Best Placement of a Mobile Antenna
Don’t Lopside Your Pattern!
Source: Larsen Amateur Catalog (PDF) - Written by Aaron Logan.
We have provided you some guidelines for mobile antenna selection. When selecting a mobile antenna, there are a number of factors that significantly affect the ultimate performance of the antenna. Gain requirements, electrical type, ground plane availability mounting style and placement, coaxial type and loss ratings, physical size, appearance, and surrounding environment are all issues that must be addressed to ensure the maximum performance from a mobile antenna installation. The electrical type or design of the mobile antenna is commonly referred to in terms of its dimensions in terms of wavelength: 1/4 wave, 1/2 wave, 5/8 wave, etc. Each electrical type has a specific radiating pattern to be considered when selecting a mobile antenna. For example, the signal radiating from a 1/4 wave antenna is directed more vertically, thus making it ideal in urban environments where buildings might obstruct the signal. The design of a 5dB collinear mobile antenna is designed to direct the signal more towards the horizon. This type of antenna is ideal for geographically flat regions where signal coverage is sparse.
Ground plane availability is another critical factor in mobile antenna performance, and must be considered when determining the location and type of the antenna. Ground plane requirements vary given the type of mobile antenna and the frequency of operation. A typical 5/8 wave antenna at 150 MHz requires a ground plane of at least 42” in diameter. At 450 MHz, 15” is required, and 800 MHz, a minimum of 8” is considered sufficient.
In terms of mounting mobile antennas on a vehicle, there are five general locations: the roof, front fender, rear fender, trunk and rear window glass (although other glass mount locations may be used). Of these, the center of an automobile roof is considered the best for mobile antenna placement, followed by the center of the trunk lid, the fenders, and then on-glass mounting. This ranking is determined by the amount of ground plane provided by the positioning, and clearance from obstruction (i.e.: the roof line), and is the reason the center of the roof is considered the ideal mounting location, provided the roof is metal. The diagram above illustrates the effective loss (at 800 MHz) due to insufficient symmetrical ground plane.
2002: found at alfenterprises.com/AntennaIndexCategory.html (dead link); 2015: located source in Larsonn Amateur Catalog; 2016: Author located! It is the policy of kv5r.com to properly credit all sources, when possible. This one just took a while!
Please correct me if I am wrong. I heard that the ground plane effects the radiation pattern of your signal. A ground plane equal on all side should have a radiation pattern of 360 degrees. A vehicle is basically rectangular. Putting the antenna on the roof, which s the best place, the longest metal is to the front of the vehicle, then comes the rear and lastly the sides. I was told that the radiation pattern is strongest to the longest metal, which in this case is the front of the vehicle. Any truth to this?
I have had the same Larsen 5/8 on two meters for decades. (really!) I have used it on multiple work vehicles, always drill a 3/4″ hole in middle of the roof and use a motorola type NMO mount. When vehicle is replaced, I just unscrew the Larsen and install it on the new vehicle. Works great!
Have also compared a mag mount 5/8 on the trunk lid of a VW Jetta VS same mag mount stuck on the center of the roof. Big, Big difference favoring the roof location.
Also, a comment on the antenna location with antenna centered in front of the windsheild, using a trunk lip mount. I have seen this in No. Wisconsin where it is common for people to use marine radios on vhf in pickups and put a 5/8 in front of the windsheild, centered. I have not tried this myself but at least it must work good in No. Wisconsin for these guys!
What would be the negatives for mounting the antenna center on the front engine hood right at the base of the front windscreen. The lip is large on most cars and would clear wiper movement ok. I plan on mounting mine here just so it will clear the garage door. I have a 40” Comet with a Diamond NMO mount. The ground plane provided by the front hood should be significant. I am using a 20 watt radio. I am interested in feedback and the thoughts of others.
Should work ok. The only potential problems might be RF exposure, with the antenna right in front of the windshield. I don’t think at 20 watts that would be a problem, but you might want to look into RF exposure guidelines for 2 meters.
Might also get RF into electronics in the dash. Might try a mag-mount before you drill a hole for the NMO mount.
73, –kv5r
Do not put a on-glass on the back screen.
Testing was completed at the Larsen Antenna manufacturing site in Vancouver, WA. This testing was done in an outdoor test field, multiple wavelengths from nearest buildings and structures in a controlled environment.
This information is directly from the Larsen catalog. (I wrote it while employed at Larsen – know Pulse Electronics.)
Ah, so now we know! see 2-3 comments below; when I found it years ago on the alfenterprises site, its source was not credited. Then a year ago Eric (below) enquired and I searched and located it and credited it Larson catalog, now I can credit it to its author. Thanks! I’ll fix it up top right now.
73, –kv5r
Wonder if there is a study of how the patter shifts with different placement on roof (middle, front center, back center, back side) of the a typical car? I had seen references in Motorola documentation that documented different patterns based on antenna position. Once it is moved off the roof of the car the are obvious obstructions. I have used a Larsen magnet mount for over 26 years and I have had wonderful results over the years. It sticks to some rental cars better than others.
Good post. I’ve been working on mobile telecom for a long time and I have realized just that.
What is your source for the picture showing relative performance of antenna placed at different locations on a vehicle? Do you have any data for such tests conducted, and how would these differ for a LDV with the antenna mounted on a bracket on the roll bar?
As the dead link at the end of the article says, I found it on alfenterprises.com, (a long time ago). Searching now, it appears in the current Larsen Antennas catalog http://www.pulseelectronics.com/download/3598/larsen_amateur_catalog_/pdf (PP 16-17). Apparently, the testing was performed by Larsen engineers.
https://www.pulseelectronics.com/wp-content/uploads/2021/01/Amateur_Radio_Catalog.pdf