I miscalculated a recent antenna install "base" by ordering 50ft.of LMR-400 and ended up needing 60ft. When temporary set up in the basement I had a perfect swr of 1.0. I later lengthened it with a barrel connector and ten more feet of LMR-400 all connectors etc are factory and supposedly top quality " I certainly paid for top quality" but anyway I now have a swr of 1.3 and with a few hundred watts flowing through the meter 1.7 with audio raising it even higher. Is 60ft. a bad length causing higher swr or is something else the culprit?
-
You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
SWR's up with 10ft. ext.
- Thread starter hammer0630
- Start date
The actual feedpoint impedance of your antenna is not a pure 50 ohms and the feedline is acting as an impedance transformer which in turn presents a different impedance at the radio end. Changing the length of cable changes the transformation ratio and that is why you see a difference in SWR with a different length of cable. If the antenna was a purely resistive 50 ohms this would not happen.You can either re-tune the antenna or change the length of cable to something that makes the SWR appear better.Either way it's a stab in the dark as the first method does not guarantee the antenna is still tuned properly and the second method is merely a bandaid masking the issue.
0630, we talked about that possibility I think, and yes 50' feet might be better than 60' feet if your antenna tune us a bit reactive.
I think I've asked you several time what were the actual lengths you used for the radiator and radials, but I don't recall you ever really saying for sure.
Read this article and check out and try and understand the table information noted there. Then we can talk more, OK?
Common-mode chokes
The Captain is right, but even so I don't think that changes the actural feed point impedance at the antenna, it just looks worse at the radio end.
I think I've asked you several time what were the actual lengths you used for the radiator and radials, but I don't recall you ever really saying for sure.
Read this article and check out and try and understand the table information noted there. Then we can talk more, OK?
Common-mode chokes
The Captain is right, but even so I don't think that changes the actural feed point impedance at the antenna, it just looks worse at the radio end.
Last edited:
two things spring to mind, reflections from the coupling between the two coax's or common mode current on the braid,
you are right eddie, the transformer action won't change the feed-point impedance or vswr, it only changes the impedance your radio sees looking into the coax,
what changes your vswr with small changes in coax length are common mode currents on the braid seen in parallel with the antenna feed-point impedance swinging about in magnitude according to the braids electrical length and its electrical path to ground,
different magnitudes of cm current = different common mode impedance in parallel with the antenna feed-point impedance = a different terminating impedance for the coax =
vswr changes with relatively small changes in coax length.
you are right eddie, the transformer action won't change the feed-point impedance or vswr, it only changes the impedance your radio sees looking into the coax,
what changes your vswr with small changes in coax length are common mode currents on the braid seen in parallel with the antenna feed-point impedance swinging about in magnitude according to the braids electrical length and its electrical path to ground,
different magnitudes of cm current = different common mode impedance in parallel with the antenna feed-point impedance = a different terminating impedance for the coax =
vswr changes with relatively small changes in coax length.