To: |
steve@karinya.net |
---|---|

Subject: |
Re: [TowerTalk] Balun Recommendation |

From: |
TexasRF@aol.com |

Date: |
Thu, 19 Apr 2012 07:42:23 -0400 (EDT) |

List-post: |
<towertalk@contesting.com">mailto:towertalk@contesting.com> |

Hi All, this is an observation from an innocent bystander: With constant power level, if the antenna Z increases, The voltage increases as well, right? Conservation of energy then requires the current to decrease, right? The rf voltage on the coax shield where it connects to the antenna would be the voltage referenced to ground at that half of the antenna. The choke current would be that voltage divided by the choke impedance (also referenced to ground), Is that right? So, one might think that a higher Z antenna = higher E = higher choke current? If so, power loss is then I^2 X whatever resistive loss is present in the choke. 2X Z = 1.414 X voltage = 1.414 X choke current = 2 X loss. if the above is true? 73, Gerald K5GW In a message dated 4/19/2012 3:08:04 A.M. Central Daylight Time, steve@karinya.net writes: Jim, Let me try to explain it more simply. Picture Roy Lewallen's "classic" diagram showing how the differential-mode current flowing on the inside of the coax braid splits two ways at the feedpoint - some flowing into the dipole leg and some flowing the CM path back along the outside of the coax braid. Suppose: Iant is the current flowing into the dipole leg Icm is the current flowing along the CM path Zant is the impedance looking into the dipole leg Zcm is the impedance looking into the CM path. We know that the current will split in inverse proportion to the two impedances, so: Icm / Iant = Zant / Zcm or, rearranging: Icm = Iant * Zant / Zcm If Zant changes, thereby changing the SWR, then Icm will also change and so will the choke dissipation. If the SWR is caused by a drop in Zant, the choke dissipation will decrease; if it's caused by an increase in Zant, the choke dissipation will increase. Either way it's quite wrong to say: "SWR has NOTHING to do with dissipation in a common mode choke." In a practical example like an HF wire dipole, you'll see that the increasing SWR as you move away from the resonant frequency results from an _increase_ of antenna impedance because the reactance changes at a faster rate than the resistance; a choke designer would be wise to allow for the extra dissipation that causes. Finally, if you don't believe my analysis, just try modelling the situation in EZNEC. I took an 80m dipole (#14 wire at 35ft) and added a 4k resistive choke to the CM path at the feedpoint. For 1000W applied power these were the choke dissipations at various frequencies: SWR=1.4:1 at resonance: 2.2W SWR=2:1 below resonance: 3.3W SWR=2:1 above resonance: 3.9W SWR=3:1 below resonance: 5.2W SWR=3:1 above resonance: 6.4W As you can see, the SWR directly affects the choke dissipation! 73, Steve G3TXQ On 19/04/2012 05:47, Jim Brown wrote: > |

Previous by Date: | Re: [TowerTalk] Balun Recommendation, Steve Hunt |
---|---|

Next by Date: | Re: [TowerTalk] Low Pass Filters: Are They Of Any Use Today ?, Bill Winkis |

Previous by Thread: | Re: [TowerTalk] Balun Recommendation, K8RI |

Next by Thread: | Re: [TowerTalk] Balun Recommendation, Steve Hunt |

Indexes: | [Date]
[Thread]
[Top]
[All Lists] |