dyrtjunkie
Member
Is there a REAL difference between antenna location? All I can think of is trunk mounting might affect forward signal detection. What do you think?
On the Trunk/On the Roof...Any Difference?
- Thread starter dyrtjunkie
- Start date
- Status
Take a look at the attached diagram from the Larsen catalog.
According to Larsen, if you assume that a permanently-mounted antenna in the center of the roof is the standard, a permantly-mounted antenna in the center of the trunk lid will be down 2.1dB from the reference.
According to Larsen, if you assume that a permanently-mounted antenna in the center of the roof is the standard, a permantly-mounted antenna in the center of the trunk lid will be down 2.1dB from the reference.
Attachments
Last edited:
The difference is the size of the ground plane. The more surface area you can get under the antenna, combined with the best height of the antenna, equals a better signal. Ideally you want a much ground plane as possible 360 degrees under the antenna, meaning an antenna mounted dead center on the roof will perform a little better than one mounted offset towards the front or rear of the roof.
dyrtjunkie
Member
So there is 2.0db of difference of gain on the roof. Got it. So my second question is, If I have a BCT15 w/ a Larsen 150/450/800 antenna, What db amount (of gain?)is there to subtract 2.0db from if I choose to use the trunk?
N
N_Jay
Guest
Those are averages and do not account for directionality. What band and what exact antennas are being compared?
I would also question accuracy to a tenth of a dB given all the variables.
It does make the point that location is everything, and that mag mounts are not as bad as people make them out to be.
I would also question accuracy to a tenth of a dB given all the variables.
It does make the point that location is everything, and that mag mounts are not as bad as people make them out to be.
N_Jay is on the money, so take these figures with a grain of salt as calculated averages. Wavelength determines physical dimensions of radiator elements and ground planes. To get specific, you'll have to then consider whether the antenna is top loaded, center loaded, or base loaded, or 1/4 wave, and how a specific design in a specific frequency range couples with the ground plane at a given location and mounting style on the vehicle.
The higher the freq, the smaller the physical dimensions of the ground plane will be, so as the frequency goes up and the wavelength goes down, the square footage of the ground plane becomes less critical in relation to available space on the vehicle because of the wavelength correlation. That means that the square footage of required ground plane at high frequencies is something that low frequencies wouldn't even acknowledge as ground plane. Remember that the propagation pattern from any place on the vehicle other than the roof will be obstructed by the roof structure and whatever else is sticking up.
One prime example recently measured with a hand held field strength meter is the typical base loaded low band VHF antenna on the NCSHP frequencies mounted on an L-bracket at the left rear corner of a Dodge Charger. The measurements were taken outside at a measured distance of 50 feet from the radiating element on the same plane as the radiating element, and with the meter sitting on a non-conductive fiberglass ladder .
Compared to the roof mounted antenna on the same frequencies on the roof of a Ford Ranger Supercab pickup, which is actually less than ideal in physical area for 42 MHz, there's a 3.2 db loss in signal strength diagonally from center off the left rear corner of the Charger, and a 3.8 db loss diagonally across the right front of the vehicle. Obviously, any gain afforded by the extended ground plane from left rear to right front of the vehicle is more than negated by the intrusion of the roof structure into the radiation center plane.
For me, that's unacceptable, but for some applications it's apparently deemed "acceptable." Anyway, for low band VHF, the loss is more than it would be for shorter wavelengths because of the lack of ground plane on the Charger. As to how much the roof structure would affect shorter wavelengths in the same mounting position is a whole other set of ballgames. Remember, what applies to the antenna during transmitting also applies to receiving.
Hope this info, though more observational and anecdotal than scientific, is somewhat real world and is helpful.
The higher the freq, the smaller the physical dimensions of the ground plane will be, so as the frequency goes up and the wavelength goes down, the square footage of the ground plane becomes less critical in relation to available space on the vehicle because of the wavelength correlation. That means that the square footage of required ground plane at high frequencies is something that low frequencies wouldn't even acknowledge as ground plane. Remember that the propagation pattern from any place on the vehicle other than the roof will be obstructed by the roof structure and whatever else is sticking up.
One prime example recently measured with a hand held field strength meter is the typical base loaded low band VHF antenna on the NCSHP frequencies mounted on an L-bracket at the left rear corner of a Dodge Charger. The measurements were taken outside at a measured distance of 50 feet from the radiating element on the same plane as the radiating element, and with the meter sitting on a non-conductive fiberglass ladder .
Compared to the roof mounted antenna on the same frequencies on the roof of a Ford Ranger Supercab pickup, which is actually less than ideal in physical area for 42 MHz, there's a 3.2 db loss in signal strength diagonally from center off the left rear corner of the Charger, and a 3.8 db loss diagonally across the right front of the vehicle. Obviously, any gain afforded by the extended ground plane from left rear to right front of the vehicle is more than negated by the intrusion of the roof structure into the radiation center plane.
For me, that's unacceptable, but for some applications it's apparently deemed "acceptable." Anyway, for low band VHF, the loss is more than it would be for shorter wavelengths because of the lack of ground plane on the Charger. As to how much the roof structure would affect shorter wavelengths in the same mounting position is a whole other set of ballgames. Remember, what applies to the antenna during transmitting also applies to receiving.
Hope this info, though more observational and anecdotal than scientific, is somewhat real world and is helpful.
Last edited:
newsphotog
Member
Wow, I have always been told to AVOID glass-mount antennas of any kind. And yet that diagram above says it's only a half-dB loss at the top of the glass, where most people mount them. It could be they are not taking into account some other variables, like rear defroster wires, but I dunno.
KevinGC
Member
And then there are some antennas that supposedly work better as a trunk lip mount rather than a roof mount. The Diamond CR-8900A quadband antenna in particular. The literature that comes with it says it is designed to be installed as a side-of-trunk lip mount. As you read thru some of the reviews/user group comments about it, this seems to be true in most cases.
knightrider
Member
If you go to the manufacturers site/catalog, or the sales group, you'll find you can buy almost any antenna with almost any kind of mount. Now, unfortunately for those who are not yet informed, this can be bad news, or a bad product. If you are not wanting to do a permanent mount, and instead opt for one that leaves no indication of having been there, you may have bad results. Again, a lot of it is the way it is mounted. As was mentioned earlier, it is the surface area in a lot of situations. Where a 1/4 wave might work in one situation, it may not work very well in another. Again, the 5/8 wave antenna may not work at all, but your buddy has a 1/4 wave that runs circles around it. It is the mount and the surface a lot of times. I have a '93 Jeep Grand Cherokee. All of my VHF/UHF Ham gear works great, but my 10m sucks! Same antenna types, just not good surface area for 10m. The Jeep is not a long term vehicle for me right now (I don't think), so I am using mag mounts. On my older BIG Blazers, I always used the permanent mounts. Fiberglass roof on the back.... I chose 1/2 wave antennas, and they seemed to work great. If I keep the Jeep, I will make permanent mounts for the higher freqs, and a different mount and antenna for 10m. I like to use a 5/8 for 2m, because I can "sneak in" 6m on it most of the time, but again, a lot of it is learning and understanding the antennas, and how they work. If all you want to do is listen locally, there is no reason to buy a $500.00 antenna, you can use a simple 1/4 wave mag mount cut to VHF, and still be able to hear UHF all the way to 800 mhz with it. Antennas are the cheapest way to improve almost any radio, but can sometimes be the most complicated and frustrating part of it.
spicerz
Member
Trunk Lip Mount Antenna Works Great When Using Laird Products
I live in the suburbs and get excellent reception with my 1/4 wave unity gain antenna. Not only do I pick up 800 MHz digital in my area but I pick up 800 MHz digital and 800 MHz analog 40-50 miles in every direction with clear reception.
I have an SRT8 and won't drill into it. Here's my post about my antenna. http://www.radioreference.com/forum...3-tested-new-mobile-antenna-my-pro-197-a.html
What you can do is order what you want and if you're not satisfied, utilize the 30 return policy until you find what you like.
I live in the suburbs and get excellent reception with my 1/4 wave unity gain antenna. Not only do I pick up 800 MHz digital in my area but I pick up 800 MHz digital and 800 MHz analog 40-50 miles in every direction with clear reception.
I have an SRT8 and won't drill into it. Here's my post about my antenna. http://www.radioreference.com/forum...3-tested-new-mobile-antenna-my-pro-197-a.html
What you can do is order what you want and if you're not satisfied, utilize the 30 return policy until you find what you like.
- Status
