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Antenna link up to coax cable
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by KG4NKX on May 27, 2001
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As I understand it, coax cable has two "wires," one the central wire and the other the braid. Is the braid just for grounding? So if I hook up some RG-58 with a BNC connector on one end to my HT and touch the center pin to a ~4' alluminum pipe, will I have a decently matched antenna? Do I have to use a coil? If so, why--to match impedances?
Thanks,
Ari
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RE: Antenna link up to coax cable
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by WB6BYU on May 29, 2001
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Feedlines will generally work best when both wires are connected
to something at the other end. It is like an electric cord - a break
in either wire and your lamp doesn't light. However, just as it is
possible to get a shock from touching only one wire of an electric
cord, it is also possible in some cases to get an antenna to work
with only one connection.
If you plug a rubber ducky into your HT, it probably doesn't use
both sides of the BNC plug (except as a mechanical support.)
The reason it works is that the ground current flows down the the
outside of the HT case instead, so the HT is acting like the other
half of the antenna. If you connect a ducky (or a single conductor
antenna) to the end of a length of coax, the current flowing on the
inside of the shield has nowhere else to go except back down the
outside of the coax. This may or may not give a good match,
and/or be a good radiator, depending on the length of the cable,
how it is bent, and what else is connected to it. However, it means
that the coax, rig, and anything else attached to it (including your
hand and body, in some cases) may now be part of the antenna.
The results are unpredictable, and often highly variable, even over
the course of a single contact. So this approach is not popular,
though you probably could coil up the coax to make an RF choke
to provide some control over the current flow.
It is the load presented to the current flowing from the center
conductor to the shield (and vice versa) which determines the SWR
on the feedline. A good match will optimize the power transfer.
Connecting the coax to one end of a 40" rod will not work very well.
The rod is about a half wavelength, and the end will have a high
impedance (around 1000 ohms perhaps.) This is not a good match
to your coax (50 ohms), even if you have the shield connected to
something. You can add an impedance matching network of a coil
and a capacitor. My favorite design is about 5 turns of coil roughly
the size of your thumb, with one end connected to the antenna and
the other end connected to the coax shield. A small trimmer
capacitor (perhaps 25pf) is connected across the coil, and the
center conductor of the coax is connected to the coil about one
turn up from the end which is soldered to the braid. Adjust the
trimmer capacitor and the point where the inner conductor attaches
to the coil for minimum SWR. Using a coil alone probably won't
give a good match in this situation.
There are other ways to match to the rod, particularly if you can
feed it in the middle instead of at one end. The most common is to
break the rod in the middle and connect the coax cable across
the break. This forms the common half-wave dipole.
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RE: Antenna link up to coax cable
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by KG4NKX on May 30, 2001
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So as I understand what you're saying, the braid/shield is just for the ground, but an end-fed design won't work without some sort of impedance matching device? If I grounded the outer shield and Created a ground plane with the radials at ~120 degrees down, and tuned (possibly also with a coil), I assume I should have a decent antenna?
When you talk about feeding a dipole, do both halves of the dipole get connected to the center conductor or does one side get the center conductor and the other side connect to the braided shield?
Thanks,
Ari Friedman
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RE: Antenna link up to coax cable
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by WB6BYU on May 30, 2001
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I think the statement that the braid/shield is "just for the ground" is
misleading: the load needs to be connected between the two wires
(just like an AC power cord.) However, there are cases where the
connection to the braid/shield is not obvious.
With a dipole, one half connects to the center conductor and the
other half connects to the shield. Even though there is no DC wire
present, current does flow between them (through the electric field
of the antenna.) If there is no wire connected to the shield, then
the current which would have flowed on it will instead try to flow
back down the outside of the coax shield, which will act as a long
random wire. Generally, this will not provide a good match, and the
performance will be unpredictable, but occasionally you might get
lucky.
If you want to use a 40" end-fed rod, your idea of adding radials
and a matching network will work. (I've converted a number of CB
mag mount antennas to 2m using this technique.) However, a
simple coil probably will NOT be adequate for impedance matching.
With any antenna, there are two components of the impedance
which must be matched to the feedline: the resistance and the
reactance. Imagine the resistance is a north/south position, and
reactance is east/west. From any random position (representing
the antenna feedpoint) to the desired position (representing the 50
ohm coax cable) will generally require a change in both directions.
A perfect coil only changes the reactance, not the resistance.
(With a very short antenna, adding a large enough coil to cancel
the reactance of the antenna usually adds loss resistance as well.)
Changing the length of an antenna varies both resistance and
reactance, but certain lengths (such as a quarter wave whip, or a
half wave dipole) when adjusted for resonance (that is, reactance
is zero) will have a resistance which is a good match to coax.
This is useful. Other lengths aren't so convenient, but any load can
be matched to coax using one coil and one capacitor (as well as
some other creative approaches.)
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RE: Antenna link up to coax cable
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by WA9SVD on September 20, 2002
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Connecting to a 40" pipe won't work well. A 19-20 " pipe would work better, but you would need to add a few radials. (Instant Vertical Ground Plane antenna!)
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