Saturday, June 22, 2019

Summer 2019 Antenna Projects

40m OCFD: I built the classical 40m version with 33% offset noted previously. For a quick "throw it up and see" test I had it in an inverted V config, the SWR and resonance results are inline with expectations.

I fired up the Ameritron AL-80B a couple weekends ago with the OCFD. On 20m propagation was the worst I've come across, managed one contact into Missouri but had to use the Utah WebSDR to receive as signals were weak. On 40m I later made two contacts into Utah, conditions were tough with static crashes from thunderstorms.

Listening to my self via the WebSDR, the 10dB gain of the amp makes a big difference. Sounded clean and punchy with the Icom IC-7300 driving at 70W with the ALC connected. The T/R switching is via a solid state relay interface I built as suggested by KV5R.

Inverted L / 43 ft vertical: Restored to 43ft vertical for now, OCFD needed the only high support I have via a tree that the horizontal wire ran to. Interesting note with the vertical, when it was last simply a vertical I had the radials laying on the ground, the SWR/match was a bit "gimmicky" in on some bands where for example on 20m, the remote ATU failed to find a match. With them buried 2" to 6" deep things are much more normalized.

What's Next?

Get 40m OCFD up in flat top config, and dialed in. At near 50ft it will cover 40m for NVIS, 20m with a good pattern and low take off angle, plus 10m and 6m. I might move the feed point to 40% and gain 15m coverage.

I got some mast sections from The Mast Company, plan to install with an apex eave mount. Put the Comet GP-15 on the top.

40m DX Option 1. The mast may provide a support for one end of a 40m Half Square. Will favor SW / NE which covers the Pacific, and Europe / Africa in the other direction. More info regarding half-squares:

Far field plot for the half square I modeled.

40m DX Option 2. Two element vertical Yagi using a single 60ft support, I got the idea from this page - I have tweaked the geometry to bring it up to 7.15MHz.

I found additional information by N6LF - Getting the Most from Half-Wave Sloper Arrays.

There are perhaps 3 ways to feed this antenna:
  • N6LF settled on resonating both elements as reflectors, and end feeding the selected driven element - the parallel tuned circuit used to match takes care of resonating the driven element.
  • Borrowing from K9YC's End-Feeding a Center-Fed Vertical Dipole, the bottom half of each element would be made from coax with good chokes at the bottom which K9YC also discusses. Leaving one side open circuit resonates the element as a reflector - the length of coax is critical, I calculated (info below) there would be enough length to form the bottom half of the dipole, the choke and connections to switching relay. Some form of strain relief may be needed for the coax half of the dipoles to stop it from stretching.
  • Using the same principal as above but center feeding with coax (just like a conventional coax fed dipole) to a mast mounted switch that short circuits the inactive element creating a closed stub which resonates it as a reflector, the length needed is much shorter hence the mast mounted switch. A balun would be needed at the feed point which adds weight.

The latter two open/closed circuit stubs will present an inductive reactance lowering the resonant frequency of the passive element turning it into a reflector. How to figure it out? W8WWV wrote a nice article on Coaxial Cable Stub Q, which lead me to an application called Transmission Line Details. Among other things this can be used to calculate the length of open or closed stubs, it also tells you how much R the stub will add at the feed-point of the passive element - this information can be plugged into MMANA to know the effect of the added R.

Another handy calculator I found is Inductive Reactance Calculator, which converts an inductance at a given frequency into an inductive reactance value. Useful in conjunction with MMANA-GAL and TLD.

It's important to note that properties of coax varies between manufactures and manufacturing runs. The length is critical, one would need to measure the velocity factor of the coax on hand using an antenna analyzer, not rely on published information. I suspect this is the reason why some struggle to get phased arrays to work properly, relying on the publish spec is not close enough to reality.

Personally I think N6LF's end fed might be the better choice as it eliminates the weight of coax, baluns, mast mounted switches etc. If using high power, capacitors need to handle several kV. The only critical part is resonating both elements as reflectors.

From the model, I like this antenna for a couple reasons, it has a nice broad beam width, and 2 dBi of gain at 10 degrees elevation, the F/B is ok at 13 dB. It can also be built using a single support.

MMANA-GAL file for 40m 2 element vertical Yagi.

View of 2 element vertical, these are half wave dipoles with the ends angled towards each other. 

Far field plot for the 2 element vertical.

3D far field plot from GAL-ANA.

What I like about this is it can be built with a single 60ft support. Has good gain at low angles, 2 dBi at 10 degrees, -3 dB beam width is slightly more than 120 degrees.

40m DX Option 3. Update 08-Aug-19, attempted to build this with my 43 ft vertical, however the vertical is too flimsy to support the tension needed on the passive elements to get them into the right shape. I'd need to rebuild the vertical with longer sections (less taper) and guy it. I did enjoy the time spent researching this antenna, modeling it, and learned a few things along the way. I'm disappointed that I cannot make a good attempt at building this antenna. Maybe another day!

Another variation of vertical Yagi from is the 3 element GP80 scaled for 40m.

Very close resemblance to the Spitfire antenna developed by K1VR and W1FV - It also appears in ON4UN’s Low Band DX book, and N6LF's Single Support Gain Antennas for 80 and 160 Meters.

What I like about this is the 43ft vertical I already have with 32 buried radials is the right length for the 40m version driven element. Makes good use of what I already have in place. All I need to do is add the passive elements, and simple matching network with series capacitance.

This antenna design is kinda neat on a few points:
  • The driven element is around 1/3 λ this raises feed-point impedance to a convenient 50 Ω, and can be adjusted as needed to be 50 Ω, series capacitance takes care of bringing it to resonance
  • The extra length also supports the passive elements and their shape, can double as guy lines.
  • The passive elements are 1/2 λ and folded (and thus independent of the ground plane), shortening one via switch/relay in the low horizontal section turns it into a director, direction could easily be made electronically reversible.

View and far field plots of the 3 element GP.

MMANA-GAL file for 3 element GP 40.

Since I have the driven element already in place, modeling it by it-self showed feed point being 70Ω, jX 108 at 7.15 MHz. I went and measured it with the analyzer and to my surprise it's very close:

Analyzer shows 72Ω, jX 104, along with the series capacitance needed for resonance - 213pF. Adding series capacitance of 200pF to the model brought it to resonance. The test and result is encouraging.

What to do for 80m due to space and tree locations, I've considered many different possibilities. The best solution I can find is a quad loop squashed into a rectangle. Feeding from one of the sides results in bi-directional low angle vertical polarization. An 160m version of this antenna gets a mention in Low-Band DXing 5th Ed - Chapter 10, pages 4 and 5.

At the other end of the spectrum, the 7 element Yagi I have in the attic for 900MHz is sub-optimal, need to relocate it or try something different.