ShopTalk Sunday: Antenna Questions Answered

Welcome to midpointing a 3-day weekend. Where we expect to get a lot done.

Let’s begin with this from a reader:

(from a dislocated W2 ham radio op in the Woodlands, TX)

“I remember you purchased a “mystery antenna” from K4TR, and were going
to evaluate it?  Any feedback on it would be appreciated.”

Oh boy!  Sea belts fastened?

Antenna Selection Soliloquy 

The K4TR W5GI “Mystery Antenna” works just fine.  As you know, these remarks are for the 40-meter version of the antenna and I also have the 20-meter version.

To describe each: the basic idea of a W5GI design is that it is a kind of poor man’s collinear made of plain wire and coax cable.  Each of the two horizontal elements consists one one side of the center insulator of about a quarter wave of plain wire, a coax section (essentially for phasing), and finally, out to the end insulator, another quarter-wave of plain wire.

In real-life applications, the antenna gets very good reviews such as W5GI Mystery Antenna – K4TR Antenna Mfg & Wireman on eHam.

The length of the two coax sections (one on each side)  is generally fed to the middle conductor only on the center insulator (on the hot) side.  On the “cold” side (to the end insulator) the inner and outer are shorted together where that outboard plain quarter-wave plain wire begins.

The “trick shot” in getting the W5GI to work well is understanding the role and trimming of the coaxial sections.  The coax must be of good quality because the single conductor in the center will be required to hold the weight of one side of the completed antenna in span.

Finding good quality coax (foam insulation is lighter than solid insulator PE material) is only part of the problem on such antennas.  The other issue is getting the trimming right and for this, accurate assessment of the velocity factor of the coax becomes very important for proper phase relationships.  These are more clear upon study of De-mystifying the W5GI Mystery Antenna at OwenDuffy.net.

Jim Stefano, W2COP, did a (2020) presentation for the Rochester Amateur Radio Association (https://www.rochesterham.org/meetings/2020-05-06%20G5RV%20Presentation.pdf) where in a later slide (#39 I seem to recall), he offered very good comments describing how the W5GI works.  The reason you want to spend some time – going through his whole presentation – is that it’s a thorough review of competing OCFD designs as well as other options.  Specifically, his remarks on the shortcomings of the G5RV versus the ZS6BKW design warrant close attention.

There are several good makers – I chose John Dube’s (K4TR) antennas, though a UK maker also gets good reviews. I also went for John’s “special extended” –  The 40-meter version of the basic (20-meter) W5GI.

Depending on your real estate situation, this may be too big for many in-city homes.  It spans about 200-feet, in contrast to the 20-meter version’s 100-feet.  Additionally, the twin-lead matching section (which enables the multiband operation) and is about 30-feet on the 20-meter version, is more like 50-feet (or a bit more) on the special super-sized 40-meter version.

Operational Results

Having been a ham (since 1963) I have come to appreciate several little thought-of aspects to antenna selection and installation.

The first is that every ham should take advantage of Ray Lewallen’s extreme generosity and download his superb antenna modeling system EZNEC.  It’s at https://eznec.com.  Antenna modeling will reveal amazing aspects to antenna selection that while all of us “kinda know ’em” is really quantifiable with Lewallen’s software.

It will take some time, sure.  But coupled with some great books, especially by the late W4RNL’s (L.B. Cebik’s landmark collection of research), very accurate predictions of field performance will be attainable.

Pay particular note that in order to gain proper insights, you will need to search for the FCC’s national ground conductivity maps and use a viable factor for local ground conductance.

The second guidance point would be to ensure you understand how to utilize antenna modeling segment declarations. The easiest way to do this is to understand that the Frequency declaration on the main page of the program drives the number of segments and that on higher bands more segments are necessary.  This translates, any time you vary a wire length of a proposed antenna, to re-segmenting the analysis using either conservative or minimal numbers of nodes.

The third point is that many popular antennas, such as the G5RV and ZS6BKW and the OCFD designs, all become end-firing on higher frequencies.  So, take a 40-meter OCFD, for example, it will have three essential operating modes at typical 50-60 foot high (and lower) installation levels.

On the 80-meter band, all of these antennas tend to operate in what may be thought of as a “ground proximity mode.”  In this space, a fraction of a wavelength above ground, the antenna will operate in what is (essentially) an “omnidirectional” mode. Lots of NVIS! (Near-vertical incident sky-wave, right?)

As the antenna is raised, to the ¼ wavelength (and higher) elevation over the ground, the ground reflections begin to cohere such that the basic antenna pattern, such as broadside to the wire direction, appears.  In other words, on 80-meters, a low-level antenna is mainly ominidirectional but at 120-feet (1/2-wave) the antenna presents a lobe of radiation on either side of – and perpendicular to – the antenna wire direction.

Thus, in Texas, an East-West hung dipole begins to fire north-south broadside, as it is raised above ground around 50-100 feet, or higher. Frequency dependent.

The final point, is this notion is exactly opposite of what happens when an antenna is operated on harmonic frequencies!  Thus, we come to see how a North-South hung antenna of the G5/ZS6/OCFD sort would likely be a good performer into Europe and Asia (corresponding to NE and NW nodes it its pattern, while 40-meters might radiate best East-West and be essentially non-directional on 80-meters.

Now we get to the main discussion of my operating experiences with both antennas.

First. The 20-meter W5GI, mounted East-West is a great antenna.  Especially when (at 60-feet up, where mine is) the antenna has begun to exhibit gain and the pattern is very much as expected.  Great signal reports.

OK, here’s where we have to get into some discernment.  This is a smaller (electrically) antenna (aperture-wise) than a full-sized antenna on 80 meters. Meaning, you won’t get the Big signal that a full-sized resonant dipole would produce on 80.  On 40-meters, it is closer to parity.

Now, the reason I have two of John’s w5GI’s is that there’s one more thing we haven’t talked about much and that is take-off angles.  Both are very good, but (paradoxically) in some instances, very good can be very bad.

Take-Off Angles Matter

We begin with the understanding that lobe positions and take-off angles are everything.  On 80-meters and 40-metrers (day) the main communications takeoff angle is NVIS – the near vertical incident skywave.  The F2 layer, which lowers and improves ionosphere reflectivity at night, is too high to be useful with high path absorption daytime.  On the other hand, at night, we want as much low takeoff angle as we can get.

I generally run a selection of antennas (low OCFD for general daytime use), the 40-meter W5GI and the 20-meter W5GI. I prefer CW and when I want to “play radio” I want to have solid contacts on whatever band I pick, regardless of the time of day.

So take the 40-meter band.  In the daytime, close in, my North-South OCFD will equal or outperform the W5GI.  Not the antenna’s FAULT.  We would be in daytime NVIS conditions says the clock.  Great out to 200-400 miles out, depending on solar conditions.  This is why reader Hank (on the Big Island of Hawai’i) uses NVIS on 40 for the Hawaiian inter-island net.

Where the W5GI 40-meter version (remember, it’s 200-feet wide and 60-feet up) absolutely creams the OCFD is when the F2 is down and DX (distant) propagation is in.  Then, especially with grayline open to the western Pacific, I am into “if I can hear it, I can work it” mode on 40-CW.

The effect is less pronounced (if you can hang a North-South OCFD made of 2” horse fence tape, hang it at 60-feet, and have it fed with 450-ohm ladder line with a low-loss antenna tuner).  The W5GI will wins, but only on some DX but also state-side contacts.

This is accounted for by the fact that the W5GI has multiple nodes, but into Europe and central Africa on a heading of 40-degrees east or north where the OCFD end-firing really struts its stuff, then the OCFD wins.

Because of size and elevation, the OCFD beats the 40-meter specific W5GI under very tightly bounded conditions.  That is, if the band is in short NVIS mode, the OCFD will do better.  But if the NVIS mode is out long, then the W5GI will work better.

On 40, though, the W5GI (CW, night, western Pacific region) will totally cream the OCFD because of the lessening of end-fire as the OCFD pattern becomes two kidney-shaped lobes on 40.

But you have brought up a key point about HF operations that most ops never really think through: Antenna selection with purpose and based on knowledge.

It is common in our hobby for people to look for the “One magic Bullet Antenna.”  There is no such animal.

Until my son gets down here and we refurb the lightning-blown trap on the AS-4-WB  beam on the tower, I get good gain with the 20-meter W5GI.  Because this is a band that is generally open for DX (via low-angle skywave) or not.

On 40 meters, the extended W5GI gives me great low angle for DX (and some gain, too, don’t forget).  But here, the OCFD will often do a bit better, as on a path length of, oh, say 430 miles (Palestine, TX to Amarillo, TX) during daytime on a “long NVIS day.”  On a short NVIS day, the OCFD will do better.

On 80-meters, the OCFD horse-tape open wire will do better when it’s up but the plain wire coax and balun version OCFD won before.  This is because of the low band focus on omnidirectional results under ½ wave above ground.

I hope this clarifies your thinking a bit.  Antenna selection and installation can be a technical problem, a financial decision tree, and all bounded by property considerations with a dollop of zoning Nazis on the side.

I don’t envy you the gauntlet to be run!

For Everything Else Worth Knowing?

…about ham antennas, please visit the ON5AU archive called the L.B. Cebik_Collection.

And learn some old-school stuff. Like about The Wouff-Hong, the Rettysnitch, and the What?

The main topic on the 3806 gathering on the 80-meter band, is offering to help north Texas hams set up rope tows and snowboard rentals for the year’s big snow (all of 1″) which is coming this week.  Bring Your Own Snow Bunny…

Write when you get rich, or can do 25 WPM CW…

Vy 73

George@ure.net  AC7x