Wednesday, February 04, 2009

80 Meter Vertical Experiment;
Bob Raynor N4JTE

I have spent most of my creative antenna energy on 40 meter wires and have had great success with those experiments. HOWEVER, the 40 meter band shuts off like a light switch here in upstate NY a couple of hours after sunset.
Having had a lot of unexpected fun with phased verticals on 40, I researched a way to get the same benefits on 80, as it seems to be the only band available after dark.
Problem is I don’t have two 60ft towers laying around, I know spiderbeam sells them for about $250 but not wanting to spend that much on an “experiment” I was forced into finding a way to use my existing 40ft poles.
I only ventured into phased verticals after figuring out an accurate way to cut feed and phaselines as detailed in previous articles. That epiphany opened a whole new area of experiments for the lower bands. The other revelation that got me going was the value of the Christman method of obtaining correct phase angles using 50ohm coax in place of complicated L/C constants which are beyond my math/circuit board experience.

Ø Utilize 2 available 40ft. pushup poles spaced 1/8 WL apart and achieve the theoretical gain of 3DBD in switched directions.
Ø Incorporate the phaselines as detailed by ON4UN in his bible, “Lowband DX’ing.” Based on the unique qualities of 135 degree phasing.
Ø Top load the 33ft. existing verticals with a (T) loading wire of appropriate length of #18 insulated wire to achieve resonance.
Ø Cut (2) phase lines at 157 degrees and (1) delay line at 39 degrees,
Ø Wire up 12v DPDT relay to achieve reverse capability.
Ø Wait for snow/ice temperature to go above 18 degrees!

My understanding of shortened verticals tells me that any loading system should be as close to the top as possible, I queried my fellow Elmers on Eham and and realized pretty early on that a coil at the base would be far from ideal. Adding wire at the top is the way to go and my lazy alternative of adding bottom fed coils was rejected.
If ON4UN is correct, and he usually is, he tells us that if the vertical is not less than 66% of 1/4WL height, you will have close to the same gain and front to back as that of a full size antenna array. You will be handed a narrow bandwidth, but that can be a matter of choice when designing the antenna and the use of a matching network at the antennas will expand your range of operation.

The first step was to top load one of the existing 40 meter verticals and achieve resonance at 3.7 megs. I cut a piece of 18 gauge insulated wire to 30 ft. and exposed the center and attached it to the vertical 33ft. wire. Before raising it back up I attached some masonry string to the ends to facilitate tying off in opposite sloping directions.

With 2 raised radials, resonance was above my target frequency so I lengthened each end of the top loading wire until I got close enough. I checked resonance, lowest swr, at the radio instead of antenna base because I don’t trust the MFJ at low frequencies due the fact we have a local am/fm station less than 1 mile away that could possibly skew the readings. A lot more trudging in the snow but worth the effort as this way the whole “system” is being measured.
The sequence was repeated on the second vertical while taking pains to keep the top loading wires parallel to the other vertical’s. Not sure why, but my instincts tell me that the mutual coupling of 33ft. should be maintained on the vertical and T wires.
I rechecked the resonance of the first vertical and noticed a slight increase in the resonant frequency. I could probably have spent a few more hours freezing my butt off to fine tune it, but I’ll save that chore for spring!

I have existing poles in an East/West orientation as it seems the best compromise for DX and stateside contacts from my home here in upstate NY. I believe the verticals are fairly broad, somewhere around 135 degrees so that’s where they going to stay for this experiment.
The relay was wired up with short alligator leads as detailed in previous articles again taking care to maintain polarity. The phase line of 39 degrees was cut out of 50 ohm coax and ended up around 15ft. The 157 degree feedlines ended up around 63ft.each.
I never rely on published VF except to get a general idea of approximate length then I use the MFJ to get the exact readings, also spelled out in previous articles.
Not shown for clarity is the feedline to the shack nor the 12volt wire from power supply in shack, 3amp Radioshack.

Remember when I said I would wait till Spring before getting both verticals exactly frequency matched? Well, preliminary testing as built was not outstanding. One antenna showed a 1.2 to one swr and the other was 1.8 to one, impossible to switch directions effectively. So before continuing on air testing, I put on the Carharts and ventured back out into the 13 degree weather to make both antennas resonant on 3.700 by adjusting the top loading wire lengths, not fun in these temperatures! That mission accomplished, I thawed out and was rewarded with a 1.2 on each wire in both directions, no tuner needed for a change.
My comparison antenna is a 40 meter EDZ at 60 ft and very close to resonance at 3.700
Did the verticals work ? Absolutely, I got significantly better reports, 3 to5 S units from various European stations as compared to the dipole and I had the same results with CA. contacts. This antenna works well but is very narrow banded. I did not lose too much in the 400 to 1000 mile range probably due to the sloping top wires. The horizontal component was not completely cancelled out as would be the case with flat top loading.

Final Thoughts;
This antenna will maximize your signal on 80 in a relatively small footprint with a very small investment as compared to store bought shortened miracle antennas. I probably will invest in a couple of 60ft. poles and eliminate the time consuming top loading and resultant narrow bandwidth. But for now I will be happy with doubling my ERP in opposite directions. And while the snow keeps burying all the mess I made in the backyard, I will be reading ON4UN’s book by the fireplace for the 5th time, looking for the next big idea. Maybe phased inverted L’s for 160 meters! Stay tuned.

Tnx for reading,


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