Mobile antenna help - single band vs dual band - roof top mounting location

[SailorDan]

TL;DR: Is there a performance benefit to using a separate 1/4 wave 2m and a 1/2 wave 70cm antennas and combining inputs with a duplexer as compared to using a single dual band antenna? What would be optimal positioning (48" x 82" roof)? What about adding a scanner antenna?

I'm finally getting around to installing my mobile (FTM-400XDR) in a Jeep Cherokee. I'm planing on drilling the roof for permanent mounts because of expected improved performance compared to a hood/fender mount. I'm considering using 2 dedicated band specific antennas instead of a single dual band because of decreased height (the dual banders I've seen are coming in 30"ish)- might I see a performance increase as well?

Mounting location - If one antenna, I'd put it in the middle of roof. If two antennas, I'd mount midline dividing roof into thirds (about 2ft between each)

--- What about adding a scanner antenna? Should I place it on the hood (about 1.5' vertical and 3.5' horizontal separation from nearest transmitting antenna) or could I place it midline/front of the roof and the transmitting antennas at the rear of the roof on left/right side?

From my research, placing the scanner antenna on hood would be the safest for the scanner, but I loose a 360 ground plane.

How would you configure these 2-3 antennas? Unfortunately a dual bander on the roof won't fit my parking garage - not sure it would be worth the inconvenience of lowering each time - and eventually forgetting.

All - I just stumbled across a post by CopperWhopper67, where mmckenna provides some great input and answers many of my questions.

I'm always open to suggestions, but I think I have a way forward.

The Jeep is gutted on the inside so I would like to install mounts now with an eye towards future flexibility - swap antennas out as use case / radios change.

I am still interested in knowing if there is a compromise in performance of a dual band antenna compared to a band specific antenna.

 

[RFI-EMI-GUY]

It has one coax connector for the antenna which makes use of a dual band antenna the simplest choice. If you want to use two antennas you will need a diplexer for 2M/70CM like a Comet model. You will probably have better performance with separate antennas in that the efficiency will be better and you will have the option of gain antennas. Spacing on a metal roof, which I assume the Jeep Cherokee has, should keep in mind that you want each antenna to have at least 1/4 wavelength of flat steel all around it. The scanner antenna will be the one most affected by the other transmitters. I would try to space it as far away as possible. If you are listening only to 700/800 MHz, get a short 1/4 wave whip for the scanner. Otherwise you might need a diode limiter (Made by HP, sold on Ebay for $25) As far a hood mounting, that seems to be a "Jeep" thing, and performance wise it is always crap.

 

[mmckenna]

TL;DR: Is there a performance benefit to using a separate 1/4 wave 2m and a 1/2 wave 70cm antennas and combining inputs with a duplexer as compared to using a single dual band antenna? What would be optimal positioning (48" x 82" roof)? What about adding a scanner antenna?

I'm finally getting around to installing my mobile (FTM-400XDR) in a Jeep Cherokee. I'm planing on drilling the roof for permanent mounts because of expected improved performance compared to a hood/fender mount. I'm considering using 2 dedicated band specific antennas instead of a single dual band because of decreased height (the dual banders I've seen are coming in 30"ish)- might I see a performance increase as well?

There are many dual band antennas that are 1/4 wave on VHF, so around 19" tall.
Larsen NMO-2/70SH was the one I ran for quite a while.
EM Wave and others make multiband antennas that will work, all around 19" tall.

Mounting location - If one antenna, I'd put it in the middle of roof. If two antennas, I'd mount midline dividing roof into thirds (about 2ft between each)

--- What about adding a scanner antenna? Should I place it on the hood (about 1.5' vertical and 3.5' horizontal separation from nearest transmitting antenna) or could I place it midline/front of the roof and the transmitting antennas at the rear of the roof on left/right side?

Mounting multiple antennas on the roof needs to take into account a few things:
-You need to consider the amount of ground plane under each antenna. The antenna will be happiest if you have a 1/4 wavelength in all directions under the antenna base.
-You want physical separation between the antennas, enough to reduce the impacts of the strong RF field getting into things you don't want.
-Consider that coaxial cable will have more loss at higher frequencies, so if you want good performance on both bands, put the UHF antenna closer to the radio (shorter feedline) than the VHF.

The scanner will be impacted by the strong RF field when transmitting with the ham radio. Best case, it'll blank out reception when transmitting with the other radio. Worse case, it'll damage the front end of the scanner from too much RF getting into it. Keep the scanner antenna well separated from the transmitting antennas.

From my research, placing the scanner antenna on hood would be the safest for the scanner, but I loose a 360 ground plane.

If done right, you'll be fine on the roof and it'll work much better/look less dorky*.

*my definition of dorky may be different than yours.

How would you configure these 2-3 antennas? Unfortunately a dual bander on the roof won't fit my parking garage - not sure it would be worth the inconvenience of lowering each time - and eventually forgetting.

19" won't fit? Then how are you going to do a 1/4 wave VHF antenna?

All - I just stumbled across a post by CopperWhopper67, where mmckenna provides some great input and answers many of my questions.

I'm always open to suggestions, but I think I have a way forward.

The Jeep is gutted on the inside so I would like to install mounts now with an eye towards future flexibility - swap antennas out as use case / radios change.

I am still interested in knowing if there is a compromise in performance of a dual band antenna compared to a band specific antenna.

Separate antennas for VHF and UHF gives you the option of controlling the radiation pattern a bit better. The problem with dual band antennas is that you are usually stuck with one band being compromised when looking at the radiation pattern of the antenna. A 19" tall dual band antenna is going to act like a 1/4 wave on VHF and have a nice globe like radiation pattern. To get a 19" tall antenna to work on UHF, it's going to be loaded in some fashion and will have a radiation pattern that sends more energy towards the horizon. Might be fine in some applications, might not in others. That's something you need to decide. But you are way ahead of most hams because you are putting a lot of thought into this. Many hams just grab an antenna, drop it on the roof and run the coax through the door and call it done….

A diplexer will result in a small amount of loss, probably not noticeable, but not zero, either.

A 1/4 wave VHF antenna will be about 19" tall. A thin whip will easily flex when driving into a garage and won't be an issue. The quarter wave will give you a nice round radiation pattern without a bunch of lobes that may or may not work in your favor. You'll want 19" of metal all around the base of the antenna to give it sufficient ground plane.

A 1/4 wave UHF antenna will be able 6" tall. A thin whip that will probably clear any garage, but if it doesn't, it'll bend out of the way. Same radiation pattern as above, nice and round. You'll want 6" of metal all around the base of the antenna for sufficient ground plane.

Plus, you can easily swap out antennas depending on what you need for a specific application.
Chances are, the Cherokee has enough roof to put each antenna in 19" from the side edge of the roof, and back from the front edge, and you'll be fine.

Figure out where your RF deck is going, and keep the coax short as possible.

Quarter wave antennas also have a LOT of useable bandwidth, so receiving outside the ham bands won't be a challenge (within the limitations of the diplexer).



Challenge you have is getting all that on the roof of the car properly spaced and leaving enough room for the scanner antenna. With a good diplexer, you don't need a lot of spacing between the VHF and UHF antennas as the diplexer will protect the other band.

You do want lots of spacing between the scanner antenna and the other transmitting antennas. A Jeep Cherokee should have plenty of roof space to do that. Put the scanner antenna towards the rear of the roof and you should be fine.

 

[SailorDan]

RFI-EMI-GUY / mmckenna - Thank you both for taking the time to provide some great responses. As I've searched the forums you have both taken the time to hand hold chuckle-heads like me, provide solid input and help us learn the ropes.

Thank you - it is much appreciated!

mmckenna - I did not know about the shorter dual-banders - 19" is great and was what was driving me in part to a separate UHF / VHF antenna (most of what I was being was 30"+). I think I'm going to plan for 3 NMO mounting points and then play with antenna selection.

 

[Project25_MASTR]

Several reasons for, several reasons against.

I recently went through this dilemma on my GMC Sierra (which is probably the most professionally comm'd out, beat up pickup in Texas running farm tags right now). Originally, I was setup for a VHF, UHF and 7/800 XTL5000's using a Motorola OEM triplexer (which is an accessory for the APX8500) to get them onto a common antenna. I never really had enough time for radio as a hobby then (and honestly I was kind of burnt out on it all) so I never finished setting up more than two at a time (just using a simple Sti-Co V/U diplexer) with a single Panorama Shakree with two different iterations of their Triband antenna.

Well, I never really liked the way triband performed on VHF and it has some certain nulls on 7/800 so I ended up saying "F*** it" a few weeks ago when gearing up to install my (big) Johnson setup mainly so I could get Phase 2 support on 7/800. Pulled the Panorama...discovered a 7 year old paint issue from when the truck was repainted (white GM, nuff said), painfully installed a Breedlove 2" diameter NMO mount in it's place with a classic Motorola branded HD Antenna Specialist VHF 1/4 wave. Then I proceeded to add two more mounts (regular NMO thankfully) and added a 7/800 MHz Motorola branded knob antenna and a wideband PCTEL UHF knob (covers 430-480 MHz).

My main goal was bandwidth and lowish profile. Thus the knobs and a HD 1/4 wave. Since one of my radios is dual band Viking radio (VHF and 7/800) I broke out the Sti-Co V-7/8 specific diplexer I had and plumbed it in backwards (i.e. single output port on the Viking mobile hits the diplexer and splits to a VHF and 7/800 antenna). The UHF antenna just goes straight through to the UHF radio. I've been debating switching the VHF whip out for a Sti-Co Flexiwhip...but I really only think about that when I drive into parking garages.

On my XJ I run a VHF 1/4 wave whip on the roof and a low band 1/4ish wave whip on the hood. I've thought about maybe upgrading that XTL5000 over to a dual band Viking radio but I don't do radio support out of that rig. The XTL5000 got used simply because I had a bunch of them and could remote mount as needed at the time.