Antenna choices Was: NX 5700 question...

[W7FDX]

I'm not sure where to put this thread so feel free to move it as the admins see fit. I bought a NX 5700 for my vehicle and want to do a fender mount antenna and was wondering if I should go with a quarter wave antenna or a half wave antenna? Drilling holes is not an option, too many sunroofs and not enough metal.

 

[kayn1n32008]

Half wave

 

[W7FDX]

That's what I'm leaning towards but I just wanted a few peoples opinions before I went and spend the money for the setup.

 

[kayn1n32008]

Buy both. A 1/4 wave is cheap. $10-$15. Make sure to buy and use NMO mounts, then you can change antennas at will.

 

[W7FDX]

I have several different 1/4 waves to try out but I've never bought a 1/2 so I was wondering the performance difference on a fender vs the 1/4 wave.

 

[kayn1n32008]

The half wave will perform better.

 

[mmckenna]

That's what I'm leaning towards but I just wanted a few peoples opinions before I went and spend the money for the setup.

In addition….

Make sure you get the Larsen NMOHF mount if you are going to install on a fender bracket.
If you use the standard mounts where the underside of the mount is exposed, water/road salts, etc. will quickly corrode the underside. Water will creep up the coax and corrode it out from the inside. In other words, you'll be replacing it every year or two.
Instead, get the HF mounts where the coax enters the underside of the mount through a protected entrance. Use some marine grade heat shrink tubing over that. That will keep the water out.

I've run across this a few times where someone used the cheap mounts that are designed to go through the roof top, only to have it fail. Save yourself the headache.

With the mount off to the side like that, it's not an ideal ground plane. Radiation pattern will be lopsided, 1/4 or 1/2 wave. You may have some challenges getting the 1/4 wave to give you low SWR on VHF frequencies, but you can likely get it low enough to be acceptable. The 1/2 wave will probably peforme a bit better since it doesn't require a ground plane (but works better with one).

 

[W7FDX]

In addition….

Make sure you get the Larsen NMOHF mount if you are going to install on a fender bracket.
If you use the standard mounts where the underside of the mount is exposed, water/road salts, etc. will quickly corrode the underside. Water will creep up the coax and corrode it out from the inside. In other words, you'll be replacing it every year or two.
Instead, get the HF mounts where the coax enters the underside of the mount through a protected entrance. Use some marine grade heat shrink tubing over that. That will keep the water out.

I've run across this a few times where someone used the cheap mounts that are designed to go through the roof top, only to have it fail. Save yourself the headache.

With the mount off to the side like that, it's not an ideal ground plane. Radiation pattern will be lopsided, 1/4 or 1/2 wave. You may have some challenges getting the 1/4 wave to give you low SWR on VHF frequencies, but you can likely get it low enough to be acceptable. The 1/2 wave will probably peforme a bit better since it doesn't require a ground plane (but works better with one).

I've always have used the NMOHF mounts when i've done fender mounts for a few friends l've helped out and they are very pleased with how they've turned out.

 

[mmckenna]

I've always have used the NMOHF mounts when i've done fender mounts for a few friends l've helped out and they are very pleased with how they've turned out.

That's good to hear.
Done right, no reason it shouldn't work. It's not an "ideal" location, but it's a bit more palatable to those that are concerned about drilling holes.

I've seen a lot of people install the standard NMO mounts that way, and in every case they've failed due to corrosion.

 

[W7FDX]

The vehicle has multiple sunroofs so it would be very difficult to find a spot to drill a hole for an amo mount, plus having to run the coax near the side curtain airbags would be a nightmare.

 

[AK9R]

I'll cast another vote for the Larsen NMOHF mounts. I have two mounted on L-brackets in the hood-fender gap of a Ford Expedition. No apparent corrosion after four Indiana winters. And, the performance with a Larsen NMO 2/70B or NMO 150HW seems to be acceptable.

I'm not opposed to drilling holes in the roof. But, I am opposed to leaving my vehicle in my driveway at night when I have a perfectly good garage. The clearance between the Expedition's roof and the bottom of the garage door when open is about 8 inches. I just don't want to subject even a 2m quarter wave to that kind of daily abuse.

 

[W5lz]

I would recommend a 5/8 wave antenna (base loaded naturally). At least around here, they are much more common than a 1/2 wave and not all that much of a clearance problem if mounted on a fender. They offer a minuscule amount of gain, where-as a 1/4 wave has a negative gain, and zero for the 1/2 wave. Not much, but some plus to that 'gain thang'. <Hey, put it on the trunk and look like one'a them LEOs! OOooo, yeah>

 

[W7FDX]

I would recommend a 5/8 wave antenna (base loaded naturally). At least around here, they are much more common than a 1/2 wave and not all that much of a clearance problem if mounted on a fender. They offer a minuscule amount of gain, where-as a 1/4 wave has a negative gain, and zero for the 1/2 wave. Not much, but some plus to that 'gain thang'. <Hey, put it on the trunk and look like one'a them LEOs! OOooo, yeah>

I'm leaning more towards the 5/8 wave just for the fact that I'll get a small amount of gain since I'm unable to do a proper NMO roof mount.

 

[mmckenna]

I'm leaning more towards the 5/8 wave just for the fact that I'll get a small amount of gain since I'm unable to do a proper NMO roof mount.

That would be something to try.
But remember, 5/8th's wave antennas require a ground plane. Mounted on the edge like that will result in a lopsided ground and you may have issues getting it to tune correctly. 1/2 wave antennas don't require a ground plane (but work better with one). A 1/2 wave with a ground plane will have a little over 2dB of gain. A 5/8th's wave will have 3dB of gain. You likely will not be able to hear that 1dB difference.

Give it a try, though and let us know how it works.

 

[W5lz]

Any vertical, if it's fed from one end, will require a groundplane as it's opposite 'pole' (+/-). That 'groundplane' may not be what you would normally expect to be a 'groundplane', but it's there somewhere. (Got to be two 'poles', a '+' and '-', or there's no current flow.)
An antenna's radiation pattern does depend on how/where it's mounted. With a typical mobile antenna moving it from the center of the roof to one fender isn't going to make a huge ~practical~ difference in the resulting radiation pattern. Just depends on how radical a change it is, where's that fender in relation to the rest of the vehicle?
Shooting for the 'perfect' antenna is something everyone does! No one ever gets there though. -Practical- is the biggy...