High SWR After Coax Replacement

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K4SRF, Jun 19, 2020.

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  1. K4SRF

    K4SRF Ham Member QRZ Page

    I have two antennas. One is a Comet GP3 for UHF/VHF and the other is an Alpha Delta DX-CC Fan. Both are about 33 feet high. I had RX 8x coax, (75') as my feedline and maintained an acceptable SWR on both UFH/VHF and HF respectively, (and an antenna tuner for HF.) I have replaced both feedlines. The UFH/VHF now has as a feedline 75' of LMR 400 and the HF has 75' of RG 213.

    Now, both the UHF/VHF are showing a SWR of 3.2 - 4.5 across the entire band spectrum. The same goes true with the HF as well. The fan antenna has the bands 80 / 40 / 20 / 15 and 10 meters. Each one of these bands have shown a considerable SWR increase since I replaced the coax.

    I am puzzled as to why the considerable increase in SWR since what I have read, these two coax types are highly recommended for the respected frequency ranges. If anything, I was expecting the SWR to decrease, especially with the 20 meter / 70 cm bands.

    Any clue(s) as what to look for? I have double checked the coax cables. Both are brand new and have no damage. I hooked a MFJ 269 D Analyzer to them prior to the install and found no problems at all. All I did was swap cables; nothing was done to the antennas themselves.
  2. WA9FZB

    WA9FZB XML Subscriber QRZ Page

    A few ideas:
    1. Can you put back the old cable? Try it, just to isolate the issue as being caused by the new cable installation.
    2. Have you checked the new cables for both shorts and opens? Either can happen, with unpredictable outcome on SWR.
    3. Is it possible that your SWR never was really low? I mean, right at the antenna. It is possible that your real SWR was close to what you now are seeing, but lossy coax was causing your meter to show closer to unity.

    Personally, I would suspect number 3. I know that is my situation at the present. My ground mounted vertical shows about a 1.2:1 SWR, but the coax is far overdue for being replaced. I am sure that once I install the direct-burial coax I have, I will need to re-tune the antenna.
    WR5B, WG7X, K7TRF and 3 others like this.
  3. N8VIL

    N8VIL Ham Member QRZ Page

    If it was a pre-made coax assembly, I would first suspect a poorly installed connector.
    PU2OZT likes this.
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I'd run a cable loss test with your MFJ on both stretches of coax (with no load connected to the far end). If the old coax was very lossy at frequencies of interest then I'd second possibility 3 listed above. Lossy coax can easily mask antenna matching problems especially at VHF and UHF.
    K8PG, W6MK, PU2OZT and 1 other person like this.
  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    ...which causes me to ask, who put the connectors on the new coax???
    PU2OZT likes this.
  6. WG7X

    WG7X Platinum Subscriber Platinum Subscriber QRZ Page

    @WA9FZB I'm betting your number three...

    Quick anecdote: At one time in the not so distant past, I acquired a six meter radio, a vertical antenna, and 100 feet of RG-8X. The radio was multi mode FM/SSB/CW. So, at any rate, I assembled all the parts together and sat back to listen to the sounds of silence. Did not hear a thing for weeks. Knowing already that a vertical antenna is not the best for local / DX weak signal work, I tried calling a local on one of our six meter repeaters. We had a nice chat and I asked him to come down to the SSB part of the band and listen for me.

    Well, he came down alright, but I had a hard time hearing him even when he was not that far away and also using a vertical. Made me think. At that point, I considered the fact that even when working on FM thru a local repeater, the received signal at my place was only about an S-5 when the other fellow reported the repeater to be full scale at his location.

    Gave it a bit more thought and went out and cut the feed line down to 50 feet, half it's original length. What a difference that made! I could now hear the local repeater full scale and the local on SSB was coming in S-9 now also. The attenuation on the 100 foot piece of coax at six meters was very high, so much so that it not only cut down the received signals, but also made the SWR nice and flat...

    Never did hear any over the horizon signals on that radio, but that's another story for another day. I guess that I lacked patience for VHF. Still do....
    K8PG and PU2OZT like this.
  7. K4SRF

    K4SRF Ham Member QRZ Page

    Factory install.
  8. K4SRF

    K4SRF Ham Member QRZ Page

    Interesting outcome.

    I decided to test both coaxes again, so I took disconnected the antennas from the coax. I tested the coaxes again, and again they tested good. I re-attached them to the antenna, but this time I just used electrical tape to cover the connections. Previously, I had used an application of electrical tape plus a coving of "Coax Seal". This time I just used electrical tape since I was out of "Coax Seal". I tested and each coax was in an acceptable SWR range.

    I am not sure what would have caused BOTH to be out of SWR range the first time, and then in range the second.
  9. W6MK

    W6MK Ham Member QRZ Page

    As the song goes "little things mean a lot."

    An imperfectly-tightend coax connection will cause problems. It's easy not to
    snug up your connectors adequately. And then you wrap them counter
    clockwise with coax seal which further loosens them.
    K4SRF likes this.
  10. N5SMO

    N5SMO Premium Subscriber QRZ Page

    I suspect the SWR was high all along, but the level of loss present in the original cable masked that fact giving you a false favorable impedance match. This is most likely to happen at the UHF frequencies. I saw the same thing when I replaced my cruddy feedline with LMR 400 and some tweaking of my antenna's gamma matches fixed my SWR.
    K4SRF and K8PG like this.

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