Saturday, November 16, 2019

A Better Off Center Fed Dipole?

Experimenting with OCFD models, I discovered adding 175 pF series capacitance at the feed point moved the resonance on 80, 40, 20 and 10 m into the phone (SSB/AM) segments of these bands.
  • 80 m - 3.8 MHz, SWR 1.6:1.
    The result falls out of the length and cap value that works best for 40, 20, 10 m.
  • 40 m - 7.2 MHz, SWR 1.33:1.
    Nicely centered across the phone segment.
  • 20 m - 14.275 MHz, SWR 1.3:1.
    Favors upper end where most activity is in North .
  • 10 m - 28.4 MHz, SWR 1.17:1.
    Favors the phone segment of the band where most activity is.

What else can we get out of this? 17 m is close, resonance is just below the band. 12 m looks good. 6 m resonance is just below the band again.
  • 17 m - SWR 2.5:1.
    ATU will take care of that one.
  • 12 m - SWR 1.36:1.
    Bonus!
  • 6 m - 50.0 MHz, SWR is 2.5:1.
    SWR goes up from there. Can't win em all.

Model parameters using EZNEC:
  • 41.3 m wire.
  • 33% offset.
  • 175 pF cap at balun / feed point.
  • 12 m / 40 ft height above ground.
  • 13 AWG, 0.5 mm PVC insulated wire (The Wireman 531 material).

This capacitively loaded OCFD relies on the antenna being physically too long. The capacitive loading network at the balun / feed point has the desired electrical shortening effect, which is greater lower in frequency where it's need more.

My goal was to find something that at a minimum moved 80 m resonance somewhere between 3.7 - 3.9 MHz, and favors phone the segments of 40, 20 and 10 m with low SWR. This satisfies my solid state rig with out needing an ATU, and gives the AL-80B a good match. It's an order better than a conventional OCFD where 80 and 40 m resonance are below the bands, 20 and 10 m end up high in my experience.

I credit the idea of adding capacitance to ON4AA's "CL-OCFD" design which moves the resonance on 80 m (and optionally adds 30 m too). This one locates the capacitor at the center of the antenna, which only affects 80 m and odd harmonic resonance. K5GP's "A broadband 80/160 meter dipole" is another example of using a center loading network.

After a lot of searching, I managed to turn up a couple of references where a capacitor is placed at the balun:

20 m 3D Far Field Plot:
20m 3D Far Field Plot - GAL-ANA

SWR Plots:
80 m SWR 40 m SWR 20 m SWR 17 m SWR 12 m SWR 10 m SWR


The modeling results are a surprise, it was literally a "what if.." thought one morning to place a cap at the balun to see what would happen. I got started building it last weekend, hopefully try it out soon time and weather permitting.

A copy of the MMANA-GAL model I started with, and the EZNEC model can be found at https://www.qsl.net/kl3no/Models/OCFD/

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This modeling exercise started out in MMANA-GAL which I've been using over the last year to teach my self with. I decided to cross check the results with a demo version of EZNEC.

EZNEC offers a high accuracy ground model (recommended with antennas under 0.2 wave length above ground), and can model the effect of wire insulation. With the ground conductivity configured for my location and insulation defined, I found I had to shorten the antenna model nearly 1 m / 3 ft and increase the value of the capacitor from 150 to 175 pF compared to the MMANA-GAL model.

After spending a few days with EZNEC and the well written manual I decided it was well worth purchasing.
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Update 01 Dec 2019:

I got the antenna assembled and up semi inverted V, with out the cap for now.

SWR sweep:



As expected resonance is low on 80, 40, 20, 17, 12 m. 15 m unexpectedly is under 3:1, 10 m pretty good.

Update 08 Dec 2019:

The trees between the front and back yard are too close together by about 15 feet for the 90+ ft long leg. The best I could do was run the long leg from the front yard tree to the tree in the back yard, with the balun hanging between the last 15 ft of the long leg and where the short leg is tied off, but the tension needed to get most of the sag out is more than I can supply which causes the short leg to get mixed up in the lower branches of a Maple tree.

Tried to find a solution with EZNEC using a 80 ft / 50 ft or so split, no dice with an offset much other than 33%.. Well, the positioning of my trees sure has let the air out of my tires on this one..

I may be-able to get the antenna situated nicely by bending the end of the long leg down and hanging the balun further out on a limb. But the weather is cold and raining all the time (Pacific North West winter), and with two Maple trees that snag the wire each time I let and end down to adjust things, I've had enough of being cold and rained on.. Making adjustments that would take 5 to 20 minutes with nothing in the way, takes 30 minutes to a couple of hours or more when things get tangled most of the time.

So for now I have shortened things to get 40 m and 20 m to fall into place. This improved 17 m and 15 m but need ATU to dial them in. 12 m looks good, 10 m is towards the upper end of 28 MHz. 80 / 75 m SWR is 1.5:1 at 3.650 MHz rising to 2:1 at 3.850, and 2.8:1 at 4 MHz. I have a harsh sounding noise level on this band up-to S9 at times, the next crusade might be see if it's originating from with-in our house and eliminating it.