Build Your Own Antenna – 1/4 Wave VHF Ground Plane Antenna

Recently I began delving into the world of amateur radio.  My interest in planespotting caused me to buy an analog scanner so I could listen to tower and aircraft communications, which is very convenient and immersive.  I can now plan a little and the advanced warning of an approaching aircraft gives me a chance to change lenses or camera settings based on what I wish to accomplish.  I reviewed the scanner earlier and you can see that review at http://www.localmeridian.com/2016/05/uniden-bearcat-bc125at-scanner-review/.

It seems that any involvement in amateur radio quickly turns to antennas.  I bought a high quality Diamond RH77CA antenna to replace the small rubber ducky that was included with my scanner.  The uptick in performance was very noticeable and works great around town or at my favorite spotting locations.  At home, I wanted a little more.  My local airport is 8.3 miles away and although I can pick-up many airborne aircraft quite well, I wanted a little more sensitivity so I could eavesdrop on tower communications.

Civil aviation operates their voice communications from 118 to 136 MHz, which is the VHF portion of the electromagnetic spectrum.  VHF require line of sight.  It works exceedingly well for its intended purpose, which is a clear line from the airport to the aircraft.  It doesn’t do well with obstructions such as those found in any good  size city.   Much can be accomplished with the right VHF antenna, but it can’t remove buildings and hills between the receiver and the source of the signal.  Armed with this knowledge, my goal was to extract every ounce of performance I could.

I did have a limitation.  I live in a townhome.  I can’t install antennas outside, but I do have access to my attic so whatever I built would have to fit into the attic above my office.  That way I can drop the down coax through the wall and have my scanner sit on my desk.  The run for the coax would be less than 15 feet, but I purchased a 25 ft length to make sure I had some flexibility in location.

I had expressed my desire for a base antenna on radioreference.com, a well-known forum for amateur radio enthusiast.   I received a lot of great information and recommendations, which included the suggestion that I build a simple ¼ wave ground plane antenna.  Sure, easy for them to say, I thought.  What the heck is a ground plane antenna?  Well, as it turns out it’s a rather simple design using a vertical antenna element cut to ¼ of the wavelength of the desired frequency and 4 radial elements that act as a reflector.  I built mine in about an hour for less than $40, and you can do it for even less.  It works great and you can build it too.  Let’s start with a list of parts.  You’ll need the following:

1ea       SO-239 UHF mount
1ea       PL-259 to “F” female adapter
1ea       25’ of RG-6U Coaxial Cable (the type used for cable TV)
1ea       “F” to BNC Male adapter (not needed if you opt for the pigtail)
11 ft     #12 Solid Copper Wire
1ea       Aluminum project box
4ea       12-14 AWG ring terminal connectors
8ea       6-32 x ½” screws, nuts and washers
1ea       Project Box (Optional)
1ea       18″ RG-174 pigtail with “F” female and BNC male connectors (optional)

The basis of the entire antenna is the SO-239 mount that has a center element that’s hollow, to which the appropriate length #12 solid copper wire can be soldered.    This becomes the antenna’s vertical element.  First you should mount the SO-239 to a project box or aluminum plate with screws or use the mount’s body to support the radials.  I chose the project box because I liked the cleaner look.  It also gave me more area to mount the radials.  Furthermore, you can secure the box to a pole with a clamp.

To determine the wavelength (λ), divide the speed of light in meters per second (c) by the frequency in hertz (ƒ).  As an example, I’ll calculate the full wave for 127 MHz, which is the center of the voice portion of the air band (118-136 MHz).  Note: the quotient of this formula is in meters.

2.361 = 299792458 / 127000000

or

92.95 inches

This is a quarter wave antenna, so the elements are ¼ of the full wave or (23.23 inches – 23 ¼ is close enough).  The vertical element and radials are the same length.  Some will tell you that the radials should be 5% longer than the vertical.  I tried it both ways and did not notice a change in performance.  You can experiment and see what works best for you.  For the radials, I soldered them to ring terminal connectors.  After drilling some pilot holes in each corner of the project box I mounted the radials using 6-32 x 1/2  screws and secured them with washers and nuts.   The radials then must be bent downward at a 45° angle, which provides  50 ohms of impendence.  That’s really about it.  Now connect your PL-259 to “F” female adapter to the bottom of the SO-259 mount.  You can then attach the “F” male connector from your RG-6 Coax.   On the receiver end, you will need an “F” female to BNC male adapter.  I used an 18” pigtail, which was an “F” female on one end and BNC male on the other.   The pigtail used a much softer and more flexible RG-174 cable, which permitted me to attach my handheld scanner to the feed-line and still have it stand up.  Otherwise the heavy and less flexible RG-6 would just knock it over.

So far, so good.  I’ve been using my project antenna for several days now and I’m quite pleased with it.  It’s more sensitive than the handheld antenna and provides strong signal reception.  As an experiment, I changed the radial element material from #12 to #8 solid copper wire.  This change is supposed to provide greater bandwidth.  If there is an improvement it’s subtle.  At worst, it’s the same as the #12.  Either way the antenna performs well and better yet, I built it myself.

It took me a while to find the appropriate information in the internet, which is why I wanted to consolidate it here.  This antenna design should work for nearly any frequency that would allow for element length supportable under their own weight.  That’s essentially VHF, UHF and shorter wavelengths.

I plane to build another antenna for use with an ADS-B ( Automatic Dependent Surveillance–Broadcast) receiver I plan to build as a Summer project with my 13 year-old using a Raspberry Pi.   At 1090 MHz, a 1/4 wave antenna will be tiny with a vertical element of a mere 2.7 inches.

Building an antenna can sound intimidating, but it’s pretty easy once you dive into it.   If I can do it, so can you, so get out there and start assembling your own 1/4 wave VHF ground plane antenna.