Antenna Selection and Placement

Alec R

Member
Sep 9, 2014
50
eLightbars
What antenna will work best for which frequencies, where on the vehicle should it be installed, and how should it be mounted?

Discuss!

Please keep it clean and organized, this forum section is heavily moderated to keep information organized. We're going to use content from these discussions to compile a helpful guide for users seeking information about products in this industry.
 
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Jarred J.

Lifetime VIP Donor
May 21, 2010
11,580
Shelbyville, TN
I find that antennas meant for the frequency of the radio you are transmitting with usually works the best...

i will find a HAM (amateur radio operator)  who has a whole site dedicated to mobile various types of radios, and post the link here.
 

Steve0625

Member
Jun 23, 2010
1,213
Northville NY
What antenna will work best for which frequencies, where on the vehicle should it be installed, and how should it be mounted?
This should answer the question of antenna placement. I've never seen a better guide in all of my years of installing radios in vehicles.

ai920.photobucket.com_albums_ad42_Steve_Collins_Misc_20radio_2a888971972a459f1aa1f506356be45f0.jpg

One of the most interesting things about the antenna placement pic is that there is minimal difference between a magnetic mount and a permanent (drilled the hole) mount in the center of the roof. That presupposes that the radio is properly grounded allowing the roof to act as ground plane. The same is true of all of these measurements.

The second point of interest is that the loss of an upper center mount on the rear window with an on-glass antenna is just -0.05 db. That won't work on a window with metal tinting in it, but for plain glass, still very attractive.

One other point, all of those measurements were taken using a quarter wave antenna for uniformity of comparison. If you were wondering, Pulse Electronics manufactures the Larsen brand of antennas.
 
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Steve0625

Member
Jun 23, 2010
1,213
Northville NY
http://www.pulseelectronics.com/download/3248/antenna_basic_concepts/pdf


I searched for that image to blow it up to find the dB loss. It's on page 8 of this PDF.
Thanks for finding that. I tried to process the pic again and get it down to a reasonable size for forum use (640x480) and it still makes the type too small to read really well. It looks OK on my screen, but not great. Others may have worse luck.

Just a caveat for everyone, even though the magnetic mount has the same minimal loss at the corner of the roof that it does in the center, the position will alter the radiation pattern. Center of the roof gives pretty much equal radiation in all directions. Corner or edge of the roof creates an offset pattern because there is less ground plane in one or more directions. A rough rule of thumb is that you strive for the 1/4 wave antenna's height in ground plane in all directions. I may not have phrased that very well so here is an example. If you're using a 17" VHF quarter wave antenna, you ideally should have 17" of ground plane in all directions. If you're using a 6" UHF 1/4 wave, you want at least 6" of ground plane in all directions.

Things get complex when you have multiple antennas of multiple frequency ranges on a single roof. Years ago, we were adding 800 mhz antennas for an MDT system to vehicles that already had 1/4 wave UHF spikes in the center of the roof. The techs found that they needed to reposition the UHF to accomodate the 800. I don't have the exact placements handy, but they told me it took a lot of experimentation to get it right.

I've got VHF and UHF quarter wave antennas on the roof of my F250 truck. Because of the construction of the cab, including side ceiling airbags, I did not want to drop the headliner to install them. But I was able to fish in from the CHMSL far enough to place the VHF antenna about 17" from the edge of the roof. The UHF is behind it with 7" of roof clear in all directions. The antennas are about 9" apart. Since I only use the UHF (GMRS) when traveling with the fifth wheeler and VHF is strictly for EMS operations when at home, I don't have interference or intermod issues. Others may have different results based in part on useage.
 

Steve0625

Member
Jun 23, 2010
1,213
Northville NY
Try this site for a lot of good info:

http://www.k0bg.com/
Those who insist on using a magnetic mount as a semi or permanent antenna because they are afraid to drill the hole might want to read the section on VHF/UHF mobile antennas on this website. There are lots of good reasons not to do so, and very few, if any, good reasons to do so.

I have one mag mount in my kit. Most of the time, it sits on a metal filing cabinet in my office so I can plug it in to any radio that I power up for programming. I'm too cheap to go buy a good VHF/UHF dummy load at this point. The mag mount lets me use VHF or UHF antennas (NMO mount) and saves damage to the radios while they are running if they are accidentally keyed up.
 
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ckgarside

Member
Jul 9, 2015
49
Los Angeles, CA
Also a pretty simple antenna install rule is to always have the highest frequency radio go to the closest antenna. Feedline losses are higher at as the frequency increases, so you want to keep the run as short as possible.
 
Oct 5, 2013
24
TX
There is not a lot you can do with a mobile antenna's performance other tahn about 3dB and direct it's pattern to "focus" its available power in one direction.

A second antenna with 1/4 wavelength(or longer) can act as a passive director and you now have a directional antenna.

The passive director doesn't have to be connected to a radio,

This can be a 2nd mag mount that now can be used to "direct" a signal, much like a directional yagi antenna.

Its also amazing how many installers forget about antenna separation and the relation to the transmitting frequency.
 
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