[Yaesu] FT102 Tuning

[pegasus@mho.net]

6/24/03 2:24:59 AM, "Geoff McCombe" <gmccombe@onthenet.com.au> wrote:

my first HF set, so if the question sounds silly I am sorry.
>
>I have tried reducing the drive to get the power down, is this right?
>
>Then, once swr is minimum and I increase the drive to the original tuned
>position, the transceiver seems to sometimes, but not always, need retuning
>from scratch.  Is this normal?

Hello Geoff...
   It should be noted that the correct position of the plate and load controls WILL change after you change the settings on the antenna tuner.
It is entirely normal to need to retune the transmitter after setting the antenna tuner.  The controls on the transmitter are matching the impedence of the radio's 
output stage to the impedence of the load (antenna).  So if you change the load by operating the antenna tuner, then the optimum settings in the radio will 
change.  That is completely normal.
  I should point out, however, that the FT-102 is MUCH less dependent on an antenna tuner than modern solid state radios.  The output circuit in the 102 can 
handle impedences on most bands from down around 15 ohms to around 150-200 ohms.  Unless your un-tuned SWR is way up over 3 or 4:1 a tuner is almost 
a waste of time with that radio.  
  Here is what I do... (with the loading control at ZERO) I set the drive level correctly, then quickly dip the plate current with the "plate" tune control.  That will 
give about 100 watts of output if the load is close to 50 ohms.  This power level is low enough to allow for tweaking an antenna tuner.  But, if you have to to a 
lot of "hunting" around in the tuner I would reduce the output using the drive control as you suggested.  The reason for this is that if you make some large 
changes in the antenna tuner you could look back at the radio and see the plate current going through the roof!  Reducing the drive to cut the output down 
to no more than, say, 50 watts would prevent this from happening.   THEN, once you have the antenna tuner set, set the drive level correctly, then 
incrementally increase LOAD whilst dipping the PLATE until the proper plate current is reading at the dip.   
   You mentioned you "tune the radio for optimum output"....  I sincerely hope you're not tuning for maximum output!  This radio is capbable of well over 200 
watts output if tuned that way.  But your signal will be considerably dirtier and you'll overheat the output tubes.   The FT-102 had the cleanest transmitter on 
the market with 3rd order products down around -40db... nothing was close and it is matched only by a few of the most expensive solid state radios sold 
today.  This number is good with the radio tuned for around 150 to 160 watts output.  That corresponds to the plate dipped at the level specified in the book.  
   As to whether or not you need an antenna tuner on a given band.... As long as you get a nice clean dip at the desired power level within the range of the 
controls then you're fine.  The position of the plate and load controls may end up different from what you're used to.  The stensiled markings for the bands are 
assuming a 50 ohm load, but they could be quite different at other impedences.  The LOAD control will probably vary the most.   I used the FT-102 for years 
with a single wire antenna connected to the back of the radio.  I had an RF Ammeter and a series capacitor in the wire.  I would tune the capacitor for highest 
reading on the ammeter, then tune the radio for proper output.  Lord only knows what the impedence was... I didn't care as long as the radio loaded correctly.  
Measuring SWR under those conditions is impossible but there was absolutely no need to.
Good luck!
73,
Dennis
N�SP