Maybe make a 1/4 wave ground plane instead of a dipole? The ground would connected to a set of 4 or more ground radials down tilted 45 degrees and a single straight up and down stick. http://ccarc.org/_misc/so-239_ant.html <http://ccarc.org/_misc/so-239_ant.html>
I was thinking about what you said about stepping the frequency as the car moved. Would successive cars entering the chain have different frequency selections? I wonder if you couldn’t choose a different frequency for the actors 49mhz, 900mhz, 2.4ghz or something, and have a central site that had a separate receiver for each microphone, and then mix the audios together into a single FM transmitter. I suspect the one-way wireless mike to receiver has been done and you may even find somebody’s maker-space article on the subject. Make 5 of those for your five actors, then a single FM transmitter with the decent antenna you are discussing would talk to the cars throughout the entire drive-through. Tadd - KA2DEW > On Sep 28, 2020, at 10:31 PM, Pete Soper via TriEmbed <[email protected]> > wrote: > > I modeled 1/2" diameter elements with EZNEC and the length of each 1/4wl > element comes out to 29.1 inches when cut for 98MHz with the antenna 10 feet > up. > > The graph of SWR vs frequency from 88 to 108MHz is below. > <http://triembed.org/images/FM-DIPOLE.png> > The closer to resonance the lower the SWR and greater proportion of RF out vs > turning the power into heat. So if you end up needing to transmit at, say, > 89MHz you'd simply make the elements 98/89 of the above dimension and it > should keep the SWR as low as it can get. If you were transmitting at 107MHz > you'd multiply by 98/107. > > But the other problem with going unbalanced into a dipole is that it doesn't > necessarily radiate like a dipole (i.e. two lobes perpendicular to the > elements). As Dan mentioned, the feedline ends up radiating and it tends to > be at wonky angles relative to the axis of the antenna elements. The coax > coil (or purpose-made balun below) solves this by isolating the feedline from > the antenna. It's a pain to have the antenna only be effective for a piece of > the azimuth range you need. From the description you probably want an omni > pattern. The gain off the ends of a horizontal dipole is terrible and the > gain falls off pretty severely more than around 40 degrees right or left of > the broadside direction. So in addition to a balun you might consider making > the dipole vertical. A vertical dipole is omnidirectional outward with the > nulls up and down. But the balun is key to getting a predictable pattern. > > You can get baluns from Digikey, by the way. The MABA-011040 > <https://www.digikey.com/product-detail/en/macom-technology-solutions/MABA-011040/1465-1702-1-ND/5131112> > is rated for 1-300MHz and doesn't look like it would be hard to solder. > This could go at the antenna and then you'd use a simple run of coax to it. > > -Pete AD4L > > <opiadpmmadeneekh.png> > > On 9/28/20 12:29 PM, Brian via TriEmbed wrote: >> Hi Folks, >> >> I know there are several radio-smart people on this list, so I hope the rest >> will forgive the noise as I pose a couple questions here. >> >> The questions first; I'll provide background afterward. >> >> I have settled on a digital FM transmitter chip (Silicon Labs' SI4721), and >> a simple dipole antenna. Two questions arise from this decision: >> >> 1. The transmitter has a single-ended RF output, but will be fully isolated >> from earth ground in operation (running on batteries or an isolated AC/DC >> supply). A dipole is a balanced load, but since "ground" of the radio >> circuit is isolated, can I just treat it as "balanced" and connect the >> circuit ground to the other half of the dipole? Or do I really need to use >> a balun for a proper balanced output? Coverage area actually needs to be >> very small (< 100'), so I'm not majorly concerned with impedance mismatch >> losses, etc. >> >> 2. I'll be using 1/2" copper pipe as the elements, held inside a larger PVC >> enclosure. What's the best way to bond wires to the pipe? Should I just >> solder them on? Tap a hole and use a screw to clamp them? Some kind of >> shark-bite approach? Does it even matter at all? >> >> Here's the background: >> >> My church does a Christmas program called the Drive-Thru Christmas, which is >> made up of five live-actor scenes distributed around our parking lot. >> Guests are typically given a narration on CD which they play inside their >> vehicles as they move from scene to scene. In order to improve our social >> isolation this year, I'm doing some R&D on the "talking sign" idea, using >> five separate short-range FM transmitters to broadcast the scene's narration >> to the guest's FM radio in their car. Each transmitter would broadcast on a >> different frequency, and some system would step each transmitter through the >> list of frequencies in time with the car's movement through the scene, so we >> can maintain our 5-car pipeline but not require the guest to re-tune their >> radios. We tried using an internet streaming option last year (for folks >> with smartphones linked to their car stereos) but that, I hear, was an >> abject failure with many people unable to access the stream. >> >> Well anyway, thanks in advance for any advice! >> >> Cheers, >> -Brian >> >> _______________________________________________ >> Triangle, NC Embedded Computing mailing list >> >> To post message: [email protected] <mailto:[email protected]> >> List info: http://mail.triembed.org/mailman/listinfo/triembed_triembed.org >> <http://mail.triembed.org/mailman/listinfo/triembed_triembed.org> >> TriEmbed web site: http://TriEmbed.org <http://triembed.org/> >> To unsubscribe, click link and send a blank message: >> mailto:[email protected]?subject=unsubscribe >> <mailto:[email protected]?subject=unsubscribe> >> > _______________________________________________ > Triangle, NC Embedded Computing mailing list > > To post message: [email protected] > List info: http://mail.triembed.org/mailman/listinfo/triembed_triembed.org > TriEmbed web site: http://TriEmbed.org > To unsubscribe, click link and send a blank message: > mailto:[email protected]?subject=unsubscribe >
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