I have made an AMAZING discovery. But, unless you’re an electronics engineer, with an emphasis on RF design, this would be gibberish without first attending “Antenna School.” So let’s do that.(..beats watching riots.)
If you’re new to ham radio, I will lay this out as directly as possible – in order – because the name of the game in ham radio is? Making every watt count. Two of three times? Well, we can do that too…
Ham Antennas 101
How long is a good antenna?
The basic answer is: It depends which band you want to run. With the Sun in rehab for another 3-7 years (or longer) three things will happen: Earth will cool more. Politicians will whine more. And the lower high frequency (3-30 MHz bands) will not be as interesting in the daytime.
Reason? At the bottom (or in present case, the extending bottom) of the Solar Cycle, 3.5 and 7 MHz can be OK for close in (500miles) daytime (YMMV on 80 meters where really 200 miles daytime is dang good). At night, 3.5 MHz “gets legs” and 7 MHz becomes amazing. I was listening – arm chair copy to a group of VK stations (Australia) the other morning on 7151.5 LSB.
20-meters is my absolute favorite band, but only “when it’s open.” Not lately. When sunspots are this low, that’s “hit and miss.” Miss, miss, miss…. which is why there’s a propagation link on the links page.
Antenna Length Forumula
The basic is the antenna frequency in MHz divided into 468 gives you the 1/2-wave length of an antenna.
In the middle of the 80 meter band (3.75 MHz) that’s 124.8-feet. While in the middle of the 40 meter band (7.15 MHz) that comes out to 65.45-feet.
Let’s Build a Dipole!
Let’s now take that 65.45-feet of wire and cut it in half. In the middle, we’re going to put a center insulator. As luck would have it, long-time
victim reader KW1B (*Bill) has designed just the goody for you:
Bill lays out key bullet points of his build:
- “Cheap, strong, nearly eternal in durability.
- Paint it flat black or camo, and you’ll never see it again once it’s up.
- PVC pipe parts.
- Nothing is electrically connected to the eyebolts internally. They’re for tension strength only.
- Don’t “RTV” around the coax hole, and you get all the “weep” you’ll ever need.
- The coax is secured inside with a small stainless screw-clamp. (Tubing or pipe clamp.)
- I solder a one megaohm 1 watt resistor across the coax as a static bleed-off. This works nicely.
- I don’t use baluns or even coax-coil baluns for common-mode block. Doesn’t seem to matter.
73 de KW1B
To get on the air? Figure out how to solder (or crimp) a PL-259 (male UHF connector) on the coaxial feedline and plug into a well-grounded radio and off you go. Assuming you hauled the antenna as high and in the clear – at least in the middle – before keying up.
Now we start chasing fine points.
Ure is a huge believer in ladder line – a kind of variant of open-wire feed line – vs coax. Lower loss under many to most conditions. A bit more picky – lower angle bends, keep away from metal gutters and whatever else would torque up the “balance.”
You can see it twisting its way up to my antenna at the top of the tower – about 55 feet up – from which a 15-foot hunk of 2 1/2″ steel goes up from the rotator (the lower whizzy) and hold up the 3-element beam which has turned cranky.
Ladder Line uses a different center insulator design.
There are plenty of designs you can download and print on a 3D printer if you wish. See Yeggi.com and Thingiverse.com for antenna insulators. (You may be lost for a week, lol.) Or, since you were ordering ladder line and wire, just toss one of these in:
(Why how proper, this being Sunday and all…)
Now we go outside to look at a support tree east of the tower that Oilman 2’s son cleared out for us.
White Scribbles? (top) A pully and halyard go up 40 feet, or so. (bottom) And from a good pulley about 2-pounds of lead fishing weights pull one side tight. Allows tree to move without breaking things. Repeat going the other direction from the Tower or center support.
Where’s that Free Decibel?
Hang in – almost there!
We can do really cool things by taking an 80-meter antenna (let’s use 135-feet) and OFF-CENTER FEEDING it.
IoW instead of feeding the wire at the 1/2-way point, how about we feed it at one end?
On 20-meters (closed most of the days now, dammit) the antenna is called (depending on salesman) A “G5RV” after one inventor. A “Windom” by another. A variant called a “Carolina Windom” is similar, and the generic term has become generalized as an “Off-Center Fed Dipole” or simply OCF.
Hang ‘Em High
Something magical happens to OCF *(and all those other names) when you hang the middle at 50 feet and the ends at 35-feet or better. Look at the gain figure over a simple dipole:
Right here, you should be saying “Wow!” But? No free lunch in antennas! Your antenna will be somewhat directional. Speak to God about where trees should really have been planted 20-years ago to be useful now…ask Him to pause the timeline and move trees around as needed. Or stomp your feet until the spouse gives in. Most often, neither will change much…
Yes, OCF Antennas Work – and very well under certain conditions. Directivity is gain. But it’s always at some cost, somewhere.
NOW We Get to Ure’s MAGIC WIRE
So there I was with the models running and I decided to try something totally counterintuitive.
Remember, now: 45-feet one side of the feed point and 90-feet on the other. Those are the wires.
“What would happen if I just added another leg connected at the center and spaced it so that it was 6-inches from the original 90-foot section at the far end?
F R E A K I N G M A G I C !!!
“Holy crap, Batman! ” “Yeah, boy-in-tights...real RF gain...”
No Free Lunch (Maybe?)
The models giveth and the models taketh away. Since (verily) there are really 3-“seasons” an HF solar/fishing cycle (top of solar cycle, bottom, the crap in between), no antenna is perfect.
This antenna is great on 20-meters when open, offering 1.57 decibels of (theoretical) gain relative to an “Un-Ure” OCF (and all those other name) antennas. Only if oriented correctly. Like north by northeast up over the pole toward Europe – that’s useful.
One “lunch payment” though is a change in SWR.
Before Ure’s “magic wire” the antenna is pretty well-suited to a 4:1 balun:
After Ure’s Magic Wire the SWR goes up a bit:
Also, one side of what was previously a more balanced pattern pulls in a bit (there’s no zero point energy hokum in our antennas – YET).
BUT, since we’re using 450-ohm ladder line, such losses are really quite trivial compared with the gain.
As always, the real proof is in the on-air operation. Unfortunately, on this front, we have to wait for Old Sol to wake up before we can test performance on some of the HF bands besides 80 and 40. Because, yeah – nearly as much gain on the Super Antenna 2 model of last weekend, as well. Yee haw!
It models brilliantly. Any time someone goes around offering almost a dB and a half of gain for the pittance of 90-feet of wire? Count me all-in, Texas style.
Write when you get rich, (or the fires die down, or the media calls ’em rioters not this namby-pamby psy-op “protesters” crap. Jesus, aren’t there any bright people left who can see through the fog of media? I’m taking the rest of the day off to let my blood pressure return to normal.)