Message 26639 in TECH.WW
From: G8MNY
Date: Sun, 26 Jul 26 15:48:00 Z
Newsgroups: TECH.WW
Subject: 2M 65W PA with 2x M6774H
Message-ID: <341_GB7CIP>
Path: N2MH4|VE3KPG|GB7CIP
R:260726/1551Z 34392@N2MH4.#SENC.NC.USA.NOAM LinBPQ6.0.24
R:260726/1550Z 27323@VE3KPG.#ECON.ON.CAN.NOAM BPQ6.0.25
R:260726/1548Z @:GB7CIP.#32.GBR.EURO #:341 [Caterham Surrey GBR] $:341_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Mar 23)
(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 unknown, although some 2m kit was
reported to use them.
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.
& case.
2 PA MATCHING PRINCIPLE
Assuming the PA blocks are identical in power level, gains, and 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 50ohm input & output, 4x 1/4 wave 75ohm coaxes
are needed to make the connections. Each 75ohm 1/4 wave transforms the 50ohm to
approx 100ohm and 2 of these paralleled up give 50ohm again.
I measured the velocity factor of a good length of the 75ohm coax (0.6 - 0.90
the speed of light depending on the type of coax) with an open circuit teed on
a signal generator and scope/(Detector) and 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
50ohm Rx coax
A tiny amount of drive RF current is fed through the 4pF to voltage doubler to
turn on the PNP (* I tried different values, and this worked with my PNP and
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 and 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 and 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
u1=== === /_\ === should be able to blow the 13A fuse.
LAYOUT
(Fan might be needed for heavy use)
RESULT
With 13.8V and just over 1W drive output was a good 68W, 13A drawn and 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 pi low pass filter @ the
aerial socket.
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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