Posted by
Bob McGraw - K4TAX on
Apr 01, 2020; 1:14pm
URL: http://elecraft.85.s1.nabble.com/75ohm-Impedance-question-K3S-and-Antenna-side-tp7659364p7659412.html
Based on TLW calculations for 1/2" CATV 75 ohm hard line, 200 ft at 28
MHz should show a 1.87:1 SWR at the transmitter. Assuming 35 ohms load
Z. This equates to 1.027 dB total loss with 0.207 dB being attributed
to the SWR on the line. Thus going through the efforts and matching
process would only net an improvement of 0.207 dB on 10 meters. Lower
frequencies would be less.
Since this is a 5 or 6 band antenna, I doubt the Z at the feed point
will always be 35 ohms. In order to determine what is actually needed,
one would need measurements for each band.
Conclusion: Feed it with 200 ft of 1/2" CATV 75 ohm hard line, use a
short jumper of RG6, or 8X or whatever to connect the hardline to the
radio and be done with it.
NOTE: I just noticed that the numbers below are somewhat in error.
468/3.540 = 132.2 which is 1/2 wavelength. Thus 132.2 X 2 = 264 ft
which is 1 wavelength, not 2 wavelengths as stated. I didn't run the
math on the others.
73
Bob, K4TAX
On 4/1/2020 7:28 AM, John K9UWA wrote:
> Ah YES I noticed it is for a multi band antenna.
>
> 2 wavelengths 3540 = 265'
> 4 wavelengths 7080 = 265'
> 6 wavelengths 10600 = 265'
> 8 wavelengths 14160 = 265'
> 12 wavelngths 21240 = 265'
> 16 wavelengths 28320 = 265'
>
> Multiply the 265' by velocity factor of the coax and you will be someplace just a bit
> over 200 feet.
>
> the 10mhz band is off a bit. So OK... other than that 80, 40, 20, 15,10 are all
> harmonically related. As I said 50 in and 50 out. If the OP runs this vertical on 17
> and 12m then yes he will have maybe a 1.5 / 1 mismatch caused by the coax. But
> the antenna isn't designed for 17 and 12.
>
> I cut all the 3/4" CATV in my system with a Hewlett Packard HP 4815A Vector
> Impedance Meter. Any 1/2 wave mult the the SWR is the same in as out. Any
> ODD 1/4 wavelength multiple creates a transformation With Multiple 4 hi stacks of
> yagi's I have many of those in the system as well. 75 ohm cable works well to put
> pairs or quads of yagi's together.
>
> The only thing that maybe won't be perfect is the actual vertical itself may not be a
> 50 ohm input. His vertical is a Hustler 6BTV.
> 73
> John k9uwa
>
>
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