IW8PGT

[Mendicino(CS)-Italy]

Login: GUEST

G8MNY > TECH 06.10.14 10:35l 147 Lines 6439 Bytes #999 (0) @ WW BID : 11942_GB7CIP Read: GUEST Subj: 2M 65W PA with 2x M6774H Path: IW8PGT<IZ3LSV<I0OJJ<GB7CIP Sent: 141006/0827Z @:GB7CIP.#32.GBR.EURO #:11942 [Caterham Surrey GBR] From: G8MNY@GB7CIP.#32.GBR.EURO To : TECH@WW By G8MNY (Updated Dec 09) (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 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 ÉÍÍÍÍÍÍÍÍÍÍÍÍµBLOCKÆÍÍÍÍÍÍÍÍÍÍÍÍ» º ¬ Wave 75ê ÀÄÄÄÄÄÙ ¬ Wave 75ê º 50ê InÄ¶ ÇÄ50ê Out º ¬ Wave 75ê ÚÄÄÄÄÄ¿ ¬ Wave 75ê º ÈÍÍÍÍÍÍÍÍÍÍÍÍµBLOCKÆÍÍÍÍÍÍÍÍÍÍÍÍ¼ 50êÀÄÄÄÄÄÙ50ê I measured the velocity factor of a good length of the 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 for my length. This meant I needed 22/145 times this length to get the same @ 2m (46cm for my white foam filled coax). PA SCHEMATIC ÚÄÄÄÄÄÄ¿ ³RF PTTÃÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ ³ Sense³ ³ ÚÄÄÄÄÄ¿ ³ ÀÄÄÄÄÄÂÙÚÄÄÁÄÄ¿ ÉÍÍÍÍÍµBLOCKÆÍÍÍÍÍ» ÚÄÄÁÄÄÄ¿ RIGÍÍÍÍÍÍÍÍÏÍµDRIVE³ º 75ê ÀÄÄÄÄÄÙ 75ê º ³OUTPUTÃ(o AERIAL 50ê ³RELAYÃÄ¶ ÇÄ´RELAY ³ SO239 Coax ÀÄÄÒÄÄÙ º 75ê ÚÄÄÄÄÄ¿ 75ê º ÀÄÄÒÄÄÄÙ º ÈÍÍÍÍÍµBLOCKÆÍÍÍÍÍ¼ º º ÀÄÄÄÄÄÙ º ÈÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ¼ 50ê Rx coax CIRCUITS 2x 1N4148 ÚÄ´>ÃÄÄÂÄÄÄÄÄÂÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´<ÃÄÄÄÄÄÄÂÄÄÄÄo\oÄÄÄÄÂÄÄo-oÄÄ< +12V ³ === ³/e ===30n //TX 1k ON ³ 13A * ÃÄ´<ÃÄÄÁÄÄÄ´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 1/100 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 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 [±±±±±] Ã¿SO 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 remains ³ ØÍÍÍÍÍÍÍÍÍto Rig quite 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 only 10A. Harmonics from these modules are quite low, I measured them at >-30dBc (near the limit of my testgear @ VHF), but an added RF low pass filtering can be used if wanted. If the modules are misterminated of course amything can happen & as with many PAs loads of sprogies could result! Through/Rx loss was 0.5dB with 1m of driver lead & BNC adaptor in circuit. 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
Read previous mail | Read next mail

11.05.2024 06:32:13l

Go back Go up