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.
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
> 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.
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?
> 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.
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.
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.
> "Ron Walters" <rwalt...@numail.org> wrote in message > news:486b84d6$0$80799$4c5efa9e@news.corp.moreusenet.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.
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
> 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?
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 ...
