What is the highest gain possible 2-meters mobile antenna that can be used with a magnet mount base

Discussion in 'Mobile Radio Systems' started by KC0BUS, Mar 21, 2018.

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  1. NE1U

    NE1U Ham Member QRZ Page

    I use Motorola mag mounts for my roof mount. It has a much stronger magnet that my local fellow hams' mounts have although they have not had a problem of their whips blowing off. Mag mounts add a bit of capacitance and will change the tuning of a vertical antenna. That said, most vertical mobile antennas have a pretty broad tuning range and will unlikely change performance. And, it only matters if the antenna is center mounted on the roof of a vehicle with a fairly large roof. I am thinking that even pickup trucks with small roofs are kind of a toss up about anything that could be called 360 degree uniform gain.

    Note that at RF a DC connection is not a requirement.

    Second, vertical mobile antennas with gain flattens the donut characteristic of 1/4 wave whips. For someone traveling in the country away from cities, I always specified gain antennas. Those in very urban areas with base station antennas on the top of buildings got the 1/4 wave whips so more of the gain pattern is more vetical and is rare for ham radio (my opinion & guess). Even the 1/4 wave whip on a car parked under a mountain top tower mounted base antenna can experience a noisy signal. Probably a rare occurance for most hams but interesting to test given the opportunity.
     
  2. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    This is not correct! Note that at RF a DC connection is not a requirement.

    RF needs a return path. Without a shield connection to the ground plane (car body in this case), there will be common mode current flow, regardless of the capacitive coupling.
     
  3. NE1U

    NE1U Ham Member QRZ Page

    Sorry. Science (maybe Gauss' law but it has been a while) and not opinion not to mention mag mounts have been used for decades.

    With a little electromagnetics understanding, grounding at the antenna is not required to have an effective RF ground plane. The radio is typically tied to the chassis providing the necessary DC return.

    Also, think about capacitive coupled antennas thru a vehicle's window glass and portables with an antenna at the end of a coax line on a mag mount on a vehicle roof. I have a Laird Phantom Elite on a Motorola mag mount fed with the as shipped length of coax to a XPR5550 radio buried in the trunk. Grounded at the base of a rear passenger door post.

    All explainable with engineering and science.
     
  4. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    The issue about not needing to be grounded is correct. However, the capacitive coupling (mag mount use) doesn't present a Øj impedance. Close perhaps, but no cigar.

    As I said, there has to be an RF return, and that is through the ground plane under the antenna. In this case, the body of the vehicle. Or radials, if a base station antenna.
     
  5. KE8FWJ

    KE8FWJ Ham Member QRZ Page

    5 pages and we no nearer to a direct reply! LOL

    America wasn't built on excuses either.

    Victor (ex ZS etc etc etc)
     
  6. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    Apparently, you didn't read the information presented! The fact is is a mag mount is immaterial with respect to gain. All the mag mount does is add ground loss. That is the same as reducing gain—less radiated power in other words.

    And... Gain isn't really what you want if you're operating mobile through repeaters. Gain antennas achieve gain by taking power from one specific portion of the antenna, and adding it to another portion. That always leaves some radiation angles with reduced radiated power. This is one reason why unit gain antennas are preferred for repeater use, especially when there is great disparity in the HAAT of the respective antennas (i.e.: mountain top/flat planes like you have in the Denver, area.

    If you're looked to work a really long distance, the beams are the way to go.

    Just answer your question directly, about 6 dBd is about the max gain possible in a mobile scenario, before height becomes an issue.

    Oh! And gain is deceptive. If you put 50 watts into an antenna, the total radiated power (less I2R losses) is still 50 watts!
     
  7. KV6O

    KV6O Ham Member QRZ Page

    20161030_110421.jpg

    Just do it.
     
    KE8FWJ likes this.
  8. K7JEM

    K7JEM Ham Member QRZ Page

    This is an old argument that is easily disproved. First off, repeaters do not use unity gain antennas, they are usually around 6dBd gain on 2M. Mobile operations also do not benefit from a unity gain antenna, unless the entire operational area is in an area where you are at a 45 degree angle to the repeater. This typically means that you are within a mile or two of a repeater that is a mile or two directly above you. This would not be typical operation, for most amateurs.

    Most people want maximum gain at the horizon, or slightly above, so that when you are many miles from the repeater, your signal has the most gain. A typical 5/8 antenna has around 2 to 3 dB gain over a QW antenna. That translates to a very effective difference when you are at a distance from the repeater. Also, the power at lower radiation angles is typically better with the 5/8 antenna.

    You can calculate the angle to the repeater at various distances, and using different heights above ground for the repeater antenna. Most of the time, most hams will be at an angle of 10 degrees or less. To put this in perspective, if the repeater was on a 1000 foot tower, and you were 1.25 miles away, you would be at a 10 degree angle. As you got further from the tower, your angle would decrease, and the 5/8 antenna would show better gain as the angle got lower, compared to the QW antenna.

    But, like all things, this is just generally speaking. There may be some azimuth over some specific automobile where the QW will perform as good, or possibly slightly better. But in the vast majority of cases, the 5/8 will do better than the QW.

    on 2M, a 6dBd gain antenna is around 20 feet in length, which is impractical for any true mobile station. You could achieve 3 dBd gain out of an antenna around 9 feet in length, this might be a practical length for a mobile, any longer and it might start being a problem.

    But the gain does concentrate the power where you want it, much as a mirror held behind a light bulb will concentrate the light. Even though the power doesn't change, the effective power, or ERP does.
     
    NE1U likes this.
  9. NE1U

    NE1U Ham Member QRZ Page

    Receive signal level also improves at the mobile
     
  10. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    And this too is a stretch: Receive signal level also improves at the mobile. The "gain" of an antenna is reciprocal—that is a given! However, the performance of an antenna IS NOT reciprocal, especially with gain antennas. We need to remember that we have no control over the incoming wave angles.

    What isn't being realized here, is the SHAPE of the resulting pattern with respect to gain. Wouldn't it be nice if we could produce a donut-shaped pattern with 10 dB of gain? Well, we can't. The pattern is also dependent on how the gain is obtained. In the case of repeaters, gain techniques are used to shape the pattern, typically in a downward angle. As I alluded to before, it deal with the differential of HAAT between the repeater set, and that of the mobile.

    There is another obscure issue at work here. The radiation pattern of gain antennas is effected by ground loss. Doubters might want to read this article. I didn't write it, although it does reside on my web site. The author is Danny Richardson, K6MHE.
    http://www.k0bg.com/images/pdf/mobile_vhf_ant.pdf
     

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