[TriEmbed] OT: RF questions

[Dan Habecker]

  Brian,

Yea, at those power levels and balun is probably not needed.  What happens
without one is you get some radiation on the coax feeding the antenna.  One
simple way though to add a little bit of a choke balun is to create a
couple a loop with a couple turns of the feed line right before the antenna.

As far as attaching to the antenna, soldering or using tap screws will work
just fine.  I've seen it done both ways.

Good luck on your project.

Dan W9EQ

On Mon, Sep 28, 2020 at 12:36 PM Brian via TriEmbed <triembed at triembed.org>
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
>
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