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RX Ant Preamps and the K3 12v Accessory Jack

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Rich-4

RX Ant Preamps and the K3 12v Accessory Jack

I had a thought for a future feature request. For those of us who use RX
antennas (active and/or preamp). It would be really cool if the 12v output
on the K3 could be triggered by the PTT. If the 12v output jack could be
set in the menus to 0v (OFF) on PTT we could power our preamps via that jack
and kill the power on TX. The K3 already has a built in delay for RF output
to amplifiers which can be set in "ms" as needed. Most modern preamps go to
ground when power is removed. This would be nice but I would assume that
the 12v output is not under control of the firmware. Just a thought for
future expansion.

Rich

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wayne burdick

Re: RX Ant Preamps and the K3 12v Accessory Jack

Administrator

Hi Rich,

You could control 12 V to an external RX antenna or preamp using the
KEY OUT signal pretty easily. This signal "floats" (open, or hi-Z)
during receive, and goes to 0 V (ground) during transmit. The timing
is perfect for such an application, because there's a delay between
activation of KEY OUT and actual RF output.

The KEY OUT signal would have to be buffered to create a "12 V on RX/0
V on TX" signal capable of powering the external device. An
alternative would be to use the signal to directly control an enable
line on the device, if available and of the correct polarity.

73,
Wayne
N6KR

On Aug 20, 2010, at 8:58 PM, K3RWN wrote:

> I had a thought for a future feature request. For those of us who
> use RX
> antennas (active and/or preamp). It would be really cool if the 12v
> output
> on the K3 could be triggered by the PTT. If the 12v output jack
> could be
> set in the menus to 0v (OFF) on PTT we could power our preamps via
> that jack
> and kill the power on TX. The K3 already has a built in delay for
> RF output
> to amplifiers which can be set in "ms" as needed. Most modern
> preamps go to
> ground when power is removed. This would be nice but I would assume
> that
> the 12v output is not under control of the firmware. Just a thought
> for
> future expansion.

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David Cutter

Re: RX Ant Preamps and the K3 12v Accessory Jack

I'm thinking this might also act as a gate to prevent excessive rf getting
into the K3 2nd rx. I'm planning to have a horizontal receive antenna
located adjacent to my vertical transmit antenna for diversity reception. I
was just investigating the ICE196, but this idea might be better.

David
G3UNA

> Hi Rich,
>
> You could control 12 V to an external RX antenna or preamp using the
> KEY OUT signal pretty easily. This signal "floats" (open, or hi-Z)
> during receive, and goes to 0 V (ground) during transmit. The timing
> is perfect for such an application, because there's a delay between
> activation of KEY OUT and actual RF output.
>
> The KEY OUT signal would have to be buffered to create a "12 V on RX/0
> V on TX" signal capable of powering the external device. An
> alternative would be to use the signal to directly control an enable
> line on the device, if available and of the correct polarity.
>
> 73,
> Wayne
> N6KR
>
>
> On Aug 20, 2010, at 8:58 PM, K3RWN wrote:
>
>> I had a thought for a future feature request. For those of us who
>> use RX
>> antennas (active and/or preamp). It would be really cool if the 12v
>> output
>> on the K3 could be triggered by the PTT. If the 12v output jack
>> could be
>> set in the menus to 0v (OFF) on PTT we could power our preamps via
>> that jack
>> and kill the power on TX. The K3 already has a built in delay for
>> RF output
>> to amplifiers which can be set in "ms" as needed. Most modern
>> preamps go to
>> ground when power is removed. This would be nice but I would assume
>> that
>> the 12v output is not under control of the firmware. Just a thought
>> for
>> future expansion.
>
>

Geoffrey Mackenzie-Kennedy

Re: RX Ant Preamps and the K3 12v Accessory Jack

If you are planning to use closely spaced Tx and Rx antennas, I would
suggest that you consider using a Reed relay or similar "fast acting" relay
to short circuit the external preamp's input during transmit periods.
Without it there is some risk that during transmit periods the external Rx
preamp's input device could be damaged, even if the preamp's DC supply is
turned off while transmitting.

Of course if diodes are used for protection instead of a relay, the diodes
could generate Tx harmonics which are then radiated by the Rx antenna.

73,

Geoff
GM4ESD

David Cutter write on Saturday, August 21, 2010 at 8:07 AM:

> I'm thinking this might also act as a gate to prevent excessive rf getting
> into the K3 2nd rx. I'm planning to have a horizontal receive antenna
> located adjacent to my vertical transmit antenna for diversity reception.
> I
> was just investigating the ICE196, but this idea might be better.

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David Cutter

Re: RX Ant Preamps and the K3 12v Accessory Jack

Hi Geoff

I did consider that and I get your point about the diode protection, I've
had first hand experience ! To reduce my bench time creating my own
amplifier and such like I was looking into using the RPA-1 from
DXEngineering which has an automatic bypass mode. I don't know if it will
cope with QSK, but as a first stop that's what is in my mind to buy. It
would be easy enough to put a high speed relay on the same line, as you say
to short the amplifier input if needed. I think the spec says the RPA
handles 30dBm signals as normal, so I'm thinking it will prevent damaging
signals getting to the rx input. I'll be doing some measurements to verify
what's going on. I think it might be worth an email to DXE for advice.

73
David
G3UNA

> If you are planning to use closely spaced Tx and Rx antennas, I would
> suggest that you consider using a Reed relay or similar "fast acting"
> relay to short circuit the external preamp's input during transmit
> periods. Without it there is some risk that during transmit periods the
> external Rx preamp's input device could be damaged, even if the preamp's
> DC supply is turned off while transmitting.
>
> Of course if diodes are used for protection instead of a relay, the diodes
> could generate Tx harmonics which are then radiated by the Rx antenna.
>
> 73,
>
> Geoff
> GM4ESD
>
> David Cutter write on Saturday, August 21, 2010 at 8:07 AM:
>
>> I'm thinking this might also act as a gate to prevent excessive rf
>> getting
>> into the K3 2nd rx. I'm planning to have a horizontal receive antenna
>> located adjacent to my vertical transmit antenna for diversity reception.
>> I
>> was just investigating the ICE196, but this idea might be better.
>
>
>

Phil Salas

Re: RX Ant Preamps and the K3 12v Accessory Jack

In reply to this post by Geoffrey Mackenzie-Kennedy

"...I would suggest that you consider using a Reed relay or similar fast
acting relay to short circuit the external preamp's input during transmit
periods..."

Take a look at the Omron signal relay, Mouser part number
653-G6A-274P40-DC12. These relays operate in 3ms typical, 5ms max. I use
them in my ALS-600 QSK conversion and they work great.

Phil - AD5X

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W8JI

Re: RX Ant Preamps and the K3 12v Accessory Jack

In reply to this post by Geoffrey Mackenzie-Kennedy

> If you are planning to use closely spaced Tx and Rx antennas, I would
> suggest that you consider using a Reed relay or similar "fast acting"
> relay
> to short circuit the external preamp's input during transmit periods.

So long as the relay is accuated before RF appears, and held until after RF
goes away.

> Of course if diodes are used for protection instead of a relay, the diodes
> could generate Tx harmonics which are then radiated by the Rx antenna.

Worse yet diodes destroy dynamic range. A typical switching silicon diode or
fast rectifier starts to conduct at about 2.5 dBm, and that can be from an
accumulation of many signals...even out of Ham bands. It takes special
circuitry to not reduce dynamic range with diodes.

<<To reduce my bench time creating my own
amplifier and such like I was looking into using the RPA-1 from
DXEngineering which has an automatic bypass mode. I don't know if it will
cope with QSK, but as a first stop that's what is in my mind to buy. >>

It wasn't planned to do that. The relay would be held up by a capacitor as
it discharges, but it could be modified.

<<I think the spec says the RPA
handles 30dBm signals as normal, so I'm thinking it will prevent damaging
signals getting to the rx input.>>

The RPA can output well over 1/2 watt, so don't expect it to act like a
limiter. It makes a good QRP transmitter.

73 Tom

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David Cutter

Re: RX Ant Preamps and the K3 12v Accessory Jack

Perhaps I should look at protecting the K3 subreceiver from the RPA-1, even
on receive mode :-(

I seem to recall Wayne saying the rx input could take 2W although not
recommended !

73

David
G3UNA
>
> The RPA can output well over 1/2 watt, so don't expect it to act like a
> limiter. It makes a good QRP transmitter.
>
> 73 Tom

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David Cutter

Re: RX Ant Preamps and the K3 12v Accessory Jack

In reply to this post by Phil Salas

Thanks Phil, you have a great site and I've often referred to it, as I do
the W8JI site.

73

David
G3UNA

> "...I would suggest that you consider using a Reed relay or similar fast
> acting relay to short circuit the external preamp's input during transmit
> periods..."
>
> Take a look at the Omron signal relay, Mouser part number
> 653-G6A-274P40-DC12. These relays operate in 3ms typical, 5ms max. I use
> them in my ALS-600 QSK conversion and they work great.
>
> Phil - AD5X
>

Geoffrey Mackenzie-Kennedy

Re: RX Ant Preamps and the K3 12v Accessory Jack

In reply to this post by David Cutter

David,

Have you measured the strength of the BC carriers in the 6 MHz and 7 MHz
"bands" after sunset as seen at the shack end of your antenna's feeder when
terminated ?

If of any use to you I could send you some screen captures using a 1600 kHz
span after sunset this evening, with the BC carrier levels marked. 7 MHz BC
carrier levels received here off a 40m dipole under normal conditions
usually reach + 5 dbm, sometimes a few db higher.

73,
Geoff
GM4ESD

David Cutter wrote on August 21, 2010 at 1:17 PM:

> Perhaps I should look at protecting the K3 subreceiver from the RPA-1,
> even
> on receive mode :-(
>
> I seem to recall Wayne saying the rx input could take 2W although not
> recommended !

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