We learned to calculate wave length from frequency for the Technician ticket.  Or at least, we memorized the questions and answers for the test. 

At the time, I thought it was an interesting tidbit of information but not critical to what I wanted.  After all, we set the transceiver to a frequency, not a wavelength.  However, thought it handy to understand when Hams use 30 meters and 10 Mhz interchangeably as they’re prone to do.

The formula to convert frequency to wavelength is 300/Frequency in MHz. 

This lay dormant in my mind until I really started operating HF portable.  I made and modified a few wire antenna’s and calculated the length of wire.  I’d do the math, cut the wire long, and twist the folded back wire. Done and done!

That is until I started band hopping with a telescoping vertical antenna.  The 17ft, quarter wave telescoping antenna is near-resonant on 20m, but to change to 17m, I had to shorten it.  I’d shorten the telescoping elements some, walk back and read the analyzer, then walk back and forth between the analyzer and antenna until I found the lowest SWR.  If I wanted to move to another band, I’d start the process over again.  To me, this was five minutes I could…. not be walking. I knew I could mark the vertical with a sharpie or layout die, but it doesn’t last. I could etch it or mark it with a file.  But I didn’t – every location and every condition is slightly different.  Soil conductivity plays a role, so soil type and moisture content can make markings misleading to an optimal tune. So, I just got my steps in.

At some point, I decided to try calculating and it worked. 

• The antenna analyzer will tell me the resonant frequency of the antenna.
• I knew the frequency I wanted resonance.

Simplified, I knew where I was and I knew where I wanted to go.  It was just a matter of determining how far – how much radiator I needed to retract or extract.  The answer is in wavelength – to convert both the resonant frequency and the desired frequency to wavelength (in meters) and the difference between the two is how far I need to shrink or grow the antenna.

For example, I want to operate on 14.060MHz but the antenna analyzer tells me the antenna’s resonant frequency is 14.250MHz.

I want to play on a frequency lower than the resonant spot so I need a longer radiator. Longer for lower – shorter for higher.

Here’s How

I convert both frequencies to wave length.

300/14.060MHz = 21.34m

300/14.250MHz = 21.05m

I find the difference between the two 21.34m – 21.05m =.29m

Looks like I need to make my radiator .29m longer. 

But wait, 300/MHz calculates a full wavelength.  In my case, I’m using a quarter wave vertical antenna.  So, I need to go a quarter of .29m. 

.29m * .25 = 0.07m.  .07m is 7 centimeters. 

I need to lengthen my telescoping radiator by about 7 pinky fingernails.

If this doesn’t make sense, read it again and take a minute to get your head around it. 

Here’s the bulleted summary:

• Longer waves for lower frequency- shorter waves for higher frequencies.
• Convert both frequencies to wavelength.
• Subtract the bigger number from the smaller and adjust the length of your radiator.
• Longer for lower – shorter for higher.

Don’t forget the calculated results are for a full wavelength.  You’ll need divide by 2 for a half-wave antenna or by 4 for a quarter-wave.

Hope this speeds your portable setup,

TNX ES 73
KA5TXN
DitWit