# Dual band 80m 40m coil-loaded coax-fed dipole

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WA7ARK, Feb 5, 2020.

Tags: 1. My friend Hall operates mostly near 3920, 7135 and 7272 kHz. He asked me to come up with a coil-loaded dipole, center-fed using RG8X, that can be deployed horizontally at 35ft agl, and will work well without a tuner.

Using AutoEz, EzNec, and the Hamwaves coil calculator, here is my design: The antenna is described by three parameters: L is the length of the wire in feet, P is the percent of L between the wire ends and the center of each coil, and XL is the nominal inductance of the coils in uH.

For modeling accuracy, the horizontal dipole wire of length L is broken into five co-linear sub-wires 1-5. The coils are placed in the center of short wires 2 and 4. The coil position can be moved with high resolution using the parameter P, which simultaneously moves all wire ends except the two outside ones, which are set at -L/2 and L/2, respectively. The source is at the center of wire 3.

The goal for optimization is to find L, P, and XL which simultaneously makes the lowest feedpoint Swr50 at 3.920, 7.135 and 7.272MHz. A first try converged on L=91.25ft, P=8%, and XL=76.6uH.

To make a suitable coil, I have some thin-wall white PVC tube to use as a form. It has an OD of 2.48" (63mm) with a wall thickness of 0.09". I have some #16awg silver-plated wire with PTFE insulation. The wire OD is 0.050" (1.27mm) and the insulation OD is 0.072" (1.83mm), making a coil with mean diameter of 64.8mm, with a tight pitch of ~1.9mm.

Using the Hamwaves Coil Calculator, at 3.92MHz, it takes 48 closely spaced turns to make a coil with an effective Inductance L_eff = 78.8uH, which has a Reactance Xeff = 1940 Ohms and has an effective series resistance R_s = 2.7 Ohms. Here are the results: Notice that the self resonant frequency of the coil is 11.4MHz, well above the 40m frequency.

If we keep all the physical dimensions the same, we can use the calculator to see how L_eff, X_eff, and R_eff change at 7.2MHz: I modified the AutoEz model to automatically switch between X_eff/R_eff at the two frequencies of interest, i.e. 3.92 and 7.2MHz, and then re-optimized for L and P. The final values are L= 86.9ft and P=7.9% (6.85 ft in from the ends). Here are the plots of Swr50 across the upper part of 80m and 40m:  This design takes into consideration the coil's variation in L_eff and R_eff at the two design frequencies.

Last edited: Feb 5, 2020
2. Isn't this what ladder line is for? 3. Or why not just use 40m traps ?

Then you have a proper half-wave dipole on both bands.

(and if you use the right L/C ratios in the traps, you have a dipole resonant on ALL the main HF Bands, a W3DZZ . . . that's what I use for 80 to 10m)

Roger G3YRO

AI3V likes this.
4. "...and will work well without a tuner."

5. There is no advantage to "a proper half-wave dipole". The goal is to get R~=50 at the same time X~=0 to make the swr50 low at the same time as having the highest possible current along most of the horizontal wire.

The as-published dimensions for the W3DZZ on the web are all over the map. Using the published dimensions on this web site, and assuming that the trap is resonant at 6.99MHz (you do not want a trap to resonate at a frequency you are going to transmit on), here are the resonant points if the trap L=6.8uH and the trap C=76.2pF (Fres=6.99MHz): Obviously, it should be fed with 70 Ohm cable, and it misses 15m.

It’s an interesting approach to a one-off antenna for specific frequencies. Nice application of the tools at hand.

That W3DZZ looks interesting as well. Does anyone have any experience with this one?

7. So what you are saying, is the traps that have been made for decades, and used by thousands of stations with perfect results are, exactly what are you trying to say?

As usual, your math looks impressive, how did you make those coils, and what did you measure the self resonant frequency as?

As usual, you toss that word assume around.

Rege

8. What might the impedance at the transmitter be?

You said without a tuner, so do you mean a fixed tuned transmitter? Or one with a built in tuner?

A vswr greater than 1.5:1, pushing 2:1 is going to either cut your power in half, or dangerously increase final transistor current in a fixed tuned rig.

Rege

9. MAR 1955 - QST (PG. 22)
Multimatch Antenna System, The

Author: Buchanan, Chester L., W3DZZ
This article describes traps with 60pF capacitors and 8.2 uH inductors (he actually used uuF back then)