So, yesterday, I decided I needed to do something about my 40m/30m dipole, as the SWR was getting a little out of hand. I really should have just rebuilt the whole thing, but I decided to see what I could do about retuning them.
The last time I had put the antenna analyzer on it, I measured a resonant point of about 6.8 MHz, so I knew I had to shorten it some. Yesterday, however, the resonant point was down around 6.5 MHz! I’m not sure that I could attribute all of that to the antenna wire (I’m using 14 ga FlexWeave wire) stretching, but the thing has been up for more than ten years, so I guess that’s possible.
On the other hand, on 30m, the antenna looks like it’s too short. The resonant point is up about 10.250 MHz. I don’t recall it being that high when I put it up, and I don’t have any records from way back then, but that just doesn’t seem right to me, either.
So, the first thing I did was to shorten the 40m legs. I shortened by about eight inches on both sides. That brought the resonant point up to about 6.9 MHz, but I’m going to need to shorten it a bit more next weekend.
Another wacky thing is the coax I’m using. Ten years ago, I purchased this roll of RG-59, and built the feedline for the original dipole from it. It worked great up until this spring, when it finally decided to fail on me. I built another 75-ft. feedline from what was left on the original roll, and that seemed to work fine up until a week or so ago.
I took my dummy load out to my yard and connected it to the far end of the feedline, then connected the other end to the antenna analyzer. The readings make really no sense to me at all. They seem to be dependent on the frequency. Now, I didn’t expect to read a 1:1 SWR, as I’m connecting a 50-ohm dummy load to 75-ohm cable, but I didn’t expect the SWR readings to be frequency dependent. If anyone can explain that to me, I’m all ears.
Fortunately, I had a 100-ft. cable that I have been using for Field Day and other portable ops sitting on the shelf. I rolled that out and connected it up and am on the air using that.
I guess what I need to do is buy some new coax. I’m looking at buying a 1000-ft. spool of RG-8X from DXEngineering. At $300 for the spool, that seems like a pretty good deal to me. I’ll then have plenty to make up whatever kind of 50-ohm feedlines that I need.
Walter Underwood K6WRU says
Yeah, I’d point a finger at the coax.
Coax is a surprisingly subtle transmission line. There are three conductors: the surface of the center conductor, the inside surface of the outer conductor (the “shield”), and the outside surface of the outside conductor.
The impedance depends on the condition of the dielectric between the first two and their condition. Water can degrade the dielectric and corrode the surface of the conductors. Bends, age, or overvoltage can also cause problems.
The two outer conductors are shorted together at connectors, which makes things more interesting. This is why a current balun at the antenna plus a choke at the shack might both be needed.
Dan KB6NU says
Points well-taken, but the previous feedline was made from coax off the same roll and worked well for nearly ten years, and I am using a balun at the feedpoint. I suppose that the coax may not have aged well on the shelf, but it was decent quality coax when I purchased it, and that seems unlikely to me. It’s possible, though, I guess.
Pat Ryan KC6VVT says
All coax is resonant, even perfectly loaded at the far end with a coaxial and purely resistive teminator (Dummy load) selected to match the cable design impedance. Sweep such a cable with a spectrum analyzer and tracking generator via a direction coupler for VSWR, and you would be surprised at what you will see. Remember, to get the antenna system (antenna, cable and matching device) VSWR measured reasonably accurate enough, the coaxial cable should be a half wavelength electrically ( or multiple thereof). With RG-59, the cable loss over distance will effect that calibration.
If at all possible, try to test the antenna itself at its feed, or driven point, or certainly with a short test cable of better quality that RG-59.
Age on the shelf, or installed with a poorly tuned antenna can degrade the replacement cable quickly with VSWR caused hot spots, too!
73 de Pat KC6VVT