Bitcoin donations to: 1CE9UfWJcHBYkWPns7iqBqZgKhd5xfqEaM thanks!
Buy Bitcoins easily by clicking HERE and get $10 in free bitcoins!

Author Topic: Super Gain 9db 40m NVIS Antenna.  (Read 2550 times)

gil

  • Administrator
  • Hero Member
  • *****
  • Posts: 2637
  • SMeter: +70/-3
    • View Profile
    • Radio Preppers
Super Gain 9db 40m NVIS Antenna.
« on: March 22, 2017, 03:07:58 PM »
I found this gem in a 1969 publication. It takes a lot of wire but worth it in light of the gain. it is also very easy and quick to set up, being hung only 7ft off the ground.









Gil.

Lamewolf

  • Sr. Member
  • ****
  • Posts: 269
  • SMeter: +5/-0
    • View Profile
Re: Super Gain 9db 40m NVIS Antenna.
« Reply #1 on: March 23, 2017, 08:31:45 AM »
I just don't see how they can claim that much gain when the antenna is basically nothing more than a half wave folded dipole.  The idea behind a folded dipole is increased bandwidth, not more gain ?

DJ6KR

  • Newbie
  • *
  • Posts: 6
  • SMeter: +0/-0
    • View Profile
Re: Super Gain 9db 40m NVIS Antenna.
« Reply #2 on: March 23, 2017, 10:00:54 AM »
I just don't see how they can claim that much gain when the antenna is basically nothing more than a half wave folded dipole.  The idea behind a folded dipole is increased bandwidth, not more gain ?

It is all about the reflector on the ground under the Antenna, I think.


Gesendet von iPhone mit Tapatalk

Quietguy

  • Full Member
  • ***
  • Posts: 164
  • SMeter: +7/-1
    • View Profile
Re: Super Gain 9db 40m NVIS Antenna.
« Reply #3 on: March 23, 2017, 09:11:28 PM »
It is all about the reflector on the ground under the Antenna, I think.

Yes, I have a copy of Terman's book that is referenced in the article.  Terman discusses a "Half-wave Antenna with Reflector" starting on page 903 and derives the gain.  He shows plots of gain compared to radiation resistance as a function of antenna to reflector spacing.  Radiation resistance (and efficiency) goes way down with small spacing, while gain stays around 8 up to a spacing of about 0.1 wavelength (about 13 feet on 40 m) before falling off with increased spacing.  He says:

"In order to prevent incidental loss resistances from making the antenna efficiency very low, the spacing S should accordingly be at least 0.05 wavelength, and preferably 0.1 wavelength."

The article uses 7 feet, which is about 0.05 wavelength at 40 meters, but Terman's chart shows you would have better efficiency without losing gain if you made the antenna 14 feet high.

Wally

gil

  • Administrator
  • Hero Member
  • *****
  • Posts: 2637
  • SMeter: +70/-3
    • View Profile
    • Radio Preppers
Re: Super Gain 9db 40m NVIS Antenna.
« Reply #4 on: March 24, 2017, 06:03:25 AM »
Thanks Wally, very informative! What's the book? I am reading "HF Antennas For All Locations" by Les Moxon right now, great book.

Gil

Lamewolf

  • Sr. Member
  • ****
  • Posts: 269
  • SMeter: +5/-0
    • View Profile
Re: Super Gain 9db 40m NVIS Antenna.
« Reply #5 on: March 24, 2017, 08:46:44 AM »
It is all about the reflector on the ground under the Antenna, I think.

Yes, I have a copy of Terman's book that is referenced in the article.  Terman discusses a "Half-wave Antenna with Reflector" starting on page 903 and derives the gain.  He shows plots of gain compared to radiation resistance as a function of antenna to reflector spacing.  Radiation resistance (and efficiency) goes way down with small spacing, while gain stays around 8 up to a spacing of about 0.1 wavelength (about 13 feet on 40 m) before falling off with increased spacing.  He says:

"In order to prevent incidental loss resistances from making the antenna efficiency very low, the spacing S should accordingly be at least 0.05 wavelength, and preferably 0.1 wavelength."

The article uses 7 feet, which is about 0.05 wavelength at 40 meters, but Terman's chart shows you would have better efficiency without losing gain if you made the antenna 14 feet high.

Wally

I can see that in my minds eye, but the gain they are speaking of in this article is going straight up !  Good for NVIS, bad for DX !

Quietguy

  • Full Member
  • ***
  • Posts: 164
  • SMeter: +7/-1
    • View Profile
Re: Super Gain 9db 40m NVIS Antenna.
« Reply #6 on: March 24, 2017, 06:33:40 PM »
Thanks Wally, very informative! What's the book?

It's "Electronic and Radio Engineering", 4th ed, by Frederick E. Terman from 1955.  It was a college textbook still in use back in the mid-1960s.  It was used in some of my courses, which is why I happen to have a copy.  Sigh, yes, I'm that old and obsolete...

I can see that in my minds eye, but the gain they are speaking of in this article is going straight up !  Good for NVIS, bad for DX !

That was pretty much the author's point - reduce interference arriving at a low angle from foreign shortwave broadcasters on 40m and favor high-angle NVIS regional contacts.  I don't believe the term NVIS was used back in the '60s when this article was written, but they certainly were aware of the effect.  As DJ6KR mentioned in his post, the significance of this article is the reflector laying on the ground underneath the antenna wire directing more of the energy upward.  Terman's book (Figure 23-36) shows "Directive Gain" of about 8 at  a spacing of 0+ to about 0.1 wavelength and then steadily dropping to about 4 at 0.3 wavelength, where the plot ends.

The same figure has "Radiation Resistance" plotted at the same spacing scale, and it gets better as spacing goes up.  The downward sloping Directive Gain plot crosses the upward sloping Radiation Resistance line at a spacing of about 0.17 wavelength.  This yields a Directive Gain of about 7 and Radiation Resistance about 50 ohms.  That is probably the sweet spot for overall efficiency, but it puts the antenna up about 22 feet, which is still NVIS territory, but is much less convenient than 7 feet.  Additionally, Terman's figure is based on a reflecting screen beneath the antenna; 3 wires 6 feet on center may be a good approximation of a (40m) screen at 7 feet height but I wonder if additional reflecting wires would be necessary with higher antenna elevations, just because of the geometry.  At 7 feet the "screen" is wider than the antenna height, so would a 22 foot high antenna require a much wider "screen"?  I suspect it would to get the maximum effect.

Wally

gil

  • Administrator
  • Hero Member
  • *****
  • Posts: 2637
  • SMeter: +70/-3
    • View Profile
    • Radio Preppers
Re: Super Gain 9db 40m NVIS Antenna.
« Reply #7 on: March 25, 2017, 04:11:50 AM »
What's great about this antenna is its simplicity. It is easy to set up and can pass for something else. Hang some underwear on the wire and you're all set ;D The poles being 7ft tall are portable. I bet it could be erected in under five minutes.

Gil.