[Elecraft] SWR Readings:Differences

[Fred Jensen]

I left your message on this, don't normally do that but I think it is 
important for context.

1.  If the feed point impedance at your antenna is equal to the 
characteristic impedance of your coax *at any given frequency,* then the 
SWR you measure at the rig will be nominally 1:1.  Note that not all SWR 
measuring instruments are created equal.

2.  Measuring the feed point impedance at an antenna is usually somewhat 
hard ... up in the air, maybe not accessible ... you probably get the idea.

3.  Measuring the SWR at the accessible bottom of the tower [I'm 
assuming that's where you're measuring it], and finding it to be 
anything other than 1:1 means that the line is not matched to the 
antenna impedance, and begins acting as an RF transformer.

4.  The SWR you will see in the shack, and remember that all SWR 
indicators are not created equal, will be whatever results in the 
impedance transformation along the length of the coax from the tower to 
the shack.

5.  A Time Domain Reflectometer will help clarify this for you if you 
know how to use one.  Sadly, they're very expensive and hard to 
interpret if you've never used one.  Check around among your friends but 
don't hold your breath.

Bottom line, you can prune or un-prune your antenna [if it's wire] a 
little, and you can adjust the length of the coax transformer to the 
shack to get acceptable SWR at the rig.

Or, if your K3 accepts the SWR, ignore it.  Honestly Mark, you'll never 
know the difference while operating.  There's also the KAT3, your SWR's 
are easily within it's range.

There was a time, in my lifetime, when "Standing Wave Ratio" was NOT an 
amateur radio term.  You "loaded the antenna" and called CQ.  Today's 
radios are significantly more particular to SWR, but if it works, it works.

73,

Fred K6DGW
- Northern California Contest Club
- CU in the 2014 Cal QSO Party 4-5 Oct 2014
- www.cqp.org

On 9/2/2014 5:06 PM, pastormg2 at verizon.net wrote:
> Good Evening, This is Mark Griffin, KB3Z and I have some questions
> regarding the SWR readings I get at my tower versus what I get on my
> K3. I will give the SWR readings that I got at my tower for a 40
> meter rotatable dipole at 55 feet.
>
> Tower:
>
> 7000  2.2 7025  1.8 7050  1.5 7075  1.3 7100  1.0 7125  1.1 7150
> 1.3 7175  1.6 7200  2.0 7225  2.2
>
> K3 Readings:
>
> 7000  3.5
>
> 7025  3.2
>
> 7050  2.9
>
> 7075  2.6
>
> 7100  2.4
>
> 7125  2.3
>
> 7150  2.4
>
> 7175  2.5
>
> 7200  2.6 7225  2.9
>
> What would cause such a big difference. The cable run from my antenna
> switch on the tower to my K3 is only an additional 75 feet. I am
> using RG-213 cable. Is there anyway that I can test the SWR reading
> that my K3 is giving me?
>
> Mark Griffin, KB3Z