2012, 05:56:50 AM »

You need to determine the TOTAL length available.

Which begs the question: Does a symmetrical antenna work any better than an asymmetrical antenna? Does the asymmetrical antenna tend to cause common-mode problems or skew the radiation pattern?

My favorite inductance calculator is at:

http://hamwaves.com/antennas/inductance.html

To convert from inches to mm, multiply by 25.4

I build coils like this by close winding #14 THHN (house wire) on a PVC form.

Do you have an estimate for the dielectric losses in the insulation and PVC form?
« Last Edit: October 04, 2012, 06:03:06 AM by W5DXP » Logged

73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
W8JI
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Posts: 9296


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« Reply #7 on: October 04, 2012, 06:56:10 AM »

Which begs the question: Does a symmetrical antenna work any better than an asymmetrical antenna? Does the asymmetrical antenna tend to cause common-mode problems or skew the radiation pattern?

A shortened antenna is far more sensitive to common mode than a full size antenna. This is because the electric field is more concentrated and intense around the antenna and the feedpoint.

Common mode would be especially problematic when asymetical, because the electric field is skewed. To one side, and not centered around the feedline. In other words the neutral electric field plane is not a slice at the feedpoint, it is skewed to one side.

I ran into this with loaded dipoles, so it was confirmed with actual measurements. I had a terrible time choking currents off the feeder.
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WX7G
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Posts: 6134




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« Reply #8 on: October 04, 2012, 07:38:44 AM »

W5DXP I use the Brian Beezley coil program and it calculates for various form materials.

Yes this asymmetrical antenna will have feedline common-mode current due to the feedline being in an asymmetrical field. That is, it is not exiting at right angles to the center of a dipole where the E-field is zero. I will run a simulation of the feedline common-mode current and see how much choking impedance is needed to reduce it by X dB. I will run this sim tonight.
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WX7G
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Posts: 6134




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« Reply #9 on: October 04, 2012, 09:16:47 AM »

And without a balun our dipole becomes a "tripole" with the feedline acting as the third antenna wire. Being asymmetrical complicates things. Placing a CM choke at the antenna will reduce the "tripole" feedline current but may not reduce the CM current caused by the feedline being in an asymmetrical field.

For the tripole mode I model the feedline as a 90 degree wire with the end open, not grounded. For the asymmetical mode I will model the feedline as 90 degree and 180 degree grounded wires and see what can be done to reduce the feedline CM current to 1/10 the antenna current.

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W8JI
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« Reply #10 on: October 04, 2012, 03:57:35 PM »

What happens to balance when you QSY off the frequency where the loaded side of the antenna element is resonant?
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N4JTE
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Posts: 1158




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« Reply #11 on: October 04, 2012, 05:18:42 PM »

I would take the extra needed wire, evidently 30 ft or so, on the short side and find a way to stretch it out at any way you can, as long as it's somewhat in the air it will out perform any overdesigned loading coil.
Bob
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K4SAV
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Posts: 1847




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« Reply #12 on: October 04, 2012, 05:31:15 PM »

What happens to balance when you QSY off the frequency where the loaded side of the antenna element is resonant?

Yes, that will change things.  Also the feedline length and its routing can change things a lot.  Just playing around with a few configurations I was able to get 60% of the antenna current to go down the feedline.  That would be a bad feedline length.  That also reduced the gain by 5 dB compared to some of the other configurations.  The antenna is also sensitive to which side goes to the coax shield.  Since the antenna has three poles, switching the coax at the feedpoint changes the current in all the wires.  It also changes the SWR, pattern, and the gain.  SWR was never very good in any of the configurations I tried.

Best chance of making this work would be to add a good choke at the feedpoint.

Jerry, K4SAV
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WX7G
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Posts: 6134




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« Reply #13 on: October 05, 2012, 07:06:15 AM »

What happens to balance when you QSY off the frequency where the loaded side of the antenna element is resonant?

The impedance of the short side will change faster than the impedance of the long side. So, the coaxial cable shield should be connected to the long side to present the lowest impedance over frequency. This will result in more current along the long antenna wire and less along the coaxial cable shield. The feedline choke will have less "work" to do.

----------------------------------------------------------------------------------------------------------------------------
Now for the simulations.

The antenna is modeled as three wires; a 32' wire with the RF source and a loading coil near the RF source, a 65' wire, and a 62' wire representing the coaxial cable shield. The antenna is placed 63' above perfect ground with the dipole horizontal and the feedline hanging down almost touching the ground. Frequency is 3.75 MHz and the loading load is adjusted after each change so the antenna is resonant. The current magnitudes, at the feedpoint, are:

32' wire 1.0 A
65' wire 0.61A
62' wire 0.67 A

Now we place a ferrite choke in the 65' wire (feedline shield) near the feedpoint. The impedance is half resistive and half reactive. With 300 + j300 ohms the currents are:

32' wire 1.0 A
65' wire 0.92A
62' wire 0.08 A

The feedline current is now about 1/10 that of the long antenna wire.
--------------------------------------------------------------------------------------------------------------------------------
Now we move the antenna down 1' so the feedline contacts ground.

32' wire 1.0 A
65' wire 0.94A
62' wire 0.06 A

This configuration does not require feedline choking. Near the feedpoint the feedline shield impedance is high and a very high Z choke would be needed to reduce feedline shield current. The choke would be better placed near the ground end. Note that the feedline shield current is low even though the feedline is along a line of zero E-field potential.

--------------------------------------------------------------------------------------------------------------------------------
Now we move the antenna upwards so it is 124' above ground and lengthen feedline so it is 124' and contacts ground.

32' wire 1.0 A
65' wire 0.90A
62' wire 0.21 A

Let's add a feedline choke near the feedpoint. The choke impedance is 300 +j300 ohms.

32' wire 1.0 A
65' wire 0.94A
62' wire 0.07 A
------------------------------------------------------------------------------------------------------------------------------

Conclusion:

To reduce the feedline shield current to <1/10 the antenna current a modest ferrite choke is required. A choke impedance of 300 +j300 is sufficient.

------------------------------------------------------------------------------------------------------------------------------
Notes:

More modeling could be performed.

1) We could explore the E-field near the feedline using the EZNEC near field feature. With this and some math we could gain a good understanding of the E-field inducing feedline shield current.

2) We could connect the feedline shield to the short side of the antenna.

3) We could take the data over the useful VSWR bandwidth of the antenna to see if more or less choking impedance is needed to maintain a 10:1 antenna-to-feedline shield current ratio.


« Last Edit: October 05, 2012, 07:53:06 AM by WX7G » Logged
WX7G
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Posts: 6134




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« Reply #14 on: October 05, 2012, 09:56:24 AM »

I made an error with this one. While the currents are correct my comment that the choke would be better placed near the ground end is not correct. If moved to the ground end this configuration becomes the first where the feedline current is excessive.

Now we move the antenna down 1' so the feedline contacts ground.

32' wire 1.0 A
65' wire 0.94A
62' wire 0.06 A
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