Why is a 5/8-wavelength vertical antenna better than a 1/4-wavelength

Back to Basics

This 'Back to Basics' may be of particular interests to you off-roaders...

The Canadian Basic Question Banks asks:

B-006-10-4  Why is a 5/8-wavelength vertical antenna better than a 1/4-wavelength vertical antenna for VHF or UHF mobile operations?

A. A 5/8-wavelength antenna has less corona loss
B. A 5/8-wavelength antenna has more gain
C. 5/8-wavelength antenna is easier to install on a car
D. A 5/8-wavelength antenna can handle more power

An ordinary 1/4 λ (wavelength) vertical is smaller and resonant without any loading coil or matching network. What's the advantage to a 5/8 wavelength vertical? Why 5/8 in particular, and not something longer or shorter?

Indeed, why? A 5/8λ isn't resonant where a 1/4λ or 1/2λ would be.

The reason is the radiation pattern. The pattern for a 1/4λ monopole is essentially a doughnut, a pretty good pattern especially for a VHF antenna used primarily for local work. Extending the antenna changes the current distribution. This flattens out the pattern, removing power from the useless (for VHF purposes) vertical dimension and giving more horizontal gain and at a lower angle. See the illustration lower left from the late L. Cebik. The 5/8λ antenna focuses energy somewhat better towards the horizon (lower radiation angle) than a regular quarter-wave antenna.

Depending on the source, they will quote anywhere from 1 dB to 3 dB gain over the 1/2λ design [3dB is a doubling!]. There has also been some discussion that in some areas (urban and mountainous terrain) the lower angle of radiation is a detriment and a standard 1/4λ or 1/2λ antenna is to be favoured. 

So, why 5/8λ? Why not long longer? After all more gain is better right? Well, inspecting the figure below, you will notice the appearance of high angle lobes. As you lengthen the antenna past 5/8λ these lobes become more pronounced and break up the pattern in undesirable ways. Making it shorter maintains a good pattern, but the gain is less. So, 5/8λ is about optimal for this style of antenna.

You may have noticed a pattern developing here. A quarter wave ground plane antenna has a radiation pattern that produces maximum gain at about 25 degrees and a half wave antenna drops that angle to 20 degrees, and the 5/8 wave antenna further drops that angle to 16 degrees. So why not just keep extending the antenna out to one full wave? Well it would be nice if it worked but unfortunately the wave pattern begins to create very high angles of radiation beyond 5/8λ. 

So we've reached the maximum gain at this point and extending the antenna any further just reduces the gain where we want it (low angles). Of course if you are interested in very short skip, extending the antenna will produce nice gains over a dipole. 
All antenna lengths depend on various factors. Some of these factors are: 

• the height above ground;
• the diameter of the wire;
• nearby structures;
• the effects of other antennas in the area; and
• even the conductivity of the soil. 

If we calculate the length of a 5/8λ antenna for our SARC repeater (147.360 MHz) the formula is 178.308/147.36 which equals 1.21m (3.97 feet).

The answer to our question therefore is 2. A 5/8-wavelength antenna has more gain.

Our next Basic Course starts September 15th.

~ John VE7TI
   08-03

Surrey Celebrates Amateur Radio Week

Surrey City Council Recognizes
The Contribution Of Amateur Radio

Again this year Surrey City Council has proclaimed June 21 to 28th, the week leading up to Field Day, as 'Amateur Radio Week".

The Mayor and council have historically been strong supporters of our role in the emergency program and several VIPs from all levels of government and our served agencies  have visited our Field Day site in past years. Unfortunately, due to COVID-19, this year will be an exception but several members of Surrey Amateur Radio Communications (SARC) and the Surrey Emergency Program Amateur Radio (SEPAR) will be participating from individual home or field stations.

Since 1933, ham radio operators throughout North America and beyond have established temporary ham radio stations in public locations during an annual exercise called Field Day to showcase the science and skill of amateur radio. 

For more than 100 years, amateur radio — also called ham radio — has allowed people from all walks to experiment with electronics and communications techniques and provide a free public service to their communities during a disaster or emergency, all without needing a cellphone or the internet. Field Day demonstrates ham radio’s ability to work reliably under any conditions from almost any location and create an independent communications network. More than 35,000 people from thousands of locations participated last year in Field Day 2019 activities.

It’s easy for anyone to pick up a computer or smartphone, connect to the internet and communicate, with no knowledge of how the devices function or connect to each other, but if there’s an interruption of service or you’re out of range of a cell tower, you have no way to communicate. Ham radio functions completely independent of the internet or cellphone infrastructure, can send messages by voice, Morse code or digital mode, even email via radio. It can interface with tablets or smartphones, and can be set up almost anywhere in minutes. That’s the beauty of amateur radio during a communications outage.

The Surrey team placed first in their Canadian category for Field Day 2019 and have consistently placed in the top three stations in Canada, even achieving an all-time high score for using low power (less power than it takes to light a Christmas tree bulb) to contact other stations. In 2014 we even spoke to an astronaut aboard the International Space Station!

Field Day

In today’s electronic do-it-yourself environment, ham radio remains one of the best ways for people to learn about electronics, physics, meteorology and numerous other scientific disciplines. In addition, amateur radio is a huge asset to any community during disasters or emergencies if the standard communication infrastructure goes down. They also provide communications for relief agencies that do not have radio communications systems of their own and offer free communications services for non-profit events to practice these skills.

Surrey Fire Service Deputy Fire Chief Mark Griffioen, who coordinates emergency response for the City of Surrey adds: “Surrey Emergency Program Amateur Radio plays a supporting role in emergency communications during any major event for the city.  They train extensively and operate a communications room out of Fire Hall 1. During a major event, they provide enhanced life safety for the citizens of Surrey by providing a communication link with the Emergency Operations Centre.”

Stan Williams, President of Surrey Amateur Radio Communications, highlighted the value of Amateur Radio in its flexibility. “Amateur Radio is not centralized. There is no single point of failure; it does not rely on the Internet, a power utility or a communications company, cell towers or other infrastructure to provide service. It works when nothing else is available. We need nothing between us but air.” 

Anyone may become a licensed amateur radio operator. Worldwide there are more than 3 million licensed amateur radio licensees, as young as 9 and as old as 100. And with groups such as SEPAR and SARC, it’s easy for anybody to get involved right here in Surrey.  We offer regular amateur radio courses, classroom or on-line, see https://ve7sar.blogspot.com/2020/03/our-basic-amateur-radio-course.html 

To learn more about Amateur Radio in the Surrey Emergency program, watch the video at http://tinyurl.com/SeparsInfo.

For a look at our previous Field Day efforts visit our YouTube channel at https://tinyurl.com/SARC-Video

When All Else Fails, Ham Radio Works