Low Band Antenna help

[smokey222]

I am looking to put up a mobile antenna as a base radio antenna and am concerned about the ground plane. I have a thought in mind on how to fabricate some radials, but....can find no info on how to calculate the length of them. Can anyone help on this? Frequency 39.80 mhz, not sure the wave length of the antenna, but it has a loaded coil and measures about 48". Thanks in advance.

 

[ka3jjz]

I'm going to hazard a guess that you have a 6 meter (50-54 mhz) antenna there - it kinda sounds like the Cushcraft magmount model. I used to use that on an old Regency scanner to listen to CHP California on 39 mhz from my work QTH in College Park Maryland when the skip was really rolling, so it should work just fine.

The standard quarter wave formula would work here; 234 / f(mhz) or in your case 234/39.8 which comes out to 5.87 feet. If you can get something metallic around 6 foot or so to stick onto, you'd be good to go

best regards..Mike

 

[smokey222]

ka3jjz,

Thank you Mike! I didn't realize it was that simple, have a great New Year! Scott

 

[WA1ATA]

The length of radials is not critical. The ideal is an infinite solid ground plan of zero impedance. Once you put up 3 radials of 1/4 wavelength or a bit longer, then there are greatly diminished returns from adding more radials and extending their length.

The most common set of radials is 3 or 4, with the length being 5 or 10% longer than 1/4 wavelength.

http://rudys.typepad.com/files/vertical-height-versus-radial-system-1.doc is an interesting doc file by Rudy Severns, N6LF, where he simulated a 1.8MHz antenna with a variety of radial systems. It has some interesting plots of gain vs number and length of radials.

While the modeled system is not the same as an elevated antenna on VHF lowband, the general conclusions should apply.