Mobile Antennas

[perrybucsdad]

I'm looking at getting a VX-6R/E once I get my license and will use this in my car. I plan on listening and transmitting to mostly repeaters and listening to anything else I might find interesting. There seems to be so many choices for antennas.

If I don't want a huge antenna on my car, but something modest, what are some things I should be looking at? What are some of the things I should be thinking about?

Thanks,

John

[n5ims]

Most of the antennas designed to work on the frequencies the VX-6R does should be fairly "modest". It's a VHF/UHF radio so those huge HF antennas wouldn't work very well on it anyway. Just find one that has the specs you want (as far as gain and height go) and you should be fine.

One suggestion though, if you stick with a standard NMO mount, you'll have hundreds of different models to swap between if you desire (obviously you won't have that many, but it's nice to have a choice) so you can get a very short model for intown use where you have to worry about garages and a longer one with good gain for highway use on trips.

I have a dual-band radio and swap out between the NMO2/70SH for around town and either the NMO2/70C or NMO150C (2 meters only) when I travel for that extra gain boost. I also use the NMO150/450/800 triband for my scanner when the dual-band radio isn't installed. The ones not in use easily fit in my trunk when not in use so swapping is easy when I get to my destination (or swapping out for the rain cap when I don't want to have any antenna on the car).

[perrybucsdad]

What kind of gain should I be looking at with the ant? Is the NMO mount the way to go? I have a hatchback, so I don't know if that will cause any limitations for mounting ideas.

Thanks for the help!

John

[n5ims]

Gain is a factor on where you are. In the city, minimal gain is OK and may actually be best. In rural areas you may need a higher gain value to reach the repeaters in the city and since there is generally less RF in the rural area, the added gain won't give you added intermod or overload your receiver's front end.

I recommend the NMO mount for the VHF and UHF bands, simply because they're the standard mount and work well, with many antennas made to use them. There are several different options for mounting them as well, from drilling a hole for the mount to ones that clamp onto a trunk lid (or hatchback in your case) to mag mounts for temporary installs. NMO stands for "New Motorola", although "New" in this case was from many years ago so it really is no longer a "new" design.

Be aware that the "NMO Mount" is simply the way to join the antenna to the antenna mount and has nothing to do with the connector on the end of the cable. An NMO mount generally has 18' (or so) of RG-58 type coax and there are many types of connectors that can be put onto the RG-58 type coax. While RG-58 does have a rather high loss at higher frequencies, the 18' run generally gives you only a small amount of actual loss.

The cheaper brands of mounts will use cheaper coax and give you more loss while better brands will use better quality coax and give you less loss. RG-58 is used since it's large enough to have minimal loss values and small enough to install easily. It also has the necessary flex to last while bouncing around on bumpy rides as well as when you open and close your trunk when using a trunk mount (for example).

I like the Larsen brand of antennas and mounts. They are quite common in commercial installations and are pretty high quality. There are "HF" and standard versions of the mounts. The "HF" style is designed to make better contact with the antenna (if it was designed to use that feature) for the 800 MHz and up antennas. They are designed so you can easily use either type of antenna when using an "HF" version, while the standard version won't work properly with an antenna designed for the "HF" version mount.

[k3td]

I agree 100% with N5IMS about the NMO mounts, and suggest a modest gain antenna so as not to overload the receiver in the portable.

[perrybucsdad]

Okay... I do live in the greater Cleveland area, about 60 miles east of the city and it is quite rural. From what I have seen on the databases here though, there are repeaters all over the place, but I drive from my home into the city every day. What would you all consider a modest gain? 1dBd?

Also, when I look at the ant's (like the NMO2/70SH), I see things split with two bits of info for the gain. Is that because it is a 2/70? What does that exactly mean? e.g. Gain: 0/1.8 dBd (2/4 dBi) . Also, what does the SH mean vs the C.

Sorry for all the dumb questions.

Thanks for the help... I really appreciate it.

[n5ims]

Originally Posted by perrybucsdad

Okay... I do live in the greater Cleveland area, about 60 miles east of the city and it is quite rural. From what I have seen on the databases here though, there are repeaters all over the place, but I drive from my home into the city every day. What would you all consider a modest gain? 1dBd?

Also, when I look at the ant's (like the NMO2/70SH), I see things split with two bits of info for the gain. Is that because it is a 2/70? What does that exactly mean? e.g. Gain: 0/1.8 dBd (2/4 dBi) . Also, what does the SH mean vs the C.

Sorry for all the dumb questions.

Thanks for the help... I really appreciate it.

"SH" indicates it's the short version. "C" indicates it's the version with the enclosed coil ("B" would indicate the open coil version). Nothing magic about this, just a code to indicate which sub-version it is.

The first gain figure is for the VHF-Hi (2 meter) part of the antenna and the second for the UHF (70 cm) part of the antenna.

Gain is reported in two general units, dBd and dBi. They differ from where the point of reference is. dBd is just over 2 units smaller than dBi when the exact same thing is being reported (it's kinda like feet vs meters). If they don't indicate what unit they're reporting the values in, assume it's dBi so you don't get fooled into getting the lower gain antenna after the marketing folks do their magic.

dB gain is an important spec on an antenna, but don't get too caught up in the "bigger is better" mess. 3 dB = double the signal, 6 dB = 4 times the signal, etc. That sounds like quite a bit of additional signal doesn't it? The fact is that 1 "s" unit (the values on many signal strength meters are shown in "s" units) is 6 dB. The extra 3dB will only change a weak signal from extremely noisy to very noisy. A strong signal will change from fully quiet to ... wait for it ... fully quiet (yes, no change at all).

The additional signal may even make things worse if you're in an area of heavy RF. The strong signals may mix together giving you what's called intermod, a signal being received on a frequency where it isn't actually being transmitted.

Generally a "no gain" antenna would be 0 dBd or 2 dBi. 3 dBd is generally pretty modest while 6 dBd is fairly high (on a mobile antenna).

[perrybucsdad]

Okay, so <= 3db is considered modest? I think most of the Larsen ant's are at that level at least on the one page I looked at.

[ab3a]

One thing I have learned over years of mobile radio use (on the job and ham radio) is that you should pay a lot of attention to ensuring that all connections at the NMO base are very clean, low resistance, solid connections.

That is more important than antenna gain. The reason I stress this is because with a vertical whip, the current going in to the antenna is at it's maximum right there at the base. Any resistance at all will result in very significant losses. Make certain that the NMO mount is cleanly installed on bare metal.

While antenna gain is interesting, and it can help, I have seen bigger problems and losses with NMO mounts that were never installed all that well in the first place.

[perrybucsdad]

Thanks Jake... so when you say the NMO base, are you talking about the ant connecting to the base or the base connecting to the vehicle (as in the case with a magnetic mount)?

[LtDoc]

I would think that the contact(s) between the mount and whatever you are connecting it to (the vehicle) would be a/the potential problem area. Making that as 'right' as you can is the idea.
NMO mounts are a little different than other types of mounts. Not really that difficult to get right, but also not 'dead easy' if you aren't familiar with them. 'Think' the installation through and at least getting 'close' to right is about all anyone can do.
- 'Doc

[perrybucsdad]

Okay, so let me ask this. I have a hatchback with a sun roof. This limits the areas that I can install to some degree, but I don't think it's too bad. I was thinking of installing on the roof just forward the hatch. So what things should I be considering with the NMO mount? Obviously, I would want the metal to be clean to get a good contact... but I see the line above about "bare metal", and I assume you are not talking about sanding the paint off correct, and just talking about making sure that both surfaces are clean... correct??

Would there be a better place to mount the ant vs where I have suggested? I do have a ant for the radio on the rear starboard (sorry, passenger) side of the vehicle, so I would assume I should try and avoid getting too close to that.

Thanks again,

jvn

[LtDoc]

That MNO mount isn't dependent on metal contact on the 'top' side as much as it is on the 'bottom' side of the mount. As long as you have a good electrical connection in one of the two places it will work just fine. I may have just been lucky but I haven't had to sand/strip the mating surfaces yet. After watching a number of other people install them, I haven't seen any of them doing anything special to get to 'bare metal'. I do know that the tool I use to cut that hole takes care of some of that paint removing in normal use. It removes maybe a 1/16" around the circumference of the hole. Oh well.
Where on the roof you mount an antenna isn't super critical either. The routing of the feed line, especially if there's other thingys in/on the roof, is the 'interesting' part. If it's possible to remove the headliner (without a huge amount of effort/$$$), that's a nice way to go about it. Otherwise, 'fishing' the feed line through all the stuff under that headliner is a very common way of going about it. (Know anyone with itty-bitty hands and arms about 5 or 6 feet long? Wish I did!)
Once you get to a 'pillar' you do a lot of routing easier (getting from the roof to the floor of the vehicle). Depending on the vehicle and the cable routing, I hope you didn't cut that 'too long' feed line yet, you know?
Good luck.
- 'Doc

(starboard/larboard, what's the difference??)

[perrybucsdad]

Hole?? Nope... not drilling or cutting holes in the car... needs to be something that leaves the car intact/

[LtDoc]

Good luck.
- 'Doc

[W2NJS]

What Lt Doc might be implying is that when you use an NMO anything short if drilling a hole represents a serious compromise. If you use a mag mount you're going to damage the car's paint underneath the mag mount, and a piece of plastic won't work because the moisture WILL get under it sooner than later. And if you put on a top rack mount you will have exposed coax with all of its eventual problems. But once you do the job right by drilling the 3/4 inch hole for the NMO and installed the unit you're home free and can forget about antenna problems for a long time. I'm in favor of using a radio shop for the job, where for maybe $100 to $150 you can get the hole drilled, NMO mount furnished, cable and correct connector installed, and the whole thing tested for proper installation. The NMO connector also usually includes a 20-inch whip that can be cut for 2M or UHF, or you can then start experimenting with dualband antennas you get from other sources such as the ham stores. The long and the short of it is that if you're serious and you wish to avoid problems in mobile radio installations you do it right the first time and then you can sit back, so to speak, and enjoy the radio while you're driving.

[biomedbob]

I've never had any antenna installation affect the resale value of an automobile.

[mark40]

After following this thread, was inspired to purchase some NMO mounts and drill. Great results followed. Had been using mag mounts and trunk lip mounts, each with their own deficits. When ordering the NMO mounts also purchased NMO caps for those times when the antennas need to come off, and also the plugs for the day when they come off. Have a 7 y/o car with 128,000 miles on it so it wasn't hard to get past the drilling part. If the car was new I may have opted for the radio shop. Good back and forth on the topic, definitely helped me to decide.

[dksac2]

A 3/8" hole drilled in the top of the car is hardly noticed when you go to sell. They make plugs, just paint it the same color as the car. If it's a lease, a drilled hole is not considered "body damage" and holes for antennas are drilled in leased cars all the time.

Thing is, the NMO mount works so much better than anything else. Some antennas like the one I use, a Comet CA-2X4SR requires a NMO drill through mount to work correctly, but it is one heck of an antenna. Moderate gain and wide banded. I get more compliments on a good signal with that antenna than any other I've had and it talks fantastic and hears as well as it talks. Pull the trigger, get what works, you will be glad you did.

John