Scanner antenna placement help please

[Railfan74]

I have a new to me 2012 Jeep Grand Cherokee. Looking for ideas on where to mount a antenna. Was thinking a lip mount on driver side front fender. I have a taller antenna that was given to me. If I recall correctly, it’s about 30 inches long. Also have the smaller length ones. The roof mount radio antenna is broken, replacing it would be around $225. Would it make sense to use that hole to mount a NMO? I usually don’t listen to AM/FM radio as I normally have the scanner on.

 

[Railfan74]

I posted on another thread, but made my mind up on the mounting location. Since the AM/FM antenna is broken off in the base of the shark fin roof mount, replacement cost is estimated at $225 for the part. I don’t normally listen to the radio as I am listening to the scanner. Not worried about losing Sirius reception as it’s spotty at best. Removal of the shark fin and placing a NMO mount through the hole seems logical. It doesn’t leak with the broken antenna in it now. This would also allow me to charge my antenna length as needed.

 

[IC-R20]

You can also get stealth antennas that look exactly the same as the shark fin, though a little bigger. They make for VHF, UHF, and 700/800/900 bands.

 

[mmckenna]

I posted on another thread, but made my mind up on the mounting location. Since the AM/FM antenna is broken off in the base of the shark fin roof mount, replacement cost is estimated at $225 for the part. I don’t normally listen to the radio as I am listening to the scanner. Not worried about losing Sirius reception as it’s spotty at best. Removal of the shark fin and placing a NMO mount through the hole seems logical. It doesn’t leak with the broken antenna in it now. This would also allow me to charge my antenna length as needed.

Are you sure the existing hole is 3/4" round? Some of those AM/FM/Satellite antennas use different size/shape mounts that may not be compatible with the NMO mount. What I can find on line shows a square hole.
Personally, I wouldn't want to mess with trying to install a round antenna mount into a square hole.

 

[Railfan74]

I have a 3/4 chassis punch. So that should work for making the hole round as long as it’s not too big of a square. I just got the NMO mount as the other one I had was worn out due to rubbing issues. I’m hoping that mounting the antenna there won’t affect the ground plane too much, as it’s not centered but more rearward.

 

[mmckenna]

I have a 3/4 chassis punch. So that should work for making the hole round as long as it’s not too big of a square. I just got the NMO mount as the other one I had was worn out due to rubbing issues. I’m hoping that mounting the antenna there won’t affect the ground plane too much, as it’s not centered but more rearward.

Ideally you want 1/4 wavelength in all directions under the antenna on your lowest operating frequency. It'll work with less, but there will be some slight degradation.

 

[Railfan74]

Forgive my silly question but what would the length of 1/4 wavelength be for a 18 inch and a 36 inch antenna?

 

[mmckenna]

Forgive my silly question but what would the length of 1/4 wavelength be for a 18 inch and a 36 inch antenna?

Depends on what frequencies you are using.

The 18" antenna is likely a 1/4 wave VHF whip, and you'd -ideally- was 18" in all directions under the antenna.

I don't know what the 36" antenna is used for, so you'd have to tell me.

 

[Railfan74]

I was told is was a UHF/VHF that ranged from 26 on the low band to 1000 on the high band. I used that length to get over the roof of my car as it was lip mounted to the driver side rear fender by the taillight so I could avoid the interference of the rear window defroster.

 

[Railfan74]

I have all the railroad frequencies, local county fire and dpw frequencies and air ambulance frequencies. The terrain varies from valley to over many hills. That’s why I was using the longer antenna.

 

[mmckenna]

I was told is was a UHF/VHF that ranged from 26 on the low band to 1000 on the high band. I used that length to get over the roof of my car as it was lip mounted to the driver side rear fender by the taillight so I could avoid the interference of the rear window defroster.

Ok, well, at 36 inches, it's not going to be great on low band. So I'd make sure it had a good ground plane on VHF high band, so 18" around the base like the other, if you can make that work. Even if it did work well on low band, you won't be able to get a perfect ground plane on a road legal vehicle. You just do the best you can.

 

[Railfan74]

Thank you. Low band is not very active in my area. So I’m just using the 36 inch antenna when I’m in hilly areas and the 18 inch for urban areas. Now I have to figure out how to get the headliner out so it can be center mounted.

 

[mmckenna]

Thank you. Low band is not very active in my area. So I’m just using the 36 inch antenna when I’m in hilly areas and the 18 inch for urban areas. Now I have to figure out how to get the headliner out so it can be center mounted.

You may not need to remove the headliner. The standard NMO mounts can be installed with only access to the outside. Reusing the odd shaped hole may complicate things, and you probably need to get to the underside of the antenna, but from what I saw online, removal of the headliner was not required. Looked like maybe some trim removal would get you what you need.

 

[Railfan74]

Let me look at that. I’m thinking if I can pull the trim over the driver’s side rear door and then the trim where the driver’s seatbelt is I could run the wire that way to get it into the compartment underneath the driver’s seat. That way I can stow the extra length out of the way. The headliner might flex enough for me to fit my arm in to get the NMO base to where I need it. Also thinking of having a local body shop cut the hole. I know the owner and a favor can be asked if you sweeten the pot one way or another.