I’ve recently been playing with a pair of low-power 440MHz UHF data transmitters (side note these are very cool and ultra cheap 100mW UHF radios) and needed to build a pair of 440 MHz directional yagi antenna to extend the range.
I stumbled upon this write up on Radio Reference about a guy who built a yagi antenna from old clothes hangers and a broom stick.
I started by plugging the frequency and length I wanted into a website developed for this specific purpose. It gave me all the numbers I needed to build my antenna: the length of each element, and the spacing between them.
I stopped by Menards to pick up two items, a 5/8″ thick dowel, and a 5 foot piece of 6 AWG wire which was on sale. I stopped by the welding supply store to pick up 1 lbs of aluminum rod.
Working with Mike ( KDØIQZ ) we used a drill press to create the holes for each element to pass through the dowel. We made sure to only drill partially through each side for the driven element (since it is cut in two).
Then I took a piece of RG58 coax and soldered the shield to one half of the driven element and the other half to the center conductor.
I went for a drive to see if I could measure the gain of the antenna. I configured a radio in my shack to act as a cross band repeater, it listened on one (VHF) frequency, and re-transmitted this on a UHF frequency that this antenna was configured to receive. Using SDR# I could measure the signal strength accurately.
First I used a 1/4 wave mag mount antenna on my car. I configured the SDR# for 8.7 dB of gain.
Then using the SDR# software I measured the signal strength. The peak was -29dB. After subtracting the 8.7dB of gain the application was telling me that the signal on the mag mount was -37.7dB
I pointed the yagi at my home and repeated the test. After turning down the gain on the receiver (the signal was overloading the SDR) I calculated the signal arriving at the yagi antenna.
With 0dB of gain SDR# measured the signal at the antenna as about -22.5dB
-37.7dB on the mag mount compared to -22.5dB on the yagi means the gain of the antenna over the other is about 15dB gain. Likely the mag mount antenna is such a poor 440 MHz antenna it has some negative gain. Therefore I think the test demonstrates the antenna I built comes pretty close to the calculated 13dBi of gain.
From Mount Mercy Hill I was able to make a transmission through the K0GH repeater in Coralville using only 100mW (low power) That is nearly 22 miles away (although it is a very good repeater)
Then I drove to the large hill at Kirkwood community college. I tried several repeaters, but using high power (3W ) I could get in to the WA0AUQ repeater in Muscatine!
That is nearly a 46 mile trip off a hand held radio! Pretty impressive for about $10 in materials to build an antenna.
Here’s a picture from Mike ( KDØIQZ ) testing the antenna near Menards on the SW side of Cedar Rapids.