Can water in the antenna/coax system, reduce the SWR reading seen on a meter?

Hi all

Most know that water in the coax/antenna system will increase the SWR reading shown on a meter.

But can it have the opposite effect and reduce the SWR?

I once had water come down a length of Westflex 103, did not notice any swr problem but had a puddle on the shack bench so knew something was up. Turned out my bad sealing of an n plug up the mast.

Hi Ray,

Most know that a SWR reading is a ratio of the power out (forward or FWD) against the power reflected back (REF). You'll recognise the FWD/REF references from most SWR meters.

I won't bother mentioning the maths nor the online calculators which are great so long as you understand what's going on.

If you had 10W going out with 0.25W or a 1/4W coming back you'd have a SWR of 1.38 (rounded up).
Change that to half the power and it'd be 5W out, 0.125W or an 1/8thW back so still 1.38 SWR.
Again if you had 100W out and 2.5W coming back you'd still have a SWR of 1.38 so now you can see the ratio going on.

Everything is hunky-dory and looks like SWR readings are infallible until you count the coax losses in.....

If you had terrible coax or a problem where you lost half the power out on it you'd also get half the power loss coming back. 

So now taking the previous example you see 10W going out on your SWR meter at your rig end, half that power is lost on the way out through the coax to the antenna (a 3dB loss) and the same loss is experienced coming back to your rig end for the reflected power reading. The ratio is now upset with you thinking you have 10W going out but far less power comes back so your SWR reading actually looks like it's improved!!!

This is why when people 'upgrade' their coax they often see worse SWR readings as something like RG213 has far less losses than RG58 giving a more accurate ratio or SWR reading. It isn't worse, it's actually better and you need to retune your antenna.
(Usually the previous installation wasn't tip-top in the first place either.)

Complicated enough yet?

Then how about factoring in other elements like coax having a certain capacitance along it's length and upsetting that by crushing  it through doors/windows or kinks & bends etc. Let alone problems with losses associated with coax length or common mode problems and the coax becoming part of your antenna upsetting everything.

(I won't make mention of taking SWR measurements at the antenna end either, bit of a bugger to be dangling off the pole on the top of the house to notice it all change again! )


You don't need to get into all the nitty-gritty though (unless so inclined) and just make sure you use the best coax you can afford, use decent terminating methods (get ready fitted PL259's etc. if you can't do them yourself) and do your very best to keep contaminants such as water out of everything.

That keeps it all much more simple.  


All the best,
Victor

(Remember as well as experiencing losses with power outputs on poor setups you'll also lose out on your reception too!)