Posted by
Ron D'Eau Claire-2 on
Nov 14, 2006; 1:02am
URL: http://elecraft.85.s1.nabble.com/Open-wire-feeder-protection-tp395274p395276.html
Stuart, K5KVH wrote:
Our radio club has operated up to 5 transmitters at Field Days, with 100,000
ohm, 2 watt carbon resistors on the 450 ohm feeders, one from each side to a
ground rod to bleed off wind static, and storm static buidup, if a
thunderstorm passes by. It has, the bleed offs worked, and we have not lost
a rig. The tuning with or without the resistors is the same on the tuners,
as long as the impedance is 10 to 100 times that on the feeder. We go for
the higher value, but it bleeds the static before it gets to arc over
potentials.
The rigs we operated at 100 watts output max with no problems with the
resistors as described. Carbon rather than wire wound prevents resonance
problems.
--------------------------------------
The larger problem often occurs trying to use chokes. An RF choke must show
a much, much higher impedance than that of the antenna circuit in order to
avoid detuning and losses.
The impedance of the choke is inversely proportional to the frequency, so it
needs to enough inductive reactance on the *lowest* frequency the rig
covers.
As the amount of inductance increases, the series-resonant frequencies also
drop. Even using sophisticated winding techniques, it's not possible to make
an RF choke that can cover from, say, 3.5 MHz to 30 MHz without series
resonances somewhere in between those frequencies. The 'trick' when
designing a Ham rig is to keep those series resonant frequencies well away
from any Ham bands.
That's possible, but difficult, when working with a 50 ohm output impedance
such as found at the antenna jack of most rigs today. Many years ago, when
RF chokes started appearing across the outputs of transmitters as a
protective device to ensure high d-c voltages* didn't appear on the antenna
jacks, they only had to ensure there were no series resonances near 80, 40,
20, 15 and 10 meters. (A lot of rigs didn't cover 160 meters to avoid the
design issues).
Nowadays, we have to include the additional "WARC" bands!
And that assumes the antenna impedance will ALWAYS be fairly low - less than
100 ohms.
When trying to use chokes on an open wire feeder that may have a high SWR,
the reactance of the chokes must be much, much higher than the highest
impedance that may appear at that point in the feed line. It's not uncommon
for that impedance to reach several thousand ohms on some bands. That means
a very choke impedance is required; 10 to 100 times larger than that used on
a 50 ohm circuit. And, the series resonant frequencies must still fall
outside any bands that will be used.
That's why the shipboard and other commercial radio gear I worked with that
covered a large range of frequencies did not use chokes across the antenna.
Now, simple transceivers such as those used on specific frequencies are a
different story, especially if they have a 50 ohm output. But we Hams like
to move around!
That's also why every shipboard radio console that covered the range of 400
kHz up into the Short waves at 10 MHz or higher that I ever worked on had a
large, heavily-insulated rotary switch at the top of the rack - often turned
with what looked a lot like a small "ships wheel". That switch selected the
antenna to be used and, very importantly, grounded the antennas not in use.
When using resistors, it's important they be "non inductive" resistors! Many
modern resistors use a resistive element that wraps around a cylindrical
form forming an inductor. Most "metal film" types are okay as well as the
older types of "carbon composition" (essentially a carbon rod) that Stuart
mentions
Ron AC7AC
* Those d-c voltages I refer to didn't come from lightning or static. When
pi-net output circuits consisting of a coil in series and capacitors to
ground started appearing shortly after WWII, transmitters still used vacuum
tube final amplifiers. The plate voltage on those tubes might be anywhere
from a few hundred to several thousand volts. A simple "blocking" capacitor
isolated that d-c voltage from the antenna jack while allowing the RF to
pass. If the cap shorted, the full d-c high voltage would appear at the
antenna jack, and on the antenna and any antenna tuner used with it. The
consequences of touching any part of the antenna circuit easily could be
fatal. The idea of the choke was to provide a d-c path that would blow the
fuses in the HV supply if that ever occurred.
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