|Antenna 012: Basic Vertical - 40 meters
Author: Frederick R. Vobbe, W8HDU
November 27, 2008
|A basic HF antenna is seen to the left. The hot side is shown in red, while the ground side is shown in blue. This fourty meter antenna can be made from something as simple as TV mast pipe, steel gas/water pipe, or even the aluminum conduit that you can get at Lowes.
The ground side is a series of radials that all group into a connection at the base of the pipe, and are insulated from the pipe (red part). These radials can be darn near any wire you can find. I've found that the best wire is something like #14 or #12 THHN or solid copper wire. But I have used doorbell wire, old discarded CATV coax (short the center and shield together at each end), discarded telephone drop wire, and even barb-wire, and hookup wire. It's always best to bury the wire under the ground to prevent people from tripping over the wire, and also to improve the ground losses. Also, you don't want the YL to step on the barb-wire in her bare feet!
I've found that the best bang for the buck is #16 "machine wire" which I have purchased from Graybar Electric. It's a smaller wire, but you can get several thousand feet of the stuff at a pretty decent price. It's an insulated wire and you can bare an end at the ground point and then stretch it out to the length you need. The way I install ground radials is simple. First, cut the grass super short, almost to the point of scalping the lawn. Next, using a flat spade, just press into the lawn about 1" and wiggle the spade back and forth till you make a "V" about 1/2" wide. Tuck the wire into the groove, and stamp it down. Water it generously, and maybe toss on some Scotts Turfbuilder, and in a few days nobody will be the wiser. I have 28 radials under my back lawn, and except for the grouping that all comes up out of the ground at the feed point, nobody but I knows there are there.
The antenna is calculated for a specific length, usually 1/4 wavelength. The radials should always be the same length, but I have found that you can vary them about,. So on a 40 meter vertical you can have a few radials that are 34 feet (ideal 1/4 wave at 7.1 mHz), and a few at 38, and a few at 30 feet.
A normal broadcast antenna uses 120 radials. I have found that for best "ham" results you can use between 18 to 40. If you go on the low side, or even an low as 6, you might notice bandwidth issues or problems with tuning. But if you can get in the 30s you'll notice it's nice and stable for ham use.
Design of a 40 meter Vertical
Let's first calculate what we need. At the end of this page you'll see a chart showing the recommended lengths. But for 40 meters we'll look at a typical element length of 33.1 foot long. Note that while this is calculated as a 50-ohm antenna, the true impedance is 37 ohms. See figure 2 and 3 below.
On the left (figure 4) you'll note that we simply go to the bottom of the vertical, and tie the center conductor of our coax to the the bottom of the vertical. The shield then is tied to the radials. Another method is to feed a 1:1 balun at the base of the antenna as seen on the right (figure 5).
The radiation pattern of this antenna is unidirectional, with the vertical component looking like this.
While not tossing a lot of radiation into the sky wave, you still realize a 2.16 dBi gain. Ground wave is almost 5.16 dBi. If you do a lot of local communications, this design is excellent.
So what happens when you increase the height of the vertical? The first thing you'll discover is that your impedance will go up significantly. Lets look at what happens with the radiation pattern if we make our 1/4 wave radiator a 1/2 wave radiator. Simply, we double our radiator from 33.1 feet to 66.2 feet long, (and do the same thing with radials).
As you can see from the purple line on the right, we have lowered the radiation into the sky, and increased it on the ground wave. This is why a lot of the 50,000 watt clear channel radio stations use 1/2 wave and even 5/8 wave antennas.
Ideas (cheap) for Vertical Antenna Insulators
On small vertical antennas, (such as 10 to 30 meter band), you can make insulate the base real easy. All you need is one beverage bottle. Personally I prefer the Coke bottles for 10 to 17 meter antennas, and thicker wine bottles for 20 and 30 meters. I've never used a bottle for 40 to 160, fearing that the downward force would crack the bottle. You will need the following....
Using aluminum tubing, drill a 1/16" hole 1" from one end of the tube. This will be the "weep hole" to allow moisture to exit, should it get inside the pipe.
Next, drill a hole 90-degrees around from the weep hole, and 1-1/2" from the end for a #6 stainless screw. Put the screw through the inside of the pipe so the head is on the inside, and the threads are on the outside. Secure tightly with a #6 lock washer against each side of the pipe
Install one of the hex nuts on the outside and tighten as best you can. (Note: this will be an electrical connection, so the tighter the better without snapping off the screw or distorting the pipe.
After tightening up the first nut against the lock washer and tube, put on the other nut and holding the first, wrench them tight together.
Install a #6 lock washer. At this point you can install some sort of lug, or make note that this is where you will install your coax or feed connection. You can use small wire lugs, or I prefer the bigger lugs such as the type electricians use in breaker boxes. Note: These will have to be water proofed to avoid corrosion. This is the FEED POINT of your antenna.
Install a #6 flat washer. Then tighten down with the remaining #6 hex nut.
Dig a hole 2" deep, and the width of the bottle's base. Place bottle in the hole, neck up.
Place the tube, with the feet over the neck of the bottle. Use non conductive rope to guy the pole straight.
Attach grounds to your radial system. To tune the vertical, start with a slightly longer tube than you need, and trim back to match in perfectly. Or install a tuner at the base of the antenna to lower the VSWR.
Be sure to see some of the other articles on Vertical antennas for some ideas.
The following is a chart for approximate lengths of radiators for vertical antennas.
* = Purple line on right represents the 3 dB point.
Frederick R. Vobbe, W8HDU
|© Copyright 2012 All Rights Reserved - W8HDU|