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Why Feedline Matters

Mike-VA3MW Administrator, FlexRadio Employee, Community Manager, Super Elmer, Moderator admin
edited May 2020 in New Ideas
Just a bit of Elmering

My 80M V was one of the first antennas I installed here about 15 years ago.  I had a run of RG8X going to it and for the most part, it was pretty good.

Like most antennas, they slowly get worse and you don't hear the difference or see any performance lose.

I could tell from some local nets that my 80M TX signal wasn't what it used to be.  The same could be true on some contests.

This morning I decided to change out the 30M of RG8X with about the same length of RG213 that was new and had been sitting around ready to go for a while.

2 Key take aways.  One, my bandwidth is narrower.  That is a good thing as it tells me that the Q of the antenna is higher.  Never trust anyone that says their SWR is flat across the band- that is the sign of big RF losses in either ground or maybe even bad feedline.  Antennas are a tuned circuit and therefore have a resonance point.  

Even better, I gained about 2-3S units of hearing.  Normally, my hearing is pretty great, but I had some birdies and broadband noise leaking in from switching power supplies and a few other things.

Those are all gone now!  Amazing.  

Here is the before and after SWR plots at the end of the feedline taken using my Flex radio and K9DUR's SWR Plotter software.  You are using it, correct?  If you do, don't forget to donate to Ray.  http://k9dur.info/smartsdr_utilities.php

Notice the lowest SWR is better and the 2.0:1 points are not as wide.  

If you are sitting on an HF antenna and you can't remember the last time you actually physically inspected it looking for broken wires and corrosion, you might want to spend some time on it.

For me, it made a big difference.

73, Mike va3mw


  • Dudley  WA5QPZ
    Dudley WA5QPZ Member ✭✭✭
    edited March 2020
    Perfect use of Ray's Plotting SW,  which you can use on and 6000 series and SmartSDR (check the instructions for SWR Plot for any restrictions)  and it's nice to save the plot of an antenna to compare later too.   Thanks for the reminder of Ray's great product.

  • John - AI4FR
    John - AI4FR Member ✭✭
    edited September 2019
    Excellent post Michael, thanks for the reminder!! Plus with K9DUR's software one can watch antenna performance over time and yearly seasons. For example, I can watch the antenna performance here change slightly when the tree's and ground is saturated with water during our wet season and slowly change again as things dry out.

  • Erika - KØDD
    Erika - KØDD Member ✭✭✭
    edited September 2019
    In 1974 at our family's business several 100 foot runs of Belden RG8u were laid out across the assembly floor. A transmitter went on one end into a wattmeter, then the coax length under test and at the end of the run went to a corresponding wattmeter connected to a dummy load.
    A couple of those runs had been on a station for several years out in the weather...  Each band was tested from 80-10 and even over a reasonable period  the loss measured in watts calculated to DB was scary.  Even the transmitter tuning was strange on the EFFECTED Lengths.

    That was the first clue there was an issue. Now imagine hanging that same hunk of coax out there for 10-15-20 years without even checking on it occasionally?  After checking the coax itself in several spots along the length by cutting it open. It became clear that water / moisture had leaked in around one end of the connector.  and had destroyed the first foot on each end and had wicked across much of the cable.  ISH.  DUMPSTER....

    Erika DD
  • Mike-VA3MW
    Mike-VA3MW Administrator, FlexRadio Employee, Community Manager, Super Elmer, Moderator admin
    edited September 2019
    I have a google document that I just paste them into.  

    The fun one to watch is the 160M inverted L move it's resonance around as the ground freezes.  It will swing about 80Khz at times resulting in having to actually cut and lengthen the vertical part.

    Erika, nice comments and very real numbers.

  • Paul - K6HR
    Paul - K6HR Member ✭✭
    edited September 2019
    Look what I found when I last checked some of my feedlines:

  • Ned K1NJ
    Ned K1NJ Member ✭✭
    edited September 2019
        Wonder what you would find if you hung a watt meter and dummy load on
    the end of that old RG8x.  2 to 3 S-Units on rcv is downright scary when you
    think about similar loss on transmit.  Try it on 6m.  ( OMG! )

         Ned,  K1NJ
  • KI4P
    KI4P Member ✭✭
    edited May 2020
    thanks so much Michael! thats a great tool. so many sites and programs, be nice if there was a list or something to look at and know where and whats there and its use, im not the power user that a lot are, but i do enjoy my flex!
    Thanks again
  • Mike-VA3MW
    Mike-VA3MW Administrator, FlexRadio Employee, Community Manager, Super Elmer, Moderator admin
    edited September 2019
    Hi Ned

    Yes, that would be a great idea and a great way to do that exact test.  Good for you.

    I am fortunate enough to have some good test equipment, and I think I did do a path loss on it.  I'll go pull the SD card off it see if I captured it earlier this year.  If I can find it, I'll post it.


  • John KB4DU
    John KB4DU Member ✭✭✭✭
    edited September 2019
    Wow, 2-3 s-units is a lot of loss, approx. 12-18 dB!. That means a 100 Watt Flex is actually putting what 3-10 watts into the antenna?
  • Mike-VA3MW
    Mike-VA3MW Administrator, FlexRadio Employee, Community Manager, Super Elmer, Moderator admin
    edited September 2019
    Yes, that is what you would think.  For me, it was a 2-3 db decrease in noise floor and less RF pickup in the receiving department.  

    When I swept the feedline about a year ago for path loss, it was about 1db at 3Mhz, so the RF exiting was not as attenuated as much. I use a Rhode and Schwarz FSH4. 

     I thought I had a screen shot of the path loss, but I guess I didn't take it.

    Short story is the feedline does have a shelf live.

  • Stan VA7NF
    Stan VA7NF Member ✭✭✭
    edited September 2019
    Your last note contained significant information; did I misread something?
    "2-3 db in noise floor and less RF pickup in the receiving department" and from your first note replacing RG-8X with RG-8U, along with references to RG-213 and LMR-400 and also 2-3 S units improved hearing, mostly from less noise pickup (i.e. better shielding)
    This is mostly from changing a 85-95% single shielded to a double shielded cable.  Strange but I cannot find ANY SPECS on RG-8U shield performance whereas LMR-400 state a shield performance of >90db.
    So a noise pickup from a plant/factory floor comparison you are not comparing cable loss more that you are measuring the advantage of single vs double shielded coax.  
    While I agree that non-distilled water infiltration, containing conductive salts, is a loss problem, only the watt meter on both ends and 1:1 SWR (to eliminate localized current at current peak positions), averaged between two runs with the watt meters swapped between the ends will demonstrate true loss.
    Part of my noise reduction guidance is to use LMR-400 with 'N' connectors or hardline and place multi-turn in ferrite current chokes on EACH END of each cable to keep noise pickup from entering the inner conductor.  Even PL/SO 259 connectors transfer outer shield current to the inner shield.
  • Mike-VA3MW
    Mike-VA3MW Administrator, FlexRadio Employee, Community Manager, Super Elmer, Moderator admin
    edited September 2019
    Hi Stan

    I think there are 2 parts to this.  

    Receiving, yes leakage from local switching power supplies (2 in particular that I know of that I used to see and now I don't see at all).   

    About a year ago, I swept the RG8X with an FSH 4 from end to end.  I know the loss was more than I expected but not surprising given its age.  I pulled the SD card to see if I saved an image, but it I guess I didn't.  I did have hand written notes that showed it was about 1.5db at 3Mhz, which is a full db more than spec which should be about 0.6db  at 3Mhz.  The feedline is 31M long. 

    The LMR400 was amazingly better of course when I swept it from end to end with the FSH 4 at 0.11db.  See the attached image.

    I do have Mix 31 chokes at each end of every feedline which I highly recommend of course.

    I have yet to measure any difference at HF frequencies using N connectors and I am not convinced I would hear the difference if I made the change to N on anything less than 30Mhz when actually operating.  However, it is an option.  We did this as a college project a long time ago (our Prof was a Ham, so he wanted to know).  :)



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