Elecraft

KPA100 rev D

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wb2srf

KPA100 rev D

Hi Wayne,

There a reason for concern for eliminating the choke in the antenna
circuit. I noticed that there is no DC path to ground to eliminate the
chance of static potenital build up on an antenna from discharging a high
voltage into the rig. This may be the reason why some builders have
experienced blown detection diodes in some ATU's etc.

I think that is a good practice to install a 47K 1/2W carbon resistor
across the antenna terminals to act as a bleeder resistor when there is a
capacitive coupled output.

73,
Bob Johansen WB2SRF
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David Cutter

Re: KPA100 rev D

In the marine set I went to sea with, 2 x 10k 25W carbon resistors in
parallel were fitted as an antenna bleed. This was on the MF/HF set: all
valves in those days. Antenna was end fed inverted L and the tx was about
150W.

David
G3UNA

----- Original Message -----
From: "Robert Johansen" <[hidden email]>
To: <[hidden email]>
Sent: Saturday, November 11, 2006 4:42 PM
Subject: [Elecraft] KPA100 rev D

> Hi Wayne,
>
> There a reason for concern for eliminating the choke in the antenna
> circuit. I noticed that there is no DC path to ground to eliminate the
> chance of static potenital build up on an antenna from discharging a high
> voltage into the rig. This may be the reason why some builders have
> experienced blown detection diodes in some ATU's etc.
>
> I think that is a good practice to install a 47K 1/2W carbon resistor
> across the antenna terminals to act as a bleeder resistor when there is a
> capacitive coupled output.
>
> 73,
> Bob Johansen WB2SRF
> _______________________________________________
> Elecraft mailing list
> Post to: [hidden email]
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
> http://mailman.qth.net/mailman/listinfo/elecraft
>
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com

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wb2srf

Re: KPA100 rev D

In reply to this post by wb2srf

Hi David,

Thank you for the info,

It is good to know how this problem was addressed on a ship board radio
installation.

I understand that when the bleeder resistor is at a high impedance point
in the antenna circuit it will need to dissipate power.

In a 50 Ohm circuit operating up to 100 W (70.7V) I think that the 47K
1/2W resistor should be sufficient with a reasonable safety factor unless
there is a high potential due to a nearby lightning strike etc.

I know that a 47K resistor will heat up from RF if it is connected across
the antenna coupler output when it is tuned to a high impedance antenna
even at QRP power levels.

The L network used in the Elecraft ATU designs provides DC coupling
through the series inductors so even though the bleeder is connected to
the 50 Ohm transmitter output it should be effective in keeping a static
charge from building up on a long wire antenna. The concern here is that
if the charge builds up to several thousand volts, dielectric breakdown
of coupling capacitors can occur which may cause damage to the diodes in
the RF bridge etc.

73,
Bob Johansen WB2SRF

On Sun, 12 Nov 2006 15:37:26 -0000 "David Cutter" <[hidden email]>
writes:

> In the marine set I went to sea with, 2 x 10k 25W carbon resistors
> in
> parallel were fitted as an antenna bleed. This was on the MF/HF
> set: all
> valves in those days. Antenna was end fed inverted L and the tx was
> about
> 150W.
>
> David
> G3UNA
>
> ----- Original Message -----
> From: "Robert Johansen" <[hidden email]>
> To: <[hidden email]>
> Sent: Saturday, November 11, 2006 4:42 PM
> Subject: [Elecraft] KPA100 rev D
>
>
> > Hi Wayne,
> >
> > There a reason for concern for eliminating the choke in the
> antenna
> > circuit. I noticed that there is no DC path to ground to
> eliminate the
> > chance of static potenital build up on an antenna from discharging
> a high
> > voltage into the rig. This may be the reason why some builders
> have
> > experienced blown detection diodes in some ATU's etc.
> >
> > I think that is a good practice to install a 47K 1/2W carbon
> resistor
> > across the antenna terminals to act as a bleeder resistor when
> there is a
> > capacitive coupled output.
> >
> > 73,
> > Bob Johansen WB2SRF
> > _______________________________________________
> > Elecraft mailing list
> > Post to: [hidden email]
> > You must be a subscriber to post to the list.
> > Subscriber Info (Addr. Change, sub, unsub etc.):
> > http://mailman.qth.net/mailman/listinfo/elecraft
> >
> > Help: http://mailman.qth.net/subscribers.htm
> > Elecraft web page: http://www.elecraft.com
>
>
>

David Cutter

Re: KPA100 rev D

On medium wave broadcast stations they fit quite large rf chokes across the
antenna terminals. I've seen some a foot long by 2" diameter. The self
resonant frequency (L plus self capacitance, in effect a parallel resonant
tuned circuit) is on the tx frequency, so they get low resistance to
discharge any build up on the antenna. Of course if there is a direct
strike, all bets are off, but this way the antenna can discharge large
amounts of charge to prevent the voltage getting high in the first place.

David
G3UNA

----- Original Message -----
From: "Robert Johansen" <[hidden email]>
To: <[hidden email]>; <[hidden email]>
Sent: Sunday, November 12, 2006 4:12 PM
Subject: Re: [Elecraft] KPA100 rev D

> Hi David,
>
> Thank you for the info,
>
> It is good to know how this problem was addressed on a ship board radio
> installation.
>
> I understand that when the bleeder resistor is at a high impedance point
> in the antenna circuit it will need to dissipate power.
>
> In a 50 Ohm circuit operating up to 100 W (70.7V) I think that the 47K
> 1/2W resistor should be sufficient with a reasonable safety factor unless
> there is a high potential due to a nearby lightning strike etc.
>
> I know that a 47K resistor will heat up from RF if it is connected across
> the antenna coupler output when it is tuned to a high impedance antenna
> even at QRP power levels.
>
> The L network used in the Elecraft ATU designs provides DC coupling
> through the series inductors so even though the bleeder is connected to
> the 50 Ohm transmitter output it should be effective in keeping a static
> charge from building up on a long wire antenna. The concern here is that
> if the charge builds up to several thousand volts, dielectric breakdown
> of coupling capacitors can occur which may cause damage to the diodes in
> the RF bridge etc.
>
> 73,
> Bob Johansen WB2SRF
>
> On Sun, 12 Nov 2006 15:37:26 -0000 "David Cutter" <[hidden email]>
> writes:
>> In the marine set I went to sea with, 2 x 10k 25W carbon resistors
>> in
>> parallel were fitted as an antenna bleed. This was on the MF/HF
>> set: all
>> valves in those days. Antenna was end fed inverted L and the tx was
>> about
>> 150W.
>>
>> David
>> G3UNA
>>
>> ----- Original Message -----
>> From: "Robert Johansen" <[hidden email]>
>> To: <[hidden email]>
>> Sent: Saturday, November 11, 2006 4:42 PM
>> Subject: [Elecraft] KPA100 rev D
>>
>>
>> > Hi Wayne,
>> >
>> > There a reason for concern for eliminating the choke in the
>> antenna
>> > circuit. I noticed that there is no DC path to ground to
>> eliminate the
>> > chance of static potenital build up on an antenna from discharging
>> a high
>> > voltage into the rig. This may be the reason why some builders
>> have
>> > experienced blown detection diodes in some ATU's etc.
>> >
>> > I think that is a good practice to install a 47K 1/2W carbon
>> resistor
>> > across the antenna terminals to act as a bleeder resistor when
>> there is a
>> > capacitive coupled output.
>> >
>> > 73,
>> > Bob Johansen WB2SRF
>> > _______________________________________________
>> > Elecraft mailing list
>> > Post to: [hidden email]
>> > You must be a subscriber to post to the list.
>> > Subscriber Info (Addr. Change, sub, unsub etc.):
>> > http://mailman.qth.net/mailman/listinfo/elecraft
>> >
>> > Help: http://mailman.qth.net/subscribers.htm
>> > Elecraft web page: http://www.elecraft.com
>>
>>
>>