Modifying top vertical element on Diamond D130 discone?

[iMONITOR]

Has anyone experimented with modifying the top vertical element on a Diamond D130 discone antenna, say for 700-800MHz? If so would it be productive or would it degrade overall performance of the discone for other bands?

I guess it would be difficult to do without some way to measure results due to the unknown influence of the loading coil, not like just cutting the element shorter.

 

[prcguy]

The Discone is an aperture antenna that launches a wave from the apex of the top disc and bottom cone like a horn antenna or V beam. There aren't really any resonant parts. Companies get by adding a low frequency base loaded whip because the base loading coil acts like an RF choke and somewhat decouples the whip from the circuit at higher frequencies where the Discone operates. However, the top whip works against the bottom cone as a ground plane that is completely inadequate and the top disc is attached to the base of the coil and whip adding lots of capacitance to the ground plane. Otherwise the low frequency coil and whip on top work at a frequency completely out of the Discone operating range and that's kind of why it works. Sort of.

Looking at adding an 800MHz whip brings up new problems like the Discone is actually working in the 800MHz range even though the radiation pattern is skewed up in the air. Adding any kind of whip for 800 seems like a problem to me since its within the Discones operating range and the amount of capacitance created by the top disc attached to the 800 whip would make a huge mess. I don't know how you would resolve these problems.

 

[iMONITOR]

The Discone is an aperture antenna that launches a wave from the apex of the top disc and bottom cone like a horn antenna or V beam. There aren't really any resonant parts. Companies get by adding a low frequency base loaded whip because the base loading coil acts like an RF choke and somewhat decouples the whip from the circuit at higher frequencies where the Discone operates. However, the top whip works against the bottom cone as a ground plane that is completely inadequate and the top disc is attached to the base of the coil and whip adding lots of capacitance to the ground plane. Otherwise the low frequency coil and whip on top work at a frequency completely out of the Discone operating range and that's kind of why it works. Sort of.

Looking at adding an 800MHz whip brings up new problems like the Discone is actually working in the 800MHz range even though the radiation pattern is skewed up in the air. Adding any kind of whip for 800 seems like a problem to me since its within the Discones operating range and the amount of capacitance created by the top disc attached to the 800 whip would make a huge mess. I don't know how you would resolve these problems.

That's what I was afraid of, but no problem, just curious.

Thanks,
Bill