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Flip Antenna Phase?

Feb 14th, 23:55

K0WUQ

Joined: Dec 3rd 2012, 11:13
Total Topics: 0
Total Posts: 0
All right, I have a weird situation. My 40M antenna works, but it doesn't do me much good. This is a 'grounded antenna' so it's 150 ohms, with a matching transformer (14:8 turns) at the base. That works fine, my homebrew antenna tuner brings it to a sharp RF power peak easily. This runs off my homebrew 30 watt amplifier.

For various reasons, the antenna cannot be made truly vertical. It leans toward the north, so radiates mostly east and west. When the ground thaws in a few weeks, I'll be able to elevate it so the lowest section will be 75-80 degrees above ground. I cannot afford to raise it onto a tower. The grounded element is 30 ft of 3/4-inch rigid copper tubing. The driven element is 33 ft of no. 12 insulated solid copper wire (house wiring). These come together with a clamp about 6 inches from the top end of the tubing, which is sealed with a sweat-soldered cap. Besides a heavy PVC pipe insulator at the base, the tube is supported 10 ft above the base with a heavy PVC insulator mounted on a steel truss support 10 ft in height. The no. 12 driven element is spaced from the tube by plastic insulators at 5 ft intervals.

Schematically, all you have to visualize is a 30ft vertical tube with a 2ft pigtail to ground at the bottom, then starting at the top a 'parallel' wire running all the way down to the secondary of the transformer. The bottom of this secondary is also grounded at the same ground rod, and the primary winding has the coax connector to the 50 ohm feed line.

My main problem is that even though we are on relatively high ground, everything around us is higher if you just go out far enough. Only to the west can I imagine seeing any horizon. So, I'm thinking it would be beneficial to 'invert' the antenna so the high current (i.e. maximum radiation) section would be at the top.

What I'd have to do is run 30 ft of new coax up to near the tip, put the matching transformer there, grounding the tube and the secondary to the braid of the coax. Then, tie the two elements together at the base insulator. To preserve the 'lightning rod' advantage of the grounded antenna type, I would rig a 'lightning gap' arrestor at the base of the tube, where the tube itself has to be insulated from ground.

Does anyone see anything wrong with this scheme? Have you ever seen an 'inverted' vertical attempted before? All advice / caution / criticism welcome!

Larry K0WUQ
Feb 15th, 08:26

W1VT

Super Moderator

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
I use a Half Square, A simple vertically polarized antenna that has a high current feed high in the air.

https://www.antennasbyn6lf.com/2005/04/halfsquare_ante.html

Zak W1VT
ARRL Senior Lab Engineer
Feb 15th, 10:58

K0WUQ

Joined: Dec 3rd 2012, 11:13
Total Topics: 0
Total Posts: 0
Zak,

Very nice. Thanks again for your help; antenna design is an area where I am woefully ignorant, as there have obviously been many innovations during the last 50 years that I haven't bothered to keep track of.

My antenna as built resembles a downward curving fishing rod leaning out over the gunwale of a boat, with the metal rings (I don't know what to call them) represented by my yellow plastic insulators spaced along the bottom side, and the slack fishing line represented by my insulated wire draped from insulator to insulator. I call it my "fly rod antenna". But as I said, it's electrically a 'grounded vertical' with two 'parallel' elements spaced a few inches apart, just re-oriented and bowed down by the weight of the copper tubing.

Larry K0WUQ
Feb 15th, 12:52

W1VT

Super Moderator

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
http://on5au.be/content/a10/wire/fomo.html
It is likely that your antenna is difficult to analyze accurately with EZNEC 7. L.B. Cebik W4RNL was quite helpful in helping hams with their antenna ideas.

Steve K6OIK has written an article on more advanced software
https://www.fars.k6ya.org/docs/k6oik
HOBBIES Software for Computational Electromagnetics [5.6 MB]
QEX Magazine
Nov/Dec 2020
Steve Stearns, K6OIK
Approximate cost of the book with software license is $200.

Another approach is to use online Internet services to measure the antenna. Reverse Beacon Network, Hamspots.net, PSK reporter and WSPR may be used.
Feb 15th, 14:41

K0WUQ

Joined: Dec 3rd 2012, 11:13
Total Topics: 0
Total Posts: 0
Zak -

You really strive to make this a helpful place. Thanks much.

Yes, what I have is more like a small piece of a single turn of a very large coil. All I can really tell about it is that it tunes to a definite, fairly sharp peak on my RF power meter. So, the INTERNAL behavior of the antenna and matching transformer seems good. I have never built an SWR meter, but that would be a good project someday soon. I still have a couple of nice large-face meters lying around.

Larry K0WUQ
Feb 16th, 07:36

W1VT

Super Moderator

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
If you want to learn about RF and have plenty of time a better approach may be to buy a NanoVNA to measure your antenna. As a 2 port device it may be possible to measure antenna gain inexpensively.
https://groups.io/g/nanovna-users/topic/negative_antenna_gain/72763916?p=

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