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
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
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