1. posterid:244139
  2. looking for an antenna/antenna simulation elmer

Re: looking for an antenna/antenna simulation elmer

Paul Butzi (W7PFB)
 

I’ve got a model for an 80m cage dipole that at 15m AGL gives a SWR of 1.01 at the sweet spot and is under 3:1 SWR from 3.5 MHz up to 4 MHz. (it’s actually just under 3:1 at both 3.5 MHz and 4.0 MHz). That’s useably close to covering the entire band. That’s for a 4 wire cage with a diameter of about half a meter. More wires or greater diameter will make it more broadband but there’s a balancing act with NVIS gain.

If you’re looking for an 80m antenna to put up at .5 wavelength I suspect you could do better still. W1AW apparently have a cage dipole for 80m, there’s a nice writeup on it in the ARRL antenna book.

A technique I just now read about to broadband a half wave dipole is to attach a shorted quarter wave stub at the feedpoint. The idea is that the reactance of the shorted stub moves almost exactly opposite the reactance of the dipole as frequency changes. I have not modeled this and am having a little trouble visualizing how it would work.

Cage dipoles are interesting for NVIS use for several reasons: they are broadband, and you can use the length of the inner taper from the cage diameter to the feedpoint to adjust the feedpoint impedance. It turns out this is more useful than you would expect because otherwise it’s quite hard to get a 50 ohm feedpoint impedance when the antenna is only .15-.2 wavelengths above the ground.


-p W7PFB
73, Don’t forget to smile and have fun!

On Feb 29, 2020, at 5:17 PM, Dan Pflugrath <dpflugrath@juno.com> wrote:

Paul,
I have been patiently waiting (decades) for a solution for an optimized (wide bandwidth) 80 meter antenna. I hope there is an Elmer out there than can help us.

Historically what hams have used for this problem is a dipole cut for 3.5 MHz fed with ladder line rather than coax using a balanced tuner like the ancient Johnson Matchbox (newer versions are now available). Even with very high Standing Wave Ratios the ladder line has almost zero loss and the match box can provide a perfect match to our transceivers. This is a very efficient antenna system for 80 through 10 meters.

Good Luck. I am curious to see how close we can come to an optimized solution.

73,
Dan KA7GPP



-----Original Message-----
From: snovarc@snovarc.groups.io [mailto:snovarc@snovarc.groups.io] On Behalf Of Paul Butzi (W7PFB)
Sent: Saturday, February 29, 2020 4:21 PM
To: snovarc@snovarc.groups.io
Subject: [SnoVARC] looking for an antenna/antenna simulation elmer

I am trying to come up with an optimized design for an 80m NVIS antenna. So far this has boiled down to hatching ideas, building an NEC model using 4nec2, and then optimizing and evaluating the design.

(side note: the entire issue of building an antenna that has enough bandwidth to cover the entire 80m band is a rathole I did not expect to fall into. Fan dipoles. Butterfly dipoles. Cage dipoles. Holy smoke, who knew there were all these dipole variants?)

I’m bumping up against the limits understanding of antenna theory, and bumping into my very limited understanding of NEC modeling really hard.

So I’m looking for an elmer to help me out. If you can help, or you can help me connect with someone who can help, I’d sure appreciate it.


-p W7PFB
73, Don’t forget to smile and have fun!





____________________________________________________________
She's 46, But Looks 25. This Is What She Does Every Day
removethewrinkles.com
http://thirdpartyoffers.juno.com/TGL3131/5e5b0d35950abd340381st02vuc
Join snovarc@snovarc.groups.io to automatically receive all group messages.