A Wire Beam Design for 80 meters

by Dave Robbins, KY1H

YCCC Scuttlebutt #117, June 1995

Over the years I have had a variety of antennas for 80m. I have tried plain inverted V’s, 1/4 wave slopers, full wave loops, a pair of full wave loops, a 4 square vertical array (48’ base loaded verticals), and a Bobtail curtain. But none of them were really that much more effective than a plain inverted V at 150’.

I wanted to build something like the pair of loops that KC1XX has such success with, or like the pair described in ON4UN’s Low Band DXing book. But I hate putting loops on my 150’ tower because of all the stuff in the way. Plus I have to be sure to keep room for the long 10m Yagi to rotate on its ring rotor. So I started playing with using inverted V elements instead of loops using K6STI’s AO program.

My design goals were to come up with a realizable boom length, a decent gain over my current inverted V, and a useful front-back ratio. After an inventory of the aluminum pile, I determined I had enough to make a decent 44’ boom so this became one constraint. I determined I could probably tie off the ends of the V’s about 250’ from the base of the tower, so that determined the slope of the wires.

Figure 1. The AO view of the beam with coordinates.

AO Plot of 80m Beam

Picking a direction switching and tuning system became the next problem. The arrangement for the loop Yagi from the ON4UN book looked like a reasonable arrangement. This loading arrangement makes use of the reactance transforming properties of a 1/4 wave piece of coax. Since I needed to put an inductance at the center of the reflector I could just put a capacitance at the other end of a 1/4 wave feed line. And since this was a parasitic element, the capacitor wouldn’t even have to handle full TX power.

For feed point impedance I decided to accept up to about a 5:1 SWR and compensate at the TX end since I was going to feed it with 3/4" CATV hardline which should keep the losses fairly low, even for the 350’ distance from the shack. This range may seem a bit wide, but as anyone who has played with short Yagi antennas knows, the feed point impedance can vary wildly with relatively small changes in the design.

Frequency (kHz) Reflector Inductance Gain (dBd) F/B (dB) Feedpoint Impedance SWR
3800 0 uH 9 13.7 50+j58 3.0
3500 5.5 uH 8.65 15.8 36-j77 5.1

Now, I have all the elements I need to model the antenna. So how long to make the wires, and how big does the inductor have to be? Well, to start the AO optimization I picked frequencies of 3500 kHz and 3800 kHz. Unfortunately I had to run each one manually since I was planning to switch the inductance when I changed from CW to SSB and the AO program can’t handle that (at least that I can see, but I am still learning some of its capabilities). I was trying to keep the gain and F/B about the same between CW and SSB; the impedance change didn’t matter much as long as it was reasonable for both modes.

After much playing I came up with the following compromise using a length of 65’ for each leg of the inverted V’s.

This makes the switching fairly simple. In SSB mode there is no inductance which would be an open circuit or very small capacitance at the other end of the 1/4 wave feed line. For CW there has to be a capacitance added at the end of the 1/4 wave line for the reflector.

To calculate the capacitance needed do the following:
XL = 2pfL = 2×3.14×3.5×106 × 5.5×10-6 = 121 W (inductive)

Now transform along 1/4 wave of 50 W line using Smith Chart, or the formula:
ZS = Zo2/ZL = 502 / 121 = 21 W (capacitive)

and then change the reactance back to capacitance at 3500 kHz:
C = 1/ (2pfXC) = 1/(2p×3.5×106×21) = 2.1×109 = 2100 pF

Figure 2: Azimuth pattern for the beam model at 3500 kHz.

Here are the detailed results of the AO calculations. First, for comparison, a simple inverted V at 150’:

Impedance 55.7 + j17.2
Wire Losses 0.10 dB
Efficiency 97.6%
At 28.0° Elevation:
Forward Gain 5.25 dBd
F/B 0.00 dB
AzimuthBeamwidth 90°

Note: the gain for an inverted V is simply due to the height.

Now, the beam model at 3500 kHz:

Impedance 34.6 - j 78.7
SWR 5.54
Total Losses 0.31 dB
Wire Losses 5.7%
Load Loss 1.3%
Efficiency 93.0%
At 28.0° Elevation:
Forward Gain 8.71dBd
F/B 15.68 dB
AzimuthBeamwidth 77°

Now, replace V’s with loops and reoptimize the wire lengths and load inductance with the same parameters to see what I am losing to KC1XX:

Impedance 134 + j 49
SWR 3.09
Total Losses 0.18 dB
Wire Losses 3.7%
Load Loss 0.4%
Efficiency 95.9%
At 28.0° Elevation:
Forward Gain 8.63 dBd
F/B 11.33 dB
AzimuthBeamwidth 74°

Note: Radiation Peak 32°Elev. (8.65dBd gain)

Hmmm..... better than KC1XX????? Must be his QTH that makes him so loud. Looking at the patterns it makes sense. The loop has radiation from wires that are much lower than the inverted V elements.

The elevation pattern of the loops shows much more at high angles; that is probably due to the lower parts of the the loops. With inverted V elements the highest current and most of the radiation comes from the highest point.

OK, so try this.... for high band quads the boom is in the middle of the loops, so the highest point is above the boom. In my test above I put the boom at the top of the loops so they were hanging below it. So, what if we built the loops like a high band quad with the center of the loops at the same height as the boom of the inverted V elements?

Impedance 41 + j 33
SWR 3.00
Total Losses 0.18 dB
Wire Losses 3.7%
Load Loss 0.4%
Efficiency 95.9%
At 28.0° Elevation:
Forward Gain 9.64 dBd
F/B 15.51 dB
AzimuthBeamwidth 74°

Note: Radiation Peak 24°Elev. (9.77 dBd gain)

Aha! There is that extra dB or so that has normally been claimed for quads over Yagi’s... unfortunately, in this case it would require a support about 35’ higher to hold the tops of the loops up, not very practical in this situation. Granted, this is not exactly KC1XX’s beam, but it shows that adding the extra wire and work to turn inverted V’s into loops isn’t always worth it.

So how does it work? This is the part of most antenna articles I hate, the "On the air tests". It is loud. Loud enough that the intentional QRMers during ARRL DX SSB gave up as we ran Europe right over them. Loud enough that the W6’s were asking what we were running during SS. And WZ1R had a run of 9 JA’s in WPX SSB. I haven’t tried the 3975 Horse Races yet, it may be too high to be really effective for them. It was so much better than the Bobtail Curtain that I took the curtain down.

Figure 3. Elevation pattern for the beam model at 3500 kHz.

Table 1. The model file used for the beam.

80-meter Inv V Beam

above Ground 

3.500 MHz

4 copper wires, feet

;basic parameters

Hgt = 150	;height of boom

Len = 65	;length of elements

boom =22	;half length of boom

slope =30/58	;drop per length of wire

slope’ =50/58	;run per length of wire

;calculated parameters

btm = Hgt-(Len*slope)	;height of bottom of ele-ment

end = Len*slope’	;horizontal distance to end of element

;layout of elements

;order of columns:

;nr of pieces, x,y,z, x’,y’,z’, size of wire

1 boom 0 Hgt boom -end btm #10

1 boom 0 Hgt boom end btm #10

1 -boom 0 Hgt -boom -end btm #10

1 -boom 0 Hgt -boom end btm #10

;location of feedpoint

1 source

Wire 1, End1

;definition of load for reflector

1 load

L1 = 5.5	;size of inductive load

Wire 3, End1 L1 uH Q=200