Since I had a few minutes after the XYL went to bad (after a *very* busy Saturday for both of us), I went into the basement and played with the antenna analyzer.

This isn’t the most ground-breaking post on my blog, but they don’t really teach much about antenna analysis in traffic engineering classes!

Scop’d

The first thing I did was drop the frequency of the analyzer to something low and see how much power the analyzer puts out.  4V peak-peak.

SWR Check – 25 Ohms

SWR is based on the mismatch between the impedance of the source and the load.  So a 2:1 SWR could mean that the load is twice or half the source impedance.  So I decided to put a few resistor arrangements on the analyzer and see if what happens is what I thought would happen.

So I had these terminators from way back when they used BNC token ring networks. I’ve never worked with anything but Ethernet (using RJ-45 connectors), so I’m not sure how I got these, but they came in handy for this.

This one is 50.3 ohms. Within a 1% tolerance.

This other one is 49.4 ohms. Just around 1% off. For what I’m doing, close enough.

This was my redneck arrangement. This basically put the two 50 ohm terminator resistors in parallel.

This doesn’t quite have the resistance I was expecting, but the SWR is correct. Of course, there’s all sorts of stuff going on with the open end, the many connectors, and the 4 foot piece of RG-58.  Given the fact that I had two 50 ohm resistors in parallel, I think I should be seeing 25 ohms R and (ideally) 0 ohms X.

This seems a little more like it. Still a nearly 2:1 SWR and close to 25 ohms R, with very little reactive ohms.

SWR Check – 100 Ohms

Since I couldn’t figure out a way to make my two BNC terminators in series, I pulled a resistor out of my parts bin.  It was really a 98.3 ohm resistor, according to my non-lab-grade Radio Shack meter, so I figure that’s close enough!

Close enough to 100 ohms.

This is how I did it.

Hey look, exactly as expected. 100 ohms resistive, and 0 ohms reactive.

Other Dummy Load Test

I had a dummy load I built for QRP uses (specifically the Softrock).  I built it a while back, which is why the callsign is wrong.

Just pretend it says “KE8P Dummy Load” 🙂

70 and 4? SEVENTY AND FOUR? WTF, it should be 50 and 0!!! I’ll just blame the extra resistance and all the reactance on the cable…

Maybe this is why it isn’t perfect. My coax isn’t exactly Belden or Times.

Capacitance Checks

So the MFJ-259B has a capacitance check on it.  This is really for the capacitance of an antenna, not for what I did in the pictures below.  I basically took a ceramic disk capacitor and clipped one end to the ground and the other I held into the center conductor of the antenna port. This is a 10,000 pF capacitor.

On 80m, this capacitor has 6,298 pf.

Up at 91 MHz, there’s only 8 pF of capacitance.

I figured the stuff above was a little more fun than me talking about how I tested every antenna I own… again.

-73-