[TenTec] Field day antennas

[Jim Brown]

Hi Don,

I did read the NCJ article. Some of it is great, but some of it is 
wrong.  Specifically this -- the standing wave pattern on the line is 
determined by the LOAD Z, which varies WIDELY with frequency.

Consider, for example, an 80M dipole. Depending on height and , it's in 
the range of 40-90 ohms at resonance, and gets increasingly reactive 
either side. On the 2nd and 4th harmonic, the load will be a very high 
Z. On the 3rd and 5th, it will be back in that low range.

On the other hand, consider an 80/40 fan -- on the second and third 
harmonics it will have a low Z in that 40-90 ohm range at resonance. 
Likewise, a tribander will be in the range of 50 ohms on the three 
bands. BUT -- in all cases, even for resonant antennas, those high 
voltage points will move up and down the line with frequency as Z varies 
around resonance.

The really excellent thing about George's piece is that he recommends 
putting stubs at the high voltage points on the line, which is dead on 
right -- if you can do it. But what seems to be missing is that they 
want to be at a high voltage point on the line AT THE FREQUENCY THAT THE 
STUB IS INTENDED TO KILL. Caps added for emphasis. Then you must measure 
both the Z at those "kill" frequencies, as well as the length of the 
length of the line, with a VNA or VIA, then export the data to SimSmith 
or equivalent to first transform the data to the load end of the line, 
then use SimSmith to find those high voltage points. To do all this, of 
course, you must also know Vf at the frequencies of interest.

BTW -- this same issue of the variability of the Z of the source and the 
load strongly affect the performance of ANY passive network, including 
bandpass filters. We tune them with a 50 ohm source and 50 ohm load, but 
use them with sources having very different impedances, especially in 
their stop-bands.

In simpler words, things are more complex than they often appear. And 
this comes not from a throne, but from staying awake in my EE classes 60 
years ago.

73, Jim K9YC

What's On Thu,4/16/2015 2:13 PM, Don Jones wrote:
> BS? Wow... I think you would benefit from reading W2VJN article 
> recently published in the NCJ. But the would require you to get off 
> the fricken throne to go find it.
> But then you have it all figured out so why bother.
> KO7i said that.