Open Wire fed lengths
 

After reading several articles both on and off the WEB regarding a
center fed antenna, only one article mentioned a recommended length for
the feed line.

My shack is located on the second floor of a new home, (24') #8 ground
wire to 8' ground stake with a further connection to the common ground
point for the power and the rebar in the pored walls and flooring of the
basement, telephone and cable. I plan on additional ground rods around
the property and at a new tower. In a new home so progress is on-going.

The 1:1 current balum used to convert the 450 ohm balance line is
external to the tuner, use about two feet of coax from the balum to the
tuner.

Antenna is cut for 80 meters and using about 60-70 ft of line to center
of the antenna. Where the antenna lines leave the shack is about four 4
ft from the location inside of the rig and floor mounted PC.

I use the antenna on all bands until such time I complete a new tower
installation. I want to minimize stray RF around the shack since I rely
on a PC and digital sound card as my primary source for ham enjoyment.
I have already had some problems with a USB keyboard that I think may
have been effected by RF when running over 50 watts on 20 meters only.

One article recommended odd multiples of a wave length is desirable at
the lowest operating frequency while other articles don't address this.

I am having no problem with a match using the tuner on all bands 80-6
meters.

Have used the center fed antenna since 1976 but always had the shack on
the ground level and had a good and effective ground system short runs
with feed lines around the 90 to 100ft in length. never had a RF
problem or surge problem.

Any recommended WEB sites or comments are welcomed.

73 de
Ron W4LDE

Open Wire fed lengths
 

Ron Walters wrote:
Any recommended WEB sites or comments are welcomed.

If your ground wire is an appreciable percentage of
a wavelength, it is a radiating element grounded at
one end, i.e. not a ground. An artificial ground
might help to reduce RF in the shack.

If your antenna is balanced, you don't need an RF
ground.
--
73, Cecil http://www.w5dxp.com

Open Wire fed lengths
 

"Ron Walters" wrote in message
t.net...
After reading several articles both on and off the WEB regarding a
center fed antenna, only one article mentioned a recommended length for
the feed line.

some antennas, like the g5rv rely on a particular length of feedline because
it becomes part of the antenna and radiates on certain bands. others use it
as an impedance transformer.

One article recommended odd multiples of a wave length is desirable at
the lowest operating frequency while other articles don't address this.

that is a myth that has been around for many years. there is no 'magic
length' that works better than others, except maybe the shortest length that
reaches from the antenna to the transmitter. The only reasons to use longer
lengths is if you need to use it as an impedance transformer because the
antenna isn't matched to the line, or to allow for future rearrangement of
the shack. what can be handy about the 1/2 wavelength line is if the
antenna isn't matched to the line impedance the antenna impedance will be
repeated every 1/2 wavelength along the line. so if you have a 300 ohm
antenna and a 50 ohm coax, every 1/2 wavelength along the line you will see
the 300 ohms again. this of course only works on harmonically related
bands.

Open Wire fed lengths
 

Cecil Moore wrote:
Ron Walters wrote:
Any recommended WEB sites or comments are welcomed.

If your ground wire is an appreciable percentage of
a wavelength, it is a radiating element grounded at
one end, i.e. not a ground. An artificial ground
might help to reduce RF in the shack.

If your antenna is balanced, you don't need an RF
ground.

Cecil:

Say I had a situation where I must use a ground wire which IS an
appreciable percentage of a wavelength ... and don't wish it to radiate.

Could I accomplish this by using coax as the ground-wire and choking the
outer braid by sufficient windings on a toroid core, and grounding the
center conductor and the braid to earth though good and deep grounding
spikes or wires?

Regards,
JS

Open Wire fed lengths
 

On Jul 2, 8:08 am, John Smith wrote:
Cecil Moore wrote:
Ron Walters wrote:
Any recommended WEB sites or comments are welcomed.

If your ground wire is an appreciable percentage of
a wavelength, it is a radiating element grounded at
one end, i.e. not a ground. An artificial ground
might help to reduce RF in the shack.

If your antenna is balanced, you don't need an RF
ground.

Cecil:

Say I had a situation where I must use a ground wire which IS an
appreciable percentage of a wavelength ... and don't wish it to radiate.

Could I accomplish this by using coax as the ground-wire and choking the
outer braid by sufficient windings on a toroid core, and grounding the
center conductor and the braid to earth though good and deep grounding
spikes or wires?

Regards,
JS

The only way to keep a wire--e.g., piece of coax--from being a
radiator is to keep net current at zero. If there's no net current,
you didn't need the wire anyway (at that frequency, at least). If
it's a protective ground for mains frequency, it will probably still
work for that purpose if you add ferrite for RF choking.

Open Wire fed lengths
 

K7ITM wrote:

...
The only way to keep a wire--e.g., piece of coax--from being a
radiator is to keep net current at zero. If there's no net current,
you didn't need the wire anyway (at that frequency, at least). If
it's a protective ground for mains frequency, it will probably still
work for that purpose if you add ferrite for RF choking.


K7ITM:

I was hoping the rf/dc/ac could reach ground via a very low
resistance/impedance to rf on the inner surface of the braid and the
center conductor ...

While it would be virtually impossible to reduce rf on the outer braid
to absolute zero, I was hoping the choke would provide sufficient
impedance to rf to where it became near negligible, at least for
practical purposes.

Regards,
JS

Open Wire fed lengths
 

John Smith wrote:
... I was hoping the choke would provide sufficient
impedance to rf to where it became near negligible, at least for
practical purposes.


In the above, change "sufficent" to low ...

Regards,
JS

Open Wire fed lengths
 

"Dave" wrote in message news:vNMak.174$P11.105@trndny06...

"Ron Walters" wrote in message
t.net...
After reading several articles both on and off the WEB regarding a
center fed antenna, only one article mentioned a recommended length for
the feed line.

some antennas, like the g5rv rely on a particular length of feedline because
it becomes part of the antenna and radiates on certain bands. others use it
as an impedance transformer.

It is also a myth that the feedline for the g5rv requires a particular length
because it becomes part of the antenna and radiates on certain bands. At the
antenna terminals the feedline becomes a transmission line with opposite
directions of current on each conductor. Thus the fields developed around each
conductor cancel resulting in no radiation from the feedline if it's dressed at
90° from the antenna.

In addition, it's also a myth that the feedline of a shortened antenna radiates.
Some believe that the portion of the feedline that makes up for the missing
length of the shortened antenna radiates--tain't so.

Walt, W2DU

Open Wire fed lengths
 

John Smith wrote:
Say I had a situation where I must use a ground wire which IS an
appreciable percentage of a wavelength ... and don't wish it to radiate.
Could I accomplish this by using coax as the ground-wire and choking the
outer braid by sufficient windings on a toroid core, and grounding the
center conductor and the braid to earth though good and deep grounding
spikes or wires?

Offhand, I would say you could accomplish a DC ground
that way but not an RF ground and not much lightning
protection.
--
73, Cecil http://www.w5dxp.com

Open Wire fed lengths
 

Cecil Moore wrote:

...
Offhand, I would say you could accomplish a DC ground
that way but not an RF ground and not much lightning
protection.

I suspect you to be correct; a "free ride" is just too much to hope for
in these times.

Well, even a dc/60hz ground which does not radiate rf can be useful.

Regards,
JS

Open Wire fed lengths
 

On Jul 2, 9:33 am, John Smith wrote:
K7ITM wrote:
...
The only way to keep a wire--e.g., piece of coax--from being a
radiator is to keep net current at zero. If there's no net current,
you didn't need the wire anyway (at that frequency, at least). If
it's a protective ground for mains frequency, it will probably still
work for that purpose if you add ferrite for RF choking.

K7ITM:

I was hoping the rf/dc/ac could reach ground via a very low
resistance/impedance to rf on the inner surface of the braid and the
center conductor ...

While it would be virtually impossible to reduce rf on the outer braid
to absolute zero, I was hoping the choke would provide sufficient
impedance to rf to where it became near negligible, at least for
practical purposes.

Regards,
JS

If there's a net RF current, there's a net RF current, and it will
radiate. It matters not a whit whether you say the net is on the
inside or the outside or distributed between them in any proportion.
If you choke things so there's no net current, you may as well not
have bothered putting the wire/coax/whatever in to begin with.
Consider what the current on the inside of the outer conductor must be
if the coax is acting as a transmission line, and consider where that
current goes at the "top" end of the piece you suggest.

To keep RF "out of the shack," put a Faraday cage around the shack and
don't turn RF loose inside that cage. Then it doesn't matter (with
respect to RF "in the shack") whether the Faraday cage is connected to
"ground" or not.

What exactly is "ground," anyway? Do you think it has magical
properties? What ARE its properties? What does it do for you?

Open Wire fed lengths
 

K7ITM wrote:

...
If there's a net RF current, there's a net RF current, and it will
radiate. It matters not a whit whether you say the net is on the
inside or the outside or distributed between them in any proportion.
...

Really, then I can throw away the current choke on my coax to my 1/2
wave vertical then? I THINK NOT!!! And, I DON'T think there is rf on
the outside of the coax shield--I DO think there is on the inside of the
braid ...

...
What exactly is "ground," anyway? Do you think it has magical
properties? What ARE its properties? What does it do for you?

Find an improperly grounded kw xmitter/liner whose case is hot with rf
and running into a noticeable SWR, now touch a sharp corner of the case
with your finger. Pay attention to the hissing noise(s), the flesh
being vaporized into a white smoke, the characteristic white, deep
penetrating, slow-to-heal rf burn(s) on your finger, etc.

Now ground the case and touch the sharp corner--notice the lack of pain?

Now, that you should get you started with what a ground is ... got
anymore questions?

Now, I can explain all that without having to bring magic into the
argument--but hell, go ahead, give me your "magical" version if you
would like ...

Regards,
JS

Open Wire fed lengths
 

On Jul 2, 3:07 pm, John Smith wrote:
K7ITM wrote:
...
If there's a net RF current, there's a net RF current, and it will
radiate. It matters not a whit whether you say the net is on the
inside or the outside or distributed between them in any proportion.
...

Really, then I can throw away the current choke on my coax to my 1/2
wave vertical then? I THINK NOT!!! And, I DON'T think there is rf on
the outside of the coax shield--I DO think there is on the inside of the
braid ...

....which is balanced by the RF on the center conductor. If it doesn't
radiate, there's no net current; the center conductor current is equal
in magnitude and opposite in phase to the current on the adjacent
inside of the outer conductor. Why would you think that what you
quoted above would imply that you could get rid of the choke on your
transmission line, if you want to keep the line from having a net
current and therefore radiating? It says just the opposite. But of
course, you already knew that, didn't you? Sorry, rhetorical
question.

Open Wire fed lengths
 

K7ITM wrote:


If there's a net RF current, there's a net RF current, and it will
radiate. It matters not a whit whether you say the net is on the
inside or the outside or distributed between them in any proportion.
If you choke things so there's no net current, you may as well not
have bothered putting the wire/coax/whatever in to begin with.
Consider what the current on the inside of the outer conductor must be
if the coax is acting as a transmission line, and consider where that
current goes at the "top" end of the piece you suggest.

To keep RF "out of the shack," put a Faraday cage around the shack and
don't turn RF loose inside that cage. Then it doesn't matter (with
respect to RF "in the shack") whether the Faraday cage is connected to
"ground" or not.

What exactly is "ground," anyway? Do you think it has magical
properties? What ARE its properties? What does it do for you?

Interesting ... suggesting that if an inductance/capacitance it added
into the shield (or center conductor) and in series with such, so that
the current voltages between the center conductor and braid are 90
degrees lagging/leading that a net current would be possible between the
inside braid and the center conductor. But, still due to the choke on
the outer braid--impossible there (well, vastly reduced.)

Hmmm ... a name for it? Perhaps "Tuned Ground" or "Tunable Ground." grin

Regards,
JS

Open Wire fed lengths
 

K7ITM wrote:

...
...which is balanced by the RF on the center conductor. If it doesn't
radiate, there's no net current; the center conductor current is equal
in magnitude and opposite in phase to the current on the adjacent
inside of the outer conductor. Why would you think that what you
quoted above would imply that you could get rid of the choke on your
transmission line, if you want to keep the line from having a net
current and therefore radiating? It says just the opposite. But of
course, you already knew that, didn't you? Sorry, rhetorical
question.

Actually, it took me awhile to put this together, your picture of "the
ground" given in your text ...

Basically, we are two blind men, you have the trunk of the elephant in
hand, me the foot ... but now that I think about it, I remember what
picture the trunk brought to my mind ...

A radiating ground I have used before, my first exposure to one instance
of it was like this:

In college, living in a couple of rooms rented to me by an individual
having a home near the campus, I resided on the third floor of an old
victorian (and, keep in mind, building codes were rather sparse back
then.) The plumbing had been "reworked" and sections of it replaced
with pvc. The old steam radiators had, had their piping removed in the
basement level. In despiration, I had to resort to a wire ran around
the base boards of the rooms and "adjusted" for various freqs/bands.
This ground DID, indeed, radiate. Indeed, the "fuzz" on the video
signal of the tv sets proved this absolutely--at times. The am band on
a radio was also, frequently, troubled by my signals/rfi.

All the above, because the owner did not want "wires ran about."

The "ground wire" I was proposing would attempt to function as a
transmission line, and simply carry the signals to a suitable ground;
Whether you made that a legitimate number of buried radials of suitable
length, a wire ran above ground and through the bushes, a wire
connecting a few ground rods, or a run to the pipe in the basement,
etc., would be at the users discretion ... however, the "ground wire"
would be made to NOT radiate, or radiate as little as possible ... that
is "the ground" (but actually, a "grounding transmission line") I was
proposing with the coax.

Regards,
JS

Open Wire fed lengths
 

John Smith wrote:
...
the current voltages between the center conductor and braid are 90
degrees lagging/leading that a net current would be possible between the
...

In the above, 90 = 180 ... just caught that ...

JS

Open Wire fed lengths
 

Ron Walters wrote:
After reading several articles both on and off the WEB regarding a
center fed antenna, only one article mentioned a recommended length for
the feed line.


There is a webpage that calculates the correct length of the feedline
for an antenna:

http://www.qsl.net/w4sat/howlong.htm


My shack is located on the second floor of a new home, (24') #8 ground
wire to 8' ground stake with a further connection to the common ground
point for the power and the rebar in the pored walls and flooring of the
basement, telephone and cable. I plan on additional ground rods around
the property and at a new tower. In a new home so progress is on-going.

The 1:1 current balum used to convert the 450 ohm balance line is
external to the tuner, use about two feet of coax from the balum to the
tuner.


Antenna is cut for 80 meters and using about 60-70 ft of line to center
of the antenna. Where the antenna lines leave the shack is about four 4
ft from the location inside of the rig and floor mounted PC.

I use the antenna on all bands until such time I complete a new tower
installation. I want to minimize stray RF around the shack since I rely
on a PC and digital sound card as my primary source for ham enjoyment.
I have already had some problems with a USB keyboard that I think may
have been effected by RF when running over 50 watts on 20 meters only.

One article recommended odd multiples of a wave length is desirable at
the lowest operating frequency while other articles don't address this.

Since you are using ladder line, you don't really want a hwole lot of
extra length, since you'd have to figure out what to do with it. Ladder
line doesn't like being wound up - note this is what I've been told, I
never experimented myself. But it stands to reason.

I am having no problem with a match using the tuner on all bands 80-6
meters.

The setup should work well. The grounding system isn't ideal, but we
work with what we have.

Have used the center fed antenna since 1976 but always had the shack on
the ground level and had a good and effective ground system short runs
with feed lines around the 90 to 100ft in length. never had a RF
problem or surge problem.

I'm not clear. Do you have an RFI problem now? Seems as if your steup
will work FB.

- 73 de Mike N3LI -

Open Wire fed lengths
 

"Michael Coslo" wrote in message
...
Ron Walters wrote:
After reading several articles both on and off the WEB regarding a
center fed antenna, only one article mentioned a recommended length for
the feed line.


There is a webpage that calculates the correct length of the feedline for
an antenna:

http://www.qsl.net/w4sat/howlong.htm


I love it, the perfect answer at the click of a button!

Open Wire fed lengths
 

Dave wrote:

...
http://www.qsl.net/w4sat/howlong.htm


I love it, the perfect answer at the click of a button!



Well, I will never accuse you two of not having a highly developed sense
of humor! (and, that calculator is actually HIGHLY ACCURATE--error
factor less than .000000000000000000000000001% grin)

At first I missed the importance of that URL, thanks for posting a reply
to the original--I got a chance to use it. LOL!

Warm regards,
JS

Open Wire fed lengths
 

"John Smith" wrote in message
...
Dave wrote:

...
http://www.qsl.net/w4sat/howlong.htm


I love it, the perfect answer at the click of a button!

Well, I will never accuse you two of not having a highly developed sense
of humor! (and, that calculator is actually HIGHLY ACCURATE--error factor
less than .000000000000000000000000001% grin)

At first I missed the importance of that URL, thanks for posting a reply
to the original--I got a chance to use it. LOL!

Warm regards,
JS

my formula is a little more complicated than the web site though. i take
the distance from the shack to the antenna, add in the width of the shack
(assuming some day i'll move the radio that it connects to somewhere else),
then add a couple feet more just for good measure.

Open Wire fed lengths
 

Dave wrote:

...
my formula is a little more complicated than the web site though. i take
the distance from the shack to the antenna, add in the width of the shack
(assuming some day i'll move the radio that it connects to somewhere else),
then add a couple feet more just for good measure.



Absolutely, I had overlooked that error myself ...

If you notify "him" of that error/bug, I should imagine he would
immediately incorporate your "upgrade!" :-)

Regards,
JS