Antenna Building 101

Well, what a weekend this was. Beautiful weather, temperature in the 70-80’s and a light breeze. I just did not put enough sun blocker on so I look like a lobster in the pot! David K1KK called and wanted to try out his new 60′ tall Spiderbeam pole and turn it into an 80 meter vertical antenna. The tapered fiberglass pole comes in 12 sections of 5′ in one tube. Once you get it then the real fun starts.You have to figure out how to cut the green 14 ga wire into the proper lengths to make up a vertical element (1/4 wavelength) that was taped to the full length of the pole and two horizontal ground wires. The ground wires were best if elevated about 1-3 feet above the ground. A 1:1 balun was used to couple the coax cable to the antenna. Initial test showed an excellent standing wave ratio was achieved by lowering the ground wires to within a foot of the ground. Theoretically I am told that the ground wires should be about 2.5% longer than the driven element at resonant frequency. (I probably got this wrong so stay tuned for corrections…) We had to wait until nightfall as there was no one on the 80M band. After dinner (home made pizza) we went out and tested to see how well we were getting out “barefoot” (100 watts, no power amplifier). Impressive results were found by the signal reports from other stations up and down the coast of USA and Europe. There was a contest going on and in 30 minutes David had 25 contacts on 80m!

While waiting for nightfall, we worked on the 40m vertical antenna that I had failed to get to resonance last fall. It consisted of a tapered 35′ fiberglass pole, a baby brother to David’s “BIG POLE!”, made by MFJ. Same construction but with the hitch that I had already cut up all of my 14 ga green wire so we patched together many pieces to get the lengths. Ultimately we had 6 ground wires. After some trimming, 1:1 SWR was achieved. It is amazing how much the height of the ground wires away from the ground makes a difference in the ratios. Laying the ground wires on the ground really messes up the SWR unless you have many wires (>10). As these antennas are designed to be portable, it cannot have a complex ground system like you can create for a fixed installation. The other interesting observation was that David would measure lengths of wire by using his arm lengths as the “tape measure” and come out about right most of the time. I would break out the tape measure and worry about the 1/4 inch variation. In my defense, I have to work with light and light has a very precise measurement where fractions of a millimeter makes a difference of “in or out” of focus. Radio waves are more forgiving. Another observation had to do with the use of the vector network analyzer hooked up the the computer. The software was running and showing the relationship of changing the lengths of the elements, their proximity to the ground and the resulting SWR while it swept over the bands. As you changes something, you could see the effect quickly and not guess what happened.

Once we were happy with both antennas, David used my IC7300 on 80m and made many contacts, I set up another station on my work bench consisting of my Elecraft K2. I immediately made a contact on 40m with Joel KE4YZ (retired lawyer in Miami) and had a long rag chew. “59’s” were passed around with sincerity. Then a breaker came in from Prince Edward’s Island in Canada asking for a signal report as Jim VY2JC had just finished his home-brew Yagi on 40m and needed a report. The three of us went on for close to an hour before we were tired and called it quits.

We got together by 11am on Sunday and calibrated the vector analyser and took another look at the 40m antenna with an eye towards adding another wire for 20m and 10m. A trip to Lowes supplied us with small PVC pipe and Tee’s. We cut the Tee’s in half then glued the small tube into the tees. We taped the tees to the pole at strategic positions to hole the 20m wire 6″ away from the 40m wire (taped onto the pole). First you trim the 40m wire and ground wires. Once that is good then trim the 20m wires and ground wires. Finally you try the 10m wire and OOPS, ran out of green wire!!! Ahhh, a project for another day.

By Sunday afternoon, we were tired and took it all down with hatching a plan to get together in 2 weeks to set up a “Vertical 80m YAGI” for the contest coming up on May 29th. David is going to get a second 60′ pole and more green wire! The more we do this, the better and quicker we can set it up.

Now that is a big pole!

73, David and Peter (contest maniacs?)


Published by DrPVH

Concerned citizen with a multitude of interests...

One thought on “Antenna Building 101

  1. Perhaps the 2.5% longer radials recommendation found in the old antenna books is due to the use of a vertical radiator made of tubing which obviously has a greater diameter than the radials made of wires. The thin wire needs to be longer to achieve the same electrical length of the thicker conductor.

    The mast has a “transportation length” of 5′-7″ (1.70 m)

    http://www.spiderbeam.us/product_info.php?info=p232_Spiderbeam%2018m%20fiberglass%20pole.html

    Peter, what a blast during the weekend! Thank you very much, OM.

    73

    David
    HK1A
    K1KK
    EC5KXA

    Like

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