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adding coax lowers SWR reading on the meter?

M

mr_fx

Sr. Member
Oct 8, 2011
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Kansas City
this goes against everything I have ever believed

well I added some coax between the meter and the antenna

tried a few different lengths, each time causing a different SWR reading on the meter... how is this possible?

BTW the longer one (about 2-3 foot) gave the lowest SWR reading
 

It's possible, and absolutely normal, because the load at the end of that feed line isn't the same impedance as that of the feed line used.
Change that "feed line" to "transmission line" and it holds true for anything/everything fed an alternating current. That means antennas, or amplifiers, or power lines, or audio devices, whatever.
- 'Doc
 
W5LZ said:
It's possible, and absolutely normal, because the load at the end of that feed line isn't the same impedance as that of the feed line used.
Change that "feed line" to "transmission line" and it holds true for anything/everything fed an alternating current. That means antennas, or amplifiers, or power lines, or audio devices, whatever.
- 'Doc
Click to expand...

If your antenna is not tuned properly then adding or removing coax to match it is essentially causing the feed line to become part of the radiator. This isn't what youi want to happen. You want the power to be radiated by the antenna and NOT the coax.
 
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What antenna it is really makes no difference. With that particular one you at least have a convenient way of changing it's input impedance. That set of rings do come in handy.
- 'Doc
 
mr_fx said:
this goes against everything I have ever believed

well I added some coax between the meter and the antenna

tried a few different lengths, each time causing a different SWR reading on the meter... how is this possible?

BTW the longer one (about 2-3 foot) gave the lowest SWR reading
Click to expand...

I would tend to think that your antenna is not resonant at the frequency being used at, plot your swr range if you have free band capability to see where your bandwidth is then try to center your antenna to accommodate the radio.
 
mr_fx said:
ALSO I tested it with an 18 foot section of coax (RG58u) and got flat SWRs all across the band
Click to expand...

Really! I would think the very opposite might happen with a short coax vs. a long coax. Plus an A99 typically has a very wide bandwidth of 3-4 mhz without radials if mounted in the clear and a wavelength high or more. With radials, the bandwidth is reduced somewhat and the curve is more pronounced. My A99 always shows a nearly flat SWR curve accross the CB bandwidth...whatever the SWR is.
 
Last edited:
Marconi said:
Really! I would think the very opposite might happen with a short coax vs. a long coax. Plus an A99 typically has a very wide bandwidth of 3-4 mhz without radials if mounted in the clear and a wavelength high or more. With radials, the bandwidth is reduced somewhat and the curve is more pronounced. My A99 always shows a nearly flat SWR curve accross the CB bandwidth...whatever the SWR is.
Click to expand...

Sounds like it has very good heat absorbing properties or does it just transfer that heat onto the coax.
 

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