I'm often asked how to match a quarter-wave vertical for 160 meters to a 50 ohm transmission line, what to do about lightning protection and even how to take RF measurements with powerful AM broadcast signals in the vicinity. This article will describe how to make a basic - but elegant - matching device for a quarter-wave vertical which does additional duty as a static bleed device and a surge arrestor with a lightning loop. I'll also explain how it's possible to take the antenna measurements needed to make the matching device and to tune the antenna correctly while in the presence of strong RF from nearby AM broadcast stations.
Antenna Matching on 160 Meters
I've seen antenna matching solutions that range from just attaching the coax to the antenna and living with the result to adding a series capacitor to allow tuning the antenna over the band or even using an L network to match the antenna's approximately 30 ohms to the feed line's 50 ohms. Let's look at a very simple solution that I've found useful.
My own quarter-wave 160 meter vertical is sort of an inverted L. It rises up from ground level to 50 feet as a free-standing aluminum tube. At that point I've attached a #12 wire that slopes upward at a 45-degree angle to my 150 foot tower. The system has 64 buried radials. Fortunately we have excellent ground here. Due to the sloping wire the antenna's feed-point impedance is lower than the theoretical 36 ohms.
What we will attempt to do is tune this antenna for a 50 + j0 ohm resonance at 1.830 MHz using an LC network. For a capacitor we will just shorten the antenna length a small amount to create the necessary "phantom capacitance." I don't like to use series capacitors, since they're prone to fail at high currents and in lightning events. Then a single inductor across the feed point is all we need to match the antenna to the transmission line. This technique is a great way to handle this sort of matching situation.
Issues with Nearby AM Broadcast Station RF
There are many AM broadcasters in my area, and one station even operates on 1700 kHz! These stations place 10V peak-to-peak RF onto my antenna during the day and even more at night. Figure 1 shows how to attach a 'scope probe to the coax stub going the Heliax feed line. Figure 2 shows the scope reading at the unterminated end of 150 feet fo half-inch 50 ohm Heliax. Trying to take an accurate measurement using any of the impedance meters available to hams is impossible. They overload, and some will even blow their diode bridges, requiring a trip to the factory. The AIM 4170B analyzer we market at Array Solutions, www.arraysolutions.com, will not blow up, but simply hooking it to the feed line won't let you take any measurements in the presence of strong RF either. It does, however, have a neat feature that can be used take this measurement accurately, and we will use it to help us tune this antenna.
The 10V peak-to-peak RF represents 0.25W of power into 50 ohms. The AIM 4170B, on the other hand, puts out microwatts of RF to enable measurements. So, how can this device override the power that's showing up in the antenna system?
When we attach the AIM 4170B to this antenna and coax and do a scan from 1.5 to 2.5 MHz, we see the plot in Figure 3. The bold line above the X axis is VSWR. The lighter line highlighted with squares is resistance(R), while the lighter line highlighted with dots is reactance(X).
Due to the RF overload, the plot is full of noise and totally useless; we need to alter our measurement technique if we are to get accurate information to allow us to adjust this antenna. What we need is a good broadcast-band high-pass filter. W3NQN makes a superb filter for this purpose, and I connected it to the RF connector of the AIM 4170B analyzer. Before using it, however, it's necessary to null out its transfer function so the measurements we take are not affected by it.
High-order filters like these have phase shifts and other linear parameters due to their design. We must normalize them. To do this the AIM 4170B has a "custom calibration" feature. This is the "neat feature" I mentioned. The software leads you through a "super" calibration using the short, open and load technique through the filter over a limited frequency range of interest. To create a very accurate calibration table requires lots of sample points. I used 500 points of measurement. The software will create a very detailed calibration table that essentially moves the measurement point from the analyzer's RF connector to the input connector of the broadcast band high-pass filter.
For the complete version of this article, with corrections of the version published in the NCJ, view the pdf version.