CARA

Calgary Amateur Radio Association

Enjoy Amateur Radio | Serve Our Community


Antenna Troubleshooting

  • 04 Sep 2021 9:23 AM
    Reply # 10980430 on 10979651
    Stephane Raimbault (Administrator)

    Hi Bud,

    Thank you for sharing your experience here.  It reinforces what I have also learnt over the years.... The most important part of your radio station is the Antenna, Transmission Line and Radio, probably in that order.

    That being said, I always encourage people to not look for that "perfect" antenna, and just build/acquire what you need to get on the air.

    I believe half the fun is to build something, and then improve on it.  Start cheap and then invest in the areas that will benefit you the most.

    You have done that with the improvements to your transmission line.

    Enjoy your setup!

  • 03 Sep 2021 5:25 PM
    Message # 10979651

    VE6BUD Antenna Troubleshooting

    For a background of the tower installation, please refer to page 8 of the November 2020 edition of Keyklix.

    So, after I got my towers and antennas all up, I couldn't wait to warm up the HF rig and start having QSO's. First thing I did was put the rig into CW mode and send out a dit on low power. Imagine my shock when the reflected power meter pegged. So, I unscrewed everything and put my NanoVNA on the Mosley Classic 33 first. It read 39:1 SWR at the lowest point. I did see three "dips" in the SWR chart near the frequencies where the antenna was supposed to resonate, so something strange was happening. I put an ohm meter on the coax and saw a DC short.

    I sent out a quick e-mail to Dick Sander, K5QY and asked him if seeing a DC short on a Mosley Classic 33 is normal. He responded quickly and mentioned that Mosley Classic antennas are capacitively coupled. I shouldn't be seeing a DC short. So, I knew there was an issue with something other than the antenna. Rich had been using this antenna for decades before me and the only thing that had changed, from a hardware perspective was the coax, so that had to be it.

    NanoVNA's have a TDR function, so I ran a ping up the coax and saw the short about 50 feet away, which told me that that the problem had to be at the SO-239 barrel joining the two 50' pieces of coax together. I saw the exact same problem with my Cushcraft A3WS antenna. The driven element of this antenna has two halves that are electrically isolated from each other, yet I saw a DC short. I knew I had to have an issue with the coax, where else could this DC short be? Again, the NanoVNA was producing puzzling readings with dips where the antenna should resonate, but with a very high SWR at the base of the dip.

    The 6m beam, a Cushcraft A50-3S seemed to have the same problem as well, but it only did it intermittently. Rotating the rotor seemed to bring it back to life, but there was always that concern, if I was having a QSO, would it develop that short again while I was transmitting?

    So, I bought a tower climbing harness and found out that I wasn't in the best physical shape to be climbing. 20' up the tower and the world started spinning. Oh boy. Perhaps part of it may have been related to the surgery that I had in late February 2020 or the fact that I was mostly homebound for the year and didn't get out and do anything physically active. I gave up trying and put out a forum post asking for the work to be done. Jose Gutierrez (VE6GTZ) answered the call and for a reasonable price, replaced all of the coax I had. His tower climbing skills and work are top notch!

    Imagine my surprise, after he unwrapped the connectors, they fell off! The ones that didn't fall off, could be easily twisted on the coax and pulled off. Only two pieces of coax seemed to test good, the segment going from the bottom of the 50' tower to the antenna and the segment inside of the shack. The rest were defective. They were all replaced with RG-213.

    So, the lessons I learned:
    - LMR400 is not the same as LMR400 Ultraflex.
    - You need a specific set of connectors and tools to work with LMR400UF. A crimping tool and connectors meant for use with RG-213 won't work with LMR400UF.
    - Just because everything was soldered and tests fine on the bench doesn't mean it'll work once it has been coiled up, uncoiled and put on the tower.
    - Don't ever be afraid to ask someone for help, even if you think it might hurt your pride. We're a club for a reason! A team of hams working together.

    2 files
© Calgary Amateur Radio Association
Powered by Wild Apricot Membership Software