Whooops........
My previous posting of this looked ok when sent but a total mess when it
arrived back from Yahoo.
I'm not saying changing it will make my ramblings any more sensible.....
but perhaps a bit more legible:-)
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In a message dated 06/05/2007 23:31:54 GMT Daylight Time, crabtreejr@...
writes:
Re. implications of the very low radiation resisitance, even at say 1 MHz -
that should be the subject of another posting. I need to give the matter
some
careful thought. I need to go through the maths, and get it right. An off
the cuff comment would not be appropriate.
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I need to give this further consideration too.
Given the relatively low input impedance of the amp I would have expected
that increase of radiation resistance with increasing frequency might have been
more of a limiting factor but suspect that's far too simplistic.
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Re the amplifier gain. If the antenna factor were unity, ie the antenna
output were 1 volt into 50 ohms for a field strength of 1 volt per meter,
then the
required amplifer gain would be 33dB. The actual antenna factor is about
-7dB, ie ca. 0.45, ie the antenna output is ca. 0.45 volts for a field
strength
of 1 volt per meter. The required antenna gain is then +33 - 7 = 26dB.
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That's what I assumed you meant before but, as previously written, it didn't
seem to make sense, with confusion perhaps mainly being due to the use of
"required".
I'd have probably still have written it here "the actual amplifier gain is
then.....26dB", rather than "required" but perhaps that's just me being
pedantic as usual:-)
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I too share your concerns about getting heat out of a transistor when it is
in potting compound. The other question is just how much current does a
2N5109
need before you get little or no improvement in IMD performance.
Conventional wisdom says as much as possible. However in his article
'Wideband Amplifier
Summary', in the November 1979 issue of Ham Radio Magazine, Dr. Ulrich Rohde
suggested that for a 2N5109 that the IMD performance was best at a collector
current of ca. 10mA. He then went on: "A typical CATV transistor, such as
the
2N5109, has a flat curve of intermodulation distortion between 20 and 80mA."
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I've always tended to follow the conventional wisdom, as have many major
manufacturers, but last year I set out to make some comparative measurements on
medium power devices, buying small batches of 2N3553s, 2N3866s, and 2N5109s
from the same UK supplier.
I was foolish enough to assume I was buying NOS devices but suspected when
they arrived that all had been recently branded from the same generic stock.
It took a while but I eventually got confirmation from the supplier that I was
right.
This practice has been going on for years, certainly it was common in the
60s, with both valves/tubes and transistors, among many of the "own brand", but
non manufacturing, companies in the UK so I should have known better and
asked first.
At least I didn't waste my time looking for non existent differences:-)
I will pursue this eventually, once my test gear comes out of storage, as I
do have enough genuine devices now to make it worth the effort but
diminishing stocks may make that more of an interesting exercise than of much practical
value.
Given the number of loops used mainly at LF I also suspect there's many
devices with lower ft that would be adequate in such circumstances but have been
ignored in the past.
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Dallas Lankford has found pretty much the same using the MRF581A. You can
run
the MRF581A at 25mA collector current without a heatsink. Given that
MicroSemi/APT (ie non-Motorola) devices are cheaper than the 2N5109 on this
side of
the Pond, it would be my first choice of device.
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I haven't seen those results but am certainly aware of genuine NOS Motorola
2N5109s hitting "premium" prices.
I've managed to stock up with what should be enough for my future needs, and
at what I consider a good price, certainly significantly better than the low
quantity prices being asked here, but still with an "ouch" factor buying in
quantity.
It'll be rather sad then if I eventually demonstrate that a 10 cent TO92
device could be just as good:-)
regards
Nigel
GM8PZR
