Bob
When I looked at your charts, I see an impedance curve hitting about 43
ohms at 1.8 Mhz (close as I can read it) and the reactance curve shows a
transition at this point, so this is just 43 ohms resistance. Using a
50 ohm reference that should give you about 1.16 SWR, not 1.7. So if
your meter says 1.7 then it is either set to 75 ohm reference or one of
the charts is incorrect.
I did an EZNEC simulation using no coax, and found 43 ohms resistance at
the antenna terminals at a resonant point of 1.79 MHz, and SWR of 1.16.
If you use 100 feet of RG8X, the resonant point stays at 1.79 MHz, R
goes to 59 ohms, and the SWR goes to 1.17. If you use 75 ohm coax, the
resonant point shifts to 1.78 MHz, the resistance goes to 135 ohms, and
SWR goes to 2.7.
If you made these measurements with 100 ft of 75 ohm coax, I don't know
how you got the answers you did. (Unless by chance the feedline length
is actually 180 feet.) If you have a large transmission line mismatch,
the SWR you measure at the end, relative to 50 ohms, can vary widely
depending on the length of the feedline.
(And before someone comments that the feedline length doesn't change the
SWR, I said "measured SWR using 50 ohms as a reference")
Your description of your soil matches what I used in EZNEC (0.0075s/m
and diel=12). Sounds like better than average RF ground. Your
measurements with resistors sounds correct, or within tolerance. Note
that carbon resistors can easily be off more than their marked tolerance
depending on their age. You can sometimes bake these things and get them
back closer to their original tolerance.
Jerry, K4SAV
Robert Chudek wrote:
>Hi Jerry,
>
>Thanks for the message, comments, and observations!
>
>BTW, the 160 meter Inv Vee is fed with 100' of RG-59/u coax. My understanding
>is an Inv Vee will present a better match to RG-58, but my roll of RG-8x
>shipped late. I will swap out the coax, cut it a multiple of half wavelength
>(electrical) at 1.830 MHz, and re-scan the antenna to see what I get.
>
>The meter is set for 50 Ohms. But I checked the analyzer calibration with 50,
>81, 100, and 560 ohm carbon resistors. I ran a sweep from 1.7 to 50 MHz.
>Here's the results:
>
>With a 50 ohm load (1%), it reads a flat 1.01:1 thru this range.
>With a 81 ohm load (1%), it reads between 1.62 ~ 1.64:1 thru this range.
>With a 100 ohm load (10%), it reads between 2.25 ~ 2.28:1 thru this range.
>With a 560 ohm load (10%), it reads 11.0:1 at the low frequencies but moves to
>12:1 and down to 10:1 in a sine wave pattern as the frequency escalates.
>
>Determining the appropriate ground conductivity has always been a challenge at
>this qth. The ground is relatively flat for miles in all directions. The first
>mile in any direction is +/- 30 feet elevation. If you look at some of the
>photos in my photo album, you will see I have a pond / stream running around
>270 degrees of my house. This area is typical wetland, mucky, marshy, you'll
>sink 18" in the slime if you try to wade through it. The ground has a high
>concentration of clay. I got both my van and tractor stuck several times when
>I first moved here (city slicker moves to the country!). Boy that stuff is
>slippery! The closest woods is about a quarter mile away.
>
>So by 160 meter standards, I classify the terrain as "flat". On 10 meters,
>that would be a different story. But I don't really know what the story is.
>
>I'm headed outside because I need to make a 40m dipole for SS!
>
>(Did I mention that I was have WAY TOO MUCH FUN with this stuff???)
>
>73 de Bob - K0RC
>
>
>Message: 6
>Date: Sat, 05 Nov 2005 00:31:18 -0600
>From: K4SAV <RadioIR@charter.net>
>Subject: Re: [TowerTalk] The AEA VIA Analyst (long)
>To: towertalk@contesting.com
>Message-ID: <436C51B6.3080905@charter.net>
>Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
>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
>_______________________________________________
>
>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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>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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