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G8MNY  > TECH     12.07.18 10:03l 156 Lines 6947 Bytes #999 (0) @ WW
BID : 28761_GB7CIP
Subj: 2M 65W PA with 2x M6774H
Sent: 180712/0619Z @:GB7CIP.#32.GBR.EURO #:28761 [Caterham Surrey GBR]

By G8MNY                            (Updated Oct 14)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

My local club had a source of these Class C VHF PA blocks. The specification
for them is a max of 500mW drive for 33W out on 12.5V, but the H type is
150-175MHz! So there usefulness on 2m was not known, although some 2m kit was
reported to use them.

Heatsink                  I tested one on a heatsink with just coax
is groundo M 6 7 7 4 H o  connected & it produced 33W, so all looked good.
           DC1 powers the 1st low current stage,
                          DC2 @ 8A Max for full output.
           in DC1  DC2  out

A 1.5W handheld can do more drive than that, so I used 2 of them to make a 2m
PA with a good power level out of junk bits.

A PCB (double sided) was cut                     
to fit over the heat plate &         M 6 7 7 4 H 
under the screws & connection                    
wires, to provide connection        
pads, through links were put         
in to. An old 900MHz phone PA        
casting was used as heatsink        
& case.

Assuming the PA blocks are identical in power level, gains, & signal phases
(time delay). Then there are a couple of ways to connect them, I have used the
simplest using just coax.

To match the 2 PA blocks to the 50 input & output, 4x 1/4 wave 75 coaxes are
needed to make the connections. Each 75 1/4 wave transforms the 50 to approx
100 & 2 of these paralleled up give 50 again.

         Wave 75   Wave 75 
50 InĶ                               50 Out
         Wave 75 Ŀ  Wave 75 

I measured the velocity factor of a good length of the 75 coax (0.6 - 0.90 the
speed of light depending on the type of coax) with an open circuit teed on a
signal generator & scope/(Detector) & adjusted the frequency to give a null
which was 22MHz for my length. This meant I needed 22/145 times this length to
get the same @ 2m (46cm for my white foam filled coax).

     RF PTTĿ
      Sense             Ŀ          
     Ŀ ͵BLOCKͻ Ŀ
   50       RELAYĶ                 ĴRELAY    SO239
  Coax         75 Ŀ 75  
                         50 Rx coax
CIRCUITS                                                  >To Handheld Rig
  2x 1N4148                        100Ŀ               
     Ĵ>Ĵ<o\oo-o< +12V
          ===  /e  ===30n           //TX     1k   ON       13A   Car lighter
   * Ĵ<ĴPNP __               LED    __               < 0V
  4pF      1n  \                            \_/=ON      12V   __
    ===           470Ŀ        LED     to
1W>>Drive  Ŀ   u1       Ŀ  __        Blocks
Rig    Relay  Output  ===     +  Drive
      Common  Relay     __   === Relay5V
              12V /_\ 10u  
                 __     1N4148

A tiny amount of drive RF current is fed through the 4pF to voltage doubler to
turn on the PNP (* I tried different values, & this worked with my PNP & relay
load @ 100mW low drive power). The 2 changeover relays are both plastic
encapsulated ones, the drive one is a smaller 5V DIL type, they are both held
in place with a local earth tag soldered to the coil connection & the coaxes
ground there too.

The RF detector & relay drive PNP transistor are mounted UGLY fashion around
the drive relay.

To stop the output relay operating when RF power is present, I delayed the
driver relay by about 1/100 second, with the 470 & 10uF. I checked the drive
relay operated on low RF power 100mW & low voltage of 11V OK, even with the
series Tx LED!

The diode across the relays stop back EMF from damaging the PNP, the u1 cap
across the output relay coil reduced VHF RF pickup on the wiring when 65W is on
the contacts!

    For each PA block
                               Power filtering capacitors are
 o   M 6 7 7 4 H   o           applied to each module keeping the
                               leads short for good VHF practice.
 75 Coax   A ferrite bead (FB) is used to feed DC1.
  __ Ŀ __            The 1N4001 is only there in case
       ~   __ ++12V     of power rail reversal & 2 of them
   u1=== === /_\ ===               should be able to blow the 13A fuse.
      6n8      1000uF
           1N4001 __

Fitting in all the coax          Ŀ
is the main problem with           []  ÿSO
this design, as my finned          _____[]239
blackened dye cast box was        PNP    O/P Tx LED
is only 18 x 13 x 3cm.           []       =]Fuse
                                 Drive       On LED
Heat conductive paste is           []  12V Lead
put under the modules &            _____  ] On Sw
the temperature remains                     to Rig
quite cool for short overs.      
(Fan might be needed for heavy use)

With 13.8V & just over 1W drive output was a good 68W, 13A drawn & with 200mW
about 50W needing only 10A.

Harmonics from these modules are quite low, I measured them at >-30dBc (near
the limit of my testgear @ VHF), but I did add a RF pie low pass filter @ the
aerial socket. 

Tx relay()))o)SO239  The 23pF caps are 2 Tx grade ceramic
       23pF===  L ===23pF       the L is 2 turns, dia & wire gauge
              select for 1:1 SWR match on test.

If the modules are misterminated of course anything can happen &
as with many PAs loads of sprogies might result!

Through/Rx loss was 0.5dB with 1m of driver coax & BNC in circuit. A switched
Mutek well filtered preamp was fitted in the Rx path, so the PA not powered up
the drive power was available, this overcomes any Rx insensivity as well as
proving good out of band Rx protection.

See my tech buls. "RF Modules List", "Scope RF Trick", "Coax Traps", "Non relay
instant RF Switching", "DC/RF Sensing PA Switching", & "2m Cavity Filter".

Why don't U send an interesting bul?

73 De John, G8MNY @ GB7CIP

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