Brainerd Area Amateur Radio Club, Inc.

"Ham Tips"    Temporary 450 MHz Antenna by Mark WØMH

This is a custom temporary 450 MHz antenna that I built in 30 minutes while on the job as a Radio Broadcast Engineer recently.  The antenna was needed as a quick replacement for a commercial antenna that had failed.  A day-long broadcast was planned at KIKV Radio in Alexandria, MN, and they could not do it without some kind of a receiver antenna at the studio, which was four miles from the remote broadcast site in town.  Their 450 MHz remote pick-up link is very similar to our ham radio equipment.  It uses +/- 5 KHz frequency modulation and runs about 20 watts. 

I first went to the local Radio Shack store to see if they had a scanner antenna, but they no longer stock them.  Then, I asked for an SO-239 connector.  That is a chassis-mounted connector that you plug a standard PL-259 UHF connector into.  Radio Shack did not have one of those either.  After rummaging around in my service truck, I found an SO-239 connector, but it did not have the traditional 4 holes so that 4-40 or 6-32 hardware could hold it to a panel.  Instead, it required a single hole and was designed to hold itself to a panel with a large nut.  That complicated things a bit, but did not prohibit the antenna from being built. 

Using the formula, 983 divided by the frequency in MHz, I determined the length of a full-wave at their frequency.  Quartering that gave me the length required to build the antenna pictured in this article.  It is a quarter-wave ground plane so the element lengths should be about 6.5 inches.  As you can see, an RG-8A/U cable is connected to a right-angle UHF adapter, which then plugs into the bottom of an SO-239 connector. 

Bare #12 copper wires are soldered onto the connector.  The four radials are a 1/4 wavelength each and droop down at about 30 degrees from the horizon, which gives a better match to the line.  Originally, I cut the center/vertical wire a little long in case the calculations were not correct.  Well, the calculations were correct and I shortened it to give the lowest VSWR at the frequency of interest.  As an added touch, I made the wires long enough so the ends could be folded back to reduce the likelihood of injury, especially eye injury,  when working around the antenna. 

For me, this was a temporary antenna, which was not rugged enough to handle a bad weather.  Also, it was not treated to keep water out.  The antenna was mounted to a flag pole and temporarily attached 20 feet up on a tower.  A month later, it was replaced with a new commercial antenna. 

Stop in again sometime.  I'll leave the soldering iron on for you.  Mark WØMH

Return to the Ham Tips page  

  Return to BAARC Main Page