In preparation for the January ARRL VHF contest, W7QQ had two of the
Transverters Store’s 222MHz transverters in his shack. These are the
inexpensive, $125 fully assembled and packaged, less as boards, transverters
available on eBay or from the manufacturer.
As the reports of the transmit quality of these units are varied, some say they
are OK, others say that the transmit quality is horrible, we decided to look at
the transmit properties of the transverters. A short description of the results
of those measurements are on the NM VHF Society Page:
< http://www.nmvhf.org/technical.html >
We found that one of the units had acceptable IMD performance and one unit was
poor. This would explain the mixed reports. Upon further investigation, we
found that the unit with poorer IMD performance had a lower final gate bias,
2.8V, than the good unit had at 3.1V. When the gate bias on the unit with the
poor performance was increased to 3.1V, its performance substantially improved.
The paper shows how the adjustment, which is straight forward, was done.
Here are the IMD test results on the two units:
Transverter 3rd order IMD Performance
Power Out (watts) Unit One Unit Two (after bias adjustment)
7.5 -12 dB -10 db
5 -20 dB -18dB
2.5 -28 dB -24 dB
Two of the locals ran theses two units in the January ARRL contest and their
signals sounded fine on the air.
If you have one of these units, or are going to purchase one make sure that the
final gate bias is set to 3.1Vdc or greater prior to putting it on the air. If
you have a spectrum analyzer, you can look at the IMD while changing the final
gate bias, if not, set it to 3.1Vdc according to the instructions in the paper.
If it is already 3.1Vdc or greater, you are probably OK. There is probably room
for further improvement as the internal settings for the drive potentiometer
and the variable inductors on the two units differed. When we get the
opportunity, we will look at this closer.
If the power is kept below 5W, these should be acceptable on the air. If you
are driving an amplifier, then you should probably keep the drive to 2.5W or
less.
The transverter boards for all of the VHF/UHF bands are similar, but we did not
make measurements on transverters for other bands. I would expect the results
to be similar, but not identical. If you have one of these units and measure
the gate bias, I would like to know what it was before and after adjusting it.
- Duffey KK6MC
--
KK6MC
James Duffey
Cedar Crest NM
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