Open wire feeder protection

[Ken Kopp-2]

Paul and others,

I didn't mean to imply the use of resistors on open-wire
feeders to drain off charges built up on the antenna.  It
would work, but the resistance would "mess up" the
tuning of the antenna unless they were of very high
resistance, which would limit their effectiveness.

I have a 2" wide strip of copper flashing that runs across the
wall of my shack just above the table top.  There are 1/4-20
brass bolts with washers and wing-nuts at interals along the
strip.  This strip runs outside under the wall atop the foundation
to several ground rods driven several feet apart.

The 2-inch spaced feed-throughs for the open wire come into
the shack just above the copper strip and there are large
RF chokes from the thru-bolts to the strip.  This places the
feeders at ground potential and drains off any unwanted
voltage to ground.  The impedence of the RF chokes is
invisible to the RF energy on the feeders.

I'm "serious" about 160M and there are 32 ground rods on the
property. They're all tied together with #8 CU and to the well's
casing.  There are 120 radials under the 80/160 tower, all tied
into the ground system.

I've also used RF chokes from the terminals of my KW
Matchboxes to the ground post just below the terminals.
Needless to say, the 2" CU strap also runs to the bolt.

Older ARRL handbooks depict a method of building a spark-
gap arrangement between the feeder insulators.  Some have
used automobile spark-plugs in this application.

Using a resistor on open-wire feeders wouldn't be ... in my
opinion ... a satisfactory arrangement.

73! Ken Kopp - K0PP
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[Stuart Rohre]

Ken, I have to respectfully disagree that properly sized resistors would
"mess up the tuning" of the antenna and feeders.

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.

Stuart
K5KVH


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[Ron D'Eau Claire-2]

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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