Instead of tearing into your antenna tomorrow, take the day off. It
appears that you have the AEA VIA Analyst set to 75 ohm reference
instead of 50 ohms. Your plots show a resistance of about 43 ohms and
and SWR of 1.7. This would be correct if the reference was 75 ohms. (A
case of too many available options?)
Nice plots from that instrument. For fun I tried to see how closely
EZNEC would match your data. I got R of 46 ohms at 1.79 MHz using
average ground, (pastoral/heavy clay, .005s/m, diel=13), pretty close.
If I change the ground to something a little better (flat marshy,
densely wooded, .0075 s/m, diel=12), I get 43 ohms, exactly what you
measured.
Jerry, K4SAV
Robert Chudek wrote:
>Hey gang...
>
>Here's the follow-up on my dead AEA SWR-121 I reported a few weeks ago. This
>unit was the original model sold by AEA about 15 years ago. Apparently it
>reached its EOL (end-of-life). The repair charges were going to be excessive
>so I traded this old unit in for their new VIA Analyst
>http://www.aeatechnology.com/html/via_analyzer.htm
>
>Here's a very quick review of my experiences using this analyzer during the
>last few days.
>
>The new 66-page user manual (on CD) is written for real-life amateur radio
>"issues", i.e., tuning HF antennas, cutting 1/4 or 1/2 matching lines,
>measuring coax attenuation, etc. I bought the full package which includes
>software for the PC, the interface cable, traveling case, etc.
>
>The manual cautions the user about taking readings in high RF environments.
>Indeed, the unit was useless at the qth of Lou, AC0X. He lives about a mile
>from 50 kW AM 1500 and has never been able to use any brand of analyzer on his
>HF antennas. Fortunately, I live in the sticks and don't have this problem.
>
>Here's the first task I put the unit to work on.
>
>I wanted to tune my 160-meter inverted vee antenna to 1.830 MHz. The apex is
>at 70 feet with one leg dropping down to 40 feet and the other leg dropping
>down to 15 feet. The wire is 14 ga hard drawn copper with a thick Teflon
>jacket. From the formula I ended up with 255.74 feet. I added a couple of feet
>for good measure so I started with 258 feet total.
>
>I ran the analyzer and saw a VSWR minimum at 1.750 MHz. I did a little more
>math and calculated that 10 feet would need to be trimmed to move the antenna
>to 1.830 MHz. I dropped one end, trimmed 5 feet, pulled it back up, and
>decided to see what effect this had. The Analyzer showed the VSWR minimum
>moved up 38 KHz to 1.788 MHz. Trimming 5 feet from the second leg moved the
>antenna to 1.830 MHz, right to the target frequency!
>
>I have posted PDF's of the charts at http://chudek.aberon.net in the AEA VIA
>Analyst Charts folder. Here you will find VSWR (long) for the first scan. All
>the other charts were created after the final 5 foot trim. You will find
>charts for VSWR, Impedance, Phase Angle, Reactance, Resistance, Return Loss,
>and a Smith Chart.
>
>Here's the question for the reflector.
>
>My next challenge is to determine why the minimum VSWR isn't closer to 1.0:1.
>It drops to 1.69:1 (RL= -11.8 dB) at best. Maybe someone with a little more
>experience in antenna matching can "see" the problem when viewing the graphs.
>I do see both the antenna reactance and phase angle make a sharp swing from
>negative to positive at 1.800 MHz. Beyond this, I haven't worked with Smith
>Charts in 20+ years!
>
>My immediate method will be the shotgun approach... remove the balun and see
>what changes... swap out the coax to see what changes... cut a 1/2 wavelength
>of coax for 1.830 MHz to get a true feedpoint measurement...
>
>Here's a little info on my purchase.
>
>At $695 retail, the AEA VIA Analyst is a pricey piece of test equipment but
>could be a good investment for a club or group of antenna enthusiasts.
>
>I am not a representative, have no monetary interest, or make any financial
>gain from AEA Technology. As a matter of fact, I will let you know the first
>unit I received from them was DOA... But their customer support was great. All
>I needed to do was provide a tracking number and they cross shipped a
>replacement unit the next day. The second unit worked perfect when it arrived.
>
>IMO, this would be a great product for a QST product review.
>
>73 de Bob - K0RC
>_______________________________________________
>
>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
>questions and ask for Sherman, W2FLA.
>
>_______________________________________________
>TowerTalk mailing list
>TowerTalk@contesting.com
>http://lists.contesting.com/mailman/listinfo/towertalk
>
>
>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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