How Will VHF Ham Radio Antenna Perform on UHF?

How Will VHF Ham Radio Antenna Perform on UHF?

It is well known among Ham Radio operators that a 7 MHz dipole antenna will work at 21 MHz as it is an odd number multiple, the third harmonic. We also know that it will not perform well at 14 MHz being an even number harmonic. What about VHF antenna on UHF? Typical UHF Ham Radio frequency of 435 MHz is the third harmonic of 145 MHz Ham Radio frequency. Some of my ham freinds told me that my VHF antenna can also be used at UHF. Another friend told me that even though it is third harmonic and SWR will be fine, performance will be poor at UHF. So I decided to test it out. My VHF antenna is a commercial, stacked two element 5/8 λ Vertical Antenna with a phasing coil in between. We do not have a UHF repeater in our region, though I can access a few local repeaters on VHF.

Went ahead and tested SWR using my cross needle SWR meter on lowest power. It gave an SWR of 1.1:1 on VHF while there was a sharp rise to 2.25:1 at UHF. Again I checked with multiple ham friends who told me that I could still work for short periods on UHF at low power. As a first step I asked my ham friend who is at about 10 km aerial distance. He had a ground plane antenna on UHF, but I could not hear anything on the UHF channel at all. As a second step, I established VHF contact with a local ham friend who is just 1 km aerial distance and fixed up a schedule for testing. This time I could hear his transmission on UHF, with low signal strength. We could establish two way contact on UHF on low power, my first ever QSO on UHF! He was using a hand-held radio. After a short QSO, I signed off, promising to come back later with a UHF antenna. So that will be my next project, to homebrew a UHF antenna!