A practical fat monopole | The MICRO-BROADCASTERS COMMUNITY Forum

Deleted
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A practical fat monopole Nov 5, 2017 19:41:24 GMT

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Post by Deleted on Nov 5, 2017 19:41:24 GMT

A fat monopole offers reduced capacitive reactance and this allows us to reduce system loss resistance while maintaining sufficient VSWR bandwidth. In an earlier post I described a base-loaded fat monopole that has the drawback of requiring very low ground resistance. Center-loading is the way to achieve similar radiation efficiency while having a higher ground resistance. Moving the loading coil from the base to the center raises the required ground loss from 8 to 18 ohms, which is much easier to achieve.

The antenna is a 3 meter monopole composed of a 1" tube topped by a 12" diameter cylinder (wire). Note that the entire antenna can be built as a 12" cylinder. The cylinder lengths above the loading coil are 4, 6, and 9' for the 55, 45, and 5% loading coil versions. The EZNEC model uses a cage structure where the 12" cylinder is a hexagon of thin wires.

Design goal: To build a practical Part 15.219 compliant antenna having maximum radiation efficiency while providing sufficient bandwidth - 20 kHz - for a high fidelity AM signal. Higher radiation efficiency can be had at the expense of bandwidth.

ANTENNAS
Ground mounted
12" diameter
Loading coil Q = 250
System Q = 84, this is calculated as 1700kHz/20kHz = 84
System R = XL/84
Coil R = XL/250
GND R = System R - Coil R, GND R is the minimum ground resistance to have a -3dB bandwidth of 20 kHz or more
Freq = 1700 kHz
-3dB bandwidth = 20 kHz

COIL PLACEMENT	COIL L	SYS R	GND R	RAD RESISTANCE	EFFICIENCY
5%	131 uH	16.7 ohms	11.1 ohms	0.12 ohms	0.72%
45%	215 uH	27.3 ohms	18.1 ohms	0.28 ohms	1.03%
55%	249 uH	31.7	21.1 ohms	0.31 ohms	0.98%

H
H
H
H
H 12" cyclinder
I loading coil
I 1" tube
I
I
I

Last Edit: Nov 5, 2017 22:32:03 GMT by Deleted

Boomer
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The Dog is listening..

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A practical fat monopole Nov 15, 2017 4:50:14 GMT

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Post by Boomer on Nov 15, 2017 4:50:14 GMT

I like this study of fat monopoles, and the increased surface area for more capacitance. It gets more capacitance for higher efficiency if you're concerned about using a top hat.

Someone had proposed 3 meters of barrels soldered together, I think that might have been a Carl(tm) idea! I thought about working with wider poles, but the problem with the mass and weight becomes insulating them from other objects. More bolts, more wood structure for a bigger pole can also drain more of the signal at high impedance for a base mounted coil. With the coil in the midsection of the antenna, you'd need a strong insulator there too.

Someone was building an antenna that was a large diameter (4-5 inch?) 3 meter PVC pipe, covered with foil for that large surface and the strength of the PVC tube. I saw pictures of it and it looked slick!

Boomer

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A practical fat monopole Nov 29, 2017 0:01:44 GMT

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Post by Deleted on Nov 29, 2017 0:01:44 GMT

Barrels soldered together would be great.

Near the top of the AM BC band our antennas are limited to a Q of 100 and no higher. A Q of 100 provides a -3dB bandwidth of F/100, where F is the frequency. At 1600 kHz that gives our antenna a bandwidth of 16 kHz and the ability to transmit sidebands out to 8 kHz. Use a higher Q and the audio bandwidth becomes more constricted. And, we need some extra bandwidth to account for the change in resonant frequency with weather (I'm encountering this right now).

The Q of our antenna is equal to the loading coil reactance divided by system resistive loss. Reduce the loss too far, by adding many radials wires and using a low-loss loading coil, and the Q becomes too high. For a CB whip type base-loaded monopole antenna at 1600 kHz the reactance is around 4000 ohms. For a Q of 100 our system loss can be reduced to 40 ohms and no lower. The radiation efficiency is equal to radiation resistance divided by system loss. Given that a 3 meter antenna at 1600 kHz has a radiation resistance of about 0.1 ohms, the radiation efficiency cannot be higher than 0.1/40 x 100% = 0.25%.

If we build our 3 meter monopole out of 12" tubing the reactance is around 1000 ohms. For a Q of 100 the system loss can then be 10 ohms. We can add radials, and use a low-loss loading coil, to achieve this. Now the antenna efficiency can be 0.1/10 X 100% = 1%. That is 4X the radiation efficiency compared to the CB whip, providing the station with 2X the range and 4X the coverage area.

Center loading doesn't help with efficiency but it makes for a less aggressive ground system. With center loading the loading coil reactance is doubled and to maintain a Q of 100 we must double the system loss. The radiation resistance is doubled to about 0.2 ohms. For 12" tubing the radiation efficiency works out to be 0.2/20 x 100% = 1%. But, our system loss can be 20 ohms and that makes the ground system easier to install (less radial wires).

I'm presently building two Part 15 AM antennas and when those are finished I'll build one using 8" or 12" galvanized ducting.

Last Edit: Nov 29, 2017 3:41:46 GMT by Deleted