Tuesday, November 20, 2018
Happy Thanksgiving, and a Minor Antenna Victory
Here's wishing you all a very Happy Thanksgiving. We'll be up in Wyoming for the day, and it looks like the weather will be gorgeous.
On the antenna front, I've been tinkering with the BuddiPole I put up a while back. When I first installed it, I went with a what I thought was an acceptable combination for a center frequency of 14.2 MHz. I used a 16.5' vertical radiator, with some 25' radials. Typically, the radials and radiator are close to the same length, and if you get the lengths correct, the antenna will be Resonant, meaning that the Inductive Reactance and the Capacitive Reactance are equal, cancelling each other out, and making the Radiation Resistance the dominant factor.
Unfortunately, I went with longer radials than required, which threw my length calculations out the window. The resonant frequency was almost 3 MHz low!
Why did I go with longer radials? WELL......I was kind of stuck on what the manufacturer of my SGC-230 remote coupler had recommended. I forgot (or didn't realize at the time) that having the coupler/matching unit at the antenna feedpoint is a different animal than having the matching unit at the radio, with a length of cable between the matching unit and the feedpoint.
And I didn't realize this interaction until I put my antenna analyzer on the cable, and swept the antenna/feedline over the frequency range I was interested in.
The first thing I did was to shorten the vertical radiator, at first by a few inches, and then by a couple of feet. This brought the resonant freq up a little over 1 MHz, still not close. Then I shortened the radials about three feet, and the resonant freq shot up past where I was shooting for. Lengthening the vertical whip about 16" put the resonant point right at 14.20 MHz, which is what I was shooting for.
Getting the antenna resonant on my frequency of interest has a minor effect on receiving, but it makes a huge difference in how effectively the antenna radiates the power from the transmitter.
By making the antenna resonant, the reactive part of the antenna impedance is minimized, and the Radiation Resistance becomes dominant, and it's the Radiation Resistance that actually couples the transmit power into the Aether.
The end result is people hear my signal better, and it's not as frustrating trying to make contacts.