[AK-VHF] Helical Antennas

[Ed Cole]

Brandon,

For satellite comm, circular pol is better as satellites change attitude 
thru a pass so their linear pol signal can vary depending on whether 
they are aligned with a linear pol ground antenna.  Some hams have 
crossed yagis which can switch from Right-handed to left-handed circular 
pol.  But most just use Right-hand circular pol (RHCP).  My 46 element 
432 x-yagi is set up for RHCP.

If you make a helical beam be sure the coil is wound in RH sense (seen a 
clockwise rotation) facing the satellite from behind the antenna.

Impedance of a helical beam is 140-ohm so one usually makes a 
quarter-wave long matching section at the antenna connector.  Solder it 
directly to the coax center pin and space it about 3/16 inch further 
from the ground plane reflector at connection with the helix coil.  1/4 
inch wide copper strap works well

I made a six turn helical feed for a 30-inch dish for 2401 MHz to 
receive the signal from AO-40.

Helical's are inheritantly wide band (15 to 20% of center frequency).

73, Ed - KL7UW

On 9/17/2021 8:45 PM, Brandon Clark via ak-vhf wrote:
> I was wondering if anyone in the group has experience with helical antennas?
> 
> I am looking into designs for 2 M and 70 cm antennas for satellite communication, and would like to use something having circular polarization. I'm also considering crossed dipole style yagis, but the helical designs seem to have an advantage of much simpler feeding arrangement, and possibly wider bandwidth.
> Brandon
> 
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