[1000mp] Mark-V and noise phasing filter (MFJ-1026, etc.)

[Tod]

George:

I was asked this question about four days ago by a friend here in Minnesota.
My response is below.

Tod KØTO 
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> 
> The manual says the MFJ-1026 handles 100 watts.  I have an FT-1000D 
> (200 watts).  Is the 100 watt rating conservative or should I consider 
> switching around it some other way?
> 73 Steve K0SR

Steve:

The schematic of the MFJ-1026 is on my web site www.k0to.us => Amateur Radio
=> Noise Notes. 

The transmitted signal goes via an NC contact (closed when there is no power
to the unit or when transmitting). My MFJ-1026 is in Idaho so I  can't look
at the relay at the moment. I would expect that you are going to use the
system on 160 through perhaps 40m and that the antenna you use on those
bands has less than 2:1 VSWR. The voltage from 200 watts is only about 100
volts across a 50 ohm resistive load and the current will be about 2 amps.
My personal opinion based upon my recall of the relay size is that you need
not be concerned about the 100 watt limitation for transmitting.

In the event that you zap the relay (which would be during the non-actuated
state) you could simply replace it with a relay from DigiKey that can handle
5 amps. Voltage is not the issue since the contacts are closed  on transmit
and the specifications for voltage between contacts and ground is much
greater than across the contacts. Such a replacement relay might cost as
much as $3.00 (plus shipping, etc.)

The one thing you do NOT want to do is to hot switch the relay --TR-Log has
provision to avoid this and I know the FT1000MP has menu settings that help
avoid such a problem. The MFJ-1026 has a TR delay that holds the relay
closed briefly after the transmit line has changed state. All you need to do
is to delay the start of RF keying for about 10-20 milliseconds after the TR
line goes low. (The line going low starts the relay switching in the
MFJ-1026).

In my MFJ-1026  I have modified the input circuit from the 'sense' antenna.
I use an FET to close a relay when the TR line goes low (Transmitting is
initiated). This added relay grounds the 'sense' antenna input and
guarantees that RF pickup on the sense antenna will not reach voltage levels
that would damage the MFJ-1026 phase shifting and nulling circuits. MFJ has
a small pilot lamp in that circuit and if your sense antenna is worth using
it will provide enough signal when running a KW to light the lamp for one or
two milliseconds before the filament vaporizes. At 100 watts the lamp will
burn very, very brightly The lamps can sometimes be found at RadioShack for
replacements, but that is not a solution -- one needs to ground that input
and MFJ should have done it. It does take one extra FET, a couple of
resistors and a cap and a relay so that might have driven the price up by
$20-30 if they did it. 

Tod, KØTO