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G8MNY  > TECH     22.11.09 09:51l 143 Lines 6153 Bytes #999 (0) @ WW
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Read: DG9NBR GUEST
Subj: 2M 65W PA with 2x M6774H
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Sent: 091121/2343Z @:GB7CIP.#32.GBR.EU #:43190 [Caterham] $:43190_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To  : TECH@WW

By G8MNY                                (New Nov 09)
(8 Bit ASCII Graphics use code page 437 or 850)
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 150-175MHz! So their
usefulness on 2m was not known, although some 2m kit was reported to use them.

         ÚÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄ¿
Heatsink ³ ³             ³ ³  I tested one on a heatsink with just coax
is ground³o³ M 6 7 7 4 H ³o³  connected & it produced 33W, so all looked good.
         ³ ³             ³ ³
         ÀÄÁÂÄÂÄÄÄÄÄÂÄÄÄÂÁÄÙ  DC1 powers the 1st low current stage,
            ³ ³     ³   ³     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 out of junk bits.
                                    ÚÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄ¿
A PCB (double sided) was cut        ÞÛÝ             ÞÛÝ
to fit over the heat plate &        ÞßÝ M 6 7 7 4 H ÞßÝ
under the screws & connection       ÞÛÝ             ÞÛÝ
wires, to provide connection        ÞÛÝÂÄÂÄÄÄÄÄÂÄÄÄÂÞÛÝ
pads, through links were put        ÞÛÝÛ ÛÞÛÛÛÝÛÞÛÝÛÞÛÝ
in to. An old 900MHz phone PA       ÞÛÝß ßÞÛÛÛÝßÞÛÝßÞÛÝ
casting was used as heatsink        ÞÛÛÛÛÛÛÛÛÛÛÛÛÛÛÛÛÛÝ
& case.

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

                 50êÚÄÄÄÄÄ¿50ê
       ÉÍÍÍÍÍÍÍÍÍÍÍ͵BLOCKÆÍÍÍÍÍÍÍÍÍÍÍÍ»
       º ¬ Wave 75ê ÀÄÄÄÄÄÙ ¬ Wave 75ê º
50ê InĶ                               ÇÄ50ê Out
       º ¬ Wave 75ê ÚÄÄÄÄÄ¿ ¬ Wave 75ê º
       ÈÍÍÍÍÍÍÍÍÍÍÍ͵BLOCKÆÍÍÍÍÍÍÍÍÍÍÍͼ
                 50êÀÄÄÄÄÄÙ50ê

I measured the velocity factor of a good length of my 75ê 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. This meant I needed 22/145 times this length to get the same
@ 2m (46cm for my white foam fill coax).

PA SCHEMATIC
     ÚÄÄÄÄÄÄ¿
     ³RF PTTÃÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
     ³ Sense³   ³          ÚÄÄÄÄÄ¿          ³
     ÀÄÄÄÄÄÂÙÚÄÄÁÄÄ¿ ÉÍÍÍÍ͵BLOCKÆÍÍÍÍÍ» ÚÄÄÁÄÄÄ¿
RIGÍÍÍÍÍÍÍÍÏ͵DRIVE³ º 75ê ÀÄÄÄÄÄÙ 75ê º ³OUTPUTÃ(o AERIAL
   50ê       ³RELAYÃĶ                 ÇÄ´RELAY ³   SO239
  Coax       ÀÄÄÒÄÄÙ º 75ê ÚÄÄÄÄÄ¿ 75ê º ÀÄÄÒÄÄÄÙ
                º    ÈÍÍÍÍ͵BLOCKÆÍÍÍÍͼ    º
                º          ÀÄÄÄÄÄÙ          º
                ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍͼ
                         50ê Rx coax
CIRCUITS
  2x 1N4148
     ÚÄ´>ÃÄÂÄÄÄÄÄÂÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´<ÃÄÄÄÄÄÄÂÄÄÄÄo\oÄÄÄÄÂÄÄo-oÄÄ< +12V
     ³    ===  ³/e  ===30n          //TX     1k   ON     ³  13A
   * ÃÄ´<ÃÄÁÄÄÄ´PNP _³_              LED    _³_          ³     ÚÄ< 0V
  4pF³     1n  ³\                           \_/=ON      12V   _³_
    ===          ÃÄÄÄÄÄÄÂÄÄÂÄ470ÄÂÄÄÄÄ¿      ³  LED     to
1W>ÄÄÁ>Drive  ÚÄÄÁÄÄÄ¿  ³u1³     ³ ÚÄÄÁÄÄ¿  _³_        Blocks
Rig    Relay  ³Output³ === ³    +³ ³Drive³
      Common  ³Relay ³  ³ _³_   ===³Relay³5V
              ÀÄÄÂÄÄÄÙ  ³ /_\ 10u³ ÀÄÄÂÄÄÙ
                 ÃÄÄÄÄÄÄÁÄÄÁÄÄÄÄÄÁÄÄÄÄÙ
                _³_     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 @ low drive power OK). The 2 change over 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 1/100S, with the 470 & 10uF. I checked the drive relay
operated on low RF power & low voltage 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 the VHF RF pickup on the wiring when 65W
is on the contacts.

     For each block
 ÚÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄ¿
 ³ ³                 ³ ³           Power filtering capacitors are
 ³o³   M 6 7 7 4 H   ³o³           applied to each module keeping the
 ³ ³                 ³ ³           leads short for good VHF practice.
 ÀÄÁÂÄÂÄÄÄÄÄÄÄÄÄÂÄÄÄÂÁÄÙ75ê Coax   A ferrite bead is used to feed DC1.
ððððÙ ³ _FB     ³   Àðððððððððððð
  _³_ ÃÄÄÄÂÄÄÄÂÄÁÄ¿ _³_            The 1N4001 is only there in case
      ³ ~ ³  _³_ +ÃÄÄÄÄÄÄÄ+12V     of power rail reversal & 2 of them
   u1=== === /_\ ===               should be able to blow the 13A fuse.
      ³6n8³   ³   ³1000uF
      ÁÄÄÄÁÄÄÄÁÄÄÄÁ¿
           1N4001 _³_

LAYOUT
Fitting in all the coax          ÚÄÄÄÄÄÄÄÄÄÄÄ¿
is the main problem with         ³  [±±±±±]  ÿSO
this design, as my finned        ³  ³_____³[]ÃÙ239
blackened dye cast box was       ³ PNP    O/PÃ Tx LED
is only 18 x 13 x 3cm.           ³[]       =ð³]Fuse
                                 ³Drive      Ã On LED
Heat conductive past is          ³  [±±±±±]  ØÍÍÍÍÍÍÍÍÍÍÍ12V Lead
put under the modules &          ³  ³_____³  ] On Sw
the temperature is quite         ³           ØÍÍÍÍÍÍÍÍÍto Rig
cool for short overs.            ÀÄÄÄÄÄÄÄÄÄÄÄÙ

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

Harmonics from these modules is quite low but added RF low pass filtering can
be used if wanted.

Through/Rx loss was under 1dB with 1m of drive lead. A preamp could be fitted
in the Rx path, but "not powered up" drive could blow it up unless extra
switching is used.


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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