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Adjusting antenna that need to be longer

NeoHam

NeoHam

New Again
Mar 27, 2012
84
9
18
MidMichigan
I am attempting to make a wire 5/8 gp but my swr is a little high. ! was calculating for 146 mhz. I cut my copper wire longer and made 10 turns around a pencil about 6" long. At 146 my SWR is 2.3 but at 158 mhz it is 1.1 indicating it is too short. If I know I am 12 mhz off is there a calculation for additional length. The standard formula won't work because of the coils.

PS I've got 4 horiz grounds. should they be horizontal or slope down.
 

If you've got the option, reach that 45 degree 'dangle' with a few stops in between. When you get to something close to what you want, quit. If you're only using an SWR meter to tune that antenna you really can't determine anything about resonance, only impedance matching. It may just be that I don't understand how you are describing your antenna, but I don't think you have a 2 meter 5/8 wave GP at all if that coil isn't tapped for feeding the thing.
Good luck with it.
- 'Doc
 
With the radials straight out on your ground plane the impedance is around 37 ohms, and by placing a 45 degree down tilt the impedance will be closer to 50 ohms. Also your radials should be 5% longer than a 1/4 wave radiating (vertical) element. So multiply a calculated vertical value by 1.05 for the radials. Your radial should be about 20.2" (using (234/146 MHz) x 1.05).

As far as the VSWR: the difference between 50 and 37 ohms is 1:1 vs 1.5:1.

See how that changes things. You my still need to lengthen your whip, but not as much as before.

73,
Mike
 

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