J-Poles

My biking and ham radio buddy, Mark, came over Monday evening. We had three objectives:

1. measure the SWR of J-pole I built a couple of months ago,
2. measure the SWR of the 2m/440 antenna I just purchased from Purchase Radio, and
3. build a 2m J-pole for Mark

To measure the SWR, Mark brought over his MFJ 259 Antenna Analyzer. In some ways, it’s much nicer than the Autek Analyzer that I have. For one thing the display is much more informative. The Autek display consists of simply a five-digit numeric display, capable of displaying only one parameter at a time. The MFJ is a full alpha-numeric display and displays the test frequency, the measured SWR, and some other information simultaneously.

The MFJ analyzer’s frequency range is also much greater. It covers the 2m and 440 MHz bands, while the Autek only goes up to 32 MHz. Mark doesn’t think it’s very accurate at 440, though.

The first thing we did was measure the SWR of the J-pole I’d built. I’ve been very happy with the antenna, so I was a little surprised that the first reading was slightly over 2.0. I did have it hanging very near to a fairly massive new barbecue grill we have, so surmising that the grill was detuning the antenna, I moved it away from the grill and the SWR did go down to about 1.5. Respectable in my opinion.

Next, we measured the SWR of the dual-band antenna. This antenna is a 5/8 wavelength antenna that I had mounted on a mast. Its SWR measured about 1.5 near the ARROW 2m repeater frequency of 14.96- MHz. I was happy with that, so we moved on to the antenna construction.

We built the J-pole from plans I found on the Net (shown above). You can build J-poles from 300 ohm twinlead, 450 ohm ladder line, and 1/2-in. copper pipe. The dimensions for each of these versions is slightly different, as the velocity factors are all different.

We opted to build ours with the ladder line because we each already had a hunk of it and it’s much easier to carve up some ladder line than solder up copper pipe. It’s not as sturdy, but it’s certainly sturdier than 300 ohm twinlead.

When I built mine, I did not have an SWR meter to tune the antenna with. So, I just built it to the recommended dimensions, and as mentioned earlier, it turned out to work pretty well. Since we had the analyzer when we built Mark’s we played around with the tuning stub, and it appears that he was able to get a slightly better match than I was.

Overall, it was mission accomplished.