March 2025 - Maple Grove Radio Club

MGRC Mission Statement

Maple Grove Radio Club provides education to current and future amateur radio operators, with the goal of serving the public. We are a reliable communications resource for events and emergencies in Maple Grove and surrounding communities.

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Zach, KØZTW

Midwinter Madness Saturday March 22nd

CQ SSB 160 Meters Contest

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Monthly Meeting

TALARC The American Legion Amateur Radio Club

Club Calendar of Events

Special Article: Dipole Antennae and Impedance

This past week, a group of people from the Maple Grove Radio Club met for a technical discussion on the topic of the Dipole antennae and its impedance testing/measurement

A dipole is a very simple, very effective and popular type of antennae all by itself, moreso, is also the foundation or basis of so many of our common antennae. Just one example is the Yagi type. A Yagi basically consists of a dipole antenna surrounded by other elements called reflectors and directors. A reflector is mounted, geometrically, in parallel to the rear of the Dipole element but is longer. Conversely, director element(s) are mounted geometrically in parallel to the front of the Dipole element but are shorter. Typically, there are several directors and one reflector. As their name implies, the directors help improve reception from the front and the reflector helps from the back. Viewing this antenna from the top, the Yagi takes the shape of a trapezoid.

A Dipole antenna has several advantages. The first is its basic simplicity, made of common electrical wire, very low cost, comparatively and easy to tune with high power capabilities. A popular length is ½ wavelength for a chosen operating band (¼ wavelength each side) and fed by a coaxial cable in its mid-point, meaning that it is a balanced antenna being fed by an unbalanced cable. E.g. A 20-meter dipole is 16’ for each element or 32’ total length, approximately. They are also directional with maximum radiation lobe perpendicular to the wire axis. See figure 2-13 below.

When choosing the wire gauge, select a stranded type of copper and a larger diameter. For starters, try 16 AWG or 14 AWG. Keep in mind the “skin effect” at higher radio frequencies, meaning stranded wire has a greater surface area. Having a larger wire diameter influences its reactance factor, i.e. ratio of R/X, hence, the “Q” or quality factor of the antenna. A higher “Q” factor will result in a narrower bandwidth, whereas a lower “Q” will result in a wider bandwidth. A wider bandwidth can be an advantage by reducing the need for transmitter re-tuning when navigating from the lower part of a band to the upper part of a band.

When choosing the height of the antenna, it should not be done at random. According to the A.R.R.L. antenna manual, graphs that show the radiation “take-off” angle, height is a leading influencer. Graphs show “lobes” which are with respect to the horizon. There are several elevations that can be chosen to give a desired “take-off” angle. Just for conversation, a sweet spot would be at ½ wavelength or slightly higher. A lower height will push the “take-off” envelope to radiate more vertically and influences or lowers its impedance. Choosing a higher elevation will give differing radiation patterns. See figure 2-29 below, graph one of six.

The impedance of this antennae topology is 73 resistive ohms with 43 ohms of inductive reactance or in rectangular format (73+j43). Adding these vectors, the resultant impedance is 84 Ohms in “free space” and at one wavelength in height. “Free space” is an unrealistic condition and in the real world, trees, buildings, surroundings and even the ground all have an effect or dramatic effect on the impedance. Measuring actual systems, the impedance is expected to be many ohms less. Realizing, maximum power transfer can only be achieved when impedance is equal, transmitter to antenna. If an antenna impedance is less than or greater than the source or transmitter, power transfer will be reduced but is typically not a concern and can be adjusted or corrected to minimize standing waves via antenna tuner, if necessary.

For class, we built a very simple 2-meter dipole. It consisted of a PL-259 connector, 8’ of coaxial cable, CPVC tubing and a short piece of hookup wire. Testing and evaluation, this antenna performed very nicely and was hardly over $5.00 in materials. One test shows 26 dB Return Loss value, which is an excellent S.W.R. value. See “2-meter dipole” oscillograph below.

Interesting Media and Links

ARRL NEWS

Upcoming State, Section, and Division Conventions

• February 22 | HAM-CON, hosting the ARRL Vermont State Convention, Colchester, Vermont
• February 28 - March 1 | BirmingHamfest, hosting the ARRL Alabama Section Convention, Trussville, Alabama
• March 7 - 8 | Greater Houston HamFest, hosting the ARRL Texas State Convention, Rosenberg, Texas
• March 15 | 69th Annual St. Patrick’s Day Hamfest, hosting the ARRL West Texas Section Convention, Midland, Texas
• March 29 | Central Dakota Amateur Radio Club Hamfest, hosting the ARRL North Dakota State Convention, Bismarck, North Dakota
• April 5 | RARSfest, hosting the ARRL North Carolina State Convention, Raleigh, North Carolina
• April 11-12 | Green Country Hamfest, hosting the ARRL Oklahoma State Convention, Claremore, Oklahoma.
• April 25 - 27 | Big Bear HamEscape, hosting the ARRL Orange Section Convention, Big Bear Lake, California
• April 26 | Delmarva Amateur Radio and Electronics Expo, host the ARRL Delaware State Convention, Georgetown, Delaware
• April 26 | Spokane Hamfest, ARRL Washington State Convention, Spokane Valley, Washington

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