Antenna – again

I first learned about antennas on my National Service signals course in the period December 1964 to February 1965 (that I mentioned in another post). I then did two further courses – my Regular Army Class 3 course followed by a Class 2 upgrade course some years later. In addition, I not only had to USE this knowledge in practical applications in the field but I also had to instruct on communications.

In order for people to understand that antenna size is dictated by the frequency that is being used we would do a lecture titled THE RELATIONSHIP BETWEEN FREQUENCY AND WAVELENGTH.

As part of this course we would do simple calculations at different frequencies. This was so that the class could have some basic understanding of why different antennas would be found in use with different types of radio according the frequency band in which that radio operated.

Without belabouring the point, the basic calculation for a WAVELENGTH is 300 divided by the frequency in MEGAHERTZ (MHz) that would give a measurement in METRES.

Very simply at a frequency of 10 MHz the full wavelength would be 30 metres.

Very simple diagram of an open dipole for HF use

At those low High Frequencies* (HF) we used what are termed HALF WAVE DIPOLES and we calculated a quarter wavelength because the radiating element of the antenna would be a half wavelength overall with a quarter wavelength on either side of the feed cable (see the simple diagram).

*(Sounds like a contradiction in terms but is correct and quite a long subject to address)

Now, in about 2005 t0 2010 I found myself regularly doing orientation lectures, where I worked in South Africa, for new staff so that they might gain SOME degree of understanding of two-way radio industry equipment.

I was doing my calculation (which we in Rhodesia had shortened somewhat) and I told the class that to calculate a quarter wavelength they needed to use the standard of 75 over the frequency in MHz – writing it out and showing the result for 10MHz as 7.5 metres. Before I could finish a radio technician (should I use that term advisedly?) who was sitting in “just for interest” interjected that that formula was WRONG.

I invited him to call out what I should do and he said that I needed to use 300 over the frequency and then divide by 4 and as I followed his instruction and it became obvious that this longer method was going to give the same result I noticed the man heading for the door – while the people in the class started to smile.

“Where did you learn that?”, about all manner of communications-related matters was a question I was quite used to and always delighted in replying “In the Army, in Rhodesia, in 1964” which by then was 40-plus years previously.