Asymmetric Bicone Antenna Design

[jonwienke]

https://1drv.ms/u/s!ApJIS-l4xqPtgvl8X-hGsUu8icErsA
The simulations indicate it should perform well from 100MHz to about 1GHz, with significantly more gain than a standard discone. It's basically a discone flipped upside-down, with the cone becoming the active element, and the disc stretched out to form a large ground plane, and then bent down a little bit to bring down lobes of the radiation pattern closer to the horizon.

Chracteristic impedance is nominally 900 ohms, but it's actually 75 ohms because I had to use 12 power feeds instead of 1, to work around some stupid bugs in NEC.

Comments & feedback welcome.

 

[Ubbe]

Could you take a screen capture instead of a nec file so we could see how it looks like?
I guess by the description it now works as a fan dipole? If you get more than 1-2dB over a discone it must surely be a bug in the nec program?

/Ubbe

 

[byndhlptom]

Asym Discone

Does it look something like this?

https://en.wikipedia.org/wiki/Biconical_antenna

$.02

 

[prcguy]

I can't open the file on the computer I'm using on travel but let us know the approx dimensions. I'm very familiar with bicones and have several commercial and military versions. See post 10 here: https://forums.radioreference.com/s...34540-my-homebrew-bicone-antenna-project.html

The way a bicone will have gain is by being many wavelengths long and having a cone angle that adds the many resulting lobes in phase at or near the horizon. A bicone with any gain would be really big at 100MHz. Here are some specs on my Harris 90-470MHz Bicone which is about 6ft tall and has about the same gain as a dipole. https://www.harris.com/sites/default/files/RF-9070_tcm26-9211.pdf

https://1drv.ms/u/s!ApJIS-l4xqPtgvl8X-hGsUu8icErsA
The simulations indicate it should perform well from 100MHz to about 1GHz, with significantly more gain than a standard discone. It's basically a discone flipped upside-down, with the cone becoming the active element, and the disc stretched out to form a large ground plane, and then bent down a little bit to bring down lobes of the radiation pattern closer to the horizon.

Chracteristic impedance is nominally 900 ohms, but it's actually 75 ohms because I had to use 12 power feeds instead of 1, to work around some stupid bugs in NEC.

Comments & feedback welcome.

 

[jonwienke]

No. The length and angle of each cone is different, which is why it a asymmetrical. You can download the free version of NEC here:
4nec2 antenna modeler and optimizer

 

[jonwienke]

The way a bicone will have gain is by being many wavelengths long and having a cone angle that adds the many resulting lobes in phase at or near the horizon. A bicone with any gain would be really big at 100MHz.

The top elements are about a meter long, and the bottom elements are about 2.8 meters long. The bottom elements slope about 19 degrees down from horizontal and the top elements are about 43 degrees from vertical. So yeah, that's pretty big.

Making the bottom elements longer than the top helps shrink the RF lobes below the horizon, which helps increase gain. I'm still running some optimization tweaks to see if similar performance can be achieved with something a bit smaller, but it doesn't look like it.

 

[prcguy]

When I mentioned big for 100MHz I meant elements that are several wavelengths long. That would be many times larger than what you are describing.

The top elements are about a meter long, and the bottom elements are about 2.8 meters long. The bottom elements slope about 19 degrees down from horizontal and the top elements are about 43 degrees from vertical. So yeah, that's pretty big.

Making the bottom elements longer than the top helps shrink the RF lobes below the horizon, which helps increase gain. I'm still running some optimization tweaks to see if similar performance can be achieved with something a bit smaller, but it doesn't look like it.

 

[Ubbe]

The way a bicone will have gain is by being many wavelengths long and having a cone angle that adds the many resulting lobes in phase at or near the horizon. A bicone with any gain would be really big at 100MHz. Here are some specs on my Harris 90-470MHz Bicone which is about 6ft tall and has about the same gain as a dipole. https://www.harris.com/sites/default/files/RF-9070_tcm26-9211.pdf

Is there a way to really pull down the loobs to the horizon, or an antenna type that does that with some gain?
I can see that the Harris antenna have -8dBi at 120Mhz and -5dBi at 180MHz and 0dBi at 300-400MHz in an angle of the horizon. All loobs point 5-10 degrees over the horizon.

Maybe several 1.25 dipoles (2x5/8), one for each key frequency connected to its own bandpass filter would be a good solution?

/Ubbe

 

[jonwienke]

Is there a way to really pull down the loobs to the horizon, or an antenna type that does that with some gain?

Download the software and antenna model I linked, and see for yourself.

 

[jonwienke]

Updated model:
https://1drv.ms/u/s!ApJIS-l4xqPtgvoBepw062esMd_uiw

 

[jonwienke]

Some further refinements:
https://1drv.ms/u/s!ApJIS-l4xqPtgvoONa5lZeAAVeNmTQ

Nominal impedance 75 ohms
Calculated SWR <3:1 from 25MHz to 1300MHz
Gain >0dBi on all scannable frequencies from 145MHz-1300MHz