On 03/20/2014 10:58 PM, Jeff Liebermann wrote:
On Thu, 20 Mar 2014 21:12:57 -0700, Jeff Liebermann
wrote:

http://802.11junk.com/jeffl/antennas/Discone/
http://802.11junk.com/jeffl/antennas/biconical/
I did some more tinkering with these two. Looks like the biquad also
has holes in the gain plot, but shows more gain at more frequencies
than the discone. I changed the characteristic impedance of the
biquad to 150 ohms to get a better looking VSWR. Real ground and
animated GIF's of the biconical antenna when I have more time.

Diamond D-130 discone antenna model borrowed from:
http://www.qsl.net/kp4md/modeling.htm
I don't recall where I found the biconical.

Neat, thanks. I've read that the discone is really a ground hugger, and
your ground plots show that perfectly.

Jon

"Jon Danniken" wrote in message
...
Thanks Ralph, I'll try that this weekend and see what the results are (I
have scads of RG6).

Speaking of cable impedance, will I see much of a difference using 70
ohm cable on an 50 ohm antenna? Assuming less than 50 feed of distance,
does it really matter that much?


YOu will not see any differance using either the 50 or 70 ohm cable due to
impedance. You may see some due to the actual loss of the cable. For
example rg-58 (50 ohm) will have more loss than rg-6, but if you go to a
lower loss 50 ohm cable such as rg-8 then the cable loss will be less. For
50 feet and receiving only, I would use the rg-6 as you said you have
plenty of it.



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On Fri, 21 Mar 2014 07:54:51 -0700, Jon Danniken
wrote:

Speaking of cable impedance, will I see much of a difference using 70
ohm cable on an 50 ohm antenna? Assuming less than 50 feed of distance,
does it really matter that much?

I use 75 ohm coax for most everything and recommend using 75 ohm
cable. It has less loss per length than the equivalent size 50 ohm
coax. It's also cheaper and more available. Like you, I have piles
of the stuff scrounged from CATV "surplus".

Biggest headache are the adapters needed to go from (waterproof)
F-connectors and BNC/UHF/N connectors. Some of my RG-6a/u cables now
have BNC connectors instead of F-connectors, which helps a little. I
still prefer the cheaper F-connectors.

Another headache is interfacing with 50 ohm test equipment. I used to
have a pile of elaborate pads, and simple 25 ohm resistor in series
adapters. Unless I'm working with very low losses and measurements to
3 decimal places, or am trying to work with low VSWR systems, mixing
impedances doesn't seem to matter much. I've also measured various
pads from my collection at both 50 and 75 ohms, and found about 0.3dB
difference, which is about the accuracy of my pads. These days, I
just ignore the problem and use the pads interchangeably.

Incidentally, the usually quoted 0.18dB mismatch loss is based on the
assumption that the antenna and the transmitter are both 50 ohms, and
that only the coax cable is 75 ohms. 0.18dB is the loss at one end of
the cable, not both. Worst case is twice the loss, or 0.36dB.
Since the antenna will be closer to 75 ohms than 50 ohms, we can
ignore that end. The receiver input impedance is also not a perfect
50 ohms, so that can also be ignored. Bottom line is to not worry
about the whole mismatch loss question.

More on 50 versus 75 ohms:
http://www.solred.com.ar/lu6etj/tecnicos/En_75.htm
http://www.belden.com/blog/broadcastav/50-Ohms-The-Forgotten-Impedance.cfm
http://www.microwaves101.com/encyclopedia/why50ohms.cfm
http://www.dkdinst.com/articles/50ohmnotes.html




--
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 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

On 03/21/2014 08:40 AM, Ralph Mowery wrote:

"Jon Danniken" wrote in message
...
Thanks Ralph, I'll try that this weekend and see what the results are (I
have scads of RG6).

Speaking of cable impedance, will I see much of a difference using 70
ohm cable on an 50 ohm antenna? Assuming less than 50 feed of distance,
does it really matter that much?


YOu will not see any differance using either the 50 or 70 ohm cable due to
impedance. You may see some due to the actual loss of the cable. For
example rg-58 (50 ohm) will have more loss than rg-6, but if you go to a
lower loss 50 ohm cable such as rg-8 then the cable loss will be less. For
50 feet and receiving only, I would use the rg-6 as you said you have
plenty of it.

Thanks Ralph, that will be my plan.

Jon

On 03/21/2014 08:49 AM, Jeff Liebermann wrote:
On Fri, 21 Mar 2014 07:54:51 -0700, Jon Danniken
wrote:

Speaking of cable impedance, will I see much of a difference using 70
ohm cable on an 50 ohm antenna? Assuming less than 50 feed of distance,
does it really matter that much?

I use 75 ohm coax for most everything and recommend using 75 ohm
cable. It has less loss per length than the equivalent size 50 ohm
coax. It's also cheaper and more available. Like you, I have piles
of the stuff scrounged from CATV "surplus".

Biggest headache are the adapters needed to go from (waterproof)
F-connectors and BNC/UHF/N connectors. Some of my RG-6a/u cables now
have BNC connectors instead of F-connectors, which helps a little. I
still prefer the cheaper F-connectors.

I have a couple dozen BNC ends, but I need to get some female/bulkhead
connectors to go along with them.

Another headache is interfacing with 50 ohm test equipment. I used to
have a pile of elaborate pads, and simple 25 ohm resistor in series
adapters. Unless I'm working with very low losses and measurements to
3 decimal places, or am trying to work with low VSWR systems, mixing
impedances doesn't seem to matter much. I've also measured various
pads from my collection at both 50 and 75 ohms, and found about 0.3dB
difference, which is about the accuracy of my pads. These days, I
just ignore the problem and use the pads interchangeably.

Incidentally, the usually quoted 0.18dB mismatch loss is based on the
assumption that the antenna and the transmitter are both 50 ohms, and
that only the coax cable is 75 ohms. 0.18dB is the loss at one end of
the cable, not both. Worst case is twice the loss, or 0.36dB.
Since the antenna will be closer to 75 ohms than 50 ohms, we can
ignore that end. The receiver input impedance is also not a perfect
50 ohms, so that can also be ignored. Bottom line is to not worry
about the whole mismatch loss question.

More on 50 versus 75 ohms:
http://www.solred.com.ar/lu6etj/tecnicos/En_75.htm
http://www.belden.com/blog/broadcastav/50-Ohms-The-Forgotten-Impedance.cfm
http://www.microwaves101.com/encyclopedia/why50ohms.cfm
http://www.dkdinst.com/articles/50ohmnotes.html

Great links, thanks Jeff.

Jon

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 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?