Mobile antenna feedpoint impedance

[6.0 Liter]

I didn't want to take away from the mobile gamma thread, so here goes something for discussion. My 55 antenna shows around 36 ohms at the feedpoint, sounds normal right? When my full length antenna shows resonance, it is showing around 63 ohms. Just seems a bit backwards to me. SWR is showing 1.3 like it should, just has me scratching my head a bit....... Not a biggie thing, just one of those why things. Maybe more bonding? I don't know............

 

[Captain Kilowatt]

I didn't want to take away from the mobile gamma thread, so here goes something for discussion. My 55 antenna shows around 36 ohms at the feedpoint, sounds normal right? When my full length antenna shows resonance, it is showing around 63 ohms. Just seems a bit backwards to me. SWR is showing 1.3 like it should, just has me scratching my head a bit....... Not a biggie thing, just one of those why things. Maybe more bonding? I don't know............

Ground losses are added to the radiation resistance and shows up as total impedance. With lower ground losses the impedance will go down. Ground losses can be lack of a groundplane or proximity of the antenna to ground itself. This is why a typical 1/4 wave mobile antenna which has a feedpoint impedance of around 35 ohms will show an impedance of 50 ohms in a typical installation. Ground losses of at least 10 ohms are normal and greater than 25 ohms are more than possible. I am assuming he 55 is loaded? Any loaded antenna will have an impedance lower than that of a 1/4 wave and I suspect that is one reason you are seeing a lower impedance but that could be just one reason. Another reason could be proximity to metal. Are both antennas mounted in the exact same place?

 

[Marconi]

I didn't want to take away from the mobile gamma thread, so here goes something for discussion. My 55 antenna shows around 36 ohms at the feedpoint, sounds normal right? When my full length antenna shows resonance, it is showing around 63 ohms. Just seems a bit backwards to me. SWR is showing 1.3 like it should, just has me scratching my head a bit....... Not a biggie thing, just one of those why things. Maybe more bonding? I don't know............

55, may tell you that radiators with big coils raised up above the base tend to deal better with the poor ground planes that auto bodies present, and that the 36 ohms you're seeing is more efficient, and operates with far less matching loss that the full length whip you tell us about, regardless of the nice SWR you see.

I think it was Walter Maxwell, W2DU, that described this situation as follows: "...a low SWR for the wrong reasons."

CK tells it better.

 

[BOOTY MONSTER]

1/4 wave impedance is also effected by the angle/shape of the ground element/vehicle body .

Antenna Notes

"As shown in the following graph, when the "radials" are at 0 degrees, the impedance is about 70 ohms, since we are really dealing with a vertical dipole. As the radials are raised to an angle of 90 degrees, the impedance drops to around 20 ohms. Notice that when the radials are at an angle of about 45 degrees, the impedance is very close to 50 ohms, which is similar to the inverted Vee."

 

[6.0 Liter]

I like the way you explained that CK......yes the 55 is loaded, both antennas were placed in the same spot, puckmount on the top of a pickup cab about 18" from the back of the cab. Truck is a 4 door F 250 so there should be pretty good ground plane toward the front. That graph is good info Booty, thinking about the shape of the truck and how it would react according to that scale is an interesting thot. And Macaroni, I wondered why that full length radiator always tuned a little longer, coming in around 112", mabe trying to compensate for a lack in ground plane of sorts........mabe some extra bonding would be in place.