coax feedline length and SWR

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Bill Allen:
Good Morning,

I have questions concerning SWR and coax feedline length.  I am using a homebrew SuperJ antenna at home for 2 meter operations.  The antenna is mounted on the top of my tower at about 40ft.  The existing coax to this antenna is about 55ft.  When I xmit a 50wts, I get an SWR range of 1.4 to 1.8 depending where on the band I am.  However, when I lengthen the coax run by coupling in another piece of coax (I am using RG8U, by the way) that is about 80 feet long, my SWR drops to a range of 1.1 to 1.4 depending where I am on the band.  Is my original feedline simply too short or am I actually having a real problem with the antenna?  I am also wondering if my SWR is dropping with the longer coax run because I may be creating a resonant feedline that is radiating.  Finally, if my feedline is significantly radiating at 50 watts, am I running risk of bad RF exposure?  Would RF burns on the bottom of one's foot be even possible?

I know these are probably elementary kinds of questions, but I could sure use some input/advice.

thx & 73
Bill Allen

Mike Gilmer:
While these may be "elementary" questions, they are still very legitimate.

Improved SWR readings with longer cable is usually caused simply by the increased loss of the cable.  RG-8/RG-213 coax has approx 2.5dB loss per 100feet at 2meter frequencies (this loss is worse at higher freq, better at lower freq).  So any reduction in reflected power (for the same forward power) will make the SWR look better.  In most cases, as in your case, I'm sure, the SWR meter is near the transmitter end, so by adding 80 feet to the line, any reflected power seen at the radio/SWR meter is reduced by the loss of 160 feet of coax and the forward power measured is NOT reduced. This is because the forward power is reduced by the loss of 80 extra feet of coax on the way TO the antenna, then once reflected, it sees another 80feet of loss on the return trip to the radio/SWR meter.

If you located your SWR bridge near the antenna, you'd get a much more accurate reading.

Formulas and graphs get a little cumbersome in this forum, so try to look up SWR in a handbook.

And no, your coax is not radiating - all the loss is in heat form.

Hope this helps.

73 Mike N2MG

Bill Allen:
Thanks for the reply!  :-))

I am going to have to do some work with that antenna.

thx & 73


An SWR reading of 1.4 - 1.8 to 1 is not, in my estimation, worth worrying over.  There is a phenomenon associated with a coaxial transmission line's length in relationship to wavelength which will affect standing wave ratio and some operators mistakenly fret over getting the "right" length cable (relative to wavelength, not line loss) in hopes of bringing SWR to 1:1.  Assuming that you're not using a long run, and that your coax is low-loss (forget the RG-58 stuff for VHF!), if you sufficiently tinker with your antenna enough to actually go from 1.4:1 down to 1:1, you will see virtually NO difference in either transmit or receive performance.  Most modern transceivers do not start "folding back" the output power until an SWR of 2:1 or greater is reached, and many rigs are hearty enough to withstand even greater mismatches.  Of course, this all assumes you're not doing "milliwatt moonbounce" or something like that!

Bob, K5IQ

A length (any length, theoretically) of coax of high integrity will exhibit a constant impedance along that length... PROVIDED... that the coax is terminated at both ends in its characteristic impedance such as 50 ohms. In the case of 50 ohm coax, if the rig's output impedance is 50 ohms, the ONLY instance of a vswr (or iswr) of 1 as measured by an swr bridge...(lets measure at the rig end of the circuit  for convenience)... is when the impedance presented at the load (antenna or matching device) end of the circuit is 50 ohms resistive.
While it's true that power loss varies linearly with the length of the coax, any swr indication greater than one, especially if it changes with differing coax lengths, suggests that there is a reactive component at work... possibly an antenna which was improperly tuned OR which was relocated after proper tuning and is now being adversely affected by nearby objects :)
Bill, N4QA


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