43′ Vertical Final Tuning

 

43′ Vertical Final Tuning

Filed in Antennas on Jun.28, 2011

After acquiring an Antenna Analyzer (MFJ-269B), I set the matching network on the bench and began playing with it.  I had a theory that I could simulate the antenna load at various frequencies by placing a capacitor in series with a resistor of various values for the different frequencies.  Then by attaching that to the matching network where the antenna would attach, I could pre-tune the matching network without having to erect the antenna in my back yard.

After some research on the AD5X site (What a great resource), I found that a vertical has a pretty consistant capacitance to ground that is determined by the formula:

Cpf = 3.5pf/ft X Actual length

So for a 43 foot vertical, it should have a capacitance of about (3.5 x 43) = 150.5 pf.

Now that I knew this, I could put a 150 pf capacitor in series with a resistor that susbstitutes for the radiation resistance of the antenna at the frequencies of interest. But what should the radiation resistance be at 160 meters and at 80 meters?

Well, again AD5X comes through with the formula:

Rr = (Actual length · ¼ wave length)^2 x 36 ohms

So at 160 meters, the radiation resistance should be about (43 · 128)^2 x 36 = 4 ohms.

I cobbled together a 3.9 ohm resistor and a 150pf capacitor in series, and attached them to the output of the matching network.

When I attached the antenna analyzer to the input of the matching network and started playing with the taps on the coils,  I was getting very strange results.  It turns out that doing this bench test was a great idea, because it allowed me to find my final mistake with the project. I had placed the 80 meter tap position for the coil on a normally closed contact of the relay, rather than the normally open contact.  Once that was corrected, everything adjusted as expected.

I knew I might have to slightly readjust when I got the real antenna hooked up, but this at least gave me confidence that the matching network was going to work. The 80 Meter adjustments were done with the 150 pf capacitor and a 15 ohm resistor.

So, on my next trip to the ranch, I hooked the matching network up to the 43′ vertical. I only had 4 radials installed when I did this, but it didn’t seem to matter much when I actually used the network later at full power, with all 32 radials attached. So it seems it was close enough.

I had to go through two iterations on 160 meters, first setting the resonant frequency point, then adjusting the tap for lowest SWR, then readjusting the resonant frequency point, as there was some interactance between the two taps.

For 80 meters, the thing worked with the same tap positions used on the bench… Cool!

Anyway, here is a video of the final tuning procedure. I don’t know how I got anything done without my antenna analyzer!