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<div class="moz-cite-prefix">> Maybe make a 1/4 wave ground plane
instead of a dipole? The ground would connected to a set of 4 or
more <br>
</div>
<div class="moz-cite-prefix">> ground radials down tilted 45
degrees and a single straight up and down stick. <a
href="http://ccarc.org/_misc/so-239_ant.html" class="">http://ccarc.org/_misc/so-239_ant.html</a></div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">Yes. This is a dead simple antenna that
works pretty well. It's designed to be fed unbalanced, just like
the diagram. You can adjust the impedance match (and the power
out) by bending the four lower diagonals closer to or farther away
from the coax, although 45 degrees is usually about right. You can
use any panel mount connector (BNC, N, etc.) and, if the connector
is made of the correct material, you can just solder the lower
wires to the base of the connectors, rather that use screws as the
diagram shows. Also, the 14-guage conductors from the inside of a
piece of Romex work well as the radiating elements. <br>
</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">Kevin<br>
</div>
<div class="moz-cite-prefix"><br>
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<div class="moz-cite-prefix"><br>
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<div class="moz-cite-prefix">On 9/29/20 1:18 AM, Tadd Torborg via
TriEmbed wrote:<br>
</div>
<blockquote type="cite"
cite="mid:BEA7323F-4BE4-45F7-938C-53849E2A1C06@mac.com">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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. <a
href="http://ccarc.org/_misc/so-239_ant.html" class=""
moz-do-not-send="true">http://ccarc.org/_misc/so-239_ant.html</a>
<div class="">
<div class=""><br class="">
</div>
<div class="">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? </div>
<div class=""><br class="">
</div>
<div class="">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. </div>
<div class=""><br class="">
</div>
<div class="">
<div class="">
<div style="color: rgb(0, 0, 0); font-family: Helvetica;
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<div style="orphans: auto; widows: auto;" class=""><span
style="text-align: -webkit-auto;" class="">Tadd -
KA2DEW</span></div>
<div class=""><br class="">
</div>
</div>
<br class="Apple-interchange-newline">
<br class="Apple-interchange-newline">
</div>
<div><br class="">
<blockquote type="cite" class="">
<div class="">On Sep 28, 2020, at 10:31 PM, Pete Soper via
TriEmbed <<a href="mailto:triembed@triembed.org"
class="" moz-do-not-send="true">triembed@triembed.org</a>>
wrote:</div>
<br class="Apple-interchange-newline">
<div class="">
<meta http-equiv="Content-Type" content="text/html;
charset=UTF-8" class="">
<div class="">
<p class="">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 class="">
</p>
<p class="">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"
class=""><br class="">
</a></p>
<p class="">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
class="">
</p>
<p class="">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 class="">
</p>
<p class="">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"
class="">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 class="">
</p>
<p class="">-Pete AD4L</p>
<p class=""><span
id="cid:part3.EDC1387B.3C1A0A35@soper.us"><opiadpmmadeneekh.png></span></p>
<div class="moz-cite-prefix">On 9/28/20 12:29 PM,
Brian via TriEmbed wrote:<br class="">
</div>
<blockquote type="cite"
cite="mid:b7632d10-7924-e9d6-ac02-e19070ef364c@undecidedgames.com"
class="">Hi Folks, <br class="">
<br class="">
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 class="">
<br class="">
The questions first; I'll provide background
afterward. <br class="">
<br class="">
I have settled on a digital FM transmitter chip
(Silicon Labs' SI4721), and a simple dipole
antenna. Two questions arise from this decision: <br
class="">
<br class="">
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 class="">
<br class="">
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 class="">
<br class="">
Here's the background: <br class="">
<br class="">
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 class="">
<br class="">
Well anyway, thanks in advance for any advice! <br
class="">
<br class="">
Cheers, <br class="">
-Brian <br class="">
<br class="">
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