Hi Roger, VLF,
Thanks for the discussion on the subject. There are different point of
views and probably there will never be a common opinion. Hoever it is
good to see the different point of views here.
Please comment to the argumentation shown below:
Well, about the " "feels" wrong", let us stay on the example of the QRO
station TXing legal 750W in OP8 on 160m. Nothing is wrong with it, i
think we can agree here (?). And a lambda/4 vertical antenna is used by
many stations of course. This antenna is able to radiate the 3rd
harmonic as well, anyway it isn't called a "frequency multiplier".
So what may be a typical 3rd harmonic supression in existing amateur
radio equipments? Can someome make a guess?
My guess would be 40...50 dB. When it's 50 dB, then it doesn't
"feel wrong" for us. Anyway there are some mW ERP on the 3rd harmonic,
even if we ignore it.
Everyone of us can check it with his own equipment today.
750W - 50 dB is 7.5 mW. So obviously 7.5 mW on 53m (160/3) is legel as
long as 750W is generated on 160m. This does not feel wrong, right? If
we can agree here, then this is a good first step :-) Do we?
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Next step:
If there are two stations, who are interested to study HF propagation
on 53m, they can do it legally by transmitting on 160m. (same applies
for LF or MF or VHF (e.g. 145 MHz => 435 MHz!) of course). But they
will quickly ask themselves "why do we have to waste so much power on a
frequency that is of no interest for us and cause QRM for other
stations on 160m?". So what can they do? Running the transmitter on 53m
directly is not legal, but QRO is legal on 160m, with a few mW on 53m!
Now, one of them has the idea to add an element which reduces the
fundamental frequency (matching for the fundamantal frequency) so that
the power on 160m becomes lower. Now if the power of the 3rd harmonic
rises, they can reduce the transmitter power, so that the ERP of the
3rd harmonic is still the same as before. Now they have reduced the
"QRM" for other 160m stations but the situation on 53m is still the
same. Just the supression of the 3rd harmonic becomes lower (ratio
3f/1f), let's say it is 10 dB now. This means 40 dB less power
on 160m, i.e. 75mW, and still 7.5mW on 53m. Stations on 160m will be
happy with this decision and nothing has changed on 53m. Does this
feel wrong now?
Your comments please.
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Now a next step for VLF:
Imagine someone is transmitting on 5755 Hz, in combination with a
neutral coupling element, a big ferrite transformer which has no narrow
band matching to the antenna. There is no power limitation on VLF, so
he can generate 10 kW on 5.755 kHz. Would this feel wrong then?
We know 1:9 "baluns" or "magic baluns" for the HF bands which are doing
the same! They are "matching" a high Z antenna to a low Z output of a
transmitter and this is independent of the frequency, at least on HF.
If the 3rd harmonic supression would be 50 dB again, then the power on
17265 Hz is 1 mW! As long as there are no interferences to other
services, this does not feel wrong, or does it? And if so, why?
Comments please! Thus i think the analogy is NOT "leaving your car
across the exit from the local fire station" Or, explain which service
is in analogy to the local fire station here??
Next: The radiation resistance of an amateur antenna is much higher on
17265 Hz than on 5755 Hz, 9x higher! So there is a "gain" of 8 dB for
the 3rd harmonic. Now, would you call a piece of wire a "frequency
multiplier"?? That would feel wrong to me :-)
73, Stefan/DK7FC
Am 03.02.2014 14:47, schrieb Roger Lapthorn:
Like many, I am more than a little concerned about the
intentional use of 3rd harmonic reception of a VLF signal. Although I
cannot really argue with Stefan's logic, somehow it jsut "feels" wrong.
Very happy to hear of Stefan's 5kHz tests however. Good luck.
73s
Roger G3XBM
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