Get ready for Field Day Everyone!

David HK1A, and I met at the waffle house on Saturday, had to have a good breakfast before we start doing what we were trying to get done. I went over to get antennas for field day from his storage unit taking a trailer but of course the trailer had a flat tire. A new tire and new permanent tags on the trailer were placed and finally got on the road again. After good breakfast we went digging through his storage unit finding all kinds of antennas that he had built. some very interesting designs, after all he is an RF engineer.

We went back to the farm and unloaded a lot of aluminum tubing as well as towers. We decided to make a 20 m half wave length J pole out of a 40 foot fiberglass mast. We taped a green wire to the side of the mast and erected it. We then took 450 ohm ladder line and connected one end to the green wire. We strung it out to the side for about 16 feet. We then stripped some of the insulation away and connected alligator clips to the antenna analyzer. That’s when the fun started. We had a few problems and demons kept coming up, the first one being David’s antenna analyzer decided not to work after many tries. That’s all right we have one on the computer in the ham shack and hook that up. We started getting somewhere but every now and again we get really squirrelly kind of results. Taking another look at the connection to the antenna, the N connector was faulty and the wire was breaking inside the coax cable giving spurious results. I made a pigtail to eliminate the connection demons and David took apart antenna analyser and discovered a broken wire. After managing to chase those demons out of our findings, we found close to residence and worked on tuning it until we got a beautiful graph of how it was resonating right in the band with a high “Q” value. 

It was time to put the antenna on the air and low and behold a spontaneous signal report from Spain was “10 over 9” when we didn’t even ask for a signal report. The gentleman in Spain was really impressed with our antenna of performance, especially as it was only 100 watts. We had other reports from Slovenia and other countries on 20 m that were telling us we were over 9 S units on their radios. Maybe the band conditions were good but we were breaking pile ups with this antenna. We even put a second vertical wire approximately 13 feet in front of the first one as a Director. The reports got better. 

During a dinner break, we talked about the physics, the math and of course Smith charts of J poles. I got about 50% of what he was saying but realized that my electrical learning that I had done all my life was all in DC circuit. This is really was hampering my understanding of AC circuits because shorting out both ends with a small loop of wire doesn’t seem like you would get any current at all if you think in DC terms. In the AC circuit world, This works great because happy electrons going back-and-forth million times a second and going really nowhere is exactly what AC circuits are. Those happy electrons push RF energy off the wire into space. So electrons going nowhere but radio frequency energy going out into space is of course the mission. 

Electrical Musings at Dinner with Smith charts and math!

I am totally amazed that a 40 foot fiberglass pole with a little green wire and ladder line can generate such a good signal reports part way around the world. With or without the Director this antenna works really well and does not need radials as it is a full wave length J pole on 20 m. 

Sunday we are going to get back to getting into the trailer and figuring out what the rest of the antennas are going to be for field day. I don’t know about the rest of you but I’m having a blast testing developing and designing new antennas. I don’t really care if we get a good score or bad score at field day, I’m going to have a blast, I hope you all join in and have fun. Let’s make this field day a field day to remember especially after last year’s lock down!
73 Peter N4PVH. 


David’s Comments:

The J-pole antenna is a half wavelength vertical antenna fed at the end. Radiation-wise is like a vertical dipole but not fed at the center where the feed point has a low impedance of 50-70 ohms. At the end of the wire the impedance is high, around 2000 to 3000 ohms. To match that to 50 ohm a transforming matching quarter wavelength stub is used. The matching stub was made of 450 ohm open wire line, commonly known as ladder line. The matching stub can be bent or curved as long as it doesn’t get close to another section of itself or near metal or laying on the ground.



Published by DrPVH

Concerned citizen with a multitude of interests...

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