Loss on short coax for portable or mobile due to SWR

The amount of power lost in a short coax is pretty small with a 2:1 SWR. In fact I doubt if you could even measure it. Look at the chart I included. The matched loss of your coax is going to be very low if short. That’s on the X axis. If it were, say, 0.4 dB then a 2:1 SWR would only cause 0.1 dB additional loss. 100 foot of RG8X has about 1.5 dB loss per 100 feet at 30 MHz, so even in this case, using 100 feet of cable on 30 MHz and having a 2:1 SWR will only give you an additional loss of maybe 0.25 dB. 

If only using 20 ft of RG8X at 30 MHz, your matched loss would be 0.3 dB and the loss due to  2:1 SWR is so low it’s not on the chart. It would take a SWR of 15 to have even almost 1 dB additional loss caused by high SWR.




Comments

  1. MartyJanuary 8, 2025 at 8:10 AM

    What is your preferred coax for use at the QTH for runs under and runs over 100 ft.?

    ReplyDelete
  2. DandyPointJanuary 8, 2025 at 2:18 PM

    I use RG-X (DXE-8X from DX Engineering). On 160, 80 and 40 I have my longest runs and the total loss is low. With a little over 150 feet on 160 the loss is insignificant. Loss is 0.65 dB/100 feet at 5 MHZ. Only 1 dB at 10 MHz so I figure even if my coax is 200 feet on 40 meters I am still OK. Loss at 30 MHz is 1.5 dB/100 feet. So 100 feet on that band is not too bad either. My 10 meter feed line is probably just under 100 feet. I only run 500 watts now but I did run as much as 750 watts during contests with no issues. I have not used anything else for at least 20 years.

    ReplyDelete
  3. DandyPointJanuary 8, 2025 at 2:20 PM

    I meant to put a 8 before the X in the post above. (RG8X)

    ReplyDelete

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