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

"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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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

"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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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

"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 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 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 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

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