Vertical, Vertical and so Very Practical

Aero 'lectrics.

2

So, here we are with fly-in season upon us. And fire season. And lots of reasons to have an aircraft com station on the move to get to the location of interest. Radios move easily. Not so most of our antennas.

The order in which a solder-style BNC male connector should be assembled. From right to left: The body or shell, the male pin, the braid grommet, the compression gasket, the ring washer and the gland nut.

Please, whatever you do, do not call that little rubber resistor that came with your handheld an antenna. A piece of limp spaghetti in a copper septic tank would give better results. I’ve done numerous side-by-side comparisons on a plain old single vertical zero gain antenna versus a rubber ducky, and the results are somewhere between -10 and -20 dB of loss on the duck versus the vertical. To translate that to power or range, let’s say we have a 5-watt handheld transmitter with a 1-microvolt receiver. Or if you like, your handheld is trying to talk to an airplane 10 miles away. What is the effect of a simple vertical antenna versus the duck?

The assembled parts prior to final assembly .

A loss of 10 dB on a 5-watt transmitter means that you are actually radiating half a watt (0.5 watts, or 500 milliwatts). A 20 dB loss means that you are actually radiating 50 milliwatts, which isn’t enough power to blow your nose.

On the receive side, that 10 dB loss means that your 1-microvolt receiver is now a 3-microvolt receiver, and the 20 dB loss puts that up to 10 microvolts.

For distance, 10 dB of loss means that 10-mile airplane sounds like it is 30 miles away, and 20 dB puts it 100 miles away.

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In either case, you’ve spent a fat chunk of change buying a nice radio with a crappy (that’s a technical term) antenna. Just for bouncing around the airport, that may not seem like a big deal, but if you are actually trying to communicate air to ground at a fly-in or during an emergency, you need something a bit better.

The male pin is soldered to the coax center conductor. Below it is about 50 mils of center conductor insulation, then the coax braid bent back around the braid grommet, then the rubber compression gasket, the ring washer and the gland nut. You can couple two male BNC connectors together with what is called a BNC barrel. You use one of these between the BNC male connector on the antenna and the BNC male connector on the long coax cable.

We will do a couple of designs, neither of which will cost you more than a Starbucks Caramel Macchiato Grande ($10). Both of them will be verticals because verticals are what we use for com in the aircraft band. One of them will truly be an in-your-pocket portable, and the other one will fit easily into the luggage space of a C-150 (or tucked into the back seat of a Long-EZ).

We will also make provisions to haul them up into a handy antenna “mast” like a tree branch or roof overhang. It is true that getting an antenna up into the air and away from the ground makes them perform a whole lot better. So, without further ado, let’s get onto it.

A drawing of the small RG-174 coax cable antenna.

The first thing we need to do is pick our operating frequency since that will determine how long to make the antenna metal elements. I’m going to pick the frequency 122.725 MHz because that’s the frequency of my home drome: Nevada County Intentional Airpatch Unicom. Your frequency might be different, but the difference will be in fractions of an inch from one end of the aircraft com band to the other. And I’m going to do it in plain old eighth-grade arithmetic and not the algebraic notation stuff. All you need is a simple four-function calculator.

This is my workhorse for analyzing antennas. It was purchased in 1995 for $150 and has analyzed over 200 antennas here at the RST labs. It is my right hand for antennas.

Here we go. The speed of light is 186,282 miles per second. 122.725 MHz can be written 122,725,000 cycles per second. Wavelength is speed divided by cycles, or 186,282 divided by 122,725,000 or 0.00151788 miles. Times 5280 feet per mile gives us 8.0144 feet. Times 12 gives us 96.17 inches. See? That wasn’t all that hard, was it? Now, antennas are built on the “quarter wave” principle, so a quarter wave is 96.17 inches divided by 4, or 24.04 inches.

Permit me to add some black magic. For half a dozen reasons, antenna designers start off with a 90% quarter wave when using thin (#10 or smaller) wire. If this magazine were 200 pages long, I’d be glad to explain this phenomenon in excruciating detail, but if you will allow me to hand wave it, this means antenna elements 24.04 times .90 or 21.6 inches long. That’s where we will start. See the images for my antenna tuning tool.

The coax vertical (right). It is nearly invisible both in the photo and in actual use. It makes a great stealth antenna.

Since this first antenna is going to be shirt-pocket small, we are going to make it out of coax cable shirt-pocket small: RG-174, my coax of choice for anything below 1000 MHz and 10 feet of run. At the com band frequencies it has a loss of 1.5 dB per 10 feet, or a power loss of about 20% for 10 feet. If this is not acceptable, you can use RG-58 with a loss of 10% for 10 feet. Same construction technique.

I’m going to take a length of RG-174 coax 72 inches long and a piece of RG-58 24 inches long. Strip 22 inches of the outer black insulation off of the RG-174. Strip all of the outer black insulation off of the RG-58. Cut all but 1 inch of the braid at the remaining black RG-174 insulation. Peel all of the braid from the RG-58, slip it over the black insulation of the RG-174 and solder it to the short 1 inch of braid remaining on the RG-174. Solder the two braids together.

Install a BNC connector onto the 2 feet of coax remaining and tune as described in the second to last paragraph. You can attach the tuned RG-58 antenna braid to the RG-174 black insulation by shrink sleeving or the homebuilder’s perennial favorite adhesive—hot glue.

The advantage to this antenna is that you don’t have to run the coax away from the antenna for a quarter of a wave like you do with a vertical dipole. The disadvantage is that it is relatively fragile and somewhat susceptible to contamination with water and other pollutants over time. It is also a bit more sensitive to being located close to the earth—it likes to be high up.

The coax at the T fitting on the PVC vertical. The braid is soldered to the copper wire element on the left, and the center conductor is soldered to the element on the right.

The alternative is a true vertical dipole with an arm that keeps the feed line away from the antenna. This one is made out of ¾-inch PVC water pipe and Romex house wiring for elements. The cost of making this antenna is 10 times that of the shirt-pocket antenna: $0.10 for the shirt antenna and almost (gasp) a dollar for this one.

The PVC vertical. Cyndi is holding it horizontally for the picture, but in use it must be held vertical. Note the short stub of coax coming out of the arm by Cyndi’s right hand.

Please note…I’ve shown this design on PVC water pipe, which is what I had in the lab to work with. Cardboard? Plastics? Plywood? Any insulator? Not a problem and no modification necessary.

As with the shirt-pocket antenna, the adhesive of choice to keep the wire onto the surface of the water pipe is hot glue, and the sealant to keep the coax from becoming contaminated is the same hot glue.

The antenna resonance tool for wire antennas (left). Make it too long and cut off what you don’t need. I leave a short stub of coax on my portable antennas, then add a coax cable with connectors. My cables range in length from a couple of feet to a hundred feet (right). If you try to cut the coax on the antenna exactly to your current need, you will have to make another antenna for the next job. It’s much easier to make a coax cable than a whole new antenna.

Both antennas will work quite well with all of the elements at 21.5 inches long, but if you can get your hands on a VSWR meter, you can prune the antennas to precise length with the tuning tool shown in the images.

There you go, folks, two highly portable antennas for less than $2. Not a bad deal. See you next month, and until then…Stay tuned…

Photos: Jim and Cyndi Weir.

2 COMMENTS

  1. Jim’

    A VSWR meter is fine but at the moment You can an antenna analyser with a range till 1000 MHz for less than 50 $

    Eddy

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