Towertalk
[Top] [All Lists]

## Re: [TowerTalk] Coax, conduit and toroids

 Subject: Re: [TowerTalk] Coax, conduit and toroids Steve Hunt Wed, 27 Jun 2012 21:34:05 +0100 mailto:towertalk@contesting.com>
 ```For anyone without access to a modelling tool, let me try to give you a simple qualitative analysis. Say we have a perfectly symmetrical dipole fed with coax that drops away exactly at right angles from the dipole. An extremely high-impedance common-mode choke has been fitted to the coax at the feedpoint, which reduces the CM current to a negligible amount. Let's say the differential-mode voltage across the dipole feedpoint is 100v. The presence of the choke ensures that the balance of the system is preserved; so, measured with respect to ground, one side of the feedpoint will be at 50v and the other side will also be at 50v but opposite phase. So we know that there is a voltage of 50v wrt ground trying to "drive" CM current through the choke and back along the coax braid. Now, for Jim's claim to be true that there is zero CM voltage across the choke, the CM voltage at the radio side of the choke must also be at 50v wrt ground. Ask yourself how that can be, when there is negligible current flowing along the braid; where does that 50v "come from"? By the way, this wouldn't be the first time that someone tried to discredit NEC because it produced answers that disproved their claim! Steve G3TXQ On 27/06/2012 21:05, Steve Hunt wrote: > Jim, > > I'm not going to waste my time in long debate with you again! > > I'll just leave other readers to check what I have said using NEC. You > might believe that NEC can't model the common-mode currents, but that's > the first time I've heard that claimed, and there are plenty of other > well-respected folk - Roy Lewallen, VK1OD and W8JI for example - who I > expect would disagree with you. Look at Tom's website and you'll see how > he uses EZNEC to model common-mode situations. > > I encourage other readers not to believe me or Jim but to check it for > themselves; model a simple coax-fed symmetrical dipole and see if Jims > claim that "a perfectly balanced antenna system must have zero voltage > across the choke and zero current through it" holds up. > > And remember, if the choke has zero voltage across it and zero current > through it, you can safely remove it and nothing will change :) > > Steve G3TXQ > > _______________________________________________ > > > > _______________________________________________ > TowerTalk mailing list > TowerTalk@contesting.com > http://lists.contesting.com/mailman/listinfo/towertalk > > _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
 Current Thread Re: [TowerTalk] Coax, conduit and toroids, (continued) Re: [TowerTalk] Coax, conduit and toroids, K8RI Re: [TowerTalk] Coax, conduit and toroids, Ian White GM3SEK Re: [TowerTalk] Coax, conduit and toroids, Jim Lux Re: [TowerTalk] Coax, conduit and toroids, Ian White GM3SEK Re: [TowerTalk] Coax, conduit and toroids, Jim Brown Re: [TowerTalk] Coax, conduit and toroids, Steve Hunt Re: [TowerTalk] Coax, conduit and toroids, Jim Brown Re: [TowerTalk] Coax, conduit and toroids, Steve Hunt Re: [TowerTalk] Coax, conduit and toroids, Jim Brown Re: [TowerTalk] Coax, conduit and toroids, Steve Hunt Re: [TowerTalk] Coax, conduit and toroids, Steve Hunt <= Re: [TowerTalk] Coax, conduit and toroids, Jim Brown Re: [TowerTalk] Coax, conduit and toroids, Steve Hunt Re: [TowerTalk] Coax, conduit and toroids, Jim Brown Re: [TowerTalk] Coax, conduit and toroids, K8RI Re: [TowerTalk] Coax, conduit and toroids, Steve Hunt Re: [TowerTalk] Coax, conduit and toroids, K8RI Re: [TowerTalk] Coax, conduit and toroids, Steve Hunt Re: [TowerTalk] Coax, conduit and toroids, Steve Hunt Re: [TowerTalk] Coax, conduit and toroids, Rick Kiessig Re: [TowerTalk] Coax, conduit and toroids, Rick Kiessig