Posted by
ae4pb on
Apr 03, 2020; 8:00pm
URL: http://elecraft.85.s1.nabble.com/75ohm-Impedance-question-K3S-and-Antenna-side-tp7659364p7659472.html
Hi Frank,
I don’t know the manf or part number. This is ½ hard cable from the cable company that I received as a gift from very fine OM K4MQG, Gary Dixon.
I just guessed at the freq for 10m. I don’t use the band so I just put something in.
The extra digits are a result of the number used as the numerator in the calculation and I didn’t want to round it because I’ll convert that to inches and add a bit before I actually cut.
I used 80% for the vf
Thank you Frank !!
My antenna purchase (Hustler 6BTV ) is slightly delayed (holding onto cash until after the pandemic).
I can still get the radial and coax work done.
Jerry – AE4PB
73
..
From:
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Sent: Friday, April 3, 2020 2:27 PM
To:
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Subject: Re: [Elecraft] 75ohm Impedance question K3S and Antenna side.
Hi Jerry,
What coax do you plan to use (manufacturer and part number)?
Did you intentionally use 27 MHz in the last row?
Your other calculations look good but you carried them to far too many
significant digits when the Vf of most coax (especially CATV coax)
is only approximate.
Cut your coax ten percent too long, leave the end of the coax open circuited.
Your antenna analyzer will then set the length very accurately.
73
Frank
W3LPL
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Sent: Friday, April 3, 2020 12:53:40 PM
Subject: Re: [Elecraft] 75ohm Impedance question K3S and Antenna side.
So I found a formula for feedline length and just need some eyes on it to make sure I applied it correctly.
I want to measure twice and cut once. Then I can use my antenna analyzer with a dummy load on the line to trim it.
The columns are band, freq in Mhz, Multiplier to give multiples of the 1/2 wave length.
You have to view the email full screen width or the tables wrap making it hard to read.
It looks like 111.7694545 feet is the calculated half wavelength for 80m (3.53Mhz), not sure what will happen on 30m as it's closest length appears to occur at 3 wavelengths where the feedline would be about 4.801946936 Feet too long.
Not sure what the SWR related losses will be but it will probably be ok.
BAND Freq 1 2 2.5 3 3.5 4 4.5 5 5.5 6 6 6.5 7 7.5 8
80 3.52 111.7694545 223.5389091 279.4236364 335.3083636 391.1930909 447.0778182 502.9625455 558.8472727 614.732 670.6167273 726.5014545 782.3861818 838.2709091 894.1556364
40 7.04 55.88472727 111.7694545 139.7118182 167.6541818 195.5965455 223.5389091 251.4812727 279.4236364 307.366 335.3083636 363.2507273 391.1930909 419.1354545 447.0778182
30 10.125 38.85713383 77.71426765 97.14283457 116.5714015 135.9999684 155.4285353 174.8571022 194.2856691 213.714236 233.142803 252.5713699 271.9999368 291.4285037 310.8570706
20 14.08 27.94236364 55.88472727 69.85590909 83.82709091 97.79827273 111.7694545 125.7406364 139.7118182 153.683 167.6541818 181.6253636 195.5965455 209.5677273 223.5389091
15 21.4 18.38450841 36.76901682 45.96127103 55.15352523 64.34577944 73.53803364 82.73028785 91.92254206 101.1147963 110.3070505 119.4993047 128.6915589 137.8838131 147.0760673
10 27 14.57142519 29.14285037 36.42856296 43.71427556 50.99998815 58.28570074 65.57141333 72.85712593 80.14283852 87.42855111 94.7142637 101.9999763 109.2856889 116.5714015
Formula used: L = (S * W * Vf)/f
S 983.5712
W 0.5
VF 0.8 983.5712
L = (S * W * Vf)/f
S speed of light in meters or feet
W number of wavelengths desired
VF velocity factor
Jerry Moore
Cell: 803-431-1870
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