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.





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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.



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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.




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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