(Continued from Part 1.)
A typical Trucker’s dual-mount CB antenna has 2 coaxes down to one connection to the radio. We want the signal from both antennas, unified exactly for a nice strong signal / in phase voltage pulse, into the radio. Because the antennas are on the sides, like mirror mounts, the strongest, best signal (in phase), will come from the front and rear of the truck, while signals from either side, will Cancel each other out (a little bit).
The same thing happens with our Vertical antennas, 2-3-4-5-6-7-8 of them. We want to get rid of the noise and interference to hear cleanly in one direction. These verticals can be arranged in a circle, straight line, triangle, all kinds of shapes. While most people *think* the signal is being concentrated (it is), the biggest advantage is Nulling out the unwanted junk. By combining the antennas, switching in shorter / longer / equal coaxes which tie them together, we can change the shape of our broadcast / receive signal. An example…
At a recent Hamfest, we set up three 20-meter vertical antennas. Everyone who commented, said, “That’s not gonna pick up anything but that noisy transformer”, some 100 feet away. True, you could hear it crackling, like it has a corroded connector. With the right switching, we had 10 different receive / broadcast patterns. Omnidirectional (all directions), Broadside (3 directions), 2 Inline (6 directions). Without looking at the controller, we just rotated the pattern, and in 2 of the positions, the transformer was totally silent. For the rest of the day, we made contact after contact, with ease, the transformer still humming and buzzing away. It’s why the NULL is so important! That could spare your relationship with a neighbor who has a cheap TV, lacking proper filtering. By the way, every electronic device is required to be properly filtered to avoid cranking out electrical noise or receiving it outside of its intended frequencies! If the neighbor says you’re bleeding over their TV set, and your radio is ‘stock’, as the manufacturer designed and built – it’s their problem, not yours. Still, I’d offer them some help fixing, grounding, etc.
That’s the fun of ham radio for us, we love building and experimenting, with the minimal tools at our disposal.
What is a “Dipole” Antenna? You’ve probably seen the parallel wires that form a “T” and hook up to a FM radio. We can do the same thing, with a ham band antenna – they get up to 360′ long on 160 meters, but pretty short as the bands / frequency go up… Again, we can combine signals to a single feed point, and tune the antenna to maximize certain frequencies on desired Bands. A Fan Dipole is one such antenna, stacking 2-3-4-5 horizontal wires, each cut for a different band, to maximize desired signal and minimize interference.
What’s interesting about a Dipole antenna, but similar to all antennas, they are Deaf, or NULL when looking at the end, down the length of the wire(s). Strongest signal is broadside – the receive / transmit pattern looks like a figure-8, at a good elevation. The lower the antenna, the more energy is focused directly UP instead of OUT sideways. Dipoles are generally cut as 2x 1/4wl antennas, or 1/2wl overall. Dipoles, like Verticals and Long wires are easy and quick to make.
A note on materials ad junctions / connectors. Anytime dissimilar metals are stuck together, like Stainless Steel (makes a poor antenna) to copper, there are inherent losses. Each joint, held together by soldier, crimped, formed, stainless steel screws, adds to the loss. The ideal antenna is one continuous length of wire, pipe, whatever. There is a thing we call “Skin Effect”. The signal flows over the top layers of the metal, like on the surface. Lower frequencies go deepest into the metal, highest frequencies more on the surface. Before someone suggests, “Oh, my, those losses are so minimal!!!” In 2013, we won 3 divisions in the Virginia QSO party (a contest) with an antenna I built for mobile, cut to tune on a single frequency. After making multiple contacts with QRP (low power, <5 watts), another guy following about a mile or two behind, broke in, “Who are you talking to? I can’t even hear them!” He had a $500 commercial antenna, a big one which covered ALL the Bands … poorly.
If you can’t hear ’em, you can’t work ’em! All of these expensive antennas are built by fellow hams, some are good, most are just made to attract buyers. The real performers are built by people like yourselves, who take a little time, understand ‘how it works’, and build accordingly. The test tool we started with was a $150 Standing Wave Ratio (SWR) meter (a waste of time and money, plotting all those data points of graph paper), and then we purchased a $500 AIM UHF antenna analyzer (hooks up to a computer). The time-testing and experimenting was compressed from weeks to hours. It did all the graphing, showed us Resonance, Impedence, SWR at the same time, and best of all, we could save and load multiple scans over the top of each other. For the experimenter, the learning curve was immediate, ‘If I do THIS, I see THAT affects this and that… Okay, let’s try this instead…’.
So, each antenna typically has a single ‘sweet spot’, a Resonant frequency, what it’s tuned to, hears the best. SWR is a funny thing, Standing Wave Ratio – the amount of signal reflected back from the antenna towards your Transmitting Radio (Transceiver). The Transmitter likes to see a 50 ohm load, but also in phase to the antenna. If we cut the Coax at random lengths, it changes the Impedance the Radio sees, regardless of the actual Impedance of the antenna. That translates into LOSS of signal, or poorer Signal to Noise. We want the signal to be much louder / stronger than the noise, static, snap-crackle-pops.
The ideal SWR is 1.0:1, we can build a good antenna, couple it to a frequency-insensitive Coax, and get performance. The goal is to make the Coax (transmission line) invisible between the Radio and Antenna… then we can use 75 ohm TV coax, or 50 ohm CB/Ham coax – just gotta cut it to the length that favors our Frequency / Band. SHORT is not better, Tuned is better. Another example.
A guy from the UK is calling CQ (hello, is anyone out there?), on 80 meters. I answer him with 4 watts, running to 2x 80 meter verticals, phased and pointed right at him. He’s stunned. I confirm, via my SWR/Watt meter (an expensive digital unit), just under 4 watts. “Sounds like several hundred – what you got there?” “Two 80 meter verticals, phased, aimed right towards the Europe and United Kingdom.” We talked for a while, but I couldn’t hear anyone else north or south of me, the radiation pattern was tight. The antenna spacing, both antennas were right on frequency, the coax was cut for 1/2wl of the frequency – near perfect match, all my power was aimed right at him, all 4 watts. The point is, give yourself every advantage you can, a little knowledge & experimenting can give you a superior system.
What is all this darn Black, sparkly stuff everywhere around the house… I wondered. While hooking up an extension cord for the parents motor home, I pulled out the VOM (volt-ohm-meter), and checked for voltage. One of the leads fell to the ground, the meter immediately showed near 100 volts! The tip was just in the dust, this black dust… Grass didn’t like growing in it, but, it makes an excellent ‘Ground Plane” (like a grounding system) for Vertical Antennas. Magnetite! A ham’s dream come true! It also explained the frequent lightning strikes. This altered or modified a lot of plans – do I fight this terrible ground, or use it to some advantage? I love Vertical Antennas, my ground loves Vertical antennas. Since the trees around the house are +400’ away, that’s gonna be big wire, lots of weight, and with the Ice Storms, lots of broken wire.
“What works for me, might not work for you, at your location!” No antenna seems ‘perfect’ for every location.
Early on, we built OMTA 40-20-15 meter Vertical Antennas. 3 Bands, a rather unique arrangement. Just one of the verticals are actually tied to the Coax, the other 2 verticals are free floating, but spaced at the optimal distance to ‘re-radiate’ their signal into the other antenna. These are tremendous performers for us. We’ve talked around the Globe on all 3 Bands, Thousands of contacts. All Aluminum pipe. They work Great at my QTH (home location), but utterly suck at another location – didn’t make sense… I’ve got good ground for antennas, he has Rock… everywhere. So bad, we gave up on the Vertical, and tried a 160 meter Horizontal Loop, also known as a “Cloud Burner”. “Hey man, you gotta try this!!!” I did, and it sucked at my QTH (my house). To help him with correct matching, we had to build a Balun, a RF Transformer. At the time, I had a bunch of 10 gauge Silver wire, with a Teflon insulation – it came from a MRI Imaging machine install. This transformer weighed close to 10lbs, and had to hang in the air. He liked to run ‘lots of power’. One fateful night, he calls me on the phone, “It blew up! The Balun blew up! I looked out the window, noticed a strange red/orange glow while transmitting (excess power), and “BANG!”” I thought that Balun / Transformer could handle 5Kw (5000 watts), sure it could, when ‘on frequency’. Closer examination, the Loop broke, one end hit the ground, nothing was working as intended, he was lucky his massive Amplifier was still intact.
The lesson: There are limits to everyone and everything, with the exception of Jesus Christ.
Okay, so that’s the idea, how about the nuts and bolts, how to build something inexpensive for my CB radio, that performs like a $250 antenna…? Yes, we can do that! The main point to remember, Build for Resonance, SWR is the match (can be done later), and use Coax that’s invisible to the system, (Cable TV coax has some real advantages!)
(To be continued tomorrow, in Part 3.)
