On Fri, 21 Mar 2014 08:14:01 -0700, Jon Danniken
wrote:

Thanks Jeff, I had not considered a biconical, but it looks interesting.
The discone I am building is of a "spoke" variety, with the cone built
separate from the disk, so it would be simple to duplicate another cone
and invert it on top of the other cone.

If you want to be cheap and sloppy (like me), consider that a bowtie
antenna, commonly found on broadband TV antennas, is nothing more than
a flattened biconcial antenna. While it is directional, it's
functions much the same way as a biconcical and is much easier to
build (out of aluminum roof flashing).

I do like aircraft, although I am starting to think that I might be
better off with a dedicated airband antenna (looking at j-poles right
now) along with a wideband antenna for general scanning.

I don't have any professional experience with aircraft antennas.
However, I have done some ADS-B 1090 MHz designs and tests. The basic
idea is that the antenna should have the most gain at the horizon and
somewhat above the horizon to get the most range. Commercial jets fly
at about 5 miles altitude maximum, so gain in the upwards direction is
less critical. That's quite opposite of what the discone and
biconical will do.

Speaking of multiple antennas, I know that some antennas use multiple
elements tuned to different bands, but can you connect two antennas to
the same feedline? Like, say, a discone/biconical and a j-pole?

No. The best you can do is insert a diplexer at the feedpoint
junction, and separate the operating frequencies. Putting two
antennas in parallel doesn't work. If both antennas received the same
signal, the antenna pattern would be a conglomeration of both
antennas, which could just as easily result in a null as it could a
peak (also known as a mess).

I have such an arrangement at a site. 120ft of very expensive 2"(?)
Heliax going between the tower and the building. One triplexer and
three antennas, each on a different band, on top of the tower. Another
triplexer and 3 radios at the other end. Works so-so as intermod and
desense are a problem on some frequencies due to insufficient
triplexer isolation.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Jeff Liebermann wrote:
Jon Danniken wrote:

Thanks Jeff, I had not considered a biconical, but it looks interesting.
The discone I am building is of a "spoke" variety, with the cone built
separate from the disk, so it would be simple to duplicate another cone
and invert it on top of the other cone.

If you want to be cheap and sloppy (like me), consider that a bowtie
antenna, commonly found on broadband TV antennas, is nothing more than
a flattened biconcial antenna. While it is directional, it's
functions much the same way as a biconcical and is much easier to
build (out of aluminum roof flashing).

I do like aircraft, although I am starting to think that I might be
better off with a dedicated airband antenna (looking at j-poles right
now) along with a wideband antenna for general scanning.

I don't have any professional experience with aircraft antennas.
However, I have done some ADS-B 1090 MHz designs and tests. The basic
idea is that the antenna should have the most gain at the horizon and
somewhat above the horizon to get the most range. Commercial jets fly
at about 5 miles altitude maximum, so gain in the upwards direction is
less critical. That's quite opposite of what the discone and
biconical will do.

I thought the discone/biconicals were more of a horizon-looking antenna,
at least from what I have read on them?

Speaking of multiple antennas, I know that some antennas use multiple
elements tuned to different bands, but can you connect two antennas to
the same feedline? Like, say, a discone/biconical and a j-pole?

No. The best you can do is insert a diplexer at the feedpoint
junction, and separate the operating frequencies. Putting two
antennas in parallel doesn't work. If both antennas received the same
signal, the antenna pattern would be a conglomeration of both
antennas, which could just as easily result in a null as it could a
peak (also known as a mess).

I have such an arrangement at a site. 120ft of very expensive 2"(?)
Heliax going between the tower and the building. One triplexer and
three antennas, each on a different band, on top of the tower. Another
triplexer and 3 radios at the other end. Works so-so as intermod and
desense are a problem on some frequencies due to insufficient
triplexer isolation.

Maybe I'll look at a switcher of some sort eventually, and plan on just
manually doing it for now, thanks.

Jon

On Sat, 22 Mar 2014 08:47:56 -0700, Jon Danniken
wrote:

I don't have any professional experience with aircraft antennas.
However, I have done some ADS-B 1090 MHz designs and tests. The basic
idea is that the antenna should have the most gain at the horizon and
somewhat above the horizon to get the most range. Commercial jets fly
at about 5 miles altitude maximum, so gain in the upwards direction is
less critical. That's quite opposite of what the discone and
biconical will do.

I thought the discone/biconicals were more of a horizon-looking antenna,
at least from what I have read on them?

After I ran the NEC2 models, that seems true for the low end of the
frequency range. They are suppose to look something like a broadband
version of a vertical dipole. However, as the frequency goes up,
additional lobes appear until at the top of the frequency range, most
of the RF is going straight up. A Biconical is somewhat better than a
discone at retaining a sane looking pattern and reasonable gain, but
not much better.

My point about listening to aircraft is that there's little difficulty
hearing aircraft that are overhead, and plenty of difficulty hearing
aircraft near the horizon. Therefore, the antenna should have most of
its gain towards the horizon, and less gain above the horizon to near
overhead. At low frequencies, the discone does that. At the high end
of the range, it's quite the opposite.

Maybe I'll look at a switcher of some sort eventually, and plan on just
manually doing it for now, thanks.

Ummm... climbing the tower to rotate a manual switch doesn't sound
like a good idea.



--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On 3/20/2014 2:06 AM, Jon Danniken wrote:
Hi all, I'm cooking up a discone antenna for receiving, and I have a
question about feedline grounding. From everything I have seen, the
discone has an impedance of about 50 ohms, and everyone seems to just
connect the feedline right to the antenna; center conductor to the disc,
and shield to the cone.

My question is about what happens when I ground the shield at the
arrestor block just before the cable comes into the house; without a
balun, won't I just be turning the cone into a ground plane? I know
that there are ground plane antennas, but even though I am new to all of
this I thought that a discone was not a ground plane.

What am I missing here?

Thanks for any suggestions,

Jon

Jon, you are making this much more complicated than it really is.

A discone is a vertically polarized omnidirectional antenna. Just like a
quarter-wave vertical. The difference is that the discone supposedly has
a wide bandwidth.

For receiving purposes, do you care about grounding the shielding to
something if you can receive what you are listening for? Grounding the
shield may cause your receive pattern to change, but that can happen
with nearby trees and nearby structures as well.

Your best bet is to put it up and try it out. You can try grounding and
ungrounding the shield to suit your reception desires, if it even makes
a difference.

Cheers,
John KD5YI

John S wrote:

Jon, you are making this much more complicated than it really is.

A discone is a vertically polarized omnidirectional antenna. Just like a
quarter-wave vertical. The difference is that the discone supposedly has
a wide bandwidth.

For receiving purposes, do you care about grounding the shielding to
something if you can receive what you are listening for? Grounding the
shield may cause your receive pattern to change, but that can happen
with nearby trees and nearby structures as well.

Your best bet is to put it up and try it out. You can try grounding and
ungrounding the shield to suit your reception desires, if it even makes
a difference.

Hi John, my sole reason for grounding the coax at the entry point of the
building is to be code compliant WRT lightning protection.

Jon

On 3/22/2014 12:02 PM, Jon Danniken wrote:
John S wrote:

Jon, you are making this much more complicated than it really is.

A discone is a vertically polarized omnidirectional antenna. Just like a
quarter-wave vertical. The difference is that the discone supposedly has
a wide bandwidth.

For receiving purposes, do you care about grounding the shielding to
something if you can receive what you are listening for? Grounding the
shield may cause your receive pattern to change, but that can happen
with nearby trees and nearby structures as well.

Your best bet is to put it up and try it out. You can try grounding and
ungrounding the shield to suit your reception desires, if it even makes
a difference.

Hi John, my sole reason for grounding the coax at the entry point of the
building is to be code compliant WRT lightning protection.

Jon


I understand that. But, does it prevent you from doing an experiment
while the weather is nice?

If so, then go for it and see if you like it. What are your alternatives?