"pneumatic-traps" dipole
 

Hi,

For various reasons I am considering building a trapped dipole
without traps. Well, not really. I would use pneumatical
(compressed-air) switches in place of the traps.

What I don't know is if the switches rated for 16A 250V AC will
stand the voltage at the edge of the antenna when open. Power
would be 100W, but I wouldn't mind reducing to 50W or even 25W if
that would make the switches work. Of course I would always
operate the switches without RF on the antenna. I know normal
voltages an antenna edges are much higher, but I also think that
250V working voltage probably means much higher isolation
voltage. Just don't know how much more.

Thanks for answers / ideas.

Toni - EA3FYA

On Wed, 10 Mar 2004 08:20:11 +0100, Toni
wrote:

Thanks Richard. I was already thinking nobody would bother to
answer.

Morbid debates (flaming) have a way of captivating other's attention.

Your point about Q is a good one. I was already thinking on how
to lower the Q for better bandwidth -cage dipole or similar- but
I had not thought it also reduces voltages.

Hi Toni,

You have to be careful about interpreting switch ratings. They are
almost always referenced to a resistive load at DC or low frequency
AC. The 250V specification presumes you are switching a live circuit,
and the specification is more concerned with the arc that will be
developed breaking the circuit. The opening contacts will try to
maintain a current with a plasma until you get to the extinguishing
potential (if ever). This current causes point erosion with metal
migrating from one switch face to the other (a pit in one, a point on
the other). This worsens the problem because of the sharp point
developed with reduced spacing.

The simple solution to this is to change the switch setting when your
transmitter is not keyed. It thus becomes a problem of that same
contact separation distance being sufficiently wide so that an arc
does not develop and jump the gap when you key down.

73's
Richard Clark, KB7QHC

The
apparently easier and safest way to go is not switching at the
very edge but at some 70% of the element.


Why do you want use pneumatic switches to disconnect part of the antenna?
Why not use the LC traps? They provide "automatic" switching.

Yuri, K3BU

En Yuri Blanarovich va escriure en 10 Mar 2004 14:47:54 GMT:

The
apparently easier and safest way to go is not switching at the
very edge but at some 70% of the element.


Why do you want use pneumatic switches to disconnect part of the antenna?
Why not use the LC traps? They provide "automatic" switching.

Because, appart from being dificult to build and tune (to me)
they restrict a lot the useful bandwidth. I have not found a
trapped design that didn't present a very narrow SWR curve.

With a switched diople for 10/15/17/40 I should be able to
operate full-band moderatelly well on all of them (specially, all
of 28-30 MHz), even if the antenna detunes a bit with aging.

As for pneumatic operation it is for simplicity. I would have
used relays but then you have to start thinking on RF chokes and
decoupling both the feeding and the control line. Compressed air
tubes are non-conductive = no-problem. Again, I like the
simplicity of it. Price is not very diferent.

Toni - EA3FYA

To stop the arcing, why not put a condenser across
the points, like on my old Morris Minor contact breaker points ?

Kevin
i
M3 TW T





(Sorry - couldn't resist that)

En nick smith va escriure en Wed, 10 Mar 2004 15:38:45 -0000:

To stop the arcing, why not put a condenser across
the points, like on my old Morris Minor contact breaker points ?

Kevin
i
M3 TW T

Not that bad an idea. Had already thought so for a rotable 10/15
rigid dipole. With a 15 pF capacitor / short at 50% on each leg
you get both a good 10 and 15 meters dipole with a little (very
little) extra gain for 10 meters. Operation voltage on capacitor
with 100W is only 170V - No arcing. SWR is about 2 for 28-30 and
1.7 for 15m


(Sorry - couldn't resist that)

(Sorry I couldn't resist either)

Toni - EA3FYA

On Wed, 10 Mar 2004 16:59:43 +0100, Toni wrote:

En nick smith va escriure en Wed, 10 Mar 2004 15:38:45 -0000:

To stop the arcing, why not put a condenser across
the points, like on my old Morris Minor contact breaker points ?

Kevin
i
M3 TW T

Not that bad an idea. Had already thought so for a rotable 10/15
rigid dipole. With a 15 pF capacitor / short at 50% on each leg
you get both a good 10 and 15 meters dipole with a little (very
little) extra gain for 10 meters. Operation voltage on capacitor
with 100W is only 170V - No arcing. SWR is about 2 for 28-30 and
1.7 for 15m


(Sorry - couldn't resist that)

(Sorry I couldn't resist either)

Toni - EA3FYA

(I can resist it)

Hi All,

The reason for the condenser in the breakers contacts was to RESONATE
the system so that current dumped through the (this time I will call
it by its more modern name) capacitor. Given this dump was a damped
one cycle wave (the spark current) it both enhanced the boost to the
ignition coil and opened the circuit effectively in the DC path. If
you chose the wrong value, it snubbed the spark and eroded the contact
faces: in other words, worse than useless.

You should take stock of the terms RESONATE and DC, two situations
that are antithetical to the application. If you want to open an AC
circuit path, the addition of this capacitor is counter-intuitive. It
is also a tuning complexity: too big a value, it makes the switch
useless; too small a value, it retains the arc potential. Any value,
it causes a shift of the operating frequency - you are back in trap
city, without the necessary inductor, with an added switch, and
nothing seems to be gained.

Much simpler to not key-down while changing the switch.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Wed, 10 Mar 2004 16:59:43 +0100, Toni wrote:

En nick smith va escriure en Wed, 10 Mar 2004 15:38:45 -0000:

To stop the arcing, why not put a condenser across
the points, like on my old Morris Minor contact breaker points ?

Kevin
i
M3 TW T

Not that bad an idea. Had already thought so for a rotable 10/15
rigid dipole. With a 15 pF capacitor / short at 50% on each leg
you get both a good 10 and 15 meters dipole with a little (very
little) extra gain for 10 meters. Operation voltage on capacitor
with 100W is only 170V - No arcing. SWR is about 2 for 28-30 and
1.7 for 15m


(Sorry - couldn't resist that)

(Sorry I couldn't resist either)

Toni - EA3FYA

(I can resist it)

Hi All,

The reason for the condenser in the breakers contacts was to RESONATE
the system so that current dumped through the (this time I will call
it by its more modern name) capacitor. Given this dump was a damped
one cycle wave (the spark current) it both enhanced the boost to the
ignition coil and opened the circuit effectively in the DC path. If
you chose the wrong value, it snubbed the spark and eroded the contact
faces: in other words, worse than useless.

You should take stock of the terms RESONATE and DC, two situations
that are antithetical to the application. If you want to open an AC
circuit path, the addition of this capacitor is counter-intuitive. It
is also a tuning complexity: too big a value, it makes the switch
useless; too small a value, it retains the arc potential. Any value,
it causes a shift of the operating frequency - you are back in trap
city, without the necessary inductor, with an added switch, and
nothing seems to be gained.

Much simpler to not key-down while changing the switch.

73's
Richard Clark, KB7QHC



I think Richard missed that one then !!!!

On Wed, 10 Mar 2004 18:34:09 -0000, "nick smith"
wrote:
I think Richard missed that one then !!!!

Why? Because I had a Hillman Minx, or was it the MG-TD?

"Toni - EA3FYA" wrote in message
...
Hi,

For various reasons I am considering building a trapped dipole
without traps. Well, not really. I would use pneumatical
(compressed-air) switches in place of the traps.

What I don't know is if the switches rated for 16A 250V AC will
stand the voltage at the edge of the antenna when open. Power
would be 100W, but I wouldn't mind reducing to 50W or even 25W if
that would make the switches work. Of course I would always
operate the switches without RF on the antenna. I know normal
voltages an antenna edges are much higher, but I also think that
250V working voltage probably means much higher isolation
voltage. Just don't know how much more.

Thanks for answers / ideas.

Toni - EA3FYA

I would think the switch should work well as lonfg as you do not transmit
while opening a switch. Most switches are rated for opening under full load.
If you rate the same swich for opening under no load it can be rated at
several times its normal rating. I experimented with something similar to
what you are doing with smaller relays and it worked fine. There were some
inherent problems usuing relays but your idea of using pneumatic switches
would over come all the problems I had..