Mobile scanner ground plane/antenna questions

[TommyAce]

I've just installed two mobile scanners in my car. One will be strictly for monitoring 800MHz trunked systems, and the other will be used mainly for VHF frequencies.

First of all, for the trunking scanner, I happen to have an 800MHz antenna. It was not advertised as a "scanner antenna" when I bought it. It was probably made for a Motorola radio. If I connect some RG6 to it, will this antenna work okay as an 800MHz receiving antenna?

Second, my car is Mazda hatchback which has one of these plastic "sport wings" above the back window. I would like to mount the 800MHz antenna to the top of that plastic wing, approximately five inches from the metal roof of the car. After reading a bit, I understand that this antenna, like all antennas, should have a ground plane. Would you recommend that I cut a piece of sheet metal and fit it inside the wing so that the antenna has something metal underneath to which to mount?

Lastly, I'm sure that whatever VHF antenna I buy, due to its length, will need to be mounted at the bottom of the hatch. I've seen hatchback NMO mounts, but since the hatchback is vertical, and the antenna will be vertical, I assume I have to suffer the consequences of no ground plane?

I'm not super picky about crystal clear reception. (The Radio Shack all-band mobile scanner antenna that I have in my other car does good enough for me on all bands.) But, before I start drilling holes, I want to at least make sure I'm hopefully starting off on the right foot.

Thanks!

-Tom

 

[Freq_n_Hertz]

Tom,
The 800 MHz antenna should work well with your scanner if properly installed.

You are correct it should have a metal ground plane. At 800 MHz the ground plane should be 7" in diameter for best performance. I don't know how wide your wing is but if it is smaller you'll suffer some loss in gain and directionality in the front to back direction.

For the VHF antenna, you don't say if its low band or high band. If its the 136-174 MHz range then I would mount an 18" antenna on the roof for best performance.
From your concerns with length I'm guessing you're looking at a 30-50 MHz antenna in which case its all tradeoffs, You have to fit it wherever you can. They can be mounted to fenders, bumpers, hoods... and the more metal around the base the better gain you'll have, but you won't be able to get the >9' diameter it needs for optimum performance.

Freq_n_Hertz

 

[jackj]

Freq_n_Hertz is correct in most of what he states. Let me add that, since the ground plane supplies the other half of the antenna, it should extend least 1/4 wavelength from the base of the antenna in all directions. It also has to be attached to the antenna system as it is part of the antenna. Usually this attachment is accomplished by mounting the base of the antenna through the ground plane. If you are unable to satisfy this measurement, you can still get a good match on the antenna as long as you have 1/4 wavelength of ground plane in one direction but the radiation pattern will be directional toward the longest portion of the ground plane.

As for mounting your VHF antenna on the hatchback, you will have a problem. Assuming that the hatchback is metal, it will shield the antenna towards the front of the car and act to de-tune the antenna. It won't act as a ground plane. A 1/4 wave antenna needs to be mounted as nearly vertical to it's ground plane as possible for best results.

 

[TommyAce]

Thank you both for your answers.

The VHF antenna will be for the 136-174 MHz range. Actually, I only need to receive 154.000-156.000. Would a roof-mounted 150-168 MHz Laird Phantom Antenna (it looks like a 3" tall black cylinder) work okay? I don't need to pull in anything more than 10-15 miles away.

Because the VHF-Hi bandwidth I need to monitor is very narrow, would I be able to buy special coax and custom-make a VHF-Hi no-ground-plane antenna? Or would I be better of fitting a piece of sheet metal into the wing (max size about 32" x 8") and grounding a regular antenna to it?

Thanks,

-Tom

 

[jackj]

There are several types of antennas that don't require a ground plane to work, most common is a 1/2 wave length antenna. It would be possible to build a VHF Hi-band antenna that was a center-fed 1/2 wave antenna to mount on a car. The hatch back is still going to act as a shield and block reception toward the front of the car. That's not to say you won't pick anything up towards the front but it will be weaker than the same signal from the back. If you can mount it on the roof, that would be the best place as you could use a 1/4 wave antenna and the pattern would be close to omnidirectional. That said only you can determine the compromises you are willing to make.

 

[canav844]

Thank you both for your answers.

The VHF antenna will be for the 136-174 MHz range. Actually, I only need to receive 154.000-156.000. Would a roof-mounted 150-168 MHz Laird Phantom Antenna (it looks like a 3" tall black cylinder) work okay? I don't need to pull in anything more than 10-15 miles away.

Depends on your terrain if it's relatively flat and it's something that's using base or mobile stations + repeaters it should do fine, if you're looking to monitor simplex handhelds putting out low wattage then it might be too sketchy, additionally as there's not much physical antenna to catch the signal so if the VHF has to get over or around mountains or sometimes in large cities then you might be more prone to dead spots but it should do better than a scanner with a duck in that same area.

I use a hatch mount for my CB and have a Comet SSB-2 on my roof in place of the factory AM antenna (and it actually sits about 2in shorter than the factory antenna did as well, so it might be worth you checking out, it's tuned for the ham bands but does really well on nearby VHF high and if you needed to transmit you could tune it to do so). With the CB I don't have any issues with blockage from the vehicle or issues with ground plane. Because the 27mhz wave is so big the antenna does not suffer from partial vehicle blockage with only about 1/3 of the antenna clearing the roofline, with a VHF antenna your wavelength is about 9 meters smaller so if a VHF antenna was blocked by the metal of the vehicle it would reduce reception in that direction. Also the hatch mounts come with set screws that make contact with the metal on the lip of the body panel and use the body panel, which is grounded to the chassis as the "ground" with no real significant ground plane on my vehicle (small hatchback with large sunroof) well grounded antenna connections have made it so that I don't have any ground plane issues.

 

[K8TEK]

RG6 is 75Ω. Use RG58.