Discone: measured loss when wrong size

[nonposter]

Have any measurements been published for the gain for incorrectly sized discone antennas for different frequencies?

To elaborate: a discone is expected to have "adequate" gain for a certain range of frequencies when built to certain dimensions, eg the disc diameter for an antenna which has a lower frequency cutoff of 100 MHz is 21 inches (source). What would be the signal loss at 100 MHz and 200 MHz if the disc was 12 inches? How about 8 inches? (Assume the length of the radials is appropriately matched to the disc size.)

Also, does the RF energy travel perpendicularly to the disc? The radials, due to their angle, reflect the energy from/to the disc to the horizon. If this is true, wouldn't it make sense to always have radials below and above the disc, to reflect all of the possible energy, instead of only half?

 

[kb2vxa]

Hi Non and all,

First of all a discone has unity gain meaning no gain at all.

Second, you need antenna modeling software or someone with a Masters degree in RF engineering to answer that one.

Third, refer to number two.

The long and short of it is even if someone here can properly answer your questions you wouldn't understand the answers. It's FAR too deep a subject for this forum and your computer can't handle the Greek letters that make up the equations.

OK guys give it a try, I'd LOVE to see what you come up with. (;->)

 

[nonposter]

First of all a discone has unity gain meaning no gain at all.

At what frequency? It can't be unity for every frequency. That's my question: at what frequency does the gain start to roll off for a discone antenna with certain dimensions?

Second, you need antenna modeling software or someone with a Masters degree in RF engineering to answer that one.

Measuring signal strength at various frequencies doesn't require a master's degree.

It's FAR too deep a subject for this forum and your computer can't handle the Greek letters that make up the equations.

This is the antenna forum, right? I would expect this to be the place to discuss it.

And, my computer can display Greek letters. Why would you think that it can't?

 

[Don_Burke]

Have any measurements been published for the gain for incorrectly sized discone antennas for different frequencies?

Not that I have seen.

To elaborate: a discone is expected to have "adequate" gain for a certain range of frequencies when built to certain dimensions, eg the disc diameter for an antenna which has a lower frequency cutoff of 100 MHz is 21 inches (source). What would be the signal loss at 100 MHz and 200 MHz if the disc was 12 inches? How about 8 inches? (Assume the length of the radials is appropriately matched to the disc size.)

The fastest way to figure that out would be to build one of each size and take the measurements. I would expect the dropoff to be substantial.

Also, does the RF energy travel perpendicularly to the disc? The radials, due to their angle, reflect the energy from/to the disc to the horizon. If this is true, wouldn't it make sense to always have radials below and above the disc, to reflect all of the possible energy, instead of only half?

A discone is an inverted version of an inverted cone, which is really a 1/4 wave monopole with some odd-looking things done for wideband impedance matching. Those are active elements, not reflectors.

Captain Paul H. Lee N6PL(SK) gave a good rundown on wideband verticals in his book, "The Amateur Radio Vertical Antenna Handbook." I can not even come close to doing it justice here, so you will have to read the book.

 

[jonny290]

The best way to summarize a discone electrically is that it's a high-pass filter with good SWR starting around 2F, where F is the design frequency. Below that, the impedance slopes but you'll still have a bad match.

And yes, the double discone does exist and will give you more gain and bandwidth, but it's physically inconvenient to build and rather ugly, that's all.

 

[prcguy]

Some of the common antenna programs like EZNEC will model the Discone and give you the gain and pattern at out of band frequencies. I have EZNEC but don’t want to spend a few hours building up the file. Using Google I found several others that have modeled Discones for other reasons and you can easily search and sift through their findings. One thing for sure is the VSWR goes nuts below the cutoff frequency.
A Double Discone as (I know it) is a Bicone or Biconical and they have much less bandwidth than a Discone but can have more gain at the higher freqs. I have a Singer measurement/EMI version that covers 30-200MHz and Harris that covers 100-400MHz (works really good) and a few military OE-254 30-90Mhz units. I also made the mistake of buying a Nevada Double Discone scanner piece of crap that is no better than two coat hangers.
prcguy