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THE AM BULLETIN BOARD => Technical Forum => Topic started by: K8DI on August 07, 2021, 10:09:13 PM

Title: 20m antenna thoughts?
Post by: K8DI on August 07, 2021, 10:09:13 PM
I want to put something up for 20. Right now for HF I have a dual dipole, fan-like setup. The top is 80m, the next is 40m. I read an article that suggested that if the centers were stacked 5-6Ē apart and the lower dipole were droopy so the wires were a meter apart at the end of the shorter segment, their tunings would be relatively independent compared to a standard fan with spacers. This was supported by some research from Stanford U and, well, it seems to be so. I can pull this down and add a 20m dipole, but itís a hassle to do so. Important detail: the center is 24 feet up, thatís it.
ORÖ. I could put up a 20m 1/4 wave vertical. Itíd be 16í of copper pipe, on a pvc spacer in a rooftop tripod mount, and four radials on the sloping roof. The feed point would be about 21 feet above ground. This will be more money but not so much that it is a real factor. It will be simpler, I can put it up by myself. The wire dipole takes two due to how itís hung.

Pros and cons???
What do you all think??


Title: Re: 20m antenna thoughts?
Post by: k7mdo on August 08, 2021, 12:23:17 AM
I'm sure my simple half wave dipole up only 15 feet could be a good example of a minimum solution.  I get good reports from Japan to eastern US.  The dipole is strung north to south. 100 watts.

Use this as a sample of "minimum" results!  I think your vertical idea sounds like a good one to start and would like to hear about your results.

73, Tom

Title: Re: 20m antenna thoughts?
Post by: w9jsw on August 08, 2021, 07:43:35 AM
2 things -

My current fan diople has 80, 40 and 20 sections with the center about 30ft high. It also resonates (sort of) on 15M. I built it in a X configuration. From a common center point, the 80M wire goes off in a N-S direction. The 40/20 part goes off approximately 20 degrees off of N-S. The 40/20 is comprised of the 40 wires, with the 20M wires suspended about 1 ft away on some black plastic irrigation piping. Works well. I ran it with the Nano-VNA and the plots look fine. Was easy to tune. Fed with RG-213. I have run the 80/40 on my 813 rig FB. I ran 20M SSB at around 80W and that worked well. I plan to get the 813 rig to run 20M once I have time in the fall to mess with some issues with neutralization. Right now it is a very large oscillator. Overall, very pleased with the antenna.

My second antenna is a planned vertical doublet. Approx 20ft of wire on each side, fed with some 300 ohm ladder wire. Only restriction is that the feed line has to come away from the antenna perpendicular. Will run this with a tuner to get 20M and 17M. Maybe even 10M if it ever opens up enough on my operations schedule. Every time I have listened, it is dead.

Title: Re: 20m antenna thoughts?
Post by: WA4WAX on August 10, 2021, 06:58:03 PM
8JK 25 feet up is not bad on 20 meters!

Title: Re: 20m antenna thoughts?
Post by: WU2D on August 11, 2021, 07:41:57 AM
Its just wire! How about a vertical square loop for 20M? That is what I have up now. I did the 75 Ohm coax matching section off the bottom to 50 Ohm Coax back across to the shack.
I had two trees spaced far enough apart. It looks like two dipoles in phase broadside.
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