[TowerTalk] Understanding Antenna Impedance Measurements

[Tod]

 In the end it is transferring power to the antenna that counts. Unless the
VSWR is awfully high and/or the line length is very long I think it is quite
stuffiest to put a tuning network at the station so the transmitter is happy
and then start sending. You most likely ( and the folks you contact) will
never know that your antenna is neither resonant nor matched at the
frequency you are operating -- and you signal will not be noticeably
impaired either.

Tod, KØTO




> -----Original Message-----
> From: towertalk-bounces at contesting.com 
> [mailto:towertalk-bounces at contesting.com] On Behalf Of Jim Brown
> Sent: Tuesday, November 15, 2005 3:50 PM
> To: Tower Talk List
> Subject: Re: [TowerTalk] Understanding Antenna Impedance Measurements
> 
> On Tue, 15 Nov 2005 15:38:28 -0700, Tod wrote:
> 
> >Those values look like they are within about 2% of expected 
> values for 
> >the R and within 20% or so on the X. Is the analyzer accuracy any 
> >better than that if there is NO coax between the load and 
> the analyzer?
> 
> What Jim Lux has shown by cranking the numbers is that when 
> the VSWR is small, the effect of the transmission line is 
> small. That would be true whether the line was a half wave or 
> not. But when it's a half wave length, it has almost the 
> effect of moving the analyzer to the antenna terminals AT 
> THAT FREQUENCY. 
> I say "almost" because the loss in the line will reduce the 
> VSWR slightly (but VERY little on topband). 
> 
> Tom can tell you a lot more about analyzer errors. But 
> consider this -- it isn't really possible to stick the 
> analyzer at the antenna and read it without a very tall 
> ladder, and your presence next to the analyzer will detune it 
> a bit. There's also the interaction of the antenna with the 
> coax. Other errors
> -- the analyzer has some finite input R and C. For small 
> VSWR, neither mean much, especially at 2 MHz, but they sure 
> do at higher frequencies and high VSWR. As I recall, W4EF 
> said that the AEA CIA-HF has input Z on the order of 10-12K 
> in parallel with 12 pF. That's an older box, and the new one 
> may or may not be better. 
> 
> Jim K9YC
> 
> 
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