Coupling a scope to transmitter output

Coupling a scope to transmitter output

[Haines Brown]

I have the feeling this question will be a bit OT, and so let me first
ask to what forum it should have been directed.

I have a Tektronix 2235 I'd like to use to monitor the PSK31 signal
from a FT-817. I googled a bit and looked in the Handbook, but no
clue. 

The simplest coupling would be a coax T connector between the FT-817
and the antenna coax, and a length of coax to the scope's External
Input BNC connector.  

This connector is 1 meg-Ohm, 20 mFd, 400 V peak max, about 100 mV/div.  

But I have no reason to think this simple coupling would work. Any
advice?

Haines Brown KB1GRM 

Re: Coupling a scope to transmitter output

[Jeff Laughlin (RV Melville)]

I wouldn't couple directly. 400V sounds like a lot but even a low-power 
transmitter can produce very high voltages. I would just connect a 
length of wire (IE antenna) to the scope and I'd bet dollars to donuts 
you'd be able to pick up your transmit signal over the air no problem, 
unless your antenna is really far away from the scope .

If that doesn't work, then perhaps you could try measuring the voltage 
drop across a very small load, like a 1/10th ohm non-inductive resistor, 
in series with your transmitter and antenna. I think thats actually how 
el-cheapo power meters work. I think the better power meters use a 
transformer, which you could also try, but I'd have no idea how to wind it.

#hamradio on irc.freenode.net is a good resource.

73 de n1ywb

Haines Brown wrote:
> I have the feeling this question will be a bit OT, and so let me first
> ask to what forum it should have been directed.
>
> I have a Tektronix 2235 I'd like to use to monitor the PSK31 signal
> from a FT-817. I googled a bit and looked in the Handbook, but no
> clue. 
>
> The simplest coupling would be a coax T connector between the FT-817
> and the antenna coax, and a length of coax to the scope's External
> Input BNC connector.  
>
> This connector is 1 meg-Ohm, 20 mFd, 400 V peak max, about 100 mV/div.  
>
> But I have no reason to think this simple coupling would work. Any
> advice?
>
> Haines Brown KB1GRM 
> -
> To unsubscribe from this list: send the line "unsubscribe linux-hams" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>   

Re: Coupling a scope to transmitter output

[Geoff Blake]

On Sat, 4 Nov 2006, Haines Brown wrote:

> I have the feeling this question will be a bit OT, and so let me first
> ask to what forum it should have been directed.
>
> I have a Tektronix 2235 I'd like to use to monitor the PSK31 signal
> from a FT-817. I googled a bit and looked in the Handbook, but no
> clue.
>
> The simplest coupling would be a coax T connector between the FT-817
> and the antenna coax, and a length of coax to the scope's External
> Input BNC connector.
>
> This connector is 1 meg-Ohm, 20 mFd, 400 V peak max, about 100 mV/div.
>
> But I have no reason to think this simple coupling would work. Any
> advice?
>
Well, it may work, but it is also an efficient way to damage the
scope input attenuator. The FT817 is perhaps 10W output or about
20V., but after passing through a unknown length of co-ax, then it
could be over quite a wide range of voltages.

If you terminate the connection in 50 ohms, then you will mismatch
the output of the FT817.

There are 'professional' samplers, for that is what you want to
use. They consist of a short length of 50 ohm line, generally a
rigid coaxial line between a pair of UHF, BNC or N connectors and
a third connector fitted to the side of the line. This connector
is inductively coupled or capacitively coupled to the line. You
could make one of these, or perhaps use a BNC T-piece with the
inner of the branch-line removed. what you have then is a
capacitive divider which frequently serves the task well.

HTH

Geoff

-- 
    Geoff Blake G8GNZ      located  near  Chelmsford, Essex, U.K.
    Please reply to:       geoff  (at) palaemon (dot) co (dot) uk
    Using Linux on Intel & Linux or NetBSD on Sun Sparc platforms

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Re: Coupling a scope to transmitter output

[Wilson G. Hein]

How about a simple pickup loop? Tak a piece of coax, on onme end put
your connector to go to the spec an, and on the other strip the coax
back enough to have enough of the insulated inner conductor to make 3 or
more loops, strip about 1/4 inch of insulation off of the end of the
center conductor and solder it back onto the shield where it goes back
into the outer insulator. Put the loop around the coax between the
transmitter and antenna. Should be satisfactory coupling to read the
tx'd signal.

Willie, WJ3G

On Sat, 2006-11-04 at 15:14 -0500, Haines Brown wrote:
> I have the feeling this question will be a bit OT, and so let me first
> ask to what forum it should have been directed.
> 
> I have a Tektronix 2235 I'd like to use to monitor the PSK31 signal
> from a FT-817. I googled a bit and looked in the Handbook, but no
> clue. 
> 
> The simplest coupling would be a coax T connector between the FT-817
> and the antenna coax, and a length of coax to the scope's External
> Input BNC connector.  
> 
> This connector is 1 meg-Ohm, 20 mFd, 400 V peak max, about 100 mV/div.  
> 
> But I have no reason to think this simple coupling would work. Any
> advice?
> 
> Haines Brown KB1GRM 
> -
> To unsubscribe from this list: send the line "unsubscribe linux-hams" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
-- 
Wilson G. Hein <wilson.hein@verizon.net>

Re: Coupling a scope to transmitter output

[Haines Brown]

Thanks to all for the advice.

It turned out that the unadorned scope probe picked up enough of the
signal to get some sense of its quality, and if need more, the loop at
the end of the coax will do just fine.

Haines Brown KB1GRM