HAM Antenna Placement. Height vs Coax Length?

[Brit55]

Guys:
I need a little help with antenna placement on my E-350 van. I’m installing a Kenwood 2M radio and 40” whip antenna. The van will have a fiberglass high top and is used off-road, so a roof or gutter mount is out of the question.
Looks like I have a choice of front or rear bumper mount options. A front mount would put the tip of the antenna about the same height as the roof of the van. A rear bumper mount would put it higher than the metal body of the van, but just below the height of the fiberglass top and double the length of coax needed.
So, the question becomes is it better to have the antenna higher, or would signal loss from the longer coax negate the height advantage?

Thanks for your opinions.
Andy

 

[libarata]

Do you have rough estimates of the actual length of coax? High quality wire should not have THAT much of a loss for you.

 

[1911]

You are not going to have any significant loss from the length of the coax; a van is not that long.

 

[65BAJA]

The further away from the body you can get the antenna the better. Front bumper. Probably right side so it isn't always in your line of sight as you are driving.

 

[Dougherty03]

You are not going to have any significant loss from the length of the coax; a van is not that long.

This

 

[mpinco]

The further away from the body you can get the antenna the better. Front bumper. Probably right side so it isn't always in your line of sight as you are driving.

Not a good idea ..............

http://www.k0bg.com/ground.html

..........Since amateurs deal with a variety of grounds (earth ground, DC ground, RF ground, ground plane, chassis ground, isolated ground, etc.), it's no wonder so many are confused about which ground is which. This said, in a mobile scenario, there is only one other (antenna) ground we need to deal with, and that's a proper RF return for the coax shield. Remember, RF must flow back to its source. It will do so in the shortest path it can find (the one with the least resistance). Ideally that's within the superstructure of the vehicle the antenna is mounted on. However, improper mounting (atop long posts, extended brackets, clamps, and luggage racks as illustrated in the drawing) causes an inordinate amount of RF to flow on the outside of the coax (common mode current), or down inadequately-choked motor control leads if so equipped...........


Mounting Location Issues

The best place to mount an HF mobile antenna, is in the center of the roof. This places it as far away from the surface the vehicle is sitting on, and as far away from the vertical surfaces of the vehicle as possible. With respect to system losses, any other position on the vehicle will exhibit more loss. And contrary to popular belief, DC or RF round straps will not negate this premise!...........

 

[Brit55]

Do you have rough estimates of the actual length of coax? High quality wire should not have THAT much of a loss for you.

If I put the antenna on the front bumper coax length will be 13ft. To put it on the rear will need a 10th extension, so 23ft in all. (These are purchased lengths, not something I cut to length as needed)

 

[mpinco]

Without a ground plane (simple metal sheet), the RF return path is the coax and any connectors. Can be ugly introducing multiple "radiating" points. Use a small metal sheet / cookie can antenna mounted to fiberglass roof?

 

[Brit55]

Without a ground plane (simple metal sheet), the RF return path is the coax and any connectors. Can be ugly introducing multiple "radiating" points. Use a small metal sheet / cookie can antenna mounted to fiberglass roof?

Unfortunately any kind of roof mount is not an option due to the potential of damage from trees while on the trail. The van has Aluminess bumpers front and rear so I'm hoping that they will act as the ground plane to some degree.

 

[jwhutch]

I see many vans with their antenna mounted on an L bracket on the hood.

 

[DaveInDenver]

At 145MHz RG-58 has about 0.05 dB loss per foot (it varies), so the difference between 15 feet and 30 feet is around 0.75 dB.

That's about the equivalent of 5~6 watt difference in power at 50 W (17 dB) input. Realize that this a difference of 0.6 dB but the coax at 30 feet will still be losing 1.8 dB, which is not insignificant. A loss of 1.8 dB is 17 watts with 50 watts input while 1.2dB is still a loss of 12 watts. Point being that the difference is fairly small but loses are important to consider. I run RG-58 for a short run but anything more than a few feet gets RG-8X or full blown RG-8 or RG-213 over 50 or 100 feet, like the antenna on the roof at home.

Bumpers will provide very uneven ground planes at VHF, you're better off near or on the hood or roof, so the front bumper is probably better if you don't mind looking at it all the time.

 

[Idahoan]

You can do a roof mount if you want. They make an adhesive foil so you can make a ground plane by adhering on the underside. If it were me I'd get a half wave / no ground plane antenna and put it anywhere you like it.

Best choice is roof, next if hood and then everything else. Most vehicles.

 

[Brit55]

Thanks for all the input guys. I will be using a half-wave antenna. I'm leaning toward a front bumper mount at this point as it will be a simpler install. If the results are not up to par then I will have to do something else.

Andy

 

[Charlie56]

So what type of fiberglass top?

The Photo below was my E350 Sportsmobile with the penthouse top. With that arrangement there was enough room in front of the fiberglass top to mount an antenna. I used a 1/2 wave that did stick up a little high for off-roading in the forest but was great for the desert. If you look closely at the photo there is also a small (Comet B10) mounted to the drivers side fender. I used that one for a dedicated APRS rig. It did much more damage to the brush and limbs than it received. There were times in the forest when I would move the B10 to the top. Even with that smaller antenna I still had decent signal reports.

Then if you look really close at the passenger rear you will see a HF screwdriver that is mounted to the Aluminus bumper rack. That one took a beating on washboard roads. I would not recommend that placement. At least with a screwdriver that has moving parts.

If you do go with the fender mount, make sure your hood is metal. Ford made some E350's with fiberglass hoods. That would throw your pattern way off.

 

[Brit55]

Charlie:
Thanks for your input. I'm going with a full length fixed fiberglass top, and as mentioned the trees will damage anything up that high anyway. For now I'll try a front bumper mount and see how that performs.
Andy