K1TTT Technical Reference

An 80m Beam From Scratch

David Robbins, K1TTT

An article written for the YCCC Scuttlebutt

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.

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.

As 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 may seem a bit wide of a range, 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.

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 the frequencies of 3500khz and 3800khz. 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:

        Reflector                       Feedpoint
Freq    Inductance      Gain      F/B     Impedance    SWR
3800    0 uH            9dbd      13.7db  50+j58       3.0
3500    5.5uH        8.65dbd      15.8db  36-j77       5.1
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 = 2*pi*f*L = 2*3.14*3.5*10^6*5.5*10^-6 = 121 ohms (inductive)
    
    transform along 1/4 wave of 50 ohm line using Smith Chart or the formula:
    ZS = Zo^2/ZL = 50^2 / 121 = 21 ohms (capacitive)
    
    and then change the reactance back to capacitance at 3500khz
    C = 1/ (2*pi*f*XC) = 1/(2*pi*3.5*10^6*21) = 2.1*10-9 = 2100 pf

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

Figure 2 is the model file used for the beam.

Figure 3 is an azimuth pattern.

Figure 4 is an elevation pattern.

Detailed results of AO calculations:

For comparison a simple inverted v at 150'
              Impedance          55.7 + j 17.2 _
              SWR                 1.41
              Wire Losses         0.10 dB
              Efficiency         97.6%
      At 28.0 Elevation:
              Forward Gain        5.25 dBd
              F/B                 0.00 dB
              Azimuth Beamwidth     90
note: the gain for an inverted V is simply due to the height.

The beam model at 3500khz
              Impedance          34.6 - j 78.7 _
              SWR                 5.54
              Total Losses        0.31 dB
              Wire Loss           5.7%
              Load Loss           1.3%
              Efficiency         93.0%
      At 28.0 Elevation:
              Forward Gain        8.80 dBd
              F/B                15.68 dB
              Azimuth Beamwidth     77
Replace v's with loops and reoptimize the wire lengths and load inductance with the same parameters to see what I am loosing to KC1XX:

              Impedance         134 + j 49 _
              SWR                 3.09
              Total Losses        0.18 dB
              Wire Loss           3.7%
              Load Loss           0.4%
              Efficiency         95.9%
      At 28.0 Elevation:
              Forward Gain        8.63 dBd
              F/B                11.33 dB
              Azimuth Beamwidth     74
              Radiation Peak        32 Elevation (8.65 dBd gain)

Hmmmm.. 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 are. 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         141 + j 33 _
              SWR                 3.00
              Total Losses        0.18 dB
              Wire Loss           3.7%
              Load Loss           0.4%
              Efficiency         95.9%
      At 28.0 Elevation:
              Forward Gain        9.64 dBd
              F/B                15.51 dB
              Azimuth Beamwidth     74
              Radiation Peak        24 Elevation (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 Bob Tail Curtain that I took the curtain down.