Discone and feedline grounding
 

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

Discone and feedline grounding
 

"Jon Danniken" wrote in message
...
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

The discone is not a ground plane antenna.

Often the shield of the coax is connected to the cone part. That part is
also often connected to the mounting point which may go to the earth ground.

The cone part isolates the shield of the coax from being part of the antenna
(in simple terms) so what hapens on the shield does not mater to the
antenna.

Maybe you are confusing what an actual ground plane antenna is ? Being
connected to the earth ground has nothing to do with a ground plane antenna.





---
This email is free from viruses and malware because avast! Antivirus protection is active.
http://www.avast.com

Discone and feedline grounding
 

On 03/20/2014 07:23 AM, Ralph Mowery wrote:
"Jon Danniken" wrote in message
...
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

The discone is not a ground plane antenna.

Often the shield of the coax is connected to the cone part. That part is
also often connected to the mounting point which may go to the earth ground.

The cone part isolates the shield of the coax from being part of the antenna
(in simple terms) so what hapens on the shield does not mater to the
antenna.

Maybe you are confusing what an actual ground plane antenna is ? Being
connected to the earth ground has nothing to do with a ground plane antenna.

Thanks Ralph, I guess that is exactly what is what I do not understand
yet. I was just assuming that connecting the cone/shield to earth
ground would change the characteristics of the cone into (what I had
assumed would be) a ground plane.

Extending this concept outside of discones, and applying it to a dipole,
could you also directly feed a dipole, ground the shield, and still have
it behave as a 1/2 wave dipole?

Additionally, when does the need for a balun to transition between a
balanced line and coax arise?

Jon

Discone and feedline grounding
 

"Jon Danniken" wrote in message
...
On 03/20/2014 07:23 AM, Ralph Mowery wrote:
Thanks Ralph, I guess that is exactly what is what I do not understand
yet. I was just assuming that connecting the cone/shield to earth
ground would change the characteristics of the cone into (what I had
assumed would be) a ground plane.

Extending this concept outside of discones, and applying it to a dipole,
could you also directly feed a dipole, ground the shield, and still have
it behave as a 1/2 wave dipole?

Additionally, when does the need for a balun to transition between a
balanced line and coax arise?

Jon

For the halfwave antenna I am going to assume that you mean one that is
horizontal and up in the air some distance,then you use coax cable to come
into the shack. You can connect the shield of the coax anywhere from right
at the feed point (which would not be practical) to a point near the
transceiver to the earth.

A balun is mainly used to connect a ballanced antenna to an unballanced line
like coax cable. The balun is a contraction of BALanced to UNbalance.

They are not always needed, but may or may not help. A simple 1/2 wave
dipole is a balanced antenna as each side is the same. Theory says to use a
balun to keep the feed line from becomming a part of the antenna. I and
many others have up dipoles that do not have baluns and they work fine.

Baluns are often used on beam antennas so the radiation patern will not be
distorted.

Unless using an antenna tuner that has a built in balun or is designed for
the open wire feedlines a balun is used to feed the coax connector of the
transceiver. Most often it will be a 4:1 ratio to change the 300 to 600 ohm
feedline to closer to 50 ohms to match the transceiver.

A ground plane is unbalanced as the elements are not equal and so a balun
would not do any good. Same as the discone you were asking about , no balun
is needed as this is an unbalanced antenna.

There is another thing that is often referred to as a choke balun, which is
not actually a balun. It can be several turns of coax coiled up or a piece
of coax with some of the ferrite beads over it.



---
This email is free from viruses and malware because avast! Antivirus protection is active.
http://www.avast.com

Discone and feedline grounding
 

Ralph Mowery wrote:
"Jon Danniken" wrote:

Thanks Ralph, I guess that is exactly what is what I do not understand
yet. I was just assuming that connecting the cone/shield to earth
ground would change the characteristics of the cone into (what I had
assumed would be) a ground plane.

Extending this concept outside of discones, and applying it to a dipole,
could you also directly feed a dipole, ground the shield, and still have
it behave as a 1/2 wave dipole?

Additionally, when does the need for a balun to transition between a
balanced line and coax arise?



For the halfwave antenna I am going to assume that you mean one that is
horizontal and up in the air some distance,then you use coax cable to come
into the shack. You can connect the shield of the coax anywhere from right
at the feed point (which would not be practical) to a point near the
transceiver to the earth.

I was actually thinking of a vertical dipole, as it's what I have on my
roof right now (an old set of rabbit ears, so I can listen to airband
while I get the discone built and figure out a proper mast setup).

A balun is mainly used to connect a ballanced antenna to an unballanced line
like coax cable. The balun is a contraction of BALanced to UNbalance.

Aha, thanks, I've been wondering about that for awhile now.

They are not always needed, but may or may not help. A simple 1/2 wave
dipole is a balanced antenna as each side is the same. Theory says to use a
balun to keep the feed line from becomming a part of the antenna. I and
many others have up dipoles that do not have baluns and they work fine.

Good to know, thanks. I'm using a 300:75 converter up there right now
(twinlead from the rabbit ears to the coax), I think I'll take it off
and see if it makes any difference.

Baluns are often used on beam antennas so the radiation patern will not be
distorted.

Unless using an antenna tuner that has a built in balun or is designed for
the open wire feedlines a balun is used to feed the coax connector of the
transceiver. Most often it will be a 4:1 ratio to change the 300 to 600 ohm
feedline to closer to 50 ohms to match the transceiver.

So they'll mostly be located by the transceiver instead of up on the mast?

A ground plane is unbalanced as the elements are not equal and so a balun
would not do any good. Same as the discone you were asking about , no balun
is needed as this is an unbalanced antenna.

There is another thing that is often referred to as a choke balun, which is
not actually a balun. It can be several turns of coax coiled up or a piece
of coax with some of the ferrite beads over it.

Yeah, I've seen the chokes on some antennas made by forming a coil from
the coax.

Thanks Ralph,

Jon

Discone and feedline grounding
 

"Jon Danniken" wrote in message
...
I was actually thinking of a vertical dipole, as it's what I have on my
roof right now (an old set of rabbit ears, so I can listen to airband
while I get the discone built and figure out a proper mast setup).


Ok on the vertical dipole. For this antenna you need to run the feedline
horizontal from it for a couple of feet and then down.

The impedance of this antenna should be around 70 ohms and if I were you , I
would use some 70 ohm rg-6 coax back to the receiver.




Good to know, thanks. I'm using a 300:75 converter up there right now
(twinlead from the rabbit ears to the coax), I think I'll take it off
and see if it makes any difference.

The 300:75 converter is actually a balun that has a 4 to 1 ratio. It
normally does 2 things, changes a 300 ohm to 75 ohm inpedance such as many
TV antennaas were set for 300 ohms so the twin lead could be used. As
things changed over the years, the newer TV sets had a 70 ohm input for the
coax cable. The 300:70 could be used either way, 300 ohm antenna to coax or
coax to the old 300 ohm input of the TV.

Removing it from the vertical dipole (70 ohm inpedance) and using coax to
the receiver will probably help.




So they'll mostly be located by the transceiver instead of up on the mast?


The baluns can be used either place. Most often at the antenna if coax is
used and at the transceiver if open wire (twinlead) is used,


I have been using the 70 and 75 ohms without paying much attention. They
are close enough it does not mater.




---
This email is free from viruses and malware because avast! Antivirus protection is active.
http://www.avast.com

Discone and feedline grounding
 

On Thu, 20 Mar 2014 00:06:50 -0700, Jon Danniken
wrote:

Hi all, I'm cooking up a discone antenna for receiving,

I don't want to discourage you, but if you're building something from
scratch, may I suggest you build an antenna that works better than a
discone. There was a discussion in this newsgroups recently that
drifted over to discones. To illustrate some of the problems, I ran
simulations of a Diamond D-130 discone (without the low frequency
vertical section) to illustrate how the antenna pattern is less than
ideal at the higher frequencies. See:
http://802.11junk.com/jeffl/antennas/Discone/index.html
An animated slide show is at:
http://802.11junk.com/jeffl/antennas/Discone/slides/discone-animated.html
The frequency is shown in the upper left. The gain in the lower
right. Note that the gain is still there, but at higher frequencies
is mostly pointing almost straight up.

Unless you're interested in listening to satellites and aircraft, I
recommend a biconcial instead. It's very much like the common "fan
dipole" used for multiband HF operation without requiring traps.

Unfortunately, my only biconical model that might be suitable seems to
have a problem. (I don't recall where I found the model). There are
some nasty nulls at several frequencies between 50 to 1000 MHz that
need to be fixed. I'll play with it some more when I have time.
http://802.11junk.com/jeffl/antennas/biconical/
I set the characteristic impedance to 75 ohms, which seems to work the
best. However, anything between 50 and 200 ohms should work. For 75
ohms, a 1:1 broadband balun (i.e. a transformer) is necessary. I'll
create an animated GIF file for the patterns at various frequencies
when I have more time.



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

Discone and feedline grounding
 

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.

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

Discone and feedline grounding
 

Ralph Mowery wrote:

Ok on the vertical dipole. For this antenna you need to run the feedline
horizontal from it for a couple of feet and then down.

The impedance of this antenna should be around 70 ohms and if I were you , I
would use some 70 ohm rg-6 coax back to the receiver.

The 300:75 converter is actually a balun that has a 4 to 1 ratio. It
normally does 2 things, changes a 300 ohm to 75 ohm inpedance such as many
TV antennaas were set for 300 ohms so the twin lead could be used. As
things changed over the years, the newer TV sets had a 70 ohm input for the
coax cable. The 300:70 could be used either way, 300 ohm antenna to coax or
coax to the old 300 ohm input of the TV.

Removing it from the vertical dipole (70 ohm inpedance) and using coax to
the receiver will probably help.


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?

The baluns can be used either place. Most often at the antenna if coax is
used and at the transceiver if open wire (twinlead) is used,


I have been using the 70 and 75 ohms without paying much attention. They
are close enough it does not mater.

Jon

Discone and feedline grounding
 

On 03/20/2014 09:12 PM, Jeff Liebermann wrote:
On Thu, 20 Mar 2014 00:06:50 -0700, Jon Danniken
wrote:

Hi all, I'm cooking up a discone antenna for receiving,

I don't want to discourage you, but if you're building something from
scratch, may I suggest you build an antenna that works better than a
discone. There was a discussion in this newsgroups recently that
drifted over to discones. To illustrate some of the problems, I ran
simulations of a Diamond D-130 discone (without the low frequency
vertical section) to illustrate how the antenna pattern is less than
ideal at the higher frequencies. See:
http://802.11junk.com/jeffl/antennas/Discone/index.html
An animated slide show is at:
http://802.11junk.com/jeffl/antennas/Discone/slides/discone-animated.html
The frequency is shown in the upper left. The gain in the lower
right. Note that the gain is still there, but at higher frequencies
is mostly pointing almost straight up.

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.

Unless you're interested in listening to satellites and aircraft, I
recommend a biconcial instead. It's very much like the common "fan
dipole" used for multiband HF operation without requiring traps.

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.

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?

Unfortunately, my only biconical model that might be suitable seems to
have a problem. (I don't recall where I found the model). There are
some nasty nulls at several frequencies between 50 to 1000 MHz that
need to be fixed. I'll play with it some more when I have time.
http://802.11junk.com/jeffl/antennas/biconical/
I set the characteristic impedance to 75 ohms, which seems to work the
best. However, anything between 50 and 200 ohms should work. For 75
ohms, a 1:1 broadband balun (i.e. a transformer) is necessary. I'll
create an animated GIF file for the patterns at various frequencies
when I have more time.

I'll look forward to that, thanks!

Jon

Discone and feedline grounding
 

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

Discone and feedline grounding
 

"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.



---
This email is free from viruses and malware because avast! Antivirus protection is active.
http://www.avast.com

Discone and feedline grounding
 

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

Discone and feedline grounding
 

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

Discone and feedline grounding
 

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

Discone and feedline grounding
 

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

Discone and feedline grounding
 

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

Discone and feedline grounding
 

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

Discone and feedline grounding
 

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

Discone and feedline grounding
 

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?

Discone and feedline grounding
 

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

Discone and feedline grounding
 

Jeff Liebermann wrote:
Jon Danniken wrote:


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.

Aha, okay I got you now, thanks for that. I still need to figure out
how to read the "lobe pattern" charts.

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.

Hehe, indeed it doesn't. I was thinking more along the lines of a relay
box, if such a thing is possible, or maybe there is actually a gadget
that does something similar.

Along those lines, if connecting antennae of differing frequencies
together is not something that works, how does an antenna with multiple
different elements, like something like a scantenna
(http://i.imgur.com/D3Aeb58.jpg) get away with it?

Jon

Discone and feedline grounding
 

On 03/21/2014 07:54 AM, Jon Danniken wrote:
Ralph Mowery wrote:

Ok on the vertical dipole. For this antenna you need to run the feedline
horizontal from it for a couple of feet and then down.

The impedance of this antenna should be around 70 ohms and if I were you , I
would use some 70 ohm rg-6 coax back to the receiver.

The 300:75 converter is actually a balun that has a 4 to 1 ratio. It
normally does 2 things, changes a 300 ohm to 75 ohm inpedance such as many
TV antennaas were set for 300 ohms so the twin lead could be used. As
things changed over the years, the newer TV sets had a 70 ohm input for the
coax cable. The 300:70 could be used either way, 300 ohm antenna to coax or
coax to the old 300 ohm input of the TV.

Removing it from the vertical dipole (70 ohm inpedance) and using coax to
the receiver will probably help.


Thanks Ralph, I'll try that this weekend and see what the results are (I
have scads of RG6).

Well, I got up on the roof and reconfigured the antenna today, and got a
nice improvement from my setup. Unfortunately I wasn't very scientific
about figuring out what made the big difference, but I did remove the
4:1 balun and ran the coax out horizonally for ~1/4 wavelength. I also
soldered the old crimp connection at the base of the elements; they were
reading about six ohms from the twinlead to the tips before, now the
resistance is low enough to not be measured by my DMM.

After the work today, noise is down ~10dB, and I can make out a lot more
transmissions, with much higher clarity, than I could before. I still
occasionally get a transmission with a strong signal that sounds
garbled, but everything else is coming in very nicely.

Thanks for the suggestions, they paid off.

Jon

Discone and feedline grounding
 

On 3/22/2014 7:13 PM, Jeff Liebermann wrote:
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.


There is? I remember back in the 70's a United Airlines pilot who would
regularly work 146.52. I had no trouble chatting with him from almost
300 miles away, even though he was only using a 1.5W HT. And out here
on the east coast, it's impossible to use an HT on any 2M repeater
frequency without bringing up multiple repeaters, even when using 1W
at about 3,000 feet or above. From 10,000 feet, forget it almost
anywhere in the country.

And BTW - commercial planes generally fly at around 7mi (35,000-37,000
ft.) high - not 5Mi (26,000 ft.) as you claimed.

--
==================
Remove the "x" from my email address
Jerry, AI0K

==================

Discone and feedline grounding
 

On Sun, 23 Mar 2014 09:24:09 -0400, Jerry Stuckle
wrote:

On 3/22/2014 7:13 PM, Jeff Liebermann wrote:
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.

There is? I remember back in the 70's a United Airlines pilot who would
regularly work 146.52. I had no trouble chatting with him from almost
300 miles away, even though he was only using a 1.5W HT. And out here
on the east coast, it's impossible to use an HT on any 2M repeater
frequency without bringing up multiple repeaters, even when using 1W
at about 3,000 feet or above. From 10,000 feet, forget it almost
anywhere in the country.

Yep. You can be heard for quite a distance from an airplane. The
problem is hearing anything as the chances of co-channel interference
is high.

I help maintain an ADS-B listening station (1090 MHz) with a good
view of the ocean. 200 Nm average range. I designed the antenna
specifically for the purpose and location:
http://802.11junk.com/jeffl/antennas/AMOS-5-1090MHz/index.html
The actual antenna system is somewhat more complex.

Also a VHF AIS receiver at a local hilltop. Great view from near the
coast from about 2,000 ft.
http://www.marinetraffic.com/ais/details/stations/112
(Note that it's NOT on Mt Umunhum. The receiver was moved to Bonny
Doon to eliminate weather xmitter interference). Average range is
about 200 Nm, and much more when ducting is available. If you're near
the coast, listen on 161.975 MHz and 162.025 MHz for the AIS traffic.

And BTW - commercial planes generally fly at around 7mi (35,000-37,000
ft.) high - not 5Mi (26,000 ft.) as you claimed.

True. The problem is that near the coast, most of the aircraft
traffic is on takeoff or approach and at much lower altitudes. Here's
a typical altitude profile for an KLAX to KSFO flight:
http://802.11junk.com/jeffl/crud/flight-profile.jpg
Data capture was from:
http://flightaware.com/live/flight/VRD935/history/20140323/1600Z/KLAX/KSFO/tracklog
which will change throughout the day. Notice that for a 500 Nm
flight, it doesn't stay above 30,000 ft for very long. By the time I
see the incoming data, it's usually between 10,000 to 15,000 ft. Of
course for cross country, they stay above 30,000 ft for much longer. I
just took a look at the raw data from the local ADS-B receiver. One
flight at 30,000 ft and everything else below about 15,000 ft over a
10 minute period.

Incidentally, until you mentioned radio range in an aircraft, I never
bothered to check how much TX power the aircraft was using for ADS-B
(1090 MHz). Looks like they deliver some serious power at altitude:
http://www.ads-b.com/PDF/UAT%20SARP.pdf
Table 12-1: Transmitter power levels
Transmitter Minimum power Maximum power Intended minimum
type at PMP at PMP air-to-air ranges
Aircraft (Low) 7 watts (+38.5 dBm) 18 watts (+42.5 dBm) 20 NM
Aircraft (Med) 16 watts (+42 dBm) 40 watts (+46 dBm) 40 NM
Aircraft (High) 100 watts (+50 dBm) 250 watts (+54 dBm) 120 NM
Note that the range is for air to air, not air to ground.

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