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I am doing some temporary FM broadcasting for car events such as COVID-safe car parades and drive-in movies. The terrain is horrible, urban areas with lots of buildings. The cars snake around a city, with the transmit car in the middle.

Understandably, there are not many purpose-built, car-mounted FM radio transmission antennas.

  • I have a 1/4λ Pulse Larsen PulseLarsen NMOQ88C on a 5" NMO mag mount.(tuned by instructions) It works okay, but I wonder if there are better options.
  • It seems there aren't any 5/8λ antennas with NMO mounts in this range, would it be possible to modify a 102" CB Whip into a 5/8λ antenna at 88mhz? Can I use a base loaded coil from a 144-174MHz NMO 5/8λ system with a longer whip?
  • Is a 1/2λ worth exploring?

I am transmitting with 30W. I have no antenna analyzer nor an SWR meter. I am most interested in what I realize is mostly impossible, better NLOS (Non line-of-sight) transmission so cars can continue to hear after they have turned a corner.

Is the 1/4λ as good as it gets for my application?

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  • $\begingroup$ I understand the convenience of a mag-mount antenna. But, are you driving around during the movie? If not, you could consider an omnidirectional antenna intended for fixed operation, like a J-pole mounted to a tripod. $\endgroup$ – Brian K1LI May 13 at 18:41
  • $\begingroup$ Hello John, and welcome to ham.stackexchange.com! And thanks for a nice first question. We look forward to seeing more of you here. :-) Also, I may have been too picky; "Best" in the context of your question was okay, and feel free to change it back if you like. $\endgroup$ – Mike Waters May 14 at 20:37
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First, some technical information. According to W8JI, who knows more about antennas than I likely ever will, a 5/8λ vertical antenna potentially offers a few decibels of horizontal gain over a 1/4λ vertical, if you have a large-enough ground plane; specifically, a ground plane with a radius of about 1.5 m or larger. That's a ground plane 10' across, in imperial units, which means a big vehicle. It might be possible to home-brew such a 5/8λ vertical or some other fancy antenna, but not without essential tools, including an SWR meter at a bare minimum. An antenna analyzer is really helpful if you're trying to do home-brew any antenna, especially a tricky one like the one you describe.

But I question your desire for fancy antennas that might get you a couple more decibels of horizontal gain. 30 W broadcast through a 1/4λ vertical would give you a fairly large broadcast area, much larger than a drive-in movie theater, even in urban terrain. Also I think you underestimate the tendency of signals to get around obstacles such as found in cities; signals diffract, reflect, and pass around and through many obstacles like buildings. When I'm in a city, I hear FM broadcast signals through my car radio quite well, even when I'm in an alley between tall buildings, unless the signal is weak to begin with.

It is quite illegal to operate a 30 W FM broadcast station from a vehicle in the US, but I see that you are in Namibia, where rules may be very different.

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    $\begingroup$ Thanks! I don't have my 30W transmitter yet, but when I did tests with my 1/4λ mag-mount and a 7W transmitter, I got about 0.4km range line-of-site in an urban environment. As soon as another car turned a corner, it was instant static. I am under the (naïve) impression that NLOS operation is best when the transmitter is very far from the receiver so that the Fresnel Zone is larger. $\endgroup$ – John Tibie May 13 at 21:17
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A quick note: If the transmit system described in the OP will be or is unlicensed by the regulating body having jurisdiction for such (e.g. the FCC in the USA), then some risk will be involved in its use. In the US, such unlicensed systems in the FM broadcast band are authorized under title 47CFR §15.239, but limit the maximum field intensity 3 meters in all directions from the transmit antenna to 250 µV/m.

That free space field at that distance is produced by a simple 1/2-wave dipole radiating about 11 nanowatts (0.000 000 011 watts).

The maximum useful range of such an FCC-compliant system to a good FM receive system on an interference-free FM channel is on the order of 300-500 feet.

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Height is king in urban environments; the higher the better.

A 5/8 wavelength antenna will reduce the vertical beam-width, not what you want.

The optimal antenna for your application is a vertical dipole(folded or not) with a flat reflector, located well above and slightly behind the screen as high as possible with a very slight forward tilt(assuming there is a building opposite of the transmitter building to bounce off of, the primary users should receive the first bounce) downward at the top to aim the antenna.

I assume you want the viewers to hear the movie, this tends to indicate line of sight, not the whole town. Your 7 watt transmitter will work perfectly, assuming a good impedance match(and decent coax) to the antenna.

Using a flat reflector behind will give you a >2db gain in the forward direction with a wide,160 to 180 degree, pattern on the front of the antenna.

Make sure you use a good bandpass filter to make sure you are not interfering with other adjacent stations.

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  • $\begingroup$ Thanks for the answer! I should've been specific, drive-in movies were an example, but there are also events that are more like parades with messaging about COVID safety. By broadcasting FM, each car can use their stereo as a portion of a distributed PA system for the messaging. In this case, the car is in the middle of a snaking line of cars. With the 7W, there was very poor performance when LOS is not maintained. From these answers, in this use-case, I get the sense that the 1/4λ is good enough, and that antenna type is not going to magically improve my NLOS performance. is this right? $\endgroup$ – John Tibie May 15 at 4:35
  • $\begingroup$ Maximizing height(most important) and delivering optimal power to the antenna is the solution. With a 1/4 wave antenna is half of the power is is distributed in a non-uniform way. The gain of an antenna is proportional to its directivity, for example, the power is focused in a certain direction. Imagine a un-popable-balloon, that can be squeezed into many shapes, this shape represents the signal gain in any direction The source power is the same, but the power density can be shaped to increase in a certain direction; So for an omni-directional signal: Add height and power. $\endgroup$ – KX4UQ May 15 at 5:56
  • $\begingroup$ Height is more important than power in an urban environment. A 5 meter high antenna pushing 100 watts has a much less effective area than a 200 meter high antenna pushing 1 watt. $\endgroup$ – KX4UQ May 15 at 6:20
  • $\begingroup$ Use a vertical dipole or folded dipole, instead of a 1/4 wave antenna for predictable results. A 1/4 wave antenna will couple to the nearest ground plane or travel back to the transmitter, whichever path has the least impedance. $\endgroup$ – KX4UQ May 15 at 6:29
  • $\begingroup$ If you must transmit from the same height as the receivers, then the 5/8 wavelength will be your best bet to maximize gain on the horizontal plane. Make sure you have adequate radials and push no more than about 30 watts at 88 MHz to ensure the safety of the cars occupants. $\endgroup$ – KX4UQ May 15 at 6:43

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