Topband: Was: Handheld Impedance Analyzer
tod at k0to.us
Tue Mar 29 10:33:19 EDT 2016
I am in complete agreement with Guy.
I have been using the AIM 4170 for at least five years and now have replaced it with the VNA 2180. Absolute overkill, but I tell myself I 'need' it because I want to build some filters, some single band antenna matching networks and some common mode chokes. Besides, I feel I am a better judge of how to spend money than my children are, so as a favor to them so I had better spent it now instead of leaving them a problem.
Characterizing the feedline from your station to the antenna feed point Is very straight forward with these instruments. That should eliminate tower climbing after the initial measurements calibration is done on the feed line from the station to the feed point.
After you get this equipment, and you start measuring everything you can think of, spend a few minutes saying, "what difference does it make".
If you have been smart enough to install relatively low loss feed line, the additional loss associated with a 4:1 SWR simply will not add enough additional loss to justify much beyond an L network in series with the feed line to assure that the transmitter is happy with the load it is sending power to. One setting of the L network is probably sufficient for providing a good load for the whole band. If not, use one for CW and another for SSB. Simply relay switch between them and forget the expensive remote tuners.
Resonance is totally unimportant as far as emission of RF is concerned. If you put power into the antenna system it will either be radiated or heat stuff up. Just use big enough components so that the loss in them is small compared to power being radiated! It probably is a good idea to check out the design so you don't have very high circulating currents in the L network tuners. Low Q will help here.
Sent from my iPhone 6
> On Mar 29, 2016, at 7:49 AM, Guy Olinger K2AV <k2av.guy at gmail.com> wrote:
> THIS is the way that you do it. The newer graphing boxes (most of them
> since AIM4170) will allow you to pre-calibrate a transmission line, and
> then see all results as if you were really up there.
> Get an instrument (they are getting less and less expensive) that will
> graph R and X (both below and above the horizontal graph axis) and see what
> is really going on. Trying to extend SWR as a prime instrument is
> ultimately doomed. SWR is around because in ancient days it was all that
> could be afforded and built from junk box parts. Since embedded CPU's are
> in everything from gas grills to washing machines to telephones to little
> kid's toys, the programs and the IC chips to make the essential math really
> simple have become very cheap.
> Resonance is where the graph line for X crosses zero. Just use your SWR
> like the oil pressure lamp in your car. Let it tell you where out of bounds
> for your amp is, tell you when something got broke in your antenna because
> it is not the same any more, etc. Don't try and use it for the deep stuff
> or initial adjustment.
> We do not have to use our hammers to fix our Swiss watch any more. Having
> the box tell you the R and X values at the FAR end of a transmission is a
> gift way, way too good not to be the first analytical line of attack.
> 73, Guy K2AV
> On Tue, Mar 29, 2016 at 2:07 AM, Jim Brown <jim at audiosystemsgroup.com>
>>> On Mon,3/28/2016 10:29 PM, Henk PA5KT wrote:
>>> You can calibrate it with your feedline connected so you do real antenna
>> One point of clarification. If you make a measurement that accurately
>> provides magnitude and phase, including the SIGN of the phase, then
>> accurately determine the electrical length of the feedline and subtract it
>> out with correct math, that measurement is just as "real" as if you took
>> your analyzer to the antenna and measured there with a zero length cable.
>> In other words, there is nothing "better" about measuring at the antenna if
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