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<p>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. <br>
</p>
<p>The graph of SWR vs frequency from 88 to 108MHz is below.<a
moz-do-not-send="true"
href="http://triembed.org/images/FM-DIPOLE.png"><br>
</a></p>
<p>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.<br>
</p>
<p>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.<br>
</p>
<p>You can get baluns from Digikey, by the way. The <a
moz-do-not-send="true"
href="https://www.digikey.com/product-detail/en/macom-technology-solutions/MABA-011040/1465-1702-1-ND/5131112">MABA-011040</a>
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. <br>
</p>
<p>-Pete AD4L</p>
<p><img src="cid:part3.EDC1387B.3C1A0A35@soper.us" alt=""></p>
<div class="moz-cite-prefix">On 9/28/20 12:29 PM, Brian via TriEmbed
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:b7632d10-7924-e9d6-ac02-e19070ef364c@undecidedgames.com">Hi
Folks,
<br>
<br>
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.
<br>
<br>
The questions first; I'll provide background afterward.
<br>
<br>
I have settled on a digital FM transmitter chip (Silicon Labs'
SI4721), and a simple dipole antenna. Two questions arise from
this decision:
<br>
<br>
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.
<br>
<br>
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?
<br>
<br>
Here's the background:
<br>
<br>
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.
<br>
<br>
Well anyway, thanks in advance for any advice!
<br>
<br>
Cheers,
<br>
-Brian
<br>
<br>
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