Posted by Mike Staal, K6MYC on 7th Mar 2016
A half wave dipole on an 80m is about 130 feet or 40 meters long. Most of us humans think an element that long is difficult to build an rotate much less keep up in the air reliably even for a single winter. So what size would be worthy to try? Something 75 to 100 feet might be manageable. So how do we shrink the length and maintain most of the performance and efficiency?
Having built linear loaded 80m Yagi’s since 1980, I know the concept works. I physically modeled the first dual driven, linear loaded 4 element Yagi at a frequency of 144 MHz. Scaling the element sizes was difficult but I was able to optimize spacing and linear loading location. Once the model worked at 144 MHz, I then measured the resonance of each element individually and scaled the results by 38:1 and I had a full size starting point. It turned out to need very little tweaking.
80M4ll was built for Arnold Tamchin, (W2HCW).
Now along comes “Computer Modeling”. Fortran based in the beginning and then in basic. Nec and mininec followed and Brian Beezley. K6STI produced YO (Yagi Optimizer) and AO (Antenna Optimizer), mininec based programs. Roy Lewellen (W7EL), followed with eznec and elnec, nec based programs.
This started a modeling frenzy. There was one problem however, nec based programs do not model linear loading accurately. But many modelers using nec based modeling built linear loaded antennas and found they did not work?? Substituting coils for the linear loading did the job however.
I was the proud owner of many versions of YO and AO. Linear Loading in mininec based programs does work so many linear loaded antenna designs followed with good performance results. Some others built linear loaded antennas as well but for many reasons they did not work well so linear loading started to lose its credibility. All sorts of half baked theories filled the airwaves about current cancellation and whatever caused the loss of front to back and again.
To most antenna designers coils seemed like the logical solution. Mechanical design issues are important with both coil and linear loading designs. Efficiency is a serious issue when doing a coil design. A Few perceptive designers realized quickly that coil Q was extremely important, particularly at 40M and 80M! 160M is another story for another time.
Here is an interesting side note. When coils are used in a dipole the coil Q is not much of a factor when related to efficiency. Poorly designed coils still work reasonably well. But, when the dipole placed, physically and electrically, close to another similar element, the current in the element goes up dramatically and losses can completely kill the gain! If the modeling program either does not calculate final efficiency or the modeling ignores it, the low Q coil design looks great but it doesn’t work well in the field.
Linear loading is much less critical to wire and tubing diameter losses but it still does show up once the antenna becomes a parasitic, directional structure.
So to put this into perspective, extensive modeling with AOP (antenna optimizer, professional) shows that linear loading designs using decent diameter loading component work very well and are very efficient. Coil loading using wire size and fabrication techniques that maintain a Q of at least 300 works very well and are very efficient.
The results of the multiple years of simultaneous, on the air testing shows no detectable difference in forward gain or front to back performance using linear loading on one antenna and coils with a Q of 500 on the other antenna.
Modeling of each antenna showed virtually identical results meaning gains within .2 dB and F/B of 24 dB plus/ minus 2 dB. So it comes down to personal choice based on your local weather and esthetics.
The new concept in the coil fabrication that Matt Staal here at M2 came up with allow us to machine the coil from 1/8 wall aluminum tubing leaving a ½”solid tube section on each end of the coil. This makes for extremely low loss, high reliability coil to element connections, because the machining is accurate, the inductance value is the same from one coil to the next.
It is one thing to wind a high Q coil on very good, low loss dielectrically only to see that beautiful coil compromised with small area, dissimilar metal connections to the element sections.
The physical covering and joining of the coil to the element is equally important to longevity and performance. M2 coil ends are CNC turned from 4” diameter aluminum billet and further CNC milling to remove excessive weight. A special 360 degree clamping connection insures maximum strength and reliability of the joints. Internally the coil floats on 4 thin strips of machined polyethylene, internally threaded, cover. This fabrication technique is a bit pricey but produces an almost indestructible inductor.