Michael,
I am glad you enjoyed the cursory look at antennas at BVARC. Not enough time to cover it all really. One of these days we’ll have to do another antenna seminar on a Saturday. That was a lot of fun about 10 years ago. OK….L3 antenna currents on the feedline – the feedpoint split is not the only way it can happen and it does not happen at the feed point all of the time. For example, if the outside shield presents a high Z at the feedpoint location then that will hinder current flow. This is caused by the shield being designed to be ¼ or ½ wl , shorted or open respectively. Antenna currents can also happen due to the radiation itself coupling back in to the shield. That is why it is recommended that you run the feedline perpendicular from the feed point to the ground….This way prevents minimal coupling, although depending on the power level, it can still happen. Even if you do everything to prevent it, it is best to choke any feedline for common mode currents where ever there is a transition, like from in the air to ground or at the entry to your house. Choking eliminates RF hot spots in the shack AND more importantly receive common mode noise that the feedline picks up from just being in the city , etc. Be sure though that the choke you use has enough resistance to stop the current flow at the frequency of concern. Google ‘Yankee Clipper Contest Club’ and search their technical articles about chokes, etc. It is very enlightening. If you can’t find it let me know and I will look it up for you. GL and 73 and thanks for the note…..I want to see your antenna presentation some day too! Rick – W5RH From: BVARC [mailto:[email protected]] On Behalf Of Michael Rapp via BVARC Sent: Friday, September 11, 2015 8:35 PM To: BRAZOS VALLEY AMATEUR RADIO CLUB <[email protected]> Cc: Michael Rapp <[email protected]> Subject: Re: [BVARC] Presentation Reference Sheet -- Antennas: Tools for Design, Building and Troubleshooting Rick, I really enjoyed your presentation last night. The traffic with the rain was so bad coming from UH that I almost turned around, but I am so glad I didn't. I've only been licensed for three and a half years so there are still many simple things I don't understand well and one of those things is common-mode current on coax feedlines. I mean, I know what it is....and I know that a current/choke balun mitigates it, but I've never understood why it is there in the first place. Your explanation of the skin effect and the splitting of the signal on the shield finally clicked with me. I get it now! Woohoo! I was so thrilled to understand something that's baffled me for three years that I went home and immediately explained it to my wife....although she didn't seem as excited about RF signals as I am. :D Thanks again for a great presentation. 73, -- /*/-=[Michael / KT5MR]-=/*/
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