Antenna Considerations

 

Because you are tying a plastic insulator to the end of the antenna leads to connect to paracord, should you add extra length to the legs of your dipole? 

When tuning a multiband HF antenna should you start by adjusting the length of the lowest received frequency band, or the highest received frequency band? Tune the lowest band first (longest element 

What is the purpose of adding a balun at the feed point of a dipole antenna? The main purpose of adding a balun (short for balanced-to-unbalanced transformer) at the feed point of a dipole antenna is to match the balanced antenna to an unbalanced transmission line (like coaxial cable) and reduce unwanted currents.

What are the expected input impedances of half wave dipole antennas? A center-fed half-wave dipole is usually close to 70–75 Ω, making it a good match to 50 Ω coax, especially if a 1:1 balun is used to reduce mismatch effects.

What is a unun? A unun is just an unbalanced-to-unbalanced transformer, typically used to feed non-resonant or high-impedance unbalanced antennas with coax, whereas a balun is for balanced antennas.

When is a unun used instead of a balun?

·  1:1 – isolates coax shield from antenna, no impedance transformation

·  4:1 – matches 200 Ω feed point to 50 Ω coax

·  9:1 – matches ~450 Ω feed point to 50 Ω coax (common for end-fed half-wave antennas)

What is the function of an Ugly Balun, or 1:1 air coil? t’s typically a 1:1 current balun made from coax wound into a coil or bundle. The coil acts as an inductive choke for RF currents on the outside of the coax. a 1:1 air-core coil using bare wire or coax, which functions exactly the same way: it creates a choke to common-mode current.

How does the performance of a horizontally mounted half wave dipole change when you install it as an inverted vee? The inverted vee has the additional characteristic that it is far more omni-directional than the dipole.

Why might the inverted vee be preferred to the dipole? A vee has a smaller bandwidth than a dipole. A 120 degree vee has a wider bandwidth than the 90 degree vee, but less than the 180 degree dipole. If you are looking for a general purpose antenna, with an omni-directional pattern, with a very good match to a 50 Ohm transmission line, the inverted vee may be a good choice. There is no magic to the antenna, and maximum gain will usually suffer a little compared to a flat dipole. Then again, the more rounded pattern will provide more uniform coverage and expectation.

Which antenna presents a better match to a 50ohm feed line, a dipole or an inverted vee? The inverted vee makes a better match to the typical 50 Ohm feed line.  The resonant half wavelength dipole has an impedance of approximately 72 Ohms. The word resonant indicates that the feed point reactance is 0 Ohms. The impedance is composed of pure resistance. In other words, the impedance of the resonant dipole, expressed as a complex number, is 72 + j0 Ohms. In order to have a resistance near 72 Ohms, the dipole cannot be too close to the ground.

How is the impedance of a dipole antenna affected by height above ground? As the dipole is brought closer to the ground, the impedance will drop.

The vee has a smaller bandwidth for a constant SWR

Even though this dipole is resonant, its SWR in a 50 Ohm system is not 1.0. Its SWR is 72/50 = 1.44

To create an inverted vee from a dipole, we simply have to lower each end in equal amounts. As you lower the ends the feed point resistance drops. When we reach an angle of approximately 90 degrees, when the ends of the vee are not too far above the ground, the feed point impedance of the resonant vee can end up very close to 50 + j0 Ohms. This is now a perfect match to a 50 Ohm transmission line. The SWR at resonance is 1.0.

So, the typical vee is simpler to support, and provides a nearly perfect match to a 50 Ohm transmission line.

The inverted vee does give up 3 dB at high take-off angles. Other than the gain difference, at high angles, both antennas have basically round patterns. Due to the relatively low height above ground, both antennas shoot straight up. At lower take-off angles, however, the inverted vee maintains a uniform azimuth pattern, whereas the dipole becomes increasingly bi-directional.

Giving up some gain with the vee is usually not much of a problem when making NVIS (near vertical incidence skywave) local contacts. A bit more bothersome is the fact that the vee has a smaller bandwidth for a constant SWR

In order to achieve the largest possible bandwidth with the dipole, it is necessary to match it to a 75 Ohm transmission line. This is because the dipole impedance at resonance is close to 72 Ohms, which is much closer to 75 Ohms, as opposed to 50 Ohms. When the dipole is driven with a 50-ohm line, its 2:1 SWR bandwidth is almost identical to the vee (150 KHz).