antenna resistance
Thread Starter
Drifting
Joined: Nov 2002
Posts: 1,665
Likes: 1
From: Ontario
antenna resistance
I just removed my car's antenna. The coax of the antenna had a lot of sharp kinks and I wanted to make sure there was continuity between the antenna and the centre conductor of the coax. I measured 17k ohm, but I thought it should be a dead short. My reception is only okay, it could be better.
Should my antenna be a dead short or is 17k ohm a normal amount of resistance?
EDIT - I've been doing some research - for maximum power transfer, and therefore best reception, you should have the impedance of the antenna equal to the input impedance of the reciever. I think in most cases a radio's input impedance is 50 ohms. Some antennas also have a capacitor in series with the line. Its looking like my antenna is not 100%.
[Modified by Boofers, 5:59 PM 4/11/2003]
Should my antenna be a dead short or is 17k ohm a normal amount of resistance?
EDIT - I've been doing some research - for maximum power transfer, and therefore best reception, you should have the impedance of the antenna equal to the input impedance of the reciever. I think in most cases a radio's input impedance is 50 ohms. Some antennas also have a capacitor in series with the line. Its looking like my antenna is not 100%.
[Modified by Boofers, 5:59 PM 4/11/2003]
Thread Starter
Drifting
Joined: Nov 2002
Posts: 1,665
Likes: 1
From: Ontario
Re: antenna resistance (Cookwithvette)
To make this measurement I set my DVM to resistance, put one lead on the antenna and the other lead on the centre conductor of the coax. Now that I think about it I'm not sure my measuring method was correct. The DVM uses DC to find the resistance, but impedance is frequency dependant. At FM frequencies the impedance just might be 50 ohms. But what about Am frequencies, they are much lower than FM.
I'm sure that about 7 years ago when I was in college I knew this...
I'm sure that about 7 years ago when I was in college I knew this...
Burning Brakes
Joined: Oct 2001
Posts: 1,185
Likes: 0
From: New Wilmington PA
Re: antenna resistance (Boofers)
I just went out to my car and noticed I could get to the antenna plug by the radio rather easily so I went ahead and tested as you described. Doing so I found out why my radio reception has been kinda lousy... my antenna isn't hooked up. I get no continuity at all between the coax plug at the radio and the antenna. :rolleyes:
I did measure the antenna itself and got only around 1 Ohm which I'm assuming is just dirt.
I did measure the antenna itself and got only around 1 Ohm which I'm assuming is just dirt.
Le Mans Master
Joined: Jun 2002
Posts: 8,339
Likes: 24
From: I can walk to MA
Re: antenna resistance (Boofers)
I didn't think that there are any passive components actually IN the antenna assembly. I may be wrong. Sometimes an inductor (coil) is placed in series
with the antenna (usually at the base) to increase it's electrical 'length'.
This could add to the DC impedence - but 17K Ohms is kinda high for a small coil.
You would need some pricey equipment to check it for sure. Might be better
off buying a new antenna instead.
Mine is the removable mast type. It is a solid steel mast - no capacitor.
Now, the base that it fits into may have internal components, but I have
never heard nor seen such a thing.
Usually the shape and length of the mast itself - coupled with the surrounding
ground plane will determine the dynamic impedence (at frequency).
multiples of 1/4 wavelength are usually preferred. That puts a 2-3 ft mast
inside the FM band.
You just may have a loose connection somewhere in the coax. However, unless
you get static and noise that is intermittent, then I don't think it is an issue.
The current flowing through that Coax is miniscule. The DC impedance
does nothing to drop the voltage. It's all RF - so the multimeter will just give
an idea whether it's connected at all.
:seeya
with the antenna (usually at the base) to increase it's electrical 'length'.
This could add to the DC impedence - but 17K Ohms is kinda high for a small coil.
You would need some pricey equipment to check it for sure. Might be better
off buying a new antenna instead.
Mine is the removable mast type. It is a solid steel mast - no capacitor.
Now, the base that it fits into may have internal components, but I have
never heard nor seen such a thing.
Usually the shape and length of the mast itself - coupled with the surrounding
ground plane will determine the dynamic impedence (at frequency).
multiples of 1/4 wavelength are usually preferred. That puts a 2-3 ft mast
inside the FM band.
You just may have a loose connection somewhere in the coax. However, unless
you get static and noise that is intermittent, then I don't think it is an issue.
The current flowing through that Coax is miniscule. The DC impedance
does nothing to drop the voltage. It's all RF - so the multimeter will just give
an idea whether it's connected at all.
:seeya
Instructor
Joined: Mar 2003
Posts: 162
Likes: 0
From: Niagara Falls Ontario
Re: antenna resistance (NHvette)
The resistance from the antenna lead to the antenna whip should be zero or one or two ohms .The resistance from the centre lead to the ground should be very high in the mega ohms scale. Dave :cool:
