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#11
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Does a HF vertical antenna need a balun?
Nick wrote:
Thanks for all the great responses guys! That is exactly what I had thought, that since the outer coax braid is grounded at the antenna, and at the shack, and (in my case) at the center with a lightening protector, then I just couldn't figure out why some antenna companies, such as reliable DX engineering with their DXE-VFCC H05-A Vertical Feedline Current Choke, strongly recommend such a line isolator for verticals. Nick, think about it. If you ground the feedline at exactly a common-mode standing wave voltage node, the ground accomplishes absolutely nothing. You certainly don't want to use "plumber's delight" techniques in your ground system. -- 73, Cecil http://www.w5dxp.com |
#12
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Does a HF vertical antenna need a balun?
On Jun 24, 3:30*pm, Cecil Moore wrote:
Nick wrote: * *Thanks for all the great responses guys! *That is exactly what I had thought, that since the outer coax braid is grounded at the antenna, and at the shack, and (in my case) at the center with a lightening protector, then I just couldn't figure out why some antenna companies, such as reliable DX engineering with their DXE-VFCC H05-A Vertical Feedline Current Choke, strongly recommend such a line isolator for verticals. Nick, think about it. If you ground the feedline at exactly a common-mode standing wave voltage node, the ground accomplishes absolutely nothing. You certainly don't want to use "plumber's delight" techniques in your ground system. -- 73, Cecil *http://www.w5dxp.com Thanks Richard and Cecil. Cecil, I understand what you are saying, but still why not use the outer shield of the coax as one of the radials, and simply choke off the coax just before it enters the shack? I also don't want to add what may be a useless choke at the antenna's feedpoint, since that would decrease the antenna system's efficiency due to the choke's resistive losses... Thanks! -Nick |
#13
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Does a HF vertical antenna need a balun?
On Tue, 24 Jun 2008 17:41:31 -0700 (PDT), Nick
wrote: I also don't want to add what may be a useless choke at the antenna's feedpoint, since that would decrease the antenna system's efficiency due to the choke's resistive losses... Hi Nick, A choke at the feedpoint does NOT add loss. The Choke (which may be resistive insofar as the bulk material's property) is in the Common Mode circuit. The load is in the Differential Mode circuit. You want a very, very high Z (or R, it makes no difference) in the Common Mode circuit. The choke is entirely transparent to the Differential Mode circuit. If you don't understand the difference between the two modes (and it is not a common topic outside of electronics design) then, no doubt, others will fill in (or simply go to wikipedia). 73's Richard Clark, KB7QHC |
#14
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Does a HF vertical antenna need a balun?
On Tue, 24 Jun 2008 17:41:31 -0700 (PDT), Nick
wrote: On Jun 24, 3:30Â*pm, Cecil Moore wrote: Nick wrote: Â* Â*Thanks for all the great responses guys! Â*That is exactly what I had thought, that since the outer coax braid is grounded at the antenna, and at the shack, and (in my case) at the center with a lightening protector, then I just couldn't figure out why some antenna companies, such as reliable DX engineering with their DXE-VFCC H05-A Vertical Feedline Current Choke, strongly recommend such a line isolator for verticals. Nick, think about it. If you ground the feedline at exactly a common-mode standing wave voltage node, the ground accomplishes absolutely nothing. You certainly don't want to use "plumber's delight" techniques in your ground system. -- 73, Cecil Â*http://www.w5dxp.com Thanks Richard and Cecil. Cecil, I understand what you are saying, but still why not use the outer shield of the coax as one of the radials, and simply choke off the coax just before it enters the shack? I also don't want to add what may be a useless choke at the antenna's feedpoint, since that would decrease the antenna system's efficiency due to the choke's resistive losses... Thanks! -Nick When I was into 2-meters 40+ years ago, I preferred gamma-matched verticals. ...Jim Thompson -- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | | | | America: Land of the Free, Because of the Brave | | | | Due to excessive spam, googlegroups, UAR & AIOE are blocked! | |
#15
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Does a HF vertical antenna need a balun?
On Jun 24, 6:16*pm, Richard Clark wrote:
On Tue, 24 Jun 2008 17:41:31 -0700 (PDT), Nick wrote: I also don't want to add what may be a useless choke at the antenna's feedpoint, since that would decrease the antenna system's efficiency due to the choke's resistive losses... Hi Nick, A choke at the feedpoint does NOT add loss. The Choke (which may be resistive insofar as the bulk material's property) is in the Common Mode circuit. *The load is in the Differential Mode circuit. *You want a very, very high Z (or R, it makes no difference) in the Common Mode circuit. *The choke is entirely transparent to the Differential Mode circuit. If you don't understand the difference between the two modes (and it is not a common topic outside of electronics design) then, no doubt, others will fill in (or simply go to wikipedia). 73's Richard Clark, KB7QHC Thanks for the further info Richard! I guess I don't really fully understand why the pure resistive losses in the coax choke wouldn't affect the efficiency of the monopole. If I understand you correctly (and I may not!), then even if we increase the impedance of the coax choke to insane levels, such as by making it 1000 feet long and winding it on a six inch diameter PVC form, then the huge losses normally inherent in such a long run of coax would not be seen at all by the vertical because it is wound on the PVC form? (I'm not arguing with you at all, I just really want to know). 73, -Nick |
#16
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Does a HF vertical antenna need a balun?
On Tue, 24 Jun 2008 22:12:55 -0700 (PDT), Nick
wrote: Thanks for the further info Richard! I guess I don't really fully understand why the pure resistive losses in the coax choke wouldn't affect the efficiency of the monopole. If I understand you correctly (and I may not!), then even if we increase the impedance of the coax choke to insane levels, such as by making it 1000 feet long and winding it on a six inch diameter PVC form, then the huge losses normally inherent in such a long run of coax would not be seen at all by the vertical because it is wound on the PVC form? (I'm not arguing with you at all, I just really want to know). 73, -Nick Hi Nick, Well, no argument perceived. 1000 feet would come with its own loss. This is published in dB attenuation/100 feet for almost any type and grade of coax in the market (and those that are no longer made). This is truly Differential Mode loss. Winding too much into a coil would not necessarily achieve the results expected due to self-resonance (it could act like a shorted choke, and not very useful). You can achieve astonishingly high Zs with a coiled transmission line with very little effort (roughly 8-12 turns on a liter pop bottle - empty of course - for HF). It will peak at some frequency and provide adequate isolation in nearby bands; however, the ferrite chokes can be made to cover a decade of frequency - or much more (3-30 MHz), if you care to really go whole hog (3MHz-3GHz) with 1000 Ohms of Z (principally R) without too much effort either (more cost than effort I should add). One proviso should be observed: Whatever problems find their way onto the shield of the coax (Common Mode circuit) can arrive over the air. If they do, they can induce themselves into the below the choke point as easily as at the antenna. The solution here is to repeat the choke construction 1/4 wavelength from the feedpoint (a second choke, that is). If you are choking a wide band, you might try distributing the ferrite beads over a greater length of coax than the 1 foot it would normally inhabit. I have a 20 foot ferrite choking section that I can insert, inline, into my feeds. It has one bead every 3 or 4 inches. I wouldn't advise this as replacing the feedpoint choke, but rather the second choke some short distance away. Note that I made a distinction about interference coming in "over the air." This is not the only way as most home based interference comes into the receiver by conduction - sharing a poor ground. Most folks who swear that chokes don't work (or at least theirs don't so none do), are suffering from ground loops. A choke can help, but not nearly as well as combining it with conventional grounding solutions. Poor advice abounds in this topic too. 73's Richard Clark, KB7QHC |
#17
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Does a HF vertical antenna need a balun?
I guess I don't really fully understand why the pure resistive losses in the coax choke wouldn't affect the efficiency of the monopole. A simple way to think of it is: Ferrite beads over the outer of a coax has no effect on the signals flowing 'normally' up and down 'inside' it. The beads only effect signals that are flowing on the outer (treating it like a piece of wire rather than a coax transmission line). I know that this is not quite the full story but it is a simple explanation of the situation under discussion. Jeff |
#18
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Does a HF vertical antenna need a balun?
Nick wrote:
Cecil, I understand what you are saying, but still why not use the outer shield of the coax as one of the radials, and simply choke off the coax just before it enters the shack? I also don't want to add what may be a useless choke at the antenna's feedpoint, since that would decrease the antenna system's efficiency due to the choke's resistive losses... Unless the coax is up and in the clear, common-mode power is probably wasted whether you choke or not. If your choke at the shack happens to be placed at a common- mode voltage-maximum/current-minimum, it will be ineffective. One reason for placing a choke at the antenna feedpoint is that is probably the maximum current point where the choke has a good chance of being most effective. -- 73, Cecil http://www.w5dxp.com |
#19
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Does a HF vertical antenna need a balun?
Nick wrote:
Thanks for all the great responses guys! That is exactly what I had thought, that since the outer coax braid is grounded at the antenna, and at the shack, and (in my case) at the center with a lightening protector, Wad is dis? Is that something to protect your coax from becoming too light? Jes kiddin'! then I just couldn't figure out why some antenna companies, such as reliable DX engineering with their DXE-VFCC H05-A Vertical Feedline Current Choke, strongly recommend such a line isolator for verticals. Since they make them, they're happy to sell them to you. A direct quote from DX Engineering's Website: "2/5 kW Vertical Feedline Current Choke If your antenna SWR is already low and you wish to reduce feedline radiation and improve reception, a Feedline Current Choke is recommended. I am really at a loss as to how this isolator improves reception. Adding a DX Engineering Feedline Current Choke at the base of a vertical antenna will substantially reduce unwanted feedline radiation (RFI), reducing the need for improved station grounding. Am I a hopeless noob, or are there a lot of things wrong here? Feedline radiation = RFI. and almost a lure toward poor station grounding? When quarter-wave antennas are constructed over a good radial system, they have a feedpoint impedance of about 36 ohms. When they are constructed over less than a good radial-system there is a loss introduced into the feed system that adds to the 36-ohm figure. This improves the SWR but there is a loss in the efficiency of the antenna, signals transmitted and received have a higher take-off angle and often there is current introduced on to the feedline. Are there tests showing this somewhere? Anyway, you are probably best served by placing as many radials as you can handle - but more than 4. I just went till my knees and back said "no more", then came back the next day and laid more. Have as good a grounding system as you can get, and good lightning protection. IMO, if that isolator is helping any, it is just trying to mitigate a problem that should be taken care of elsewhere. - 73 de Mike N3LI - |
#20
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Does a HF vertical antenna need a balun?
On Jun 25, 8:04*am, Michael Coslo wrote:
Nick wrote: * *Thanks for all the great responses guys! *That is exactly what I had thought, that since the outer coax braid is grounded at the antenna, and at the shack, and (in my case) at the center with a lightening protector, Wad is dis? Is that something to protect your coax from becoming too light? Jes kiddin'! * then I just couldn't figure out why some antenna companies, such as reliable DX engineering with their DXE-VFCC H05-A Vertical Feedline Current Choke, strongly recommend such a line isolator for verticals. Since they make them, they're happy to sell them to you. * *A direct quote from DX Engineering's Website: "2/5 kW Vertical Feedline Current Choke If your antenna SWR is already low and you wish to reduce feedline radiation and improve reception, a Feedline Current Choke is recommended. I am really at a loss as to how this isolator improves reception. Adding a DX Engineering Feedline Current Choke at the base of a vertical antenna will substantially reduce unwanted feedline radiation (RFI), reducing the need for improved station grounding. * Am I a hopeless noob, or are there a lot of things wrong here? Feedline radiation = RFI. and almost a lure toward poor station grounding? When quarter-wave antennas are constructed over a good radial system, they have a feedpoint impedance of about 36 ohms. When they are constructed over less than a good radial-system there is a loss introduced into the feed system that adds to the 36-ohm figure. This improves the SWR but there is a loss in the efficiency of the antenna, signals transmitted and received have a higher take-off angle and often there is current introduced on to the feedline. Are there tests showing this somewhere? Anyway, you are probably best served by placing as many radials as you can handle - but more than 4. I just went till my knees and back said "no more", then came back the next day and laid more. Have as good a grounding system as you can get, and good lightning protection. IMO, if that isolator is helping any, it is just trying to mitigate a problem that should be taken care of elsewhere. * * * * - 73 de Mike N3LI - Thanks again guys. I'll have to study common versus diff mode currents on coax cable again to gain a better understanding of the situation! And as was stated by many, a good radial field should mitigate the situation anyway. 73, -Nick |
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