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Re: [TowerTalk] analyzing antenna systems - some questions

To: towertalk@contesting.com
Subject: Re: [TowerTalk] analyzing antenna systems - some questions
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Mon, 20 Mar 2017 12:10:05 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On Mon,3/20/2017 10:32 AM, k7wxw wrote:
I am building my own antennas and have a couple of questions about using 
antenna analyzers properly. Here's the setup:

I have a 133' doublet, fed with  46' of 450 ohm window line, into a Johnson 
matchbox.  My rig is a FT-897D. I mostly run QRP though I venture up to 100W on 
SSB.  Because my inline SWR meter is on the repair bench, I depend upon the 
rig's SWR indicator.

That's probably good enough.

To characterize the antenna/tuner system from the radio, I've connected a Comet 
CAA-500 on the radio side of the matchbox to sweep through the bands by hand. 
Basically I set the frequency on the analyzer, and tune for lowest SWR.  So 
each measurement consists of the bandswitch/tune/match settings on the matchbox 
and the SWR and ohmic readings on the analyzer.

That's also probably plenty good enough.

So, three questions: does an analyzer like the CAA-500 have enough drive to 
properly drive the matchbox/feedline/antenna system?

If there are no strong signals (like nearby AM broadcast stations) nearby, yes.

  If it doesn't, what other options do I have for characterizing the antenna 
system?

My favorite is the VNWA 3e. http://sdr-kits.net/VNWA3_Description.html Depending on your technical background, you may find that you need a lot of self-study to take full advantage of it. For your needs, what you have is probably good enough if it will generate a Touchstone file that you can export to SimSmith.

And lastly, can anyone point me to a good tutorial on Smith charts? I'd like to 
see if I can convert the impedance I am seeing at the matchbox to what is 
happening up at the dipole feedpoint.

The ARRL Handbook and ARRL Antenna Book are great for this sort of thing, and ought to be on every ham's bookshelf. For SimSmith (free, runs in Java), there are both text-based and video tutorials, but they assume that you know the fundamentals of transmission lines and antennas. That's where the ARRL books come in.

73, Jim K9YC
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