Helge wrote:
Ian, I guess you know this already, so it's for information.
"only -19dB and quite irresponsible by implying that level of IMD is fit to
be put on the air".
I may agree, but it depends.
You can not only look at the 3rd.IMD product but should measure how the IMD
products fall off.
There is a big difference in how products fall off. If it falls off quickly
it is of no big concern.
An excample is the TS-2000 tranceiver that has got bad reputation (?) in the
UK based on the bad 3rd. and 5th products presented in Radcom.
However looking at the IMD spectrum in QST, it falls of quickly and on air
it sounds acceptable to me.
Using a speech processor, you will typically measure bad 3rd and 5th, but it
sounds OK off frequency.
I'd agree with that - higher-order IMD covers much more of the spectrum.
But the fact remains that a single 4CX250B, driven to the 380W PEP
output level that Eimac recommends, is a potent source of both low and
high order IMD. The best exciter and power supplies in the world won't
change that - it's the tube that is responsible... or rather, the greed
for too much output power.
About the 4CX250B, that I have used in a W2GN Arcos type 144MHz amp and
replaced with 4CX400A, the latter one
is clearly better off frequency. It can produce more power.
If it has a bigger HV supply - 2.5kV on-load at 400mA per tube. (A
4CX250B will often go well at the higher voltage, but not at the higher
current too.)
If the operator is greedy for more power the 400 is also capable for a lot
of IMD in SSB.
4C250B can sound terrible locally, using a typical tranceiver that reduces
power from abt 50W to 5W with ALC curcuit.
A typical ALC circuit will easily give a lot of overshoot and overdrive the
tube at speech transients.
This is most often the reason to the splatter you hear from local stations
using small tetrodes.
A solution to this is to check the modulation envelope with a oscilloscope.
If it overshoots, compared to
whistling a tone, use a power attenuator between the tranceiver and amp.
Adjust anttenuator so that transmitting full power will give full power from
the amp.
Yes, all agreed... there are so many ways to get something wrong, and so
few to get everything right.
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
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