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Sunday, June 3, 2018

Understanding Antenna Directivity -- Help from Canada



I am in the process of repairing my beloved 17 meter fishing-pole Moxon.  It was taken out of service by the last Nor'easter of the winter.  This repair has caused me to review the theory behind antenna directivity.  I find there is a lot of "hand waving" in the explanations of how directivity happens:  "You put a reflector element next to the antenna.  And it REFLECTS!"   You are left wondering how that reflection happens.  

The Royal Canadian Air Force made a video that does a pretty good job of explaining how the reflector reflects.  See above.  Thanks Canada!  

While we are talking about antennas,  I wanted to alert readers to a really nice antenna modeling program that is available for free.  It is called 4nec2.  You can find it here: 
http://www.qsl.net/4nec2/   There is a bit of a learning curve, and I am still climbing it, but I can see how this software would be very useful.  It has an optimization feature that runs the antenna through many versions and tells you how to optimize for F/B,  gain,  SWR, or whatever you want to prioritize.  

I have discovered that my Moxon was resonant below the 17 meter band.  In other words, the antenna elements were too big.  About 3.6% too big according to my calculations.  This may be the result of my using insulated wire for the antenna elements.  Apparently the MOXGEN software assumes the use of uninsulated wire.  I'm thinking that an easy way to deal with this would be to use the frequency 3.6% above my target frequency and then use the dimensions given my the MOXGEN program.  Any thoughts on this plan? 

What a shame that Cebik's web sites have all disappeared.      

2 comments:

  1. Thanks for that, Bill! I'd already envisaged much of that, but it was good education... Now how do Directors work, particularly multiple at VHF/UHF? SLY beams and the like :) It would also be good to envisage ground effect ... a challenge for the knowledgeable?
    73

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  2. I have the fiberglass poles, hub, and 14 guage insulated stranded house wiring to build a Moxon and have been slowed a little by the same issue you mentioned. I did find find that the 4nec2 file which defines the antenna shape can be configured to use insulated wire. I haven't modeled it yet, but the following looked useful:
    https://kk7s.net/2012/07/12/modeling-insulation-and-other-tricks-in-4nec2-4/

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