Some notes on my 70 MHz transverter:
- radio amateurs in Finland are waiting patiently for access to the "new" four meter band, hopefully already this year (2009)
- a very good general source of 70 MHz information with lots of good links: the Four Meter Website
- the website of Bo, OZ2M describes the "OZ 70 MHz transverter kit" by OZ2M & OZ7IS
- having already built and used the original OE9PMJ 50 MHz transverter the OZ transverter kit is based on, I decided to go for the full kit, including the PA module kti (by OZ2m)
- some notes, comments and ideas that may be of benefit to others:
- pretinned the PA PCB with solder paste and a hot air
blower - much nicer to solder to and keeps the PCB nice and shiny after all that fingering
- I use a large heatsink to allow long duration keying - the PA is broadband and high gain, but also high dissipation
- tried to orientate the PA so that it is topmost in the module and
thermally far away from the transverter in order to avoid thermal drift
in the 42 MHz XTAL LO. The main influence now is only thermal
radiation, but the modules are still at opposite ends of the casing. No
drift has been evident so far in the test lab, so hopefully no problems
when we finally get on the air ;-)
- replaced the module BNC's with SMA's to save space (smaller connectors, thinner PTFE coax, personal preference).
- I decided to go for the split IF operation mode and to use BNC connectors for connecting to the exciter with 6 mm/terminal screws for DC power and PTT control (note: no ALC at this stage !)
- replaced the two PA trim pots with new ones as the supplied
ones were badly oxidized (I think) - it was simply impossible to solder
them. Could the contacts somehow have become tarnished due to some
vinyl or silicone material in the vicinity ?
- replaced three obviously wrong resistors, OZ2M sent a mail circular on this later - I did not realize this may have effected
several kits, rather thought it was only my shipment - excellent service from Bo !
- so with all my amateur and professional experience and confidence
(particularly strong in this department) I just went and cracked a
Neosid ferrite slug (again). I have a specially made fibreglass tuning
tool that I could not find in all the excitement so I reverted to the
no-no watchmakers screwdriver (again) - snap. OK, so I removed the
Neosid coil from the PCB, recalled how difficult i.e. impossible it is
to purchase a single Neosid coil here in Finland and decided to replace
the coil and can with an axial inductor. Fortunately the coil was not
in a critical place so I got away with this quick n' dirty trick
(again). Just WHEN am I gonna learn ???
- I measured the transverter 2. harmonic
and it is now >61 dB down (from a 40 W carrier) without the separate
LPF, better of course at a more sensible output power :-)
- here is a photo of the transverter interior: the RF directional coupler for VWSR measurent and the DC current measuring shunt have not been installed yet
- the additional shielding of the 42 MHz crystal oscillator is
absolutely necessary to fulfill the 60 dB requirement for spurious
emissions in our Finnish regulations (withought the shielding the 42
and 84 MHz components are only about 50 dB down from the main signal)
- and a second photo of the transverter with the transverter PCB and PA unit RF shielding covers removed
(SMA T/R switch in the middle - this microwave switch is serious
overkill, but it came from my junkbox and after all, I am a microwaver...)
- one additional modification was to add a fourth feedthrough
capacitor to the transverter module: this is to give a true RX ON LED
indication on the front panel of the transverter; other LED's indicate
general power ON, TX ON and Fuse Blown
- the front panel multifunction analogue meter and selection switch
allow quick measurement of input voltage, input current via a 22 mohm
current shunt, and also forward RF power and reflected RF power
- I did a lot of work with my Yaesu FT-897D with the objective of
using it for driving this and other transverters, finally deciding to
use a Kenwood TS430 instead. This radio uses the driver stage only and
disables the PA when in transverter mode - this mode is activated via a
mechanical switch physically on the transverter DIN (!) connector on
the rear panel and grounding one of the pins. Perfect solution.
- To briefly describe what the Yaesu radio does: you can select
between two transverter modes, each of which you can program the
display the correct on-air frequency respectively (i.e 70.000,00 MHz or
e.g 5760.000.00 MHz on the display, taking into account transverter LO
and LO error). This is a very nice feature. The transverter operation
is very poorly documented by Yaesu. The sad thing (my personal opinion)
is that the output power is reduced to 5 W via the radio PA ALC (in
practise my radio output was somewhere between 3 and 9 W depending on
the transverting band being used). Incidentally, another nice feature
is that there is no limitation as to what your transverting IF is).
Strangely, CW, FM etc. where pretty OK, but on SSB it was possible to
capture initialization peaks of almost full power before the ALC
settled. This would call for a couple of protection diodes or maybe
even a PIN diode arrangement for protecting the transverter. I used a
very fast analogue spectrum analyser and PTT triggering to analyse the
ALC behaviour.
- I also did some experimenting with the predriver in the radio.
There are two switchable bridges in the predriver that allows bias
current measurement with a DVM when setting the switch to break the
consecutive FET amps Drain lines. I inserted suitable resistors (about
1k) in the Drain lines (a really dirty trick, but I was desperate and
anyway - this is amateur radio !) amazingly still leaving the stages
operating in a adequately linear fashion. This allowed me to get the
output power level dropped the Watt level with the PA still in the
chain. I could then use the radio normally via the antenna connection,
but I really hated taking a couple of Watts from a 100 W amplifier. and
without ALC action any more too.
- The next thing I did was to locate a pair of those (Hirose ?)
ubiquitious Japanese PCB RF coaxial connectors. These have been de
facto industry standards in Japanese radios for a couple of decades.
The FT-897D PA drive and receiver input use these connectors. My idea
was to use the Battery A/B selection switch to activate a miniature RF
relay to bypass the radio T/R switch in receive and the PA in transmit.
This actually worked just great and would have allowed the radio to be
returned to original shape if desired. The biggest problem was to
actually find the space for two RF connectors on the side or rear of
the radio without having to drill any holes. I even tried to fit two
SMB connectors, but now way. Until I completely reform my mindset and
decide to drill two SMB-sized holes in the side of my Yaesu, I am going
to stick to my TS430...
- the OZ transverter kit in itself is recommendable; the RF
performance is very good, especially with the PA module. Some
experience is needed in assembly and it is always wise to check your
handywork with some test equipment at your local club or like here in
Finland at the annual VHF meeting test lab
-
here is a photo of the transverter
in use (80k) with the old Kenwood TS-430S as exciter
- latest addition was adding a dual diode negative detector (13k) to provide about -7V DC for 25 W PEP at the
transverter rear (90k). This is for controlling the TS-430 external ALC input
and it works in all modes
- here is a drawing of the ultimate IF interface (18k) for attaching transverters to the TS-430S and it should work for the TS-940S too (28.12.2009)
- there are many different interface circuits for connecting your radio and your PC for Machine Generated Modes, here is mine (18k) for the TS-430S (I use it for e.g. 70 MHz WSJT and HF RFSM8000) (30.12.2009)
- Finnish hams gained access to the band on November 5th 2009 and during
the first few days I already made several nice Tropo and Meteor Scatter
contacts 25 - 1400 km with this 25 W transverter kit using my broadband
7 element Log Periodic (59k)
25 - 80 MHz. This antenna fits in a medium sized kitbag along with 6 m
of mast and tethering, spikes, feedline etc. Perfect for portable operation ;-)
Created 31.08.2009 and latest update 13.01.2019/OH2AUE
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