Communications Antennas mounted on front bumper

[jwhutch]

Hey Everyone,

Total newb here. I am wondering why I see people mounting antennas to the front of the vehicle. Is this a preferred location for overland travel? This is a serious question. I have been a ham for a long time and have never noticed anyone mounting an antenna up front until I spent a couple of weeks in Africa this summer. Just curious.

On a second note, I am wondering why people mount a cable from the front bumper to the roof rack. I would guess this is to deflect branches. Again curious. Thanks in advance.

[mountainpete]

First, don't think I have had a chance to say Welcome to ExPo

I'll start with the easy one first. The cables you see running from bumper bars to roof racks are called limb risers. They are often used in heavily treed areas to deflect large branches up and over the vehicle. Very useful in the forest or jungle, probably a little more farkle (functional sparkle) in the desert. Search "limb risers" and you will find a few excellent discussions on how to install them and a recent one on spring loading.

For antennas mounted on bull bars, they are very common in Australia and Africa but less common in North America. Here is some very light details - others will chime in with more

One of the main reasons you see them so often in Australia is the type of antenna that is commonly used. CB radio usage is very different there compared to North America. Included are thick, heavy, special purpose antennas which are regularly used for long distance communications. These antennas need a very solid mounting point and an ARB bumper, etc, provide an excellent solid point. Due to the length and rigidity of the antenna, roof mounts are inconvenient and prone to failure.

In North America, CB radio is only used for short distances and antennas are light enough to be placed virtually anywhere. Some people do place CB antennas on bumpers, but it isn't as common. The next step up, VHF amateur, requires a wider ground plane then a bumper typically provides - therefore a fender or roof mount provides better reception. The antennas are also less rigid and could be distracting to a driver if placed in front (swaying, sleepy )

Hope that helps.

Pete

[AndrewP]

The front bumper is just about the worst place to mount an antenna of any type. The grounding there is poor, and there are usually sites on the vehicle with a good body ground and more height. A place with more metal up high to reflect signal off is usually good as well.

Remember, everything else being equal, height is king. Electrically, the center of your roof is king. Generally though, compromises get made and a lip mount on a hatch is pretty darn good.

For most uses , whether 2m/440 or 2m only, whip antennas are the toughest. and electrically they are pretty good too. Plus, they are cheap! I am partial to the Larsen antennas but there are other brands out there too.

I have not really followed the radio threads here, but I'll bet someone has published Lip mount pictures before.

KI6MIE

[jwhutch]

I guess I should sort of clarify. I have been a ham for 20 years. When I asked the question initially I thought that the antennas that I saw mounted on the front bumper were HF antennas, (or at least CB antennas). The trucks I saw in Uganda were usually UN or government type trucks, and the antenna was pretty substantial, (8-12 feet long, and very heavy duty). I was not aware of the difference in US CB and Australian CB, which is UHF, (477 mhz vs HF or 27 mhz). I looked and found a site that sells that stuff. I would think that a DC ground would be easy since the bumper is mounted to the frame. I would also think you may aleviate some of the issues of a vertical metal surface being in close proximity to an HF antenna. I currently have a Jeep Wrangler and have my HF antennas mounted on the spare tire carrier. It is a Hustler setup where you change the radiator out to change bands. I was wondering if a front bumper mount might do better for that. The first response to my initial post answered many of those questions, especially having a large whip, whipping around in front of my face. Anyway keep the responses coming especially regarding HF setups. I guess I am looking for the perfect HF antenna setup. Thanks again.

P.S. X2 on Larsen Antennas

Antennas L-R, Larsen NMO2/70 on Teraflex Mount, Hustler HF, Firestick 11meter on a Teraflex Mount

[Luke]

FWIW you mentioned that the antennas you saw were mounted on UN/Government vehicles. If they are anything like the antennas we have mounted on our US Army HMMWV's all they are for is FM communications.

[DaveInDenver]

I had my CB antenna on the edge of the fender, on the bull bar, then on the roof and now back on the bull bar. The fender was OK, but the flopping back and forth wore the paint off the edge of the hood as the mount flexed back and forth. The roof was just too tall, always hitting stuff, even with a Larsen NMO27, a relatively short low band antenna. I have a 5/8 wavelength stainless, bottom loaded whip that with it mounted on the bull bar the tip is about the same height as the tip of my dual band Comet mounted on the roof. Sure, the bull bar is electrically not an ideal spot, the middle of a big counterpoise like the roof would be. Thing is it's going to be hard to find anywhere on a car that has a big enough surface for HF, anyway. But with some grounding braid, the bumper can be well grounded to RF and so to the back and sides, the field is fine. Also some antennas are better than others at dealing with a lack of a ground plane. I don't rely on the CB for long range comm, but forward looking it's good for a few miles reliably and sometimes it seems the soil is just right and I'll get a pretty uniform pattern. It's doesn't seem completely deaf to the front anyway. But my reasons are mostly that it was the most stable spot, the mount is about as rigidly fixed as anywhere.

[jwhutch]

Once again thanks to everyone, I hope others will continue to post their mobile antenna farms. Dave how does the Larsen NMO27 work out. I wanted to put one of those on the right side but my local Larsen dealer went belly up. I already had the Firestick so I kept it on that mount. Luke, you may be right about the FM antennas. How tall are those. My dream setup at this point would be the ICOM IC-7000 with an Icom AH-4 tuner, tuning a whip antenna. The problem with most of the CB whips or even the Icom AH-2b is that they are too "whippy", especially off road. Maybe a surplus military antenna would be stouter. I think I am inventing words now. Anyway good to hear from everyone. '73

Luke thank you for your service!

[Grim Reaper]

In a perfect world center of the largest highest piece of steel is best. Ideally ground plane should be the same distance from the base of the antenna as the antenna is tall.

Think of signal as a doughnut around the mast. Ground plane helps flatten that doughnut out and push it further away from the mast. Then you can load int cool things like Phasing coils and really get it to push out. The problem is the more it pushes out the shorter the height of the signal becomes especially at higher frequency's (where I have most of my RF experiance).

Anytime you mount an antenna on the edge of the vehicle it is going to cause the signal to twist. On the side on the edge it will drop off and loose its distance. On the side with the ground plane it will pull up at an angle.

I was trying to find some examples on the web and I ran across this site. About midway down it has some vehicle mounted antenna information on it. including a slick way to make a ground plane to increase transmitting distance when parked and a lot of "Skip" tricks and info for you 2M guys.
http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=11213

I found that off this site that had a lot of good related info.
http://www.dxzone.com/catalog/Antennas/Theory/

As for where to mount. Either end of the vehicle is just as bad. When you get clutter like roof and roll bars it cuts off the bottom of the signal in that direction and shortens the antennas effective length. The front may be the better option as the hood will make the better ground plane on a truck.

I suspect it is as much about getting it out of the way as anything. With it on the rear you are more likely to find it in the way when loading or unloading the truck. I know on my last truck I ran it on the rear for a while and I did find when loading I ran into it often.


My personal preference is a Mag mount on the hood in front of the passenger (drives my wife nuts LOL ). My truck has a sunroof and the rear section of the roof is fiberglass. The hood offers the best ground plane on my vehicle.

[Luke]

I don't have a measuring tape handy but I'd guesstimate they are about 8ft.

[DaveInDenver]

Ideally ground plane should be the same distance from the base of the antenna as the antenna is tall.

According to guys like Laport and Kraus, the ideal ground plane is 1/2 wavelength minimum regardless of the physical antenna height. Some practical references, like the ARRL, say you only need 1/4 wavelength for the radials of an artificial ground plane. Personally, I think an ideal manufactured counterpoise will use at least 1/2 wavelength radials. But I think since that can be a problem in the field on the very low bands (like 160m would require radials that are 250 feet long), the practical application has been amended to 1/4. Anyway, so for an USA CB, my ideal ground plane is about 36 feet across the hood or roof for the counterpoise to properly couple. It's impossible to get a good counterpoise like that for a half dipole whip on a car, I believe you really aren't doing yourself much good by worrying too much about it. What is more important is finding ways to reduce the feedpoint impedance at the antenna. So a very solid and well grounded mount, good connections, tuning the whip, that sort of thing. You also want to bond the body panels and exhaust (if you're really worried about it) as well as possible. I just took the time to bond the hood, front fenders, bumper and firewall to the frame and didn't worry about the bed or exhaust. You want a very efficient radiator and a minimum of parasitic currents on the car itself. Basically you are going to give up the sympathetic half of the wave (usually called the image, which is the mirror of the source in a dipole) and you want the real ground to get as much energy from your source so that you might generate a reasonable image in the Earth itself. That's my thoughts at least.