Antenna – Page 4 – AB4UG Radioblog

Hamfest antenna

One of the bundles of wire I picked up at the Charleston Hamfest last weekend turned out to be a dipole cut for 10m, according to the paper tag that was tied to it.

Each leg of the dipole is about 2.8m (9.2 feet) long and there is what appears to be a trap near the end of each leg.

At the center is 6.9m (22.6 feet) of BNC terminated coax.

It appears to have seen a few seasons out in the elements, judging by the amount of oxidation on the wires and the condition of the coax. Still seems to work pretty decently. Using an old painter’s pole and some of the extra rope tying off the current antenna, I was able to measure an SWR of 1.3 at 28.5 MHz with the center about 2m off the ground.

I didn’t check to see if it was resonant anywhere else. Once I figure out how to suspend it a reasonable distance off the ground, I’ll check it out a little more thoroughly.

10m Moxon antenna is here!

Thanks to the amateurradio subreddit and K5WL and, I now have a 10m Moxon antenna (MFJ-1890).

According to K5WL, the antenna got blown down and sustained some damage, but nothing too severe from what I can see. A few of the aluminum tubes are bent a little bit, and one has about 2cm broken off. I think it should be easy enough to straighten the bent tubes and I could probably find a replacement for the shortened tube. If I have to, I’ll see if MFJ will sell me some replacement parts.

Very excited to get working on this and get it on the air.

Icy antenna

After a night of rain, sleet and ice build up, I was pleasantly surprised to see the antenna was still up in the tree. It was covered in ice which I think must have affected the performance of the antenna because the radio seemed to have a harder time tuning on some of the bands.

Still seemed to work ok, although I don’t think I was able to melt any of the ice while I was playing radio today. I probably need moar watts.

Antenna SWR curves

Spent some time yesterday using the MFJ-259B to collect SWR values for the antenna in the different ham bands from 160m to 6m (I skipped 60m). The end result looked like this (vertical scale on all the graphs is from 0 to 10 SWR).

So I have an antenna that’s pretty good on 6 bands: 40m, 20m, 17m, 12m, 10m and 6m. Interestingly, even with the high SWR on 15m, the radio will tune the antenna there too. Not bad for a dipole that’s straight on one side and kind of L-shaped on the other one (darn tree branches)

Playing with the antenna analyzer

Finally got around to playing with the antenna analyzer I picked up at the Atlanta Hamfest. Came with batteries installed and a wall wart so it was ready to go out of the box. First thing I tried out were the inductance and capacitance measurements. Grabbed random wire wound inductor out of the junk box and put it against the antenna connector.

13 μH at 7.06 MHz. A pretty decent amount of inductance. It’s got a lot of windings and came out of a dead laptop power supply I think. Next I put a random capacitor on it. Capacitance meter told me it was around 216 pF. At 7.06 MHz, the analyzer measured 177 pF. Not too far off and something that I might expect at high frequencies.

The analyzer will also measure the amount of coax loss at different frequencies. Put a 75′ length of RG-8X to see what I’d get. At 7.06 MHz, a respectable 0.3 dB loss. At 146 MHz, though, 2.8 dB. Will have to make sure to keep the coax relatively short for whatever VHF/UHF antenna I decide to build.

Measuring coax loss at VHF with the MFJ256B

Time to bring it in and try it out on the antenna. I know the antenna works really well on the 40m band, and the analyzer confirms it. SWR of 1.1 and 50Ω impedance at around 7.2 MHz, pretty much smack dab in the middle of the band. SWR ranges from 2.1 to 1.5 across the band.

Next I want to go through and record some more detailed measurements about the frequency performance of the antenna and maybe graph some things out. I’m sure that will look interesting.

I wonder what the analyzer will tell me about my HT antennas…Analyze ALL THE THINGS!