Adjusting antenna that need to be longer

[NeoHam]

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.

 

[binrat]

If it was me, 45 degree angle down on the ground radials before you make another one.

 

[mr_fx]

If it was me, 45 degree angle down on the ground radials before you make another one.

yep should make for a better match

 

[W5LZ]

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

 

[KC9Q]

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

 

[NeoHam]

If I google 5/8 ground plane pictures almost all of them have horizontal radials. I'm kind of making it like these:

5-8 Ground Plane

http://www.vk4adc.com/web/index.php...44mhz-antennas/50-2-metre-58th-wave-whip.html

http://www.parelectronics.com/pdf/OA432OMNITEST.pdf page 17

 

[binrat]

I think Doc has the answer. Is the coil bottom feed or connect partially the way up??

 

[NeoHam]

End fed. I know 1/4 waves should be at 45 degrees but does that apply to 5/8?