"Kash J. Rangan" wrote in
:

Is it advisable to use a combination of Coax and a ladder line to feed
a simple horizontal multiband dipole antenna?

In my new QTH it would be very easy for me to get a short run of coax
out of my shack window but I would like to use a low loss ladder line
for the main run up to the antenna feed point. Is it OK to place a 4:1
balun just outside of the window for the transition?

If your antenna is balanced, it would probably be better to simply split
the ladder line onto the center conducters of two identical short coaxes
and then run ladder line inside to your tuner. If you MUST ground the
shields that's OK and you can use lightning arrestors on both coaxes. But
remember, no lighning arrestor is as good for protecting equipment as a
foot or two of air. Disconnecting during thunderstorms is solid policy!

--
Dave Oldridge+
ICQ 454777283

"Dave Oldridge" wrote in message
0...
"Kash J. Rangan" wrote in
:

Is it advisable to use a combination of Coax and a ladder line to feed
a simple horizontal multiband dipole antenna?

In my new QTH it would be very easy for me to get a short run of coax
out of my shack window but I would like to use a low loss ladder line
for the main run up to the antenna feed point. Is it OK to place a 4:1
balun just outside of the window for the transition?

If your antenna is balanced, it would probably be better to simply split
the ladder line onto the center conducters of two identical short coaxes
and then run ladder line inside to your tuner. If you MUST ground the
shields that's OK and you can use lightning arrestors on both coaxes. But
remember, no lighning arrestor is as good for protecting equipment as a
foot or two of air. Disconnecting during thunderstorms is solid policy!

--
Dave Oldridge+
ICQ 454777283

Sounds a little backwards. The Ladder Line should transition to coax
(balanced as you pointed out) inside to hopefully reduce RF in the shack.
An arrestor outside in addition to disconnecting from equipment will avoid
arcing in the shack at least. You have to be there to even remember to
disconnect.

Dave Oldridge wrote:
"Kash J. Rangan" wrote in


If your antenna is balanced, it would probably be better to simply split
the ladder line onto the center conducters of two identical short coaxes
and then run ladder line inside to your tuner. If you MUST ground the
shields that's OK and you can use lightning arrestors on both coaxes. But
remember, no lighning arrestor is as good for protecting equipment as a
foot or two of air. Disconnecting during thunderstorms is solid policy!


If you have a direct hit, a one foot air gap isn't necessarily going to
do you much good, unless the antenna end of the gap is on the ground
surface. (i.e. the wire going from where the coax ends to your
lightning dissipation ground has some non-zero inductance/resistance)

If you're worried about induced voltages from adjacent strikes, then a
good transient suppressor will help, but almost all suppressors have
"let through" voltage that is above the damage threshold for, say, a FET
front end. Depends on what your equipment sensitivity is.

Shorting the input of the radio and tying it to chassis ground.. that
WILL protect the radio.

Jim Lux wrote in
:

Dave Oldridge wrote:
"Kash J. Rangan" wrote in


If your antenna is balanced, it would probably be better to simply
split the ladder line onto the center conducters of two identical
short coaxes and then run ladder line inside to your tuner. If you
MUST ground the shields that's OK and you can use lightning arrestors
on both coaxes. But remember, no lighning arrestor is as good for
protecting equipment as a foot or two of air. Disconnecting during
thunderstorms is solid policy!


If you have a direct hit, a one foot air gap isn't necessarily going to
do you much good, unless the antenna end of the gap is on the ground
surface. (i.e. the wire going from where the coax ends to your
lightning dissipation ground has some non-zero inductance/resistance)

If you're worried about induced voltages from adjacent strikes, then a
good transient suppressor will help, but almost all suppressors have
"let through" voltage that is above the damage threshold for, say, a
FET front end. Depends on what your equipment sensitivity is.

Shorting the input of the radio and tying it to chassis ground.. that
WILL protect the radio.

I have been through some VERY violent storms. I always disconnected all
antenna and power leads from the radio. Outside arrestors will help keep
it out of the house but you need airspace to protect receiver front ends.
Also disconnect any ethernet runs.

Surge protection can only do so much. I learned the hard way about
disconnecting stuff. My neighbour across the street had a direct hit and
it danced across the phone lines into my equipment. I lost several
modems, a couple of ethernet cards and a monitor and considered myself
lucky that none of the computers was totalled. But my radios were
disconnected and unscathed.

Now the coast station I worked for had a direct hit on our Nautel 500khz
transmitter's tower. Blew out half the solid state final modules but the
damn thing kept right on ticking on the others!



--
Dave Oldridge+
ICQ 454777283
VA7CZ

MUST ground the shields that's OK and you can use lightning arrestors
on both coaxes.

This is probably not going to work in most cases. Remember an open wire
feedline can have
rather high voltages on it compared to a matched coax liine. This is
because they are often
mismatched at the antenna.
If a voltage peak should happen to occur at the place where you have the
arrestor, it can fire
the gas tube because of the rf voltage.
For example, a gas tube arrestor made for legal limit typically has a firing
voltage of about 800 volts. In a matched
50 ohm system, even with a full 1500 watts into 50 ohms the peak rf voltage
is in the neighborhood of
600 volts. But even a moderate power into a high impedance open line could
be enough to fire
the gas tubes. Neither the transmitter nor the gas tube would be happy with
this situation.

Rick K2XT

"Rick" wrote in
:

MUST ground the shields that's OK and you can use lightning
arrestors on both coaxes.

This is probably not going to work in most cases. Remember an open
wire feedline can have
rather high voltages on it compared to a matched coax liine. This is
because they are often
mismatched at the antenna.
If a voltage peak should happen to occur at the place where you have
the arrestor, it can fire
the gas tube because of the rf voltage.
For example, a gas tube arrestor made for legal limit typically has a
firing voltage of about 800 volts. In a matched
50 ohm system, even with a full 1500 watts into 50 ohms the peak rf
voltage is in the neighborhood of
600 volts. But even a moderate power into a high impedance open line
could be enough to fire
the gas tubes. Neither the transmitter nor the gas tube would be happy
with this situation.

That's true. You would probably need air-gap arrestors that can be
adjusted to not arc on normal transmitted voltages.

Still, the only really effective way of feeding a non-trap wire system on
several bands that I've encountered is to use open wire.

--
Dave Oldridge+
ICQ 454777283

In article ,
Dave Oldridge wrote:

That's true. You would probably need air-gap arrestors that can be
adjusted to not arc on normal transmitted voltages.

Still, the only really effective way of feeding a non-trap wire system on
several bands that I've encountered is to use open wire.

--
Dave Oldridge+
ICQ 454777283

One of the best "air-gap arrestors" of the home-brew variety I have seen,
was made with a couple of Wide Gapped Spark-Plugs, threaded into a 1/4
Steel Plate that was bonded to a very deep Grounding System. The fellow
used GTO15000 to connect the spark-plugs to the Wire-Feeders. Worked
very well.......

--
Bruce in alaska
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