[hpsdr] Pentalop PA Consideration

[Joe Martin K5SO]

Hi Mac,

Since the T/R relay you're presently using doesn't provide sufficient  
isolation to keep Mercury from overloading, one easy solution would be  
to simply put a second "T/R" relay between the present one and the  
Mercury input connector (i.e., in series with the Rx line out of the  
present T/R relay to Mercury) to provide additional isolation for  
Mercury.  A small coaxial relay would work well for that since coaxial  
relays typically have reasonable isolation between the ports.

If you elect to install a second relay, you might as well have the  
second relay terminate the Mercury input with a 50-ohm resistor when  
the system is in Tx to keep the Mercury input from simply being open  
(unterminated) when your system is in Tx mode.

I'm currently modifying my IC7800-to-linear amplifier setup to work  
with the HPSDR boards and I'm sure not gonna give up my present Tx  
power to do it, even if I have to install a second T/R relay to  
achieve sufficient isolation.   I don't know if it will be necessary  
or not just yet (I don't really think it will be necessary but I'll  
find out pretty soon!).  Our earlier mods to a Kenwood TS-940S running  
at 100W output into a linear amp did not require a second T/R switch;  
Mercury was happy.

This is simply one alternative for you to consider.

73,  Joe K5SO



On Jan 14, 2009, at 12:55 PM, Mac wrote:

> ***** High Performance Software Defined Radio Discussion List *****
>
> I also have T/R relay or antenna changeover issue with Mercury. I'm  
> using
> just the Mercury /Ozy combo. I have a
> transmitter running about 250 watts carrier - continuous AM. Using  
> common
> antenna so antenna removed from Mercury
> and switched to transmitter via PPT on mike. That didn't work as  
> Mercury
> front end overloads. Next I tried to short Mercury ant coax going to  
> T/R
> relay (about 2 ft long shorted coax left on Mercury) as well as move  
> ant
> back to transmit via relay. That did'nt work as S meter pinned and  
> all kinds
> of scary stuff happens to console display.
> Now I'm thinking that the input connection, pin 3 on K1 relay to pin  
> 2 on
> FL1 should be broken (interrupted) on transmit. Too bad there wasn't a
> jumper there - heh heh. (see HPSDR mercury schematic: Front End).
> But I guess I won't try that, and just accept the fact that if you  
> use an
> independant transmitter it must be low power and driven by penny under
> software control.
> Mac W6FCS
>
>
>
> ---- Original Message -----
> From: "a.groff" <k0vm at mchsi.com>
> To: <hpsdr at lists.hpsdr.org>
> Sent: Wednesday, January 14, 2009 7:55 AM
> Subject: [hpsdr] Pentalop PA Consideration
>
>
>> ***** High Performance Software Defined Radio Discussion List *****
>>
>> I am in the process of patching Mercury and Pentalope into the 100  
>> w PA
>> of a 1970's transceiver.  In the process of setting up the driver  
>> levers
>> in PowerSDR>Setup>PASetting , I observed that, on the higher bands, I
>> started to to see the Mercury ADC OVERLOAD message...
>>
>> I can measure -15dbm on 80M up to +10dbm on 10M coming from the PA
>> antenna change over relay back to the input of Mercury.
>> This may be typical (??) isolation of a antenna change over relay  
>> and it
>> may still be in the safe (??) range for the front end of Mercury,  
>> but PA
>> designers (Thor ? ) and integrators should pay at least a bit of
>> attention to this detail.
>>
>> AL, K0VM
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